Author name code: white-stephen ADS astronomy entries on 2022-09-14 =author:"White, S.M." OR =author:"White, Stephen M." ------------------------------------------------------------------------ Title: A Genetic Algorithm to Model Solar Radio Active Regions From 3D Magnetic Field Extrapolations Authors: de Oliveira e Silva, Alexandre José; Selhorst, Caius Lucius; Costa, Joaquim E. R.; Simões, Paulo J. A.; Giménez de Castro, Carlos Guillermo; Wedemeyer, Sven; White, Stephen M.; Brajša, Roman; Valio, Adriana Bibcode: 2022FrASS...9.1118D Altcode: 2022arXiv220503385S; 2022arXiv220503385J In recent decades our understanding of solar active regions (ARs) has improved substantially due to observations made with better angular resolution and wider spectral coverage. While prior AR observations have shown that these structures were always brighter than the quiet Sun at centimeter wavelengths, recent observations at millimeter and submillimeter wavelengths have shown ARs with well defined dark umbrae. Given this new information, it is now necessary to update our understanding and models of the solar atmosphere in active regions. In this work, we present a data-constrained model of the AR solar atmosphere, in which we use brightness temperature measurements of NOAA 12470 at three radio frequencies: 17, 100 and 230 GHz. The observations at 17 GHz were made by the Nobeyama Radioheliograph (NoRH), while the observations at 100 and 230 GHz were obtained by the Atacama Large Millimeter/submillimeter Array (ALMA). Based on our model, which assumes that the radio emission originates from thermal free-free and gyroresonance processes, we calculate radio brightness temperature maps that can be compared with the observations. The magnetic field at distinct atmospheric heights was determined in our modelling process by force-free field extrapolation using photospheric magnetograms taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). In order to determine the best plasma temperature and density height profiles necessary to match the observations, the model uses a genetic algorithm that modifies a standard quiet Sun atmospheric model. Our results show that the height of the transition region (TR) of the modelled atmosphere varies with the type of region being modelled: for umbrae the TR is located at 1080 ± 20 km above the solar surface; for penumbrae, the TR is located at 1800 ± 50 km; and for bright regions outside sunspots, the TR is located at 2000 ± 100 km. With these results, we find good agreement with the observed AR brightness temperature maps. Our modelled AR can be used to estimate the emission at frequencies without observational coverage. Title: Heating of the solar chromosphere through current dissipation Authors: da Silva Santos, J. M.; Danilovic, S.; Leenaarts, J.; de la Cruz Rodríguez, J.; Zhu, X.; White, S. M.; Vissers, G. J. M.; Rempel, M. Bibcode: 2022A&A...661A..59D Altcode: 2022arXiv220203955D Context. The solar chromosphere is heated to temperatures higher than predicted by radiative equilibrium. This excess heating is greater in active regions where the magnetic field is stronger.
Aims: We aim to investigate the magnetic topology associated with an area of enhanced millimeter (mm) brightness temperatures in a solar active region mapped by the Atacama Large Millimeter/submillimeter Array (ALMA) using spectropolarimetric co-observations with the 1-m Swedish Solar Telescope (SST).
Methods: We used Milne-Eddington inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions, and a magnetohydrostatic extrapolation to obtain constraints on the three-dimensional (3D) stratification of temperature, magnetic field, and radiative energy losses. We compared the observations to a snapshot of a magnetohydrodynamics simulation and investigate the formation of the thermal continuum at 3 mm using contribution functions.
Results: We find enhanced heating rates in the upper chromosphere of up to ∼5 kW m−2, where small-scale emerging loops interact with the overlying magnetic canopy leading to current sheets as shown by the magnetic field extrapolation. Our estimates are about a factor of two higher than canonical values, but they are limited by the ALMA spatial resolution (∼1.2″). Band 3 brightness temperatures reach about ∼104 K in the region, and the transverse magnetic field strength inferred from the non-LTE inversions is on the order of ∼500 G in the chromosphere.
Conclusions: We are able to quantitatively reproduce many of the observed features including the integrated radiative losses in our numerical simulation. We conclude that the heating is caused by dissipation in current sheets. However, the simulation shows a complex stratification in the flux emergence region where distinct layers may contribute significantly to the emission in the mm continuum.

The movie is available at https://www.aanda.org Title: Subarcsecond Imaging of a Solar Active Region Filament With ALMA and IRIS Authors: da Silva Santos, J. M.; White, S. M.; Reardon, K.; Cauzzi, G.; Gunár, S.; Heinzel, P.; Leenaarts, J. Bibcode: 2022FrASS...9.8115D Altcode: 2022arXiv220413178D Quiescent filaments appear as absorption features on the solar disk when observed in chromospheric lines and at continuum wavelengths in the millimeter (mm) range. Active region (AR) filaments are their small-scale, low-altitude analogues, but they could not be resolved in previous mm observations. This spectral diagnostic can provide insight into the details of the formation and physical properties of their fine threads, which are still not fully understood. Here, we shed light on the thermal structure of an AR filament using high-resolution brightness temperature (Tb) maps taken with ALMA Band 6 complemented by simultaneous IRIS near-UV spectra, Hinode/SOT photospheric magnetograms, and SDO/AIA extreme-UV images. Some of the dark threads visible in the AIA 304 Å passband and in the core of Mg ii resonance lines have dark (Tb < 5,000 K) counterparts in the 1.25 mm maps, but their visibility significantly varies across the filament spine and in time. These opacity changes are possibly related to variations in temperature and electron density in filament fine structures. The coolest Tb values (< 5,000 K) coincide with regions of low integrated intensity in the Mg ii h and k lines. ALMA Band 3 maps taken after the Band 6 ones do not clearly show the filament structure, contrary to the expectation that the contrast should increase at longer wavelengths based on previous observations of quiescent filaments. The ALMA maps are not consistent with isothermal conditions, but the temporal evolution of the filament may partly account for this. Title: Pulsar observations at low frequencies: applications to pulsar timing and solar wind models Authors: Kumar, P.; White, S. M.; Stovall, K.; Dowell, J.; Taylor, G. B. Bibcode: 2022MNRAS.511.3937K Altcode: 2022MNRAS.tmp..319K; 2022arXiv220201251K Efforts are underway to use high-precision timing of pulsars in order to detect low-frequency gravitational waves. A limit to this technique is the timing noise generated by dispersion in the plasma along the line of sight to the pulsar, including the solar wind. The effects due to the solar wind vary with time, influenced by the change in solar activity on different time-scales, ranging up to ~11 yr for a solar cycle. The solar wind contribution depends strongly on the angle between the pulsar line of sight and the solar disc, and is a dominant effect at small separations. Although solar wind models to mitigate these effects do exist, they do not account for all the effects of the solar wind and its temporal changes. Since low-frequency pulsar observations are most sensitive to these dispersive delays, they are most suited to test the efficacy of these models and identify alternative approaches. Here, we investigate the efficacy of some solar wind models commonly used in pulsar timing using long-term, high-cadence data on six pulsars taken with the Long Wavelength Array, and compare them with an operational solar wind model. Our results show that stationary models of the solar wind correction are insufficient to achieve the timing noise desired by pulsar timing experiments, and we need to use non-stationary models, which are informed by other solar wind observations, to obtain accurate timing residuals. Title: NuSTAR observations of a repeatedly microflaring active region Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.; Smith, David M.; Duncan, Jessie Bibcode: 2021MNRAS.507.3936C Altcode: 2021arXiv210900263C; 2021MNRAS.tmp.2159C We investigate the spatial, temporal, and spectral properties of 10 microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using the Nuclear Spectroscopic Telescope ARray and the Solar Dynamic Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager. We find GOES sub-A class equivalent microflare energies of 1026-1028 erg reaching temperatures up to 10 MK with consistent quiescent or hot active region (AR) core plasma temperatures of 3-4 MK. One microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has non-thermal hard X-ray emission during its impulsive phase (of non-thermal power ~7 × 1024 erg s-1) making it one of the faintest X-ray microflares to have direct evidence for accelerated electrons. In 4 of the 10 microflares, we find that the X-ray time profile matches fainter and more transient sources in the extreme-ultraviolet, highlighting the need for observations sensitive to only the hottest material that reaches temperatures higher than those of the AR core (>5 MK). Evidence for corresponding photospheric magnetic flux cancellation/emergence present at the footpoints of eight microflares is also observed. Title: Reduced Microwave Brightness Temperature in a Sunspot Atmosphere Due to Open Magnetic Fields Authors: Vrublevskis, A.; Ryabov, B. I.; White, S. M. Bibcode: 2021SoPh..296..144V Altcode: 2021arXiv210205476V Motivated by dark coronal lanes in 284 Å extreme-ultraviolet (EUV) observations from the Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory (SOHO/EIT), we construct and optimize an atmosphere model of the active region (AR) 8535 sunspot by adding a cool and dense component in the volume of plasma along open field lines determined using the potential-field source-surface (PFSS) extrapolation. Our model qualitatively reproduces the observed reduced microwave brightness temperature in the northern part of the sunspot in Very Large Array (VLA) observations from 13 May 1999 and provides a physical explanation for the coronal dark lanes. We propose the application of this method to other sunspots with such observed dark regions in the EUV or soft X-rays and with concurrent microwave observations to determine the significance of open field regions. The connection between open fields and the resulting plasma temperature and density change is of relevance for slow solar wind source investigations. Title: Nonequilibrium Flux Rope Formation by Confined Flares Preceding a Solar Coronal Mass Ejection Authors: Kliem, Bernhard; Lee, Jeongwoo; Liu, Rui; White, Stephen M.; Liu, Chang; Masuda, Satoshi Bibcode: 2021ApJ...909...91K Altcode: 2021arXiv210102181K We present evidence that a magnetic flux rope was formed before a coronal mass ejection (CME) and its associated long-duration flare during a pair of preceding confined eruptions and associated impulsive flares in a compound event in NOAA Active Region 12371. Extreme-ultraviolet images and the extrapolated nonlinear force-free field show that the first two (impulsive) flares, SOL2015-06-21T01:42, result from the confined eruption of highly sheared low-lying flux, presumably a seed flux rope. The eruption spawns a vertical current sheet, where magnetic reconnection creates flare ribbons and loops, a nonthermal microwave source, and a sigmoidal hot channel that can only be interpreted as a magnetic flux rope. Until the subsequent long-duration flare, SOL2015-06-21T02:36, the sigmoid's elbows expand, while its center remains stationary, suggesting nonequilibrium but not yet instability. The "flare reconnection" during the confined eruptions acts like "tether-cutting reconnection" whose flux feeding of the rope leads to instability. The subsequent full eruption is seen as an accelerated rise of the entire hot channel, seamlessly evolving into the fast halo CME. Both the confined and ejective eruptions are consistent with the onset of the torus instability in the dipped decay index profile that results from the region's two-scale magnetic structure. We suggest that the formation or enhancement of a nonequilibrium but stable flux rope by confined eruptions is a generic process occurring prior to many CMEs. Title: An overall view of temperature oscillations in the solar chromosphere with ALMA Authors: Jafarzadeh, S.; Wedemeyer, S.; Fleck, B.; Stangalini, M.; Jess, D. B.; Morton, R. J.; Szydlarski, M.; Henriques, V. M. J.; Zhu, X.; Wiegelmann, T.; Guevara Gómez, J. C.; Grant, S. D. T.; Chen, B.; Reardon, K.; White, S. M. Bibcode: 2021RSPTA.37900174J Altcode: 2021RSTPA.379..174J; 2020arXiv201001918J By direct measurements of the gas temperature, the Atacama Large Millimeter/submillimeter Array (ALMA) has yielded a new diagnostic tool to study the solar chromosphere. Here, we present an overview of the brightness-temperature fluctuations from several high-quality and high-temporal-resolution (i.e. 1 and 2 s cadence) time series of images obtained during the first 2 years of solar observations with ALMA, in Band 3 and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz), respectively. The various datasets represent solar regions with different levels of magnetic flux. We perform fast Fourier and Lomb-Scargle transforms to measure both the spatial structuring of dominant frequencies and the average global frequency distributions of the oscillations (i.e. averaged over the entire field of view). We find that the observed frequencies significantly vary from one dataset to another, which is discussed in terms of the solar regions captured by the observations (i.e. linked to their underlying magnetic topology). While the presence of enhanced power within the frequency range 3-5 mHz is found for the most magnetically quiescent datasets, lower frequencies dominate when there is significant influence from strong underlying magnetic field concentrations (present inside and/or in the immediate vicinity of the observed field of view). We discuss here a number of reasons which could possibly contribute to the power suppression at around 5.5 mHz in the ALMA observations. However, it remains unclear how other chromospheric diagnostics (with an exception of Hα line-core intensity) are unaffected by similar effects, i.e. they show very pronounced 3-min oscillations dominating the dynamics of the chromosphere, whereas only a very small fraction of all the pixels in the 10 ALMA datasets analysed here show peak power near 5.5 mHz.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: NuSTAR Observation of Energy Release in 11 Solar Microflares Authors: Duncan, Jessie; Glesener, Lindsay; Grefenstette, Brian W.; Vievering, Juliana; Hannah, Iain G.; Smith, David M.; Krucker, Säm; White, Stephen M.; Hudson, Hugh Bibcode: 2021ApJ...908...29D Altcode: 2020arXiv201106651D Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. This paper presents 11 NuSTAR microflares from two active regions (AR 12671 on 2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and energetic properties of each are discussed in context with previously published HXR brightenings. They are seen to display several "large flare" properties, such as impulsive time profiles and earlier peak times in higher-energy HXRs. For two events where the active region background could be removed, microflare emission did not display spatial complexity; differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high-energy excess over a single thermal model in all events. This excess was consistent with additional higher-temperature plasma volumes in 10/11 microflares and only with an accelerated particle distribution in the last. Previous NuSTAR studies focused on one or a few microflares at a time, making this the first to collectively examine a sizable number of events. Additionally, this paper introduces an observed variation in the NuSTAR gain unique to the extremely low livetime (<1%) regime and establishes a correction method to be used in future NuSTAR solar spectral analysis. Title: Probing chromospheric heating with millimeter interferometry Authors: da Silva Santos, J. M.; de la Cruz Rodriguez, J.; White, S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H.; Danilovic, S. Bibcode: 2020AGUFMSH0010001D Altcode: Observations at visible and ultraviolet wavelengths have shown that solar active regions host different kinds of small-scale, transient, bright structures that are believed to be heating events resulting from the release of magnetic energy in the low atmosphere of the Sun, especially at the early stages of flux emergence. It is of great scientific interest to be able to accurately infer temperatures and formation heights of the most localized events, which are still matter of debate, in the hope of learning about the evolution of active regions where occasionally more energetic phenomena lead to much larger outbursts that propagate across the Solar System. The millimeter (mm) continuum is a new complementary diagnostic for chromospheric heating that is now available thanks to the Atacama Large Millimeter/submillimeter Array (ALMA).

We report on the first ALMA 3 mm observations of small-scale heating events in a solar active region. In contrast with the low-amplitude brightness temperature variations in the quiet-Sun, the interferometric maps show that the active region consists of long, warm, fibril-like structures that connect magnetic concentrations of opposite polarity and often flare up along with compact, flickering mm-bursts -- reminiscent of ultraviolet bursts -- with brightness temperatures of up to 14000 K at 1.2" scales. These events also show simultaneous EUV emission observed by the Solar Dynamics Observatory (SDO). We find a weak correlation between the photospheric bright patches and the 3 mm continuum brightness and, in particular, we do not detect any mm counterpart of Ellerman bombs which confirms that they are photospheric phenomena.

Our observations and modelling highlight the diagnostic capabilities of ALMA for local heating in solar active regions and emphasize the need for coordinated observations with IRIS and DKIST in the future. Title: NuSTAR Observation of Eleven Solar Microflares Authors: Duncan, J. M.; Glesener, L.; Grefenstette, B.; Vievering, J. T.; Hannah, I. G.; Smith, D. M.; Krucker, S.; White, S. M.; Hudson, H. S. Bibcode: 2020AGUFMSH045..07D Altcode: This work presents eleven microflares observed by the Nuclear Spectroscopic Telescope ARray (NuSTAR), representing the first time that a sizable number of these events have been examined collectively. NuSTAR's direct focusing optics give it a dramatic increase in sensitivity over indirect imagers in the hard X-ray (HXR) range. HXR emission in solar flares originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, both of which are diagnostic of flare energy release. Therefore, NuSTAR solar observation campaigns can give unique insight into the energetics of faint microflares, including those that were unobservable with previous-generation HXR instruments. We discuss the temporal, spatial, and energetic properties of all eleven microflares in context with previously published HXR brightenings. They are seen to display several `large-flare' properties, such as impulsive time profiles and earlier peaktimes in higher energy HXRs. For two events where active region background could be removed, microflare emission did not display spatial complexity: differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high energy excess over a single thermal model in all events. This excess was found to most likely originate from additional higher-temperature plasma volumes in 10/11 microflares, and from an accelerated particle distribution in the last. Finally, we introduce an observed variation in the NuSTAR gain unique to the extremely low-livetime (< 1%) regime, and establish a correction method to be used in future NuSTAR solar spectral analysis. Title: Next-Generation Solar Radio Imaging Spectroscopy Authors: Gary, D. E.; Bastian, T.; Chen, B.; Saint-Hilaire, P.; White, S. M. Bibcode: 2020AGUFMSH056..07G Altcode: Recent progress using instruments such as the Expanded Owens Valley Solar Array (EOVSA) and the Jansky Very Large Array (JVLA) have amply demonstrated the power of radio imaging spectroscopy at centimeter and decimeter wavelengths for quantitative diagnostics of both flaring and non-flaring solar plasma. For example, the unique sensitivity of radio emission to the flaring coronal magnetic field has been dramatically shown in a series of recent EOVSA papers, along with accelerated electron diagnostics in the same volume. JVLA observations have been used to probe particle acceleration in a termination shock during a flare. In addition, multi-frequency full-disk imaging of the non-flaring Sun with EOVSA is showing promise for quantitative diagnostics of electron-based emission measure and active region magnetic field and temperature structure. But EOVSA and the JVLA are mere demonstrators for a far more advanced solar radio instrument, the Frequency Agile Solar Radiotelescope (FASR), that is designed to address a much more comprehensive science program with much greater precision than is possible with EOVSA or non-solar-dedicated instruments like the JVLA. This includes direct imaging of Coronal Mass Ejections (CMEs) and their associated energetic particles both on and off the solar disk, routine coronal magnetic field measurements at high cadence, and continuous sampling of the full-Sun coronal temperature and emission measure. Here we use examples from EOVSA and the JVLA to envision what FASR's advanced design will be capable of. When paired with highly complementary new X-ray-based diagnostics from space, the remote sensing of high-energy particles through radio imaging spectroscopy from the ground provides a far more complete picture of the broad range of energetic phenomena that occur on the Sun than either alone. Scientists interested in high-energy solar phenomena have ample incentive to coordinate their efforts to ensure that space- and ground-based capabilities advance together. Title: An age uncertainty model for the Cairo Nile height record, 622 to 1898 CE Authors: White, S. M.; Stine, A.; Ludlow, F.; Manning, J. Bibcode: 2020AGUFMPP033..06W Altcode: The longest instrumental climate record humans possess is the annual Nile high/low stand record from Cairo (622 C.E. - 1898 C.E.), with climatically useful records ending with construction of the Aswan Low Dam in 1899. This unique record reflects a balance between equatorial precipitation and subtropical evaporation integrated over ~30 degrees of latitude. The veracity of this record has been questioned by ancient and modern sources, but its utility was recently illustrated by demonstrating a strong association between the record of explosive volcanic eruptions in Greenland ice cores and years of low Nile flood height. High-frequency uncertainty in the age model remains because of the inconsistency in how the Islamic lunar calendar was marked in Egypt, as opposed to other regions of the Islamic world, during this 1276 year period. This uncertainty arises, in part, because of language in the Koran (Surah 9 : 37) which was interpreted to require explicit declaration from the Caliph (or after Saladin's conquest, from the Sultan) in order to rectify the lunar calendar to the solar calendar, a duty that was neglected to a greater or lesser extent during different political dynasties leading to uncertain drift between the two calendars. Here we explicitly model the uncertainty in the how the lunar calendar was historically rectified, producing a model of the full structure of the uncertainty in the dating of the Cairo Nilometer record. Uncertainty grows across lunar cycles where no independent constraints on the calendar exist, and collapses when documentary records exist that tie the calendars on a specific date. We demonstrate that application of this age-uncertainty model increases the power with which climate signals can be extracted from the Nile record. Title: Observations of Solar Spicules at Millimeter and Ultraviolet Wavelengths Authors: Bastian, T.; De Pontieu, B.; Shimojo, M.; Iwai, K.; Alissandrakis, C.; Nindos, A.; Vial, J. C.; White, S. M. Bibcode: 2020AGUFMSH004..08B Altcode: Solar spicules are a ubiquitous chromospheric phenomenon in which multitudes of dynamic jets with temperatures of order 104 K extend thousands of kilometers into the solar atmosphere. Recent progress has been made refining the observational characteristics of spicules using the Hinode Solar Optical Telescope (SOT) and the Interface Region Imaging Spectrograph (IRIS) observations at optical and ultraviolet wavelengths, respectively. Two types of spicule have been identified. Type I spicules, prevalent in solar active regions, have upward speeds of order 25 km/s and lifetimes of 3-7 min. They may be the limb counterpart to shock-wave-driven fibrils commonly seen against the solar disk in active regions. In contrast, type II spicules, more common in quiet regions and coronal holes, display upward speeds of 50-150 km/s, lifetimes of 30-110 s, and appear to be partially heated to temperatures of 105 K and higher. These observations have provoked intense interest in spicules and have led to proposals that type II spicules play a central role as a source of hot plasma in the corona. Nevertheless, their role in mass and energy transport between the lower and upper layers of the solar atmosphere remains an outstanding problem.

Here, we report imaging observations of solar spicules at millimeter wavelengths using the Atacama Large Millimeter-submillimeter Array (ALMA) with arcsecond angular resolution. Continuum millimeter wavelength radiation forms under conditions of local thermodynamic equilibrium, thereby providing a complementary tool to UV lines, which form under non-LTE conditions. The observations were made on 2018 December 24-25 at λ=1.25 mm and λ=3 mm. The ALMA observations pose special challenges, particularly at 1.25 mm, where the limited field of view of the instrument motivated us to use a novel mosaic imaging technique: multiple pointings were assembled to form a single map with an angular resolution of 1" x 0.7" on a cadence of roughly 2 min. In contrast, we were able to image at 3 mm continuously, with a map cadence of 2 s and an angular resolution of 2.3" x 1.3".

We compare and contrast the morphology and dynamics of mm-λ observations of spicules with those obtained by IRIS at UV wavelengths and place constraints on spicule temperatures and masses using the joint millimeter-wavelength observations. Title: Microwave Study of a Circular Ribbon Flare Authors: Lee, J.; White, S. M.; Liu, C. Bibcode: 2020AGUFMSH0430010L Altcode: We present a microwave study of a circular ribbon flare. Data obtained are the 17/34 GHz maps from the Nobeyama Radioheliograph (NoRH) of SOL2014-12-17T04:51 along with (E)UV and magnetic data from the Solar Dynamics Observatory (SDO). Major results and interpretations are as follows: (1) the first activation of the circular ribbons comes in the form of a gradual increase of the 17 GHz flux without a counterpart at 34 GHz, about 13 min before the impulsive phase, which we interpret as representing thermal preheating. (2) The first sign of nonthermal activity occurs in the form of stepwise flux increases at both 17 and 34 GHz about 4 min before the impulsive phase, which is likely of nonthermal activation. (3) The main impulsive flare produces a sign flip of the 17 GHz polarization state, which implies a breakout type magnetic eruption. (4) The 17 GHz flux shows quasi-periodic pulsations with a period around 1.5 min before and during the flare. While the preflare pulsation is more obvious in total intensity at one end of the flare loop, the flare pulsation is more obvious in the polarized intensity at the region near the inner spine. We interpret the latter as evidence for torsional Alfven waves generated at the magnetic reconnection. (5) The 17/34 GHz fluxes show a secondary maximum at 25 min after the impulsive phase maximum. These fluxes are more enhanced in the flare looptop than in the footpoints, which has an implication on the nature of the EUV late phase. Our results show that microwave emission of a circular ribbon flare provides new information on the activities in the fan-spine structure. Title: ALMA observations of transient heating in a solar active region Authors: da Silva Santos, J. M.; de la Cruz Rodríguez, J.; White, S. M.; Leenaarts, J.; Vissers, G. J. M.; Hansteen, V. H. Bibcode: 2020A&A...643A..41D Altcode: 2020arXiv200614564D
Aims: We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA).
Methods: We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis.
Results: We report the null detection of EBs in the 3 mm continuum at ∼1.2″ spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts - reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of > 10 kK plasma launched with high plane-of-sky velocities (37 - 340 km s-1) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.

Movie associated to Fig. 5 is available at https://www.aanda.org Title: Microwave Study of a Solar Circular Ribbon Flare Authors: Lee, Jeongwoo; White, Stephen M.; Chen, Xingyao; Chen, Yao; Ning, Hao; Li, Bo; Masuda, Satoshi Bibcode: 2020ApJ...901L..10L Altcode: 2020arXiv200911926L A circular ribbon flare (CRF) SOL2014-12-17T04:51 is studied using the 17/34 GHz maps from the Nobeyama Radioheliograph along with (E)UV and magnetic data from the Solar Dynamics Observatory. We report the following three findings as important features of the microwave CRF. (1) The first preflare activation comes in the form of a gradual increase of the 17 GHz flux without a counterpart at 34 GHz, which indicates thermal preheating. The first sign of nonthermal activity occurs in the form of stepwise flux increases at both 17 and 34 GHz about 4 minutes before the impulsive phase. (2) Until the impulsive phase, the microwave emission over the entire active region is in a single polarization state matching the magnetic polarity of the surrounding fields. During and after the impulsive phase, the sign of the 17 GHz polarization state reverses in the core region, which implies a magnetic breakout-type eruption in a fan-spine magnetic structure. (3) The 17 GHz flux around the time of the eruption shows quasi-periodic variations with periods of 1-2 minutes. The pre-eruption oscillation is more obvious in total intensity at one end of the flare loop, and the post-eruption oscillation, more obvious in the polarized intensity at a region near the inner spine. We interpret this transition as transfer of oscillatory power from kink mode oscillation to torsional Alfvén waves propagating along the spine field after the eruption. We argue that these three processes are interrelated and indicate a breakout process in a fan-spine structure. Title: Microwave and Hard X-Ray Flare Observations by NoRH/NoRP and RHESSI: Peak-flux Correlations Authors: Krucker, Säm; Masuda, Satoshi; White, Stephen M. Bibcode: 2020ApJ...894..158K Altcode: This paper presents initial results from a statistical study of solar microwave and hard X-ray flares jointly observed over the past two solar cycles by the Nobeyama Radio Polarimeters, the Nobeyama Radio Heliograph, and the Reuven Ramaty High Energy Solar Spectroscopic Imager. As has been previously demonstrated, the microwave (17 GHz and 34 GHz) peak flux shows a linear correlation with the nonthermal hard X-ray bremsstrahlung peak emission seen above 50 keV. The correlation holds for the entire rise phase of each individual burst, while the decay phases tend to show more extended emission at microwaves than is generally attributed to particle trapping. While the correlation is highly significant (coefficient of 0.92) and holds over more than four orders of magnitude, individual flares can be above or below the fitted line by an average factor of about 2. By restricting the flare selection to source morphologies with the radio emission from the top of the flare loop, the correlation tightens significantly, with a correlation coefficient increasing to 0.99 and the scatter reduced to a factor of 1.3. These findings corroborate the assumption that gyrosynchrotron microwave and hard X-ray bremsstrahlung emissions are produced by the same flare-accelerated electron population. The extent of the linear correlation over four orders of magnitude suggests that magnetic field strengths within nonthermal 17 GHz sources are surprisingly similar over a wide range of flare sizes. Title: NuSTAR Observation of a Minuscule Microflare in a Solar Active Region Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.; Smith, David M. Bibcode: 2020ApJ...893L..40C Altcode: 2020arXiv200411176C We present X-ray imaging spectroscopy of one of the weakest active region (AR) microflares ever studied. The microflare occurred at ∼11:04 UT on 2018 September 9 and we studied it using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatory's Atmospheric Imaging Assembly (SDO/AIA). The microflare is observed clearly in 2.5-7 keV with NuSTAR and in Fe XVIII emission derived from the hotter component of the 94 Å SDO/AIA channel. We estimate the event to be three orders of magnitude lower than a GOES A class microflare with an energy of 1.1 × 1026 erg. It reaches temperatures of 6.7 MK with an emission measure of 8.0 × 1043 cm-3. Non-thermal emission is not detected but we instead determine upper limits to such emission. We present the lowest thermal energy estimate for an AR microflare in literature, which is at the lower limits of what is still considered an X-ray microflare. Title: Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare Authors: Glesener, Lindsay; Krucker, Säm; Duncan, Jessie; Hannah, Iain G.; Grefenstette, Brian W.; Chen, Bin; Smith, David M.; White, Stephen M.; Hudson, Hugh Bibcode: 2020ApJ...891L..34G Altcode: 2020arXiv200312864G We report the detection of emission from a nonthermal electron distribution in a small solar microflare (GOES class A5.7) observed by the Nuclear Spectroscopic Telescope Array, with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The flaring plasma is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the "coronal thick-target" behavior occasionally observed in larger flares. This is the first positive detection of nonthermal hard X-rays from the Sun using a direct imager (as opposed to indirectly imaging instruments). The accelerated electron distribution has a spectral index of 6.3 ± 0.7, extends down to at least 6.5 keV, and deposits energy at a rate of ∼2 × 1027 erg s-1, heating the flare loop to at least 10 MK. The existence of dominant nonthermal emission in X-rays down to <5 keV means that RHESSI emission is almost entirely nonthermal, contrary to what is usually assumed in RHESSI spectroscopy. The ratio of nonthermal to thermal energies is similar to that of large flares, in contrast to what has been found in previous studies of small RHESSI flares. We suggest that a coronal thick target may be a common property of many small microflares based on the average electron energy and collisional mean free path. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating. Title: Dampened El Niño in the Early Pliocene Warm Period Authors: White, S. M.; Ravelo, A. C. Bibcode: 2020GeoRL..4785504W Altcode: El Niño-Southern Oscillation (ENSO) is the strongest mode of interannual climate variability, and its predicted response to anthropogenic climate change remains unclear. Determining ENSO's sensitivity to climatic mean state and the strength of positive and negative feedbacks, notably the thermocline feedback, will help constrain its future behavior. To this end, we collected ENSO proxy data from the early and mid-Pliocene, a time during which tropical Pacific zonal and vertical temperature gradients were much lower than today. We found that El Niño events had a reduced amplitude throughout the early Pliocene, compared to the late Holocene. By the mid-Pliocene, El Niño amplitude was variable, sometimes reduced and sometimes similar to the late Holocene. This trend in Pliocene ENSO amplitude mirrors the long-term strengthening of zonal and vertical temperature gradients and verifies model results showing dampened ENSO under reduced gradients due to a weaker thermocline feedback. Title: NuSTAR observations of the quietest Sun Authors: Hannah, I. G.; Cooper, K.; Grefenstette, B.; Glesener, L.; Krucker, S.; Smith, D. M.; Hudson, H. S.; White, S. M.; Kuhar, M. Bibcode: 2019AGUFMSH41F3335H Altcode: Observing X-rays (above a few keV) from the Sun provides a direct insight into energy release (heating and/or particle acceleration) in the solar atmosphere. Targeting the faintest X-ray emission allows the study of the smallest flares and eruption, and crucially their contribution to heating the solar atmosphere. NuSTAR is an astrophysics telescope that uses directly focusing X-rays optics to detect weak X-rays from the Sun. We have observed the Sun many times since the start of solar pointings in Sep 2014 through to our latest observations in 2019. See http://ianan.github.io/nsovr/ for an overview. During the current solar minimum, when the Sun is devoid of active regions and presenting the very quietest levels of activity, NuSTAR has targeted the Sun several times. We have detected X-rays from a variety of sources: large diffuse sources, steady compact sources, brief flares/brightenings and small eruptions. The NuSTAR X-ray images of these weak sources are related to features seen at other wavelengths, such as in softer X-rays with Hinode/XRT and EUV with SDO/AIA. Crucially, NuSTAR's imaging spectroscopy allows us to obtain and fit the X-ray spectrum from these small events determining their thermal (and potentially non-thermal) properties. We will present some of the latest solar observations with NuSTAR as we go through the current solar minimum. Title: The Slowly Varying Corona. II. The Components of F 10.7 and Their Use in EUV Proxies Authors: Schonfeld, S. J.; White, S. M.; Henney, C. J.; Hock-Mysliwiec, R. A.; McAteer, R. T. J. Bibcode: 2019ApJ...884..141S Altcode: 2019arXiv191012964S Using four years of full-disk-integrated coronal differential emission measures calculated in Schonfeld et al. (2017), we investigate the relative contribution of bremsstrahlung and gyroresonance emission in observations of F 10.7, the 10.7 cm (2.8 GHz) solar microwave spectral flux density and commonly used activity proxy. We determine that the majority of coronal F 10.7 is produced by the bremsstrahlung mechanism, but the variability observed over individual solar rotations is often driven by gyroresonance sources rotating across the disk. Our analysis suggests that the chromosphere may contribute significantly to F 10.7 variability and that coronal bremsstrahlung emission accounts for 14.2 ± 2.1 sfu (∼20%) of the observed solar minimum level. The bremsstrahlung emission has a power-law relationship to the total F 10.7 at high activity levels, and this combined with the observed linearity during low activity yields a continuously differentiable piecewise fit for the bremsstrahlung component as a function of F 10.7. We find that the bremsstrahlung component fit, along with the Mg II index, correlates better with the observed 5-37 nm spectrum than the common 81 day averaged F 10.7 proxy. The bremsstrahlung component of F 10.7 is also well approximated by the moderate-strength photospheric magnetic field parameterization from Henney et al. (2012), suggesting that it could be forecast for use in both atmospheric research and operational models. Title: Joint X-Ray, EUV, and UV Observations of a Small Microflare Authors: Hannah, Iain G.; Kleint, Lucia; Krucker, Säm; Grefenstette, Brian W.; Glesener, Lindsay; Hudson, Hugh S.; White, Stephen M.; Smith, David M. Bibcode: 2019ApJ...881..109H Altcode: 2018arXiv181209214H We present the first joint observation of a small microflare in X-rays with the Nuclear Spectroscopic Telescope ARray (NuSTAR), in UV with the Interface Region Imaging Spectrograph (IRIS), and in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These combined observations allow us to study the hot coronal and cooler chromospheric/transition region emission from the microflare. This small microflare peaks from 2016 July 26 23:35 to 23:36 UT, in both NuSTAR, SDO/AIA, and IRIS. Spatially, this corresponds to a small loop visible in the SDO/AIA Fe XVIII emission, which matches a similar structure lower in the solar atmosphere seen by IRIS in SJI1330 and 1400 Å. The NuSTAR emission in both 2.5-4 and 4-6 keV is located in a source at this loop location. The IRIS slit was over the microflaring loop, and fits show little change in Mg II but do show intensity increases, slight width enhancements, and redshifts in Si IV and O IV, indicating that this microflare had most significance in and above the upper chromosphere. The NuSTAR microflare spectrum is well fitted by a thermal component of 5.1 MK and 6.2 × 1044 cm-3, which corresponds to a thermal energy of 1.5 × 1026 erg, making it considerably smaller than previously studied active region microflares. No non-thermal emission was detected but this could be due to the limited effective exposure time of the observation. This observation shows that even ordinary features seen in UV can remarkably have a higher-energy component that is clear in X-rays. Title: The Low-Frequency Solar Corona in Circular Polarization Authors: McCauley, Patrick I.; Cairns, Iver H.; White, Stephen M.; Mondal, Surajit; Lenc, Emil; Morgan, John; Oberoi, Divya Bibcode: 2019SoPh..294..106M Altcode: 2019arXiv190710878M We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 - 240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artifact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V ) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5% to nearly 100%. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated. Title: ALMA detection of dark chromospheric holes Authors: White, Stephen M.; Loukitcheva, Maria A.; Solanki, Sami K. Bibcode: 2019AAS...23421704W Altcode: Atacama Large Millimeter/submillimeter Array (ALMA) observations of a quiet-Sun region at a wavelength of 3 mm are compared with available chromospheric observations in the UV and visible as well as with photospheric magnetograms. The ALMA images clearly reveal the presence of distinctive cold areas in the millimeter maps having temperatures of around 60% of the normal quiet Sun at 3 mm, which are not seen in the other data. We speculate that ALMA is sensing cool chromospheric gas, whose presence had earlier been inferred from infrared CO spectra. Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener, Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Gburek, Szymon; Steslicki, Marek; Allred, Joel C.; Battaglia, Marina; Baumgartner, Wayne H.; Drake, James; Goetz, Keith; Grefenstette, Brian; Hannah, Iain; Holman, Gordon D.; Inglis, Andrew; Ireland, Jack; Klimchuk, James A.; Ishikawa, Shin-Nosuke; Kontar, Eduard; Massone, Anna-maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard A.; Woods, Thomas N.; Chen, Bin; Gary, Dale E.; Hudson, Hugh S.; Kowalski, Adam; Warmuth, Alexander; White, Stephen M.; Veronig, Astrid; Vilmer, Nicole Bibcode: 2019AAS...23422501C Altcode: The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept in Phase A, is the first-ever solar-dedicated, direct-imaging, hard X-ray telescope. FOXSI provides a revolutionary new approach to viewing explosive magnetic-energy release on the Sun by detecting signatures of accelerated electrons and hot plasma directly in and near the energy-release sites of solar eruptive events (e.g., solar flares). FOXSI's primary science objective is to understand the mystery of how impulsive energy release leads to solar eruptions, the primary drivers of space weather at Earth, and how those eruptions are energized and evolve. FOXSI addresses three important science questions: (1) How are particles accelerated at the Sun? (2) How do solar plasmas get heated to high temperatures? (3) How does magnetic energy released on the Sun lead to flares and eruptions? These fundamental physics questions are key to our understanding of phenomena throughout the Universe from planetary magnetospheres to black hole accretion disks. FOXSI measures the energy distributions and spatial structure of accelerated electrons throughout solar eruptive events for the first time by directly focusing hard X-rays from the Sun. This naturally enables high imaging dynamic range, while previous instruments have typically been blinded by bright emission. FOXSI provides 20-100 times more sensitivity as well as 20 times faster imaging spectroscopy than previously available, probing physically relevant timescales (<1 second) never before accessible. FOXSI's launch in July 2022 is aligned with the peak of the 11-year solar cycle, enabling FOXSI to observe the many large solar eruptions that are expected to take place throughout its two-year mission. Title: ALMA Detection of Dark Chromospheric Holes in the Quiet Sun Authors: Loukitcheva, Maria A.; White, Stephen M.; Solanki, Sami K. Bibcode: 2019ApJ...877L..26L Altcode: 2019arXiv190506763L We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of a quiet-Sun region at a wavelength of 3 mm, obtained during the first solar ALMA cycle on 2017 April 27, and compare them with available chromospheric observations in the UV and visible as well as with photospheric magnetograms. ALMA images clearly reveal the presence of distinct particularly dark/cool areas in the millimeter maps with temperatures as low as 60% of the normal quiet Sun at 3 mm, which are not seen in the other data. We speculate that ALMA is sensing cool chromospheric gas, whose presence had earlier been inferred from infrared CO spectra. Title: Thermal and Nonthermal Evolutions of a Circular Ribbon Flare Authors: Lee, Jeongwoo; Liu, Chang; White, Stephen M.; Wang, Haimin Bibcode: 2019AAS...23431003L Altcode: We studied the activation and the extended activity of a circular-ribbon flare, SOL2014-12-17T04:51 by performing the Differential Emission Measure (DEM) inversion analysis of the extreme ultraviolet (EUV) images of the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) and analysis of the microwave data from the Nobeyama Radioheliograph (NoRH). We will present the following results. 1. The circular ribbon as an indicator of the dome-shaped separatrix is better visible in the preflare phase in the form of very narrow channel of enhance temperature. It is activated much earlier (10-15 min before the impulsive phase at 04:33 UT), and its temporal correlation with the outer spine temperature suggests the initial activation of the circular ribbon by magnetic reconnection in the outer spine. 2. The flare ribbons are active only within a limited section of the entire circular ribbon, suggesting the hyperbolic flux tube reconnection rather than null point reconnection. They are activated 4 min before the impulsive phase together with the inner spine, and involve nonthermal electrons as evidenced by the NoRH 34 GHz emission. 3. The temperature-high region and the DEM-high region coincide each other in the impulsive phase, and then separate from each other, implying that the rising magnetic fields carry hot plasma away while the high-density cooler plasma stays on the fan surface. 4. An unusually extended decay phase in this event can be understood as the maximum DEM in the impulsive phase occurs at a higher temperature and then gradually shifts to lower temperatures where EUV emission is more efficient. Namely, the extended EUV activity is due to slow cooling of the hot plasma within the confined magnetic structure. Title: First detection of non-thermal emission in a NuSTAR solar microflare Authors: Glesener, Lindsay; Krucker, Sam; Duncan, Jessie McBrayer; Hannah, Iain; Grefenstette, Brian; Chen, Bin; Smith, David M.; White, Stephen M.; Hudson, Hugh S. Bibcode: 2019AAS...23422503G Altcode: We report the detection of emission from a non-thermal electron distribution in a small solar microflare observed by the Nuclear Spectroscopic Telescope Array (NuSTAR). On 2017 August 21, NuSTAR observed a solar active region for approximately an hour before the region was eclipsed by the Moon. The active region emitted several small microflares of GOES class A and smaller. In this work, we present spectroscopy demonstrating evidence of electron acceleration in one of these microflares (GOES class A5.7) and we compare energetic aspects of the accelerated distribution to commonly studied larger flares. The flaring plasma observed by NuSTAR, with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the "coronal thick target" behavior occasionally observed in larger flares. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating. Title: Resolving the Radio Photospheres of Main Sequence Stars Authors: Carilli, Christopher; Butler, B.; Golap, K.; Carilli, M. T.; White, S. M. Bibcode: 2019BAAS...51c.243C Altcode: 2019astro2020T.243C We discuss the need for spatially resolved observations of the radio photospheres of main sequence stars. Such studies are fundamental to determining the structure of stars in the key transition region from the cooler optical photosphere to the hot chromosphere — the regions powering exo- space weather phenomena. Title: Particle Acceleration and Transport, New Perspectives from Radio, X-ray, and Gamma-Ray Observations Authors: Gary, Dale; Bastian, Timothy S.; Chen, Bin; Drake, James F.; Fleishman, Gregory; Glesener, Lindsay; Saint-Hilaire, Pascal; White, Stephen M. Bibcode: 2019BAAS...51c.371G Altcode: 2019astro2020T.371G Particle acceleration and particle transport are ubiquitous in astrophysics. The Sun offers an astrophysical laboratory to study these in minute detail, using radio dynamic imaging spectroscopy to measure coronal magnetic fields, time and space evolution of the electron distribution function. Title: Diagnostics of Space Weather Drivers Enabled by Radio Observations Authors: Bastian, Tim; Bain, Hazel; Chen, Bin; Gary, Dale E.; Fleishman, Gregory D.; Glesener, Lindsay; Saint-Hilaire, Pascal; Lonsdale, Colin; White, Stephen M. Bibcode: 2019BAAS...51c.323B Altcode: 2019astro2020T.323B; 2019arXiv190405817B The Sun is an active star that can impact the Earth, its magnetosphere, and the technological infrastructure on which modern society depends. Radio emission from space weather drivers offers unique diagnostics that complement those available at other wavelengths. We discuss the requirements for an instrument to enable such diagnostics. Title: Radio, Millimeter, Submillimeter Observations of the Quiet Sun Authors: Bastian, Tim; Chen, Bin; Gary, Dale E.; Fleishman, Gregory D.; Glesener, Lindsay; Lonsdale, Colin; Saint-Hilaire, Pascal; White, Stephen M. Bibcode: 2019BAAS...51c.493B Altcode: 2019arXiv190405826B; 2019astro2020T.493B We point out the lack of suitable radio observations of the quiet Sun chromosphere and corona and outline requirements for next generation instrumentation to address the gap. Title: Probing Magnetic Reconnection in Solar Flares: New Perspectives from Radio Dynamic Imaging Spectroscopy Authors: Chen, Bin; Bastian, Tim; Dahlin, Joel; Drake, James F.; Fleishman, Gregory; Gary, Dale; Glesener, Lindsay; Guo, Fan; Ji, Hantao; Saint-Hilaire, Pascal; Shen, Chengcai; White, Stephen M. Bibcode: 2019BAAS...51c.507C Altcode: 2019astro2020T.507C; 2019arXiv190311192C Magnetic reconnection is a fundamental physical process in many laboratory, space, and astrophysical plasma contexts. In this white paper we emphasize the unique power of remote-sensing observations of solar flares at radio wavelengths in probing fundamental physical processes in magnetic reconnection. Title: Solar Coronal Magnetic Fields: Quantitative Measurements at Radio Wavelengths Authors: Fleishman, Gregory; Bastian, Timothy S.; Chen, Bin; Gary, Dale E.; Glesener, Lindsay; Nita, Gelu; Saint-Hilaire, Pascal; White, Stephen M. Bibcode: 2019BAAS...51c.426F Altcode: 2019astro2020T.426F Quantitative measurements of coronal and chromospheric magnetic field is currently in its infancy. We describe a foundation of such observations, which is a key input for MHD numerical models of the solar atmosphere and eruptive processes, and a key link between lower layers of the solar atmosphere and the heliosphere. Title: ngVLA Observations of Coronal Magnetic Fields Authors: Fleishman, G. D.; Nita, G. M.; White, S. M.; Gary, D. E.; Bastian, T. S. Bibcode: 2018ASPC..517..125F Altcode: Energy stored in the magnetic field in the solar atmosphere above active regions is a key driver of all solar activity (e.g., solar flares and coronal mass ejections), some of which can affect life on Earth. Radio observations provide a unique diagnostic of the coronal magnetic fields that make them a critical tool for the study of these phenomena, using the technique of broadband radio imaging spectropolarimetry. Observations with the ngVLA will provide unique observations of coronal magnetic fields and their evolution, key inputs and constraints for MHD numerical models of the solar atmosphere and eruptive processes, and a key link between lower layers of the solar atmosphere and the heliosphere. In doing so they will also provide practical "research to operations" guidance for space weather forecasting. Title: Imaging Stellar Radio Photospheres with the Next Generation Very Large Array Authors: Carilli, C. L.; Butler, B.; Golap, K.; Carilli, M. T.; White, S. M. Bibcode: 2018ASPC..517..369C Altcode: 2018arXiv181005055C We perform simulations of the capabilities of the next generation Very Large Array to image stellar radio photospheres. For very large (in angle) stars, such as red supergiants within a few hundred parsecs, good imaging fidelity results can be obtained on radio photospheric structures at 38 GHz employing standard techniques, such as disk model fitting and subtraction, with hundreds of resolution elements over the star, even with just the ngVLA-classic baselines to 1000 km. Using the ngVLA Rev B plus long baseline configuration (with baselines out to 9000 km, August 2018), we find for main sequence stars within ∼ 10 pc, the photospheres can be easily resolved at 85 GHz, with accurate measures of the mean brightness and size, and possibly imaging large surface structures, as might occur on e.g., active M dwarf stars. For more distant main sequence stars, we find that measurements of sizes and brightnesses can be made using disk model fitting to the (u,v)-data down to stellar diameters ∼ 0.4 mas in a few hours. This size would include M0 V stars to a distance of 15 pc, A0 V stars to 60 pc, and Red Giants to 2.4 kpc. Based on the Hipparcos catalog, we estimate that there are at least 10,000 stars that will be resolved by the ngVLA. While the vast majority of these (95%) are giants or supergiants, there are still over 500 main sequence stars that can be resolved, with ∼ 50 to 150 in each spectral type (besides O stars). Note that these are lower limits, since radio photospheres can be larger than optical, and the Hipparcos catalog might not be complete. Our initial look into the Gaia catalog suggests these numbers might be pessimistic by a factor few. Title: Characteristics of Late-phase >100 MeV Gamma-Ray Emission in Solar Eruptive Events Authors: Share, G. H.; Murphy, R. J.; White, S. M.; Tolbert, A. K.; Dennis, B. R.; Schwartz, R. A.; Smart, D. F.; Shea, M. A. Bibcode: 2018ApJ...869..182S Altcode: We characterize and catalog 30 solar eruptive events observed by the Fermi Large Area Telescope (LAT) having late-phase >100 MeV γ-ray emission (LPGRE), identified 30 yr ago in what were called long-duration gamma-ray flares. We show that LPGRE is temporally and spectrally distinct from impulsive phase emission in these events. The spectra are consistent with the decay of pions produced by >300 MeV protons and are not consistent with primary electron bremsstrahlung. Impulsive >100 keV X-ray emission was observed in all 27 LPGRE events where observations were made. All but two of the LPGRE events were accompanied by a fast and broad coronal mass ejection (CME). The LPGRE start times range from CME onset to 2 hr later. Their durations range from ∼0.1 to 20 hr and appear to be correlated with durations of >100 MeV solar energetic particle (SEP) proton events. The power-law spectral indices of the >300 MeV protons producing LPGRE range from ∼2.5 to 6.5 and vary during some events. Combined γ-ray line and LAT measurements indicate that LPGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of LPGRE protons >500 MeV is typically about 10× the number in the impulsive phase of the solar eruptive event and ranges in nine events from ∼0.01× to 0.5× the number in the accompanying SEP event, with large systematic uncertainty. What appears to be late-phase electron bremsstrahlung with energies up to ∼10 MeV was observed in one LPGRE event. We discuss how current models of LPGRE may explain these characteristics. Title: Science with an ngVLA: ngVLA Observations of Coronal Magnetic Fields Authors: Fleishman, Gregory D.; Nita, Gelu M.; White, Stephen M.; Gary, Dale E.; Bastian, Tim S. Bibcode: 2018arXiv181006622F Altcode: Energy stored in the magnetic field in the solar atmosphere above active regions is a key driver of all solar activity (e.g., solar flares and coronal mass ejections), some of which can affect life on Earth. Radio observations provide a unique diagnostic of the coronal magnetic fields that make them a critical tool for the study of these phenomena, using the technique of broadband radio imaging spectropolarimetry. Observations with the ngVLA will provide unique observations of coronal magnetic fields and their evolution, key inputs and constraints for MHD numerical models of the solar atmosphere and eruptive processes, and a key link between lower layers of the solar atmosphere and the heliosphere. In doing so they will also provide practical "research to operations" guidance for space weather forecasting. Title: Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare Authors: Gary, Dale E.; Chen, Bin; Dennis, Brian R.; Fleishman, Gregory D.; Hurford, Gordon J.; Krucker, Säm; McTiernan, James M.; Nita, Gelu M.; Shih, Albert Y.; White, Stephen M.; Yu, Sijie Bibcode: 2018ApJ...863...83G Altcode: 2018arXiv180702498G We report the first science results from the newly completed Expanded Owens Valley Solar Array (EOVSA), which obtained excellent microwave (MW) imaging spectroscopy observations of SOL2017-09-10, a classic partially occulted solar limb flare associated with an erupting flux rope. This event is also well-covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXRs). We present an overview of this event focusing on MW and HXR data, both associated with high-energy nonthermal electrons, and we discuss them within the context of the flare geometry and evolution revealed by extreme ultraviolet observations from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory. The EOVSA and RHESSI data reveal the evolving spatial and energy distribution of high-energy electrons throughout the entire flaring region. The results suggest that the MW and HXR sources largely arise from a common nonthermal electron population, although the MW imaging spectroscopy provides information over a much larger volume of the corona. Title: Observations of the solar chromosphere with ALMA and comparison with theoretical models Authors: Brajsa, Roman; Sudar, Davor; Skokic, Ivica; Benz, Arnold O.; Kuhar, Matej; Kobelski, Adam; Wedemeyer, Sven; White, Stephen M.; Ludwig, Hans-G.; Temmer, Manuela; Saar, Steven H.; Selhorst, Caius L. Bibcode: 2018csss.confE..37B Altcode: 2018arXiv181207293B In this work we use solar observations with the ALMA radio telescope at the wavelength of 1.21 mm. The aim of the analysisis to improve understanding of the solar chromosphere, a dynamic layer in the solar atmosphere between the photosphere andcorona. The study has an observational and a modeling part. In the observational part full-disc solar images are analyzed.Based on a modied FAL atmospheric model, radiation models for various observed solar structures are developed. Finally, theobservational and modeling results are compared and discussed. Title: Modeling the Ionosphere with GPS and Rotation Measure Observations Authors: Malins, J. B.; White, S. M.; Taylor, G. B.; Stovall, K.; Dowell, J. Bibcode: 2018RaSc...53..724M Altcode: 2018arXiv180602802M The ionosphere contributes time-varying Faraday rotation (FR) to radio signals passing through it. Correction for the effect of the ionosphere is important for deriving magnetic field information from FR observations of polarized cosmic radio sources, as well as providing valuable diagnostics of the structure of the ionosphere. In this paper, we evaluate the accuracy of models commonly used to correct for its effects using new observations of pulsars at low frequencies, which provide total rotation measures (RMs) at better precision than previously available. We evaluate models of the ionosphere derived from modern digital ionosondes that provide electron density information as a function of height, as well as GPS-derived total electron content (TEC) measurements. We combine these density models with reference global magnetic field models to derive ionospheric RM contributions. We find that the models disagree substantially with each other and seek corrections that may explain the differences in RM prediction. Additionally, we compare these models to global TEC models and find that local high-cadence TEC measurements are superior to global models for ionospheric RM correction. Title: The Reported Durations of GOES Soft X-Ray Flares in Different Solar Cycles Authors: Swalwell, Bill; Dalla, Silvia; Kahler, Stephen; White, Stephen M.; Ling, Alan; Viereck, Rodney; Veronig, Astrid Bibcode: 2018SpWea..16..660S Altcode: 2018arXiv180510246S The Geostationary Orbital Environmental Satellites (GOES) Soft X-ray (SXR) sensors have provided data relating to, inter alia, the time, intensity, and duration of solar flares since the 1970s. The GOES SXR Flare List has become the standard reference catalogue for solar flares and is widely used in solar physics research and space weather. We report here that in the current version of the list there are significant differences between the mean duration of flares which occurred before May 1997 and the mean duration of flares thereafter. Our analysis shows that the reported flare timings for the pre-May 1997 data were not based on the same criteria as is currently the case. This finding has serious implications for all those who used flare duration (or fluence, which depends on the chosen start and end times) as part of their analysis of pre-May 1997 solar events or statistical analyses of large samples of flares, for example, as part of the assessment of a solar energetic particle forecasting algorithm. Title: Daily Coronal DEMs from AIA Authors: White, Stephen M.; henry, Timothy; Schonfeld, Samuel; Hock-Mysliwiec, Rachel Bibcode: 2018tess.conf22404W Altcode: Daily solar full-disk coronal differential emission measure (DEM) Title: The Focusing Optics X-ray Solar Imager (FOXSI) Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener, Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel C.; Chen, Bin; Battaglia, Marina; Drake, James Frederick; Gary, Dale E.; Goetz, Keith; Gburek, Szymon; Grefenstette, Brian; Hannah, Iain G.; Holman, Gordon; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa, Shin-nosuke; Klimchuk, James A.; Kontar, Eduard; Kowalski, Adam F.; Massone, Anna Maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard; Steslicki, Marek; Ryan, Daniel; Warmuth, Alexander; Veronig, Astrid; Vilmer, Nicole; White, Stephen M.; Woods, Thomas N. Bibcode: 2018tess.conf40444C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer (SMEX) Heliophysics mission that is currently undergoing a Phase A concept study. FOXSI will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI's primary instrument, the Direct Spectroscopic Imager (DSI), is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis-stabilized spacecraft in low-Earth orbit, DSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. DSI is composed of a pair of X-ray telescopes with a 14-meter focal length enabled by a deployable boom. DSI has a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec FWHM; it will cover the energy range from 3 up to 50-70 keV with a spectral resolution of better than 1 keV. DSI will measure each photon individually and will be able to create useful images at a sub-second temporal resolution. FOXSI will also measure soft x-ray emission down to 0.8 keV with a 0.25 keV resolution with its secondary instrument, the Spectrometer for Temperature and Composition (STC) provided by the Polish Academy of Sciences. Making use of an attenuator-wheel and high-rate-capable detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect X-ray emission from weak flares, escaping electrons, and hot active regions. This presentation will cover the data products and software that can be expected from FOXSI and how they could be used by the community. Title: Microwave Imaging of Flares and Active Regions with the Expanded Owens Valley Solar Array Authors: Gary, Dale E.; Chen, Bin; Fleishman, Gregory D.; Kuroda, Natsuha; Nita, Gelu M.; White, Stephen M.; Yu, Sijie Bibcode: 2018tess.conf21058G Altcode: The Expanded Owens Valley Solar Array (EOVSA), operating in the microwave range (2.5-18 GHz), is the first solar-dedicated radio instrument to achieve true multi-frequency imaging of the Sun. As of the time of this Triennial Earth-Sun Summit meeting, EOVSA will have completed its first year of full operation. We present some highlights of both active region and flare observations that demonstrate the exciting new capabilities of the instrument, including excellent coverage of several large flares that occurred in the 2017 September period. We also provide information for public access to the data, and for getting started with analysis. We are developing tools to simplify working with the data, as well as modeling tools for interpreting the results in the context of complementary multi-wavelength data from space- and ground-based instruments. The progress and current status of these efforts is given in this and a number of companion presentations at the meeting. Title: DoD Space Weather S&T Challenges Authors: Hock-Mysliwiec, Rachel; Starks, Michael J.; White, Stephen M. Bibcode: 2018tess.conf41204H Altcode: AFRL is a key player for transitioning space environment research into DoD operations, and feeding operational needs back to the research community Title: EUV Irradiance from the EVE MEGS-A Dataset and its Implications for F10.7 Authors: Schonfeld, Samuel; White, Stephen M.; Hock-Mysliwiec, Rachel; Henney, Carl J.; Mcateer, R. T. James Bibcode: 2018tess.conf40906S Altcode: We present analysis of the complete spectral dataset from the Extreme-ultraviolet (EUV) Variability Experiment (EVE) MEGS-A instrument. Using these data, we construct daily differential emission measures (DEMs) and use them to analyze the long-term variability of the global corona and the irradiance it produces. We identify a discontinuity in the EUV irradiance and DEMs separating solar minimum and maximum conditions. Using the DEMs we also study the relationship between EUV and F10.7, the 10.7 cm (2.8 GHz) solar activity proxy. We compare predictions of the geoeffective F10.7 from the DEMs and photospheric magnetic field observations with the commonly used 81-day averaged F10.7 to investigate their uses in parameterizing the solar EUV irradiance. We demonstrate a fundamental variability in the relationship between F10.7 and EUV associated with the two coronal sources of F10.7 emission. Title: Hunting for Hydrothermal Vents at the Local-Scale Using AUV's and Machine-Learning Classification in the Earth's Oceans Authors: White, S. M. Bibcode: 2018LPICo2085.6026W Altcode: New AUV-based mapping technology coupled with machine-learning methods for detecting individual vents and vent fields at the local-scale raise the possibility of understanding the geologic controls on hydrothermal venting. Title: NuSTAR Detection of X-Ray Heating Events in the Quiet Sun Authors: Kuhar, Matej; Krucker, Säm; Glesener, Lindsay; Hannah, Iain G.; Grefenstette, Brian W.; Smith, David M.; Hudson, Hugh S.; White, Stephen M. Bibcode: 2018ApJ...856L..32K Altcode: 2018arXiv180308365K The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2-4.1 MK and emission measures of (0.6-15) × 1044 cm-3 describe their spectra well, resulting in thermal energies in the range (2-6) × 1026 erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares. Title: Magnetic Structure of a Composite Solar Microwave Burst Authors: Lee, Jeongwoo; White, Stephen M.; Liu, Chang; Kliem, Bernhard; Masuda, Satoshi Bibcode: 2018ApJ...856...70L Altcode: A composite flare consisting of an impulsive flare SOL2015-06-21T01:42 (GOES class M2.0) and a more gradual, long-duration flare SOL2015-06-21T02:36 (M2.6) from NOAA Active Region 12371, is studied using observations with the Nobeyama Radioheliograph (NoRH) and the Solar Dynamics Observatory (SDO). While composite flares are defined by their characteristic time profiles, in this paper we present imaging observations that demonstrate the spatial relationship of the two flares and allow us to address the nature of the evolution of a composite event. The NoRH maps show that the first flare is confined not only in time, but also in space, as evidenced by the stagnation of ribbon separation and the stationarity of the microwave source. The NoRH also detected another microwave source during the second flare, emerging from a different location where thermal plasma is so depleted that accelerated electrons could survive longer against Coulomb collisional loss. The AIA 131 Å images show that a sigmoidal EUV hot channel developed after the first flare and erupted before the second flare. We suggest that this eruption removed the high-lying flux to let the separatrix dome underneath reconnect with neighboring flux and the second microwave burst follow. This scenario explains how the first microwave burst is related to the much-delayed second microwave burst in this composite event. Title: Exploring the Sun with ALMA Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu, B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson, H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S. M.; Yan, Y. Bibcode: 2018Msngr.171...25B Altcode: The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory opens a new window onto the Universe. The ability to perform continuum imaging and spectroscopy of astrophysical phenomena at millimetre and submillimetre wavelengths with unprecedented sensitivity opens up new avenues for the study of cosmology and the evolution of galaxies, the formation of stars and planets, and astrochemistry. ALMA also allows fundamentally new observations to be made of objects much closer to home, including the Sun. The Sun has long served as a touchstone for our understanding of astrophysical processes, from the nature of stellar interiors, to magnetic dynamos, non-radiative heating, stellar mass loss, and energetic phenomena such as solar flares. ALMA offers new insights into all of these processes. Title: Magnetic Diagnostics of the Solar Corona: Synthesizing Optical and Radio Techniques Authors: Casini, R.; White, S. M.; Judge, P. G. Bibcode: 2018smf..book..145C Altcode: No abstract at ADS Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Gburek, S.; Grefenstette, B.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Schwartz, R.; Steslicki, M.; Turin, P.; Ryan, D.; Warmuth, A.; Veronig, A.; Vilmer, N.; White, S. M.; Woods, T. N. Bibcode: 2017AGUFMSH44A..07C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer (SMEX) Heliophysics mission that is currently undergoing a Phase A concept study. FOXSI will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis-stabilized spacecraft in low-Earth orbit, FOXSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. FOXSI is composed of a pair of x-ray telescopes with a 14-meter focal length enabled by a deployable boom. Making use of a filter-wheel and high-rate-capable solid-state detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect x-ray emission from weak flares, escaping electrons, and hot active regions. This mission concept is made possible by past experience with similar instruments on two FOXSI sounding rocket flights, in 2012 and 2014, and on the HEROES balloon flight in 2013. FOXSI's hard X-ray imager has a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec; it will cover the energy range from 3 up to 50-70 keV with a spectral resolution of better than 1 keV; and it will have sub-second temporal resolution. Title: Modeling the Ionosphere with GPS and Rotation Measure Observations Authors: Malins, J. B.; Taylor, G. B.; White, S. M.; Dowell, J. Bibcode: 2017AGUFMSA51D..01M Altcode: Advances in digital processing have created new tools for looking at and examining the ionosphere. We have combined data from dual frequency GPSs, digital ionosondes and observations from The Long Wavelength Array (LWA), a 256 dipole low frequency radio telescope situated in central New Mexico in order to examine ionospheric profiles. By studying polarized pulsars, the LWA is able to very accurately determine the Faraday rotation caused by the ionosphere. By combining this data with the international geomagnetic reference field, the LWA can evaluate ionospheric profiles and how well they predict the actual Faraday rotation. Dual frequency GPS measurements of total electron content, as well as measurements from digisonde data were used to model the ionosphere, and to predict the Faraday rotation to with in 0.1 rad/m2. Additionally, it was discovered that the predicted topside profile of the digisonde data did not accurate predict faraday rotation measurements, suggesting a need to reexamine the methods for creating the topside predicted profile. I will discuss the methods used to measure rotation measure and ionosphere profiles as well as discuss possible corrections to the topside model. Title: Forecasting E > 50-MeV Proton Events with the Proton Prediction System (PPS) Authors: Kahler, S. W.; White, S. M.; Ling, A. G. Bibcode: 2017AGUFMSH31A2723K Altcode: Forecasting solar energetic (E > 10 MeV) particle (SEP) events is an important element of space weather. While several models have been developed for use in forecasting such events, satellite operations are particularly vulnerable to higher-energy (> 50 MeV) SEP events. Here we validate one model, the proton prediction system (PPS), which extends to that energy range. We first develop a data base of E > 50-MeV proton events > 1.0 proton flux units (pfu) events observed on the GOES satellite over the period 1986 to 2016. We modify the PPS to forecast proton events at the reduced level of 1 pfu and run PPS for four different solar input parameters: (1) all > M5 solar X-ray flares; (2) all > 200 sfu 8800-MHz bursts with associated > M5 flares; (3) all > 500 sfu 8800-MHz bursts; and (4) all > 5000 sfu 8800-MHz bursts. For X-ray flare inputs the forecasted event peak intensities and fluences are compared with observed values. The validation contingency tables and skill scores are calculated for all groups and used as a guide to use of the PPS. We plot the false alarms and missed events as functions of solar source longitude. Title: ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm Authors: Iwai, K.; Loukitcheva, M.; Shimojo, M.; Solanki, S. K.; White, S. M. Bibcode: 2017AGUFMSH43A2806I Altcode: We report the discovery of a brightness enhancement in the center of a large sunspot umbra at a wavelength of 3 mm using the Atacama Large Millimeter/sub-millimeter Array (ALMA). Sunspots are among the most prominent features on the solar surface, but many of their aspects are surprisingly poorly understood. We analyzed a λ = 3 mm (100 GHz) mosaic image obtained by ALMA that includes a large sunspot within the active region AR12470, on 2015 December 16. The 3 mm map has a 300''×300'' field of view and 4.9''×2.2'' spatial resolution, which is the highest spatial resolution map of an entire sunspot in this frequency range. We find a gradient of 3 mm brightness from a high value in the outer penumbra to a low value in the inner penumbra/outer umbra. Within the inner umbra, there is a marked increase in 3 mm brightness temperature, which we call an umbral brightness enhancement. This enhanced emission corresponds to a temperature excess of 800 K relative to the surrounding inner penumbral region and coincides with excess brightness in the 1330 and 1400 Å slit-jaw images of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial lightbridge. This λ = 3 mm brightness enhancement may be an intrinsic feature of the sunspot umbra at chromospheric heights, such as a manifestation of umbral flashes, or it could be related to a coronal plume, since the brightness enhancement was coincident with the footpoint of a coronal loop observed at 171 Å. Title: Anticipated Results from the FOXSI SMEX Mission Authors: Shih, A. Y.; Christe, S.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Gburek, S.; Goetz, K.; Grefenstette, B.; Gubarev, M.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Ryan, D.; Schwartz, R.; Steslicki, M.; Turin, P.; Veronig, A.; Vilmer, N.; Warmuth, A.; White, S. M.; Woods, T. N. Bibcode: 2017AGUFMSH43C..03S Altcode: While there have been significant advances in our understanding of impulsive energy release at the Sun since the advent of RHESSI observations, there is a clear need for new X-ray observations that can capture the full range of emission in flares (e.g., faint coronal sources near bright chromospheric sources), follow the intricate evolution of energy release and changes in morphology, and search for the signatures of impulsive energy release in even the quiescent Sun. The FOXSI Small Explorer (SMEX) mission, currently undergoing a Phase A concept study, combines state-of-the-art grazing-incidence focusing optics with pixelated solid-state detectors to provide direct imaging of hard X-rays for the first time on a solar observatory. FOXSI's X-ray observations will provide quantitative information on (1) the non-thermal populations of accelerated electrons and (2) the thermal plasma distributions at the high temperatures inaccessible through other wavelengths. FOXSI's major science questions include: Where are electrons accelerated and on what time scales? Where do escaping flare-accelerated electrons originate? What is the energy input of accelerated electrons into the chromosphere and corona? How much do flare-like processes heat the corona above active regions? Here we present examples with simulated observations to show how FOXSI's capabilities will address and resolve these and other questions. Title: Thermal and Nonthermal Emissions of a Composite Flare Derived from NoRH and SDO Observations Authors: Lee, Jeongwoo; White, Stephen M.; Jing, Ju; Liu, Chang; Masuda, Satoshi; Chae, Jongchul Bibcode: 2017ApJ...850..124L Altcode: Differential emission measure (DEM) derived from the extreme ultraviolet (EUV) lines of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory is used in the analysis of a solar flare observed by the Nobeyama Radioheliograph (NoRH). The target was a composite event consisting of an impulsive flare, SOL2015-06-21T01:42 (GOES class M2.0), and a gradual flare, SOL2015-06-21T02:36 (M2.6), for which separation of thermal plasma heating from nonthermal particle acceleration was of major interest. We have calculated the thermal free-free intensity maps with the AIA-derived DEM and compared them against the observed NoRH maps to attribute the difference to the nonthermal component. In this way, we were able to locate three distinct sources: the major source with thermal and nonthermal components mixed, a nonthermal source devoid of thermal particles, and a thermal source lacking microwave emission. Both the first and the second nonthermal sources produced impulsively rising 17 GHz intensities and moved away from the local magnetic polarization inversion lines in correlation with the flare radiation. In contrast, the thermal sources stay in fixed locations and show temporal variations of the temperature and emission measure uncorrelated with the flare radiation. We interpret these distinct properties as indicating that nonthermal sources are powered by magnetic reconnection and thermal sources passively receive energy from the nonthermal donor. The finding of these distinct properties between thermal and nonthermal sources demonstrates the microwave and EUV emission measure combined diagnostics. Title: The First ALMA Observation of a Solar Plasmoid Ejection from an X-Ray Bright Point Authors: Shimojo, M.; Hudson, H. S.; White, S. M.; Bastian, T.; Iwai, K. Bibcode: 2017AGUFMSH41A2754S Altcode: Eruptive phenomena are important features of energy releases events, such solar flares, and have the potential to improve our understanding of the dynamics of the solar atmosphere. The 304 A EUV line of helium, formed at around 10^5 K, is found to be a reliable tracer of such phenomena, but the determination of physical parameters from such observations is not straightforward. We have observed a plasmoid ejection from an X-ray bright point simultaneously with ALMA, SDO/AIA, and Hinode/XRT. This paper reports the physical parameters of the plasmoid obtained by combining the radio, EUV, and X-ray data. As a result, we conclude that the plasmoid can consist either of (approximately) isothermal ∼10^5 K plasma that is optically thin at 100 GHz, or a ∼10^4 K core with a hot envelope. The analysis demonstrates the value of the additional temperature and density constraints that ALMA provides, and future science observations with ALMA will be able to match the spatial resolution of space-borne and other high-resolution telescopes. Title: High-Energy Aspects of Small-Scale Energy Release at the Sun Authors: Glesener, L.; Vievering, J. T.; Wright, P. J.; Hannah, I. G.; Panchapakesan, S. A.; Ryan, D.; Krucker, S.; Hudson, H. S.; Grefenstette, B.; White, S. M.; Smith, D. M.; Marsh, A.; Kuhar, M.; Christe, S.; Buitrago-Casas, J. C.; Musset, S.; Inglis, A. R. Bibcode: 2017AGUFMSM33E..04G Altcode: Large, powerful solar flares have been investigated in detail for decades, but it is only recently that high-energy aspects of small flares could be measured. These small-scale energy releases offer the opportunity to examine how particle acceleration characteristics scale down, which is critical for constraining energy transfer theories such as magnetic reconnection. Probing to minuscule flare sizes also brings us closer to envisioning the characteristics of the small "nanoflares" that may be responsible for heating the corona. A new window on small-scale flaring activity is now opening with the use of focusing hard X-ray instruments to observe the Sun. Hard X-rays are emitted by flare-accelerated electrons and strongly heated plasma, providing a relatively direct method of measuring energy release and particle acceleration properties. This work will show the first observations of sub-A class microflares using the FOXSI sounding rocket and the NuSTAR astrophysics spacecraft, both of which directly focus hard X-rays but have limited observing time on the Sun. These instruments serve as precursors to a spacecraft version of FOXSI, which will explore energy release across the entire range of flaring activity. Title: The Slowly Varying Corona: Findings from DEMs with the EVE MEGS-A Dataset Authors: Schonfeld, S.; White, S. M.; Hock, R. A.; Henney, C. J.; McAteer, J. Bibcode: 2017AGUFMSH43B2812S Altcode: We present analysis of the complete spectral dataset from the Extreme-ultraviolet (EUV) Variability Experiment (EVE) MEGS-A instrument. Using these data, we construct daily differential emission measures (DEMs) and use them to analyze the long-term variability of the global corona and the irradiance it produces. We identify a discontinuity in the EUV irradiance and DEMs separating solar minimum and maximum conditions. Using the DEMs we also study the relationship between EUV and F10.7, the 10.7 cm (2.8 GHz) solar activity proxy. We compare predictions of the geoeffective F10.7 from the DEMs and photospheric magnetic field observations with the commonly used 81-day averaged F10.7 to investigate their uses in parameterizing the solar EUV irradiance. Title: Solar Flare Dynamic Microwave Imaging with EOVSA Authors: Gary, D. E.; Chen, B.; Nita, G. M.; Fleishman, G. D.; Yu, S.; White, S. M.; Hurford, G. J.; McTiernan, J. M. Bibcode: 2017AGUFMSH41A2755G Altcode: The Expanded Owens Valley Solar Array (EOVSA) is both an expansion of our existing solar array and serves as a prototype for a much larger future project, the Frequency Agile Solar Radiotelescope (FASR). EOVSA is now complete, and is producing daily imaging of the full solar disk, including active regions and solar radio bursts at hundreds of frequencies in the range 2.8-18 GHz. We present highlights of the 1-s-cadence dynamic imaging spectroscropy of radio bursts we have obtained to date, along with deeper analysis of multi-wavelength observations and modeling of a well-observed burst. These observations are revealing the full life-cycle of the trapped population of high-energy electrons, from their initial acceleration and subsequent energy-evolution to their eventual decay through escape and thermalization. All of our data are being made available for download in both quick-look image form and in the form of the community-standard CASA measurement sets for subsequent imaging and analysis. Title: First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay; Hannah, Iain G.; Grefenstette, Brian W.; Caspi, Amir; Krucker, Säm; Hudson, Hugh S.; Madsen, Kristin K.; White, Stephen M.; Kuhar, Matej; Wright, Paul J.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, William W. Bibcode: 2017ApJ...849..131M Altcode: 2017arXiv171105385M We present the first results of a search for transient hard X-ray (HXR) emission in the quiet solar corona with the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite. While NuSTAR was designed as an astrophysics mission, it can observe the Sun above 2 keV with unprecedented sensitivity due to its pioneering use of focusing optics. NuSTAR first observed quiet-Sun regions on 2014 November 1, although out-of-view active regions contributed a notable amount of background in the form of single-bounce (unfocused) X-rays. We conducted a search for quiet-Sun transient brightenings on timescales of 100 s and set upper limits on emission in two energy bands. We set 2.5-4 keV limits on brightenings with timescales of 100 s, expressed as the temperature T and emission measure EM of a thermal plasma. We also set 10-20 keV limits on brightenings with timescales of 30, 60, and 100 s, expressed as model-independent photon fluxes. The limits in both bands are well below previous HXR microflare detections, though not low enough to detect events of equivalent T and EM as quiet-Sun brightenings seen in soft X-ray observations. We expect future observations during solar minimum to increase the NuSTAR sensitivity by over two orders of magnitude due to higher instrument livetime and reduced solar background. Title: Solar ALMA Observations: Constraining the Chromosphere above Sunspots Authors: Loukitcheva, Maria A.; Iwai, Kazumasa; Solanki, Sami K.; White, Stephen M.; Shimojo, Masumi Bibcode: 2017ApJ...850...35L Altcode: 2017arXiv171003812L We present the first high-resolution Atacama Large Millimeter/Submillimeter Array (ALMA) observations of a sunspot at wavelengths of 1.3 and 3 mm, obtained during the solar ALMA Science Verification campaign in 2015, and compare them with the predictions of semi-empirical sunspot umbral/penumbral atmosphere models. For the first time, millimeter observations of sunspots have resolved umbral/penumbral brightness structure at the chromospheric heights, where the emission at these wavelengths is formed. We find that the sunspot umbra exhibits a radically different appearance at 1.3 and 3 mm, whereas the penumbral brightness structure is similar at the two wavelengths. The inner part of the umbra is ∼600 K brighter than the surrounding quiet Sun (QS) at 3 mm and is ∼700 K cooler than the QS at 1.3 mm, being the coolest part of sunspot at this wavelength. On average, the brightness of the penumbra at 3 mm is comparable to the QS brightness, while at 1.3 mm it is ∼1000 K brighter than the QS. Penumbral brightness increases toward the outer boundary in both ALMA bands. Among the tested umbral models, that of Severino et al. provides the best fit to the observational data, including both the ALMA data analyzed in this study and data from earlier works. No penumbral model among those considered here gives a satisfactory fit to the currently available measurements. ALMA observations at multiple millimeter wavelengths can be used for testing existing sunspot models, and serve as an important input to constrain new empirical models. Title: Characteristics of Sustained >100 MeV Gamma-ray Emission Associated with Solar Flares Authors: Share, G. H.; Murphy, R. J.; Tolbert, A. K.; Dennis, B. R.; White, S. M.; Schwartz, R. A.; Tylka, A. J. Bibcode: 2017arXiv171101511S Altcode: We characterize and provide a catalog of thirty >100 MeV sustained gamma-ray emission (SGRE) events observed by Fermi LAT. These events are temporally and spectrally distinct from the associated solar flares. Their spectra are consistent with decay of pions produced by >300 MeV protons and are not consistent with electron bremsstrahlung. SGRE start times range from CME onset to two hours later. Their durations range from about four minutes to twenty hours and appear to be correlated with durations of >100 MeV SEP proton events. The >300 MeV protons producing SGRE have spectra that can be fit with power laws with a mean index of ~4 and RMS spread of 1.8. Gamma-ray line measurements indicate that SGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of SGRE protons >500 MeV is on average about ten times more than then the number in the associated flare and about fifty to one hundred times less than the number in the accompanying SEP. SGRE can extend tens of degrees from the are site. Sustained bremsstrahlung from MeV electrons was observed in one SGRE event. Flare >100 keV X-ray emission appears to be associated with SGRE and with intense SEPs. From this observation, we provide arguments that lead us to propose that sub-MeV to MeV protons escaping from the flare contribute to the seed population that is accelerated by shocks onto open field lines to produce SEPs and onto field lines returning to the Sun to produce SGRE. Title: Forecasting E > 50-MeV proton events with the proton prediction system (PPS) Authors: Kahler, Stephen W.; White, Stephen M.; Ling, Alan G. Bibcode: 2017JSWSC...7A..27K Altcode: Forecasting solar energetic (E > 10-MeV) particle (SEP) events is an important element of space weather. While several models have been developed for use in forecasting such events, satellite operations are particularly vulnerable to higher-energy (≥50-MeV) SEP events. Here we validate one model, the proton prediction system (PPS), which extends to that energy range. We first develop a data base of E ≥ 50-MeV proton events >1.0 proton flux units (pfu) events observed on the GOES satellite over the period 1986-2016. We modify the PPS to forecast proton events at the reduced level of 1 pfu and run PPS for four different solar input parameters: (1) all ≥M5 solar X-ray flares; (2) all ≥200 sfu 8800-MHz bursts with associated ≥M5 flares; (3) all ≥500 sfu 8800-MHz bursts; and (4) all ≥5000 sfu 8800-MHz bursts. The validation contingency tables and skill scores are calculated for all groups and used as a guide to use of the PPS. We plot the false alarms and missed events as functions of solar source longitude, and argue that the longitude-dependence employed by PPS does not match modern observations. Use of the radio fluxes as the PPS driver tends to result in too many false alarms at the 500 sfu threshold, and misses more events than the soft X-ray predictor at the 5000 sfu threshold. Title: Magnetic Diagnostics of the Solar Corona: Synthesizing Optical and Radio Techniques Authors: Casini, R.; White, S. M.; Judge, P. G. Bibcode: 2017SSRv..210..145C Altcode: 2017SSRv..tmp...91C In this contribution we review the current state-of-the-art of coronal magnetometry, in both optical and radio domains. We address the achievable objectives and the challenges of present measurement techniques and interpretation tools. In particular, we focus on the role that these observations can play for constraining and validating numerical models of the global coronal magnetic field. With regard to optical techniques, we mainly focus on the use of M1 diagnostics, further developing the theory of the formation of their polarization signatures in the magnetized corona. Title: Microflare Heating of an Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA Authors: Wright, Paul James; Hannah, Iain; Grefenstette, Brian; Glesener, Lindsay; Krucker, Sam; Hudson, Hugh S.; Smith, David M.; Marsh, Andrew; White, Stephen M.; Kuhar, Matej Bibcode: 2017SPD....4810802W Altcode: We present the first joint observation of a GOES equivalent A0.2 microflare that occurred on the 29 Apr 2015 with Hinode/XRT and NuSTAR. During the three hours of combined observation we observe distinctive loop heating in the soft X-rays from Hinode/XRT, and the hottest channels from SDO/AIA. Crucially the impulsive phase of this microflare was also observed by NuSTAR, a highly sensitive hard X-ray (2.5-80 keV; Harrison et al. 2013) focussing optics imaging spectrometer. The NuSTAR spectrum before and after the microflare is well-fitted by a single thermal model of about 3.3 - 3.5 MK, but at the impulsive phase shows additional material up to 10 MK. This higher temperature emission is confirmed when we produce the DEM using a combination of SDO/AIA, Hinode/XRT, and NuSTAR data. During the impulsive phase of the microflare we determine the heating rate to be about 3 x 1025 erg s-1. Although non-thermal emission is not detected we find upper-limits that are consistent with the required heating rate. Title: Solar Commissioning Observations of the Sun with ALMA Authors: White, Stephen M.; Shimojo, Masumi; Bastian, Timothy S.; Iwai, Kazumasa; Hales, Antonio; Brajsa, Roman; Skokic, Ivica; Kim, Sujin; Hudson, Hugh S.; Loukitcheva, Maria; Wedemeyer, Sven Bibcode: 2017SPD....4820402W Altcode: PI-led science observations have commenced with the Atacama Large Millimeter-submillimeter Array (ALMA) following an extensive commissioning effort. This talk will summarize that effort and discuss some of the scientific results derived from the commissioning data. As the solar cycle declines, ALMA observations will mainly address chromospheric science topics. Examples of data obtained during commissioning, both from the interferometer and from single-dish observations, will be presented. The temperatures of the layers that ALMA is most sensitive to have been determined for the two frequency bands currently used for solar observations. Curious behavior in a sunspot umbra and an observations of a small chromospheric ejection will be discussed. Title: Results from NuSTAR: Dynamics and time evolution in a sub-A class hard X-ray flare Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Hudson, Hugh S.; Grefenstette, Brian; White, Stephen M.; Smith, David M.; Marsh, Andrew Bibcode: 2017SPD....4810803G Altcode: We report a NuSTAR observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the flare to be an A0.2 class flare in brightness. This flare, with only ∼5 counts s-1 detector-1 observed by RHESSI, is fainter than any hard X-ray (HXR) flare in the existing literature. The flare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR flare spectra and images. The flare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peaking time with higher energy, similar spatial dimensions to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The flare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with its arising via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the flare to be 1.8×1027 ergs. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as “flarelike” properties. Title: Science with the Expanded Owens Valley Solar Array Authors: Nita, Gelu M.; Gary, Dale E.; Fleishman, Gregory D.; Chen, Bin; White, Stephen M.; Hurford, Gordon J.; McTiernan, James; Hickish, Jack; Yu, Sijie; Nelin, Kjell B. Bibcode: 2017SPD....4811009N Altcode: The Expanded Owens Valley Solar Array (EOVSA) is a solar-dedicated radio array that makes images and spectra of the full Sun on a daily basis. Our main science goals are to understand the basic physics of solar activity, such as how the Sun releases stored magnetic energy on timescales of seconds, and how that solar activity, in the form of solar flares and coronal mass ejections, influences the Earth and near-Earth space environment, through disruptions of communication and navigation systems, and effects on satellites and systems on the ground. The array, which is composed out of thirteen 2.1 m dishes and two 27 m dishes (used only for calibration), has a footprint of 1.1 km EW x 1.2 km NS and it is capable of producing, every second, microwave images at two polarizations and 500 science channels spanning the 1-18 GHz frequency range. Such ability to make multi-frequency images of the Sun in this broad range of frequencies, with a frequency dependent resolution ranging from ∼53” at 1 GHz to ∼3”at 18 GHz, is unique in the world. Here we present an overview of the EOVSA instrument and a first set of science-quality active region and solar flare images produced from data taken during April 2017.This research is supported by NSF grant AST-1615807 and NASA grant NNX14AK66G to New Jersey Institute of Technology. Title: A comparison of solar ALMA maps with solar images obtained at other wavelengths Authors: Brajsa, Roman; Sudar, Davor; Skokic, Ivica; Benz, Arnold; Kuhar, Matej; White, Stephen M. Bibcode: 2017SPD....4820403B Altcode: We use recently released Commissioning and Science Verification data of the Sun from the observing campaigns perfomed with the ALMA radio telescope in December 2014 and in December 2015. The dataset consists of calibrated maps of the Sun recorded in ALMA observing band 3 (corresponding to a wavelength of 3 mm) and band 6 (1.2 mm) which show both bright and dark areas and a background of highly structured pattern. Solar ALMA maps are compared with images in EUV (SDO/AIA), H-alpha (NISP, Cerro Tololo) and He 1083 nm (NSO/SOLIS), as well as with magnetograms (SDO/HMI) recorded at times closest to the ALMA observations. With a special software the images are overlapped and a correspondence of identified structures is checked in both ways. The visibility of active regions, sunspots, inversion lines of global magnetic field, prominences on the disc, coronal holes and coronal bright points is investigated in ALMA images at mm wavelengths. Single-dish ALMA images of the Sun reveal large-scale structures in the solar atmosphere, while high resolution interferometric images are used to analyse the fine-scale chromospheric structure. The intensities (the brightness temperatures) of identified structures were determined and compared with selected model-based predictions. Title: NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare Authors: Glesener, Lindsay; Krucker, Säm; Hannah, Iain G.; Hudson, Hugh; Grefenstette, Brian W.; White, Stephen M.; Smith, David M.; Marsh, Andrew J. Bibcode: 2017ApJ...845..122G Altcode: 2017arXiv170704770G We report a Nuclear Spectroscopic Telescope Array (NuSTAR) observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the event to be an A0.1 class flare in brightness. This microflare, with only ∼5 counts s-1 detector-1 observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is fainter than any hard X-ray (HXR) flare in the existing literature. The microflare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR microflare spectra and images. The microflare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peak time with higher energy, spatial dimensions similar to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The microflare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with an origin via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the microflare to be 2.4 × 1027 erg. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as “flare-like” properties. Title: The Slowly Varying Corona: Findings using DEMs from the EVE MEGS-A Dataset Authors: Schonfeld, Samuel J.; White, Stephen M.; Hock, Rachel A.; Henney, Carl John; McAteer, James Bibcode: 2017SPD....4840204S Altcode: We present analysis of the complete spectral dataset from the Extreme-ultraviolet (EUV) Variability Experiment (EVE) MEGS-A instrument. With these data we construct daily differential emission measures (DEMs) and use them to analyze the long-term variability of the global corona. We identify a discontinuity in the DEMs separating solar minimum and maximum conditions that suggests a fundamental change in the coronal temperature structure with solar activity. Using the DEMs, we also study the relationship between EUV and F10.7, the 10.7 cm (2.8 GHz) solar activity proxy. We compare the F10.7 predictions from the DEMs and photospheric magnetic field observations with irradiance microwave observations to constrain the source mechanisms of F10.7 and their relative contribution as a function of solar activity. This has serious implications for the use of F10.7 as an activity proxy in terrestrial atmospheric modeling and we discuss our results in the context of previous work. Comparing the DEMs with microwave observations also allows for a determination of the coronal iron abundance and a measurement of the FIP effect. Title: Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA Authors: Wright, Paul J.; Hannah, Iain G.; Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Smith, David M.; Marsh, Andrew J.; White, Stephen M.; Kuhar, Matej Bibcode: 2017ApJ...844..132W Altcode: 2017arXiv170606108W NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its initial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/XRT on 2015 April 29 at ∼11:29 UT. This microflare shows the heating of material to several million Kelvin, observed in soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single thermal model of 3.2-3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK, emission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at ∼3 MK and falls off sharply by 5 MK; but during the microflare’s impulsive phase, the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about 2.5× {10}25 erg s-1. As the NuSTAR spectrum is purely thermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the accelerated electrons to be the source of heating, a power-law spectrum of δ ≥slant 7 with a low-energy cutoff {E}c≲ 7 keV is required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares. Title: The Slowly Varying Corona. I. Daily Differential Emission Measure Distributions Derived from EVE Spectra Authors: Schonfeld, S. J.; White, S. M.; Hock-Mysliwiec, R. A.; McAteer, R. T. J. Bibcode: 2017ApJ...844..163S Altcode: 2017arXiv170609525S Daily differential emission measure (DEM) distributions of the solar corona are derived from spectra obtained by the Extreme-ultraviolet Variability Experiment (EVE) over a 4 yr period starting in 2010 near solar minimum and continuing through the maximum of solar cycle 24. The DEMs are calculated using six strong emission features dominated by Fe lines of charge states viii, ix, xi, xii, xiv, and xvi that sample the nonflaring coronal temperature range 0.3-5 MK. A proxy for the non-Fe xviii emission in the wavelength band around the 93.9 Å line is demonstrated. There is little variability in the cool component of the corona (T < 1.3 MK) over the 4 yr, suggesting that the quiet-Sun corona does not respond strongly to the solar cycle, whereas the hotter component (T > 2.0 MK) varies by more than an order of magnitude. A discontinuity in the behavior of coronal diagnostics in 2011 February-March, around the time of the first X-class flare of cycle 24, suggests fundamentally different behavior in the corona under solar minimum and maximum conditions. This global state transition occurs over a period of several months. The DEMs are used to estimate the thermal energy of the visible solar corona (of order 1031 erg), its radiative energy loss rate ((2.5-8) × {10}27 erg s-1), and the corresponding energy turnover timescale (about an hour). The uncertainties associated with the DEMs and these derived values are mostly due to the coronal Fe abundance and density and the CHIANTI atomic line database. Title: Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric Imaging Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.; Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.; Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.; Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder, S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.; Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y. Bibcode: 2017SoPh..292...87S Altcode: 2017arXiv170403236S Observations of the Sun at millimeter and submillimeter wavelengths offer a unique probe into the structure, dynamics, and heating of the chromosphere; the structure of sunspots; the formation and eruption of prominences and filaments; and energetic phenomena such as jets and flares. High-resolution observations of the Sun at millimeter and submillimeter wavelengths are challenging due to the intense, extended, low-contrast, and dynamic nature of emission from the quiet Sun, and the extremely intense and variable nature of emissions associated with energetic phenomena. The Atacama Large Millimeter/submillimeter Array (ALMA) was designed with solar observations in mind. The requirements for solar observations are significantly different from observations of sidereal sources and special measures are necessary to successfully carry out this type of observations. We describe the commissioning efforts that enable the use of two frequency bands, the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum interferometric-imaging observations of the Sun with ALMA. Examples of high-resolution synthesized images obtained using the newly commissioned modes during the solar-commissioning campaign held in December 2015 are presented. Although only 30 of the eventual 66 ALMA antennas were used for the campaign, the solar images synthesized from the ALMA commissioning data reveal new features of the solar atmosphere that demonstrate the potential power of ALMA solar observations. The ongoing expansion of ALMA and solar-commissioning efforts will continue to enable new and unique solar observing capabilities. Title: Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota, A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales, A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki, W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.; Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.; Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann, G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y. Bibcode: 2017SoPh..292...88W Altcode: 2017arXiv170504766W The Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12 m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that uses the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions, we derive quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at λ =1.3 mm. These values have statistical uncertainties of about 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of about 25″, the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range. Active regions and plages are among the hotter features, while a large sunspot umbra shows up as a depression, and filament channels are relatively cool. Prominences above the solar limb are a common feature of the single-dish images. Title: Laser Ablation Experiments on the Tamdakht H5 Chondrite Authors: White, S. M.; Stern, E. C. Bibcode: 2017LPICo1987.6130W Altcode: High-powered lasers were used to ablate and form surfaces similar to naturally formed fusion crusts on Tamdakht and basalt. High speed camera showed ablation during heating. IR reflectances at 15, 30 and 45 degrees observation angles are compared. Title: ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm Authors: Iwai, Kazumasa; Loukitcheva, Maria; Shimojo, Masumi; Solanki, Sami K.; White, Stephen M. Bibcode: 2017ApJ...841L..20I Altcode: 2017arXiv170503147I We report the discovery of a brightness enhancement in the center of a large sunspot umbra at a wavelength of 3 mm using the Atacama Large Millimeter/sub-millimeter Array (ALMA). Sunspots are among the most prominent features on the solar surface, but many of their aspects are surprisingly poorly understood. We analyzed a λ = 3 mm (100 GHz) mosaic image obtained by ALMA that includes a large sunspot within the active region AR12470, on 2015 December 16. The 3 mm map has a 300″ × 300″ field of view and 4.″9 × 2.″2 spatial resolution, which is the highest spatial resolution map of an entire sunspot in this frequency range. We find a gradient of 3 mm brightness from a high value in the outer penumbra to a low value in the inner penumbra/outer umbra. Within the inner umbra, there is a marked increase in 3 mm brightness temperature, which we call an umbral brightness enhancement. This enhanced emission corresponds to a temperature excess of 800 K relative to the surrounding inner penumbral region and coincides with excess brightness in the 1330 and 1400 Å slit-jaw images of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial lightbridge. This λ = 3 mm brightness enhancement may be an intrinsic feature of the sunspot umbra at chromospheric heights, such as a manifestation of umbral flashes, or it could be related to a coronal plume, since the brightness enhancement was coincident with the footpoint of a coronal loop observed at 171 Å. Title: Millimeter radiation from a 3D model of the solar atmosphere. II. Chromospheric magnetic field Authors: Loukitcheva, M.; White, S. M.; Solanki, S. K.; Fleishman, G. D.; Carlsson, M. Bibcode: 2017A&A...601A..43L Altcode: 2017arXiv170206018L
Aims: We use state-of-the-art, three-dimensional non-local thermodynamic equilibrium (non-LTE) radiative magnetohydrodynamic simulations of the quiet solar atmosphere to carry out detailed tests of chromospheric magnetic field diagnostics from free-free radiation at millimeter and submillimeter wavelengths (mm/submm).
Methods: The vertical component of the magnetic field was deduced from the mm/submm brightness spectra and the degree of circular polarization synthesized at millimeter frequencies. We used the frequency bands observed by the Atacama Large Millimeter/Submillimeter Array (ALMA) as a convenient reference. The magnetic field maps obtained describe the longitudinal magnetic field at the effective formation heights of the relevant wavelengths in the solar chromosphere.
Results: The comparison of the deduced and model chromospheric magnetic fields at the spatial resolution of both the model and current observations demonstrates a good correlation, but has a tendency to underestimate the model field. The systematic discrepancy of about 10% is probably due to averaging of the restored field over the heights contributing to the radiation, weighted by the strength of the contribution. On the whole, the method of probing the longitudinal component of the magnetic field with free-free emission at mm/submm wavelengths is found to be applicable to measurements of the weak quiet-Sun magnetic fields. However, successful exploitation of this technique requires very accurate measurements of the polarization properties (primary beam and receiver polarization response) of the antennas, which will be the principal factor that determines the level to which chromospheric magnetic fields can be measured.
Conclusions: Consequently, high-resolution and high-precision observations of circularly polarized radiation at millimeter wavelengths can be a powerful tool for producing chromospheric longitudinal magnetograms. Title: Editorial: Coronal Magnetometry Authors: Gibson, Sarah E.; Rachmeler, Laurel A.; White, Stephen M. Bibcode: 2017FrASS...4....3G Altcode: 2017FrASS...4E...3G No abstract at ADS Title: The First ALMA Observation of a Solar Plasmoid Ejection from an X-Ray Bright Point Authors: Shimojo, Masumi; Hudson, Hugh S.; White, Stephen M.; Bastian, Timothy S.; Iwai, Kazumasa Bibcode: 2017ApJ...841L...5S Altcode: 2017arXiv170404881S Eruptive phenomena such as plasmoid ejections or jets are important features of solar activity and have the potential to improve our understanding of the dynamics of the solar atmosphere. Such ejections are often thought to be signatures of the outflows expected in regions of fast magnetic reconnection. The 304 Å EUV line of helium, formed at around 105 K, is found to be a reliable tracer of such phenomena, but the determination of physical parameters from such observations is not straightforward. We have observed a plasmoid ejection from an X-ray bright point simultaneously at millimeter wavelengths with ALMA, at EUV wavelengths with SDO/AIA, and in soft X-rays with Hinode/XRT. This paper reports the physical parameters of the plasmoid obtained by combining the radio, EUV, and X-ray data. As a result, we conclude that the plasmoid can consist either of (approximately) isothermal ∼105 K plasma that is optically thin at 100 GHz, or a ∼104 K core with a hot envelope. The analysis demonstrates the value of the additional temperature and density constraints that ALMA provides, and future science observations with ALMA will be able to match the spatial resolution of space-borne and other high-resolution telescopes. Title: Evidence of Significant Energy Input in the Late Phase of a Solar Flare from NuSTAR X-Ray Observations Authors: Kuhar, Matej; Krucker, Säm; Hannah, Iain G.; Glesener, Lindsay; Saint-Hilaire, Pascal; Grefenstette, Brian W.; Hudson, Hugh S.; White, Stephen M.; Smith, David M.; Marsh, Andrew J.; Wright, Paul J.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, William W. Bibcode: 2017ApJ...835....6K Altcode: 2017arXiv170107759K We present observations of the occulted active region AR 12222 during the third Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaign on 2014 December 11, with concurrent Solar Dynamics Observatory (SDO)/AIA and FOXSI-2 sounding rocket observations. The active region produced a medium-size solar flare 1 day before the observations, at ∼18 UT on 2014 December 10, with the post-flare loops still visible at the time of NuSTAR observations. The time evolution of the source emission in the SDO/AIA 335 Å channel reveals the characteristics of an extreme-ultraviolet late-phase event, caused by the continuous formation of new post-flare loops that arch higher and higher in the solar corona. The spectral fitting of NuSTAR observations yields an isothermal source, with temperature 3.8-4.6 MK, emission measure (0.3-1.8) × 1046 cm-3, and density estimated at (2.5-6.0) × 108 cm-3. The observed AIA fluxes are consistent with the derived NuSTAR temperature range, favoring temperature values in the range of 4.0-4.3 MK. By examining the post-flare loops’ cooling times and energy content, we estimate that at least 12 sets of post-flare loops were formed and subsequently cooled between the onset of the flare and NuSTAR observations, with their total thermal energy content an order of magnitude larger than the energy content at flare peak time. This indicates that the standard approach of using only the flare peak time to derive the total thermal energy content of a flare can lead to a large underestimation of its value. Title: Focusing Solar Hard X-rays: Expected Results from a FOXSI Spacecraft Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.; Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis, A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.; Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.; Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey, N. L. S.; Musset, S.; Swisdak, M. Bibcode: 2016AGUFMSH13A2282G Altcode: Over the course of two solar cycles, RHESSI has examined high-energy processes in flares via high-resolution spectroscopy and imaging of soft and hard X-rays (HXRs). The detected X-rays are the thermal and nonthermal bremsstrahlung from heated coronal plasma and from accelerated electrons, respectively, making them uniquely suited to explore the highest-energy processes that occur in the corona. RHESSI produces images using an indirect, Fourier-based method and has made giant strides in our understanding of these processes, but it has also uncovered intriguing new mysteries regarding energy release location, acceleration mechanisms, and energy propagation in flares. Focusing optics are now available for the HXR regime and stand poised to perform another revolution in the field of high-energy solar physics. With two successful sounding rocket flights completed, the Focusing Optics X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and power of direct solar HXR imaging with its vastly superior sensitivity and dynamic range. Placing this mature technology aboard a spacecraft will offer a systematic way to explore high-energy aspects of the solar corona and to address scientific questions left unanswered by RHESSI. Here we present examples of such questions and show simulations of expected results from a FOXSI spaceborne instrument to demonstrate how these questions can be addressed with the focusing of hard X-rays. Title: Signatures of Reconnection Observed in a Candle-Flame Solar Flare at the Limb Authors: Reeves, K.; Chen, B.; White, S. M.; Schanche, N.; Tian, H. Bibcode: 2016AGUFMSH31B2565R Altcode: We examine a well-observed flare that occurred on the limb of the Sun on March 7, 2015 in order to find possible signatures of a termination shock due to outflows from reconnecting magnetic fields. Images of this flare from Hinode/XRT and the SDO/AIA 131 bandpass show a cusp-shaped morphology. The IRIS slit was positioned in the region of the current sheet, above the flare loops. Fe XXI is detected in the IRIS spectra with an average Doppler velocity of about 20 km/s. The non-thermal widths in IRIS decrease steadily from 23:00 UT on the 7th until 00:20 UT the next day. This decrease correlates well with the microwave radio flux observed by the Nobeyama Radioheliograph (NoRH), which is primarily due to thermal free-free emission based on the examination of NoRH images at 17 GHz and 34 GHz. Temperatures of the loop-top source derived from RHESSI and XRT also show a steady decrease during this time. We measure downflow velocities in the cusp region in the AIA 131 A bandpass, and find that from 22-23 UT the flows are about 300-400 km/s, and they slow down to about 100 km/s after 23 UT. This work supported by NASA Grant NNX15AJ93G. Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.; Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Grefenstette, B.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Gubarev, M.; Schwartz, R. A.; Steslicki, M.; Ryan, D.; Turin, P.; Warmuth, A.; White, S. M.; Veronig, A.; Vilmer, N.; Dennis, B. R. Bibcode: 2016AGUFMSH13A2281C Altcode: We present FOXSI (Focusing Optics X-ray Solar Imager), a recently proposed Small Explorer (SMEX) mission that will provide a revolutionary new perspective on energy release and particle acceleration on the Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic range and better than 10x the sensitivity of previous instruments. Flown on a 3-axis stabilized spacecraft in low-Earth orbit, FOXSI uses high-angular-resolution grazing-incidence focusing optics combined with state-of-the-art pixelated solid-state detectors to provide direct imaging of solar hard X-rays for the first time. FOXSI is composed of two individual x-ray telescopes with a 14-meter focal length enabled by a deployable boom. Making use of a filter-wheel and high-rate-capable solid-state detectors, FOXSI will be able to observe the largest flares without saturation while still maintaining the sensitivity to detect x-ray emission from weak flares, escaping electrons, and hot active regions. This SMEX mission is made possible by past experience with similar instruments on two sounding rocket flights, in 2012 and 2014, and on the HEROES balloon flight in 2013. FOXSI will image the Sun with a field of view of 9 arcminutes and an angular resolution of better than 8 arcsec; it will cover the energy range from 3 to 100 keV with a spectral resolution of better than 1 keV; and it will have sub-second temporal resolution. Title: Gamma-Ray Observations Related to the Acceleration of Ions in the Corona Authors: Share, G. H.; Murphy, R. J.; Tolbert, A. K.; White, S. M.; Dennis, B. R.; Schwartz, R. A.; Tylka, A. J. Bibcode: 2016AGUFMSH32A..07S Altcode: High-energy gamma-rays associated with solar flares have been observed for over thirty-five years by various satellite experiments. The most sensitive observations have been made recently by the Large Area Telescope (LAT) experiment on the Fermi satellite. These observations confirm the earlier observations in which >100 MeV emission was detected coincident with the impulsive flare hard X-rays and also in the minutes and hours afterward. Spectral evidence from the largest of these latter, time-extended events, indicate they arise from the decay of neutral and charged pions produced by the interaction of protons and alpha particles with energies >300 MeV/nucl and >200 MeV/nucl, respectively. These high-energy time-extended events, are almost always associated with fast CME's and appear to begin from as short as 1 min to as long as 100 min after the onset of the CME. The events appear to last as short as 10 min to as long as 18 hr. Our analysis indicates that the number of >500 MeV protons producing the time-extended emission typically is an order magnitude larger than the number producing emission during the impulsive flare. The observed delays from the CME and energetic comparisons suggest that most of the energy in the ions producing the sustained time-extended emission came from a source other than the impulsive flare. It is likely that the particles were accelerated by shocks associated with the CME's and thus may have an origin common with SEPs observed in space. Our comparisons using GOES HEPAD and neutron monitor data, and those reported in this Session by DeNolfo et al. using data from PAMELA, suggest that the numbers of particles producing the gamma-ray emission are typically at least an order of magnitude smaller than those observed in space. We focus our discussion in this talk on the time-extended gamma-ray events that begin within minutes of the CME onset as they may reflect shock-acceleration of protons to hundreds of MeV deep in the corona. This work was supported by the SHINE/NSF, NASA Fermi GI and SR&T, Chief of Naval Research, and EU HESPERIA programs. Title: Measuring the Magnetic Field of Coronal Mass Ejections Near the Sun Using Pulsars Authors: Howard, T. A.; Stovall, K.; Dowell, J.; Taylor, G. B.; White, S. M. Bibcode: 2016ApJ...831..208H Altcode: The utility of Faraday rotation to measure the magnetic field of the solar corona and large-scale transients within is a small, yet growing field in solar physics. This is largely because it has been recognized as a potentially valuable frontier in space weather studies, because the ability to measure the intrinsic magnetic field within coronal mass ejections (CMEs) when they are close to the Sun is of great interest for understanding a key element of space weather. Such measurements have been attempted over the last few decades using radio signals from artificial sources (I.e., spacecraft on the far side of the Sun), but studies involving natural radio sources are scarce in the literature. We report on a preliminary study involving an attempt to detect the Faraday rotation of a CME that passed in front of a pulsar (PSR B0950+08) in 2015 August. We combine radio measurements with those from a broadband visible light coronagraph, to estimate the upper limit of the magnetic field of the CME when it was in the corona. We find agreement between different approaches for obtaining its density, and values that are consistent with those predicted from prior studies of CME density close to the Sun. Title: Observation of quasi-periodic solar radio bursts associated with propagating fast-mode waves Authors: Goddard, C. R.; Nisticò, G.; Nakariakov, V. M.; Zimovets, I. V.; White, S. M. Bibcode: 2016A&A...594A..96G Altcode: 2016arXiv160804232G
Aims: Radio emission observations from the Learmonth and Bruny Island radio spectrographs are analysed to determine the nature of a train of discrete, periodic radio "sparks" (finite-bandwidth, short-duration isolated radio features) which precede a type II burst. We analyse extreme ultraviolet (EUV) imaging from SDO/AIA at multiple wavelengths and identify a series of quasi-periodic rapidly-propagating enhancements, which we interpret as a fast wave train, and link these to the detected radio features.
Methods: The speeds and positions of the periodic rapidly propagating fast waves and the coronal mass ejection (CME) were recorded using running-difference images and time-distance analysis. From the frequency of the radio sparks the local electron density at the emission location was estimated for each. Using an empirical model for the scaling of density in the corona, the calculated electron density was used to obtain the height above the surface at which the emission occurs, and the propagation velocity of the emission location.
Results: The period of the radio sparks, δtr = 1.78 ± 0.04 min, matches the period of the fast wave train observed at 171 Å, δtEUV = 1.7 ± 0.2 min. The inferred speed of the emission location of the radio sparks, 630 km s-1, is comparable to the measured speed of the CME leading edge, 500 km s-1, and the speeds derived from the drifting of the type II lanes. The calculated height of the radio emission (obtained from the density) matches the observed location of the CME leading edge. From the above evidence we propose that the radio sparks are caused by the quasi-periodic fast waves, and the emission is generated as they catch up and interact with the leading edge of the CME.

The movie associated to Fig. 2 is available at http://www.aanda.org Title: F10.7 and the slowly varying corona from EVE DEMs Authors: Schonfeld, Sam; White, S. M.; Hock, R. A.; McAteer, R. T. J. Bibcode: 2016shin.confE.193S Altcode: We present a differential emission measure (DEM) analysis of the slowly varying corona during the first half of solar cycle 24. Using the Extreme ultraviolet Variability Experiment (EVE) and the CHIANTI atomic line database we identify strong isolated iron emission lines present in the non-flaring spectrum with peak emissions covering the coronal temperature range of 5.7 < log(T) < 6.5. These lines are used to generate daily DEMs from EVE spectra to observe the long term variability of global coronal thermal properties. We discuss the choice of emission lines and the implications of this data set for the relationship between EUV and the F10.7 radio flux. Title: The First Focused Hard X-ray Images of the Sun with NuSTAR Authors: Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh; Hannah, Iain G.; Smith, David M.; Vogel, Julia K.; White, Stephen M.; Madsen, Kristin K.; Marsh, Andrew J.; Caspi, Amir; Chen, Bin; Shih, Albert; Kuhar, Matej; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Forster, Karl; Hailey, Charles J.; Harrison, Fiona A.; Miyasaka, Hiromasa; Stern, Daniel; Zhang, William W. Bibcode: 2016ApJ...826...20G Altcode: 2016arXiv160509738G We present results from the the first campaign of dedicated solar observations undertaken by the Nuclear Spectroscopic Telescope ARray (NuSTAR) hard X-ray (HXR) telescope. Designed as an astrophysics mission, NuSTAR nonetheless has the capability of directly imaging the Sun at HXR energies (>3 keV) with an increase in sensitivity of at least two magnitude compared to current non-focusing telescopes. In this paper we describe the scientific areas where NuSTAR will make major improvements on existing solar measurements. We report on the techniques used to observe the Sun with NuSTAR, their limitations and complications, and the procedures developed to optimize solar data quality derived from our experience with the initial solar observations. These first observations are briefly described, including the measurement of the Fe K-shell lines in a decaying X-class flare, HXR emission from high in the solar corona, and full-disk HXR images of the Sun. Title: The smallest hard X-ray flare? Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Smith, David M.; Grefenstette, Brian; Marsh, Andrew; Hudson, Hugh S.; White, Stephen M.; Chen, Bin Bibcode: 2016SPD....4740302G Altcode: We report a NuSTAR observation of a small solar flare on 2015 September 1, estimated to be on the order of a GOES class A.05 flare in brightness. This flare is fainter than any hard X-ray (HXR) flares in the existing literature, and with a peak rate of only ∼5 counts s-1 detector-1 observed by RHESSI, is effectively the smallest that can just barely be detected by the current standard (indirectly imaging) solar HXR instrumentation, though we expect that smaller flares will continue to be discovered as instrumental and observational techniques progress. The flare occurred during a solar observation by the highly sensitive NuSTAR astrophysical HXR spacecraft, which used its direct focusing optics to produce detailed flare spectra and images. The flare exhibits properties commonly observed in larger flares, including a fast rise and more gradual decay, and similar spatial dimensions to the RHESSI microflares. We will discuss the presence of non-thermal (flare-accelerated) electrons during the impulsive phase. The flare is small in emission measure, temperature, and energy, though not in physical dimensions. Its presence is an indication that flares do indeed scale down to smaller energies and retain what we customarily think of as “flarelike” properties. Title: Science Objectives of the FOXSI Small Explorer Mission Concept Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred, Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas, Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman, Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian; Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita, Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz, Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen Bibcode: 2016SPD....47.0814S Altcode: Impulsive particle acceleration and plasma heating at the Sun, from the largest solar eruptive events to the smallest flares, are related to fundamental processes throughout the Universe. While there have been significant advances in our understanding of impulsive energy release since the advent of RHESSI observations, there is a clear need for new X-ray observations that can capture the full range of emission in flares (e.g., faint coronal sources near bright chromospheric sources), follow the intricate evolution of energy release and changes in morphology, and search for the signatures of impulsive energy release in even the quiescent Sun. The FOXSI Small Explorer (SMEX) mission concept combines state-of-the-art grazing-incidence focusing optics with pixelated solid-state detectors to provide direct imaging of hard X-rays for the first time on a solar observatory. We present the science objectives of FOXSI and how its capabilities will address and resolve open questions regarding impulsive energy release at the Sun. These questions include: What are the time scales of the processes that accelerate electrons? How do flare-accelerated electrons escape into the heliosphere? What is the energy input of accelerated electrons into the chromosphere, and how is super-heated coronal plasma produced? Title: The slowly varying corona from DEMs with the EVE data set Authors: Schonfeld, Samuel J.; White, Stephen M.; Hock, Rachel A.; McAteer, James Bibcode: 2016SPD....47.0333S Altcode: We present a differential emission measure (DEM) analysis of the slowly varying corona during the first half of solar cycle 24. Using the Extreme ultraviolet Variability Experiment (EVE) and the CHIANTI atomic line database we identify strong isolated iron emission lines present in the non-flaring spectrum with peak emissions covering the coronal temperature range of 5.7 < log(T) < 6.5. These lines are used to generate daily DEMs from EVE spectra to observe the long term variability of global coronal thermal properties. We discuss the choice of emission lines and the implications of this data set for the relationship between EUV and the F10.7 radio flux. Title: Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu, B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin, P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz, A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.; Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary, D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.; Selhorst, C. L.; Barta, M. Bibcode: 2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Title: The First X-Ray Imaging Spectroscopy of Quiescent Solar Active Regions with NuSTAR Authors: Hannah, Iain G.; Grefenstette, Brian W.; Smith, David M.; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Madsen, Kristin K.; Marsh, Andrew; White, Stephen M.; Caspi, Amir; Shih, Albert Y.; Harrison, Fiona A.; Stern, Daniel; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Zhang, William W. Bibcode: 2016ApJ...820L..14H Altcode: 2016arXiv160301069H We present the first observations of quiescent active regions (ARs) using the Nuclear Spectroscopic Telescope Array (NuSTAR), a focusing hard X-ray telescope capable of studying faint solar emission from high-temperature and non-thermal sources. We analyze the first directly imaged and spectrally resolved X-rays above 2 keV from non-flaring ARs, observed near the west limb on 2014 November 1. The NuSTAR X-ray images match bright features seen in extreme ultraviolet and soft X-rays. The NuSTAR imaging spectroscopy is consistent with isothermal emission of temperatures 3.1-4.4 MK and emission measures 1-8 × 1046 cm-3. We do not observe emission above 5 MK, but our short effective exposure times restrict the spectral dynamic range. With few counts above 6 keV, we can place constraints on the presence of an additional hotter component between 5 and 12 MK of ∼ {10}46 cm-3 and ∼ {10}43 cm-3, respectively, at least an order of magnitude stricter than previous limits. With longer duration observations and a weakening solar cycle (resulting in an increased livetime), future NuSTAR observations will have sensitivity to a wider range of temperatures as well as possible non-thermal emission. Title: Comparison of 30 THz impulsive burst time development to microwaves, Hα, EUV, and GOES soft X-rays Authors: Miteva, R.; Kaufmann, P.; Cabezas, D. P.; Cassiano, M. M.; Fernandes, L. O. T.; Freeland, S. L.; Karlický, M.; Kerdraon, A.; Kudaka, A. S.; Luoni, M. L.; Marcon, R.; Raulin, J. -P.; Trottet, G.; White, S. M. Bibcode: 2016A&A...586A..91M Altcode: 2015arXiv151201763M The recent discovery of impulsive solar burst emission in the 30 THz band is raising new interpretation challenges. One event associated with a GOES M2 class flare has been observed simultaneously in microwaves, Hα, EUV, and soft X-ray bands. Although these new observations confirm some features found in the two prior known events, they exhibit time profile structure discrepancies between 30 THz, microwaves, and hard X-rays (as inferred from the Neupert effect). These results suggest a more complex relationship between 30 THz emission and radiation produced at other wavelength ranges. The multiple frequency emissions in the impulsive phase are likely to be produced at a common flaring site lower in the chromosphere. The 30 THz burst emission may be either part of a nonthermal radiation mechanism or due to the rapid thermal response to a beam of high-energy particles bombarding the dense solar atmosphere. Title: EUV & X-ray observations of microflare heating of AR12333 Authors: Hannah, I. G.; Wright, P. J.; Grefenstette, B.; Glesener, L.; Hudson, H. S.; Smith, D. M.; Krucker, S.; Marsh, A.; White, S. M. Bibcode: 2015AGUFMSH13B2442H Altcode: We present a study of the heating in AR12333 due to small microflares between 10:30 and 13:30UT on 29 April 2015. This region is well observed in EUV by the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) as well as Hinode's X-ray Telescope (XRT) operating in a higher cadence mode, switching through the five thicker filters (sensitive to the higher temperature range). The Hinode observations were a coordinated campaign with the NuSTAR hard X-ray focusing optics telescope (Harrison et al. 2013). NuSTAR was conducting a full disk mosaic observation of the Sun and caught AR12333 several times, providing imaging spectroscopy >2 keV. We investigate the heating in the active region due to several small microflares (about A1-Class). These were visible with the thicker XRT filters and only clear in EUV once the FeXVIII component was extracted from SDO/AIA 94Å, indicating heating primarily >3MK. Using the regularized inversion method of Hannah & Kontar 2012, we recover the DEM from the SDO/AIA and Hinode/XRT data and compare this to the thermal characteristics derived from NuSTAR. Title: Probing the Sun with ALMA: Observations and Simulations Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.; Carlsson, M. Bibcode: 2015ASPC..499..349L Altcode: 2015arXiv150805686L ALMA will open a new chapter in the study of the Sun by providing a leap in spatial resolution and sensitivity compared to currently available mm wavelength observations. In preparation of ALMA, we have carried out a large number of observational tests and state-of-the-art radiation MHD simulations. Here we review the best available observations of the Sun at millimeter wavelengths.Using state of the art radiation MHD simulations of the solar atmosphere we demonstrate the huge potential of ALMA observations for uncovering the nature of the solar chromosphere. We show that ALMA will not only provide a reliable probe of the thermal structure and dynamics of the chromosphere, it will also open up a powerful new diagnostic of magnetic field at chromospheric heights, a fundamentally important, but so far poorly known parameter. Title: Capabilities of a FOXSI Small Explorer Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis, B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar, E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred, J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk, J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey, B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J. Bibcode: 2015AGUFMSH43B2456I Altcode: We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer (SMEX) concept, a mission dedicated to studying particle acceleration and energy release on the Sun. FOXSI is designed as a 3-axis stabilized spacecraft in low-Earth orbit making use of state-of-the-art grazing incidence focusing optics, allowing for direct imaging of solar X-rays. The current design being studied features three telescope modules deployed in a low-inclination low-earth orbit (LEO). With a 15 meter focal length enabled by a deployable boom, FOXSI will observe the Sun in the 3-50 keV energe range. The FOXSI imaging concept has already been tested on two sounding rocket flights, in 2012 and 2014 and on the HEROES balloon payload flight in 2013. FOXSI will image the Sun with an angular resolution of 5'', a spectral resolution of 0.5 keV, and sub-second temporal resolution using CdTe detectors. In this presentation we investigate the science objectives and targets which can be accessed from this mission. Because of the defining characteristic of FOXSI is true imaging spectroscopy with high dynamic range and sensitivity, a brand-new perspective on energy release on the Sun is possible. Some of the science targets discussed here include; flare particle acceleration processes, electron beams, return currents, sources of solar energetic particles (SEPs), as well as understanding X-ray emission from active region structures and the quiescent corona. Title: Towards a Data-Optimized Coronal Magnetic Field Model (DOC-FM): Synthetic Test Beds and Multiwavelength Forward Modeling Authors: Gibson, S. E.; Dalmasse, K.; Fan, Y.; Fineschi, S.; MacKay, D.; Rempel, M.; White, S. M. Bibcode: 2015AGUFMSH54B..04G Altcode: Understanding the physical state of the solar corona is key to deciphering the origins of space weather as well as to realistically representing the environment to be navigated by missions such as Solar Orbiter and Solar Probe Plus. However, inverting solar coronal observations to reconstruct this physical state -- and in particular the three-dimensional coronal magnetic field - is complicated by limited lines of sight and by projection effects. On the other hand, the sensitivity of multiwavelength observations to different physical mechanisms implies a potential for simultaneous probing of different parts of the coronal plasma. In order to study this complementarity, and to ultimately establish an optimal set of observations for constraining the three-dimensional coronal magnetic field, we are developing a suite of representative simulations to act as diagnostic test beds. We will present three such test beds: a coronal active region, a quiescent prominence, and a global corona. Each fully define the physical state of density, temperature, and vector magnetic field in three dimensions throughout the simulation domain. From these test beds, and using the FORWARD SolarSoft IDL codes, we will create a broad range of synthetic data. Radio observables will include intensity and circular polarization (including gyroresonance effects) and Faraday rotation for a range of frequencies. Infrared and visible forbidden line diagnostics of Zeeman and saturated Hanle effects will yield full Stokes vector (I, Q, U, V) synthetic data, and UV permitted line Hanle diagnostics will yield intensity and linear polarization. In addition, we will synthesize UV and SXR imager data, UV/EUV spectrometric data, and white light brightness and polarized brightness. All of these synthetic data, along with the "ground truth" physical state of the simulations from which they are derived, will be made available to the community for the purpose of testing coronal inversion techniques. Title: Coronal Sources of the Solar F10.7 Radio Flux Authors: Schonfeld, S. J.; White, S. M.; Henney, C. J.; Arge, C. N.; McAteer, R. T. J. Bibcode: 2015ApJ...808...29S Altcode: 2015arXiv150800599S We present results from the first solar full-disk {{{F}}}10.7 (the radio flux at 10.7 cm, 2.8 GHz) image taken with the S-band receivers on the recently upgraded Karl G. Jansky Very Large Array in order to assess the relationship between the {{{F}}}10.7 index and solar extreme ultraviolet (EUV) emission. To identify the sources of the observed 2.8 GHz emission, we calculate differential emission measures from EUV images collected by the Atmospheric Imaging Assembly and use them to predict the bremsstrahlung component of the radio emission. By comparing the bremsstrahlung prediction and radio observation we find that 8.1% ± 0.5% of the variable component of the {{{F}}}10.7 flux is associated with the gyroresonance emission mechanism. Additionally, we identify optical depth effects on the radio limb which may complicate the use of {{{F}}}10.7 time series as an EUV proxy. Our analysis is consistent with a coronal iron abundance that is four times the photospheric level. Title: Bright 30 THz impulsive solar bursts Authors: Kaufmann, P.; White, S. M.; Marcon, R.; Kudaka, A. S.; Cabezas, D. P.; Cassiano, M. M.; Francile, C.; Fernandes, L. O. T.; Hidalgo Ramirez, R. F.; Luoni, M.; Marun, A.; Pereyra, P.; Souza, R. V. Bibcode: 2015JGRA..120.4155K Altcode: 2015arXiv150506177K Impulsive 30 THz continuum bursts have been recently observed in solar flares, utilizing small telescopes with a unique and relatively simple optical setup concept. The most intense burst was observed together with a GOES X2 class event on 27 October 2014, also detected at two subterahertz (sub-THz) frequencies, Reuven Ramaty High Energy Solar Spectroscopic Imager X-rays and Solar Dynamics Observatory/Helioseismic and Magnetic Imager and EUV. It exhibits strikingly good correlation in time and in space with white-light flare emission. It is likely that this association may prove to be very common. All three 30 THz events recently observed exhibited intense fluxes in the range of 104 solar flux units, considerably larger than those measured for the same events at microwave and submillimeter wavelengths. The 30 THz burst emission might be part of the same spectral burst component found at sub-THz frequencies. The 30 THz solar bursts open a promising new window for the study of flares at their origin. Title: Coronal Magnetography of a Simulated Solar Active Region from Microwave Imaging Spectropolarimetry Authors: Wang, Zhitao; Gary, Dale E.; Fleishman, Gregory D.; White, Stephen M. Bibcode: 2015ApJ...805...93W Altcode: 2015arXiv150305239W We have simulated the Expanded Owens Valley Solar Array (EOVSA) radio images generated at multiple frequencies from a model solar active region, embedded in a realistic solar disk model, and explored the resulting data cube for different spectral analysis schemes to evaluate the potential for realizing one of EOVSA’s most important scientific goals—coronal magnetography. In this paper, we focus on modeling the gyroresonance and free-free emission from an on-disk solar active region model with realistic complexities in electron density, temperature and magnetic field distribution. We compare the magnetic field parameters extrapolated from the image data cube along each line of sight after folding through the EOVSA instrumental profile with the original (unfolded) parameters used in the model. We find that even the most easily automated, image-based analysis approach (Level-0) provides reasonable quantitative results, although they are affected by systematic effects due to finite sampling in the Fourier (UV) plane. Finally, we note the potential for errors due to misidentified harmonics of the gyrofrequency, and discuss the prospects for applying a more sophisticated spectrally based analysis scheme (Level-1) to resolve the issue in cases where improved UV coverage and spatial resolution are available. Title: Millimeter radiation from a 3D model of the solar atmosphere. I. Diagnosing chromospheric thermal structure Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; White, S. M. Bibcode: 2015A&A...575A..15L Altcode: 2015arXiv150102898L
Aims: We use advanced 3D non-local thermodynamic equilibrium radiative magnetohydrodynamic simulations of the solar atmosphere to carry out detailed tests of chromospheric diagnostics at millimeter and submillimeter wavelengths.
Methods: We focused on the diagnostics of the thermal structure of the chromosphere in the wavelength bands from 0.4 mm up to 9.6 mm that can be accessed with the Atacama Large Millimeter/Submillimeter Array (ALMA) and investigated how these diagnostics are affected by the instrumental resolution.
Results: We find that the formation height range of the millimeter radiation depends on the location in the simulation domain and is related to the underlying magnetic structure. Nonetheless, the brightness temperature is a reasonable measure of the gas temperature at the effective formation height at a given location on the solar surface. There is considerable scatter in this relationship, but this is significantly reduced when very weak magnetic fields are avoided. Our results indicate that although instrumental smearing reduces the correlation between brightness and temperature, millimeter brightness can still be used to reliably diagnose electron temperature up to a resolution of 1''. If the resolution is more degraded, then the value of the diagnostic diminishes rapidly.
Conclusions: We conclude that millimeter brightness can image the chromospheric thermal structure at the height at which the radiation is formed. Thus multiwavelength observations with ALMA with a narrow step in wavelength should provide sufficient information for a tomographic imaging of the chromosphere. Title: Forecasting Solar UV & F10.7 with ADAPT Authors: Henney, C. J.; Hock, R. A.; Toussaint, W. A.; Schooley, A. K.; Arge, C. N.; White, S. M. Bibcode: 2014AGUFMSA12A..07H Altcode: A new method is reviewed here to forecast the solar 10.7 cm (2.8 GHz) radio flux, abbreviated F10.7, and selected bands of solar ultraviolet (UV) irradiance, ranging from 0.1 to 175 nm, utilizing advanced predictions of the global solar magnetic field generated by the ADAPT (Air Force Data Assimilative Photospheric Flux Transport) model. Initial results reveal a good correlation between the absolute value of the observed photospheric magnetic field and the observed F10.7 and selected UV bands. In Henney et al (2012), the observed F10.7 signal is found to correlate well with strong magnetic field regions. In addition, we find that observed integrated full-disk solar UV signals are correlated with weaker fields. By evolving solar magnetic maps forward 1 to 7 days with a flux transport model, this new method provides a realistic estimation of the Earth-side solar magnetic field distribution used to predict the F10.7 flux and UV irradiance. The ADAPT model used in this work was developed with support by a grant from the AFOSR (Air Force Office of Scientific Research). Title: Forecasting Sep Events with Same Active-Region Prior Flares Authors: Kahler, S. W.; Ling, A. G.; White, S. M. Bibcode: 2014AGUFMSH41D..08K Altcode: Forecasting large solar energetic (E > 10 MeV) particle (SEP) events is currently based on observed solar X-ray flare peak fluxes or fluences. Recent work has indicated that the probability of a solar eruptive event in an active region (AR) is enhanced when a large flare has occurred in that AR during the previous day. In addition, peak intensities Sp of SEP events associated with fast CMEs are larger for CMEs with prior CMEs from the same associated ARs in the previous day. This suggests that the associated SEP event probability and/or Sp may be higher for a given solar X-ray flare with a recent prior major flare in the same AR. We use data sets of NOAA flares and SEP events from cycles 23 and 24 to test this idea statistically for periods of prior flares ranging from 12 to 48 hours. The occurrence probabilities of large SEP events are not higher for flares with prior same-AR major flares, hence prior flare occurrence is not a useful SEP event forecasting tool. We show an example of a flare-prolific AR for which the SEP-associated flares are spatially distinct from the numerous non-SEP associated flares, indicating how prior flares may be unrelated to SEP-associated flares. Title: The Sources of F10.7 Emission Authors: Schonfeld, Samuel Joseph; White, S. M.; Henney, C. J.; McAteer, R. T. J.; Arge, C. N. Bibcode: 2014shin.confE..62S Altcode: The solar radio flux at a wavelength of 10.7 cm, F10.7, serves as a proxy for the Sun"s ionizing flux striking the Earth and is a heavily used index for space weather studies. In principal both the coronal sources of ionizing flux and strong coronal magnetic fields contribute to F10.7 via different emission mechanisms. Recently the Karl G. Jansky Very Large Array (VLA) has added the capability to make high-spatial-resolution images of the Sun at 10.7 cm. In this work we compare a trial F10.7 image from the VLA with the radio emission predicted to be present from EUV images of the Sun acquired by the AIA telescope on the Solar Dynamics Observatory at 6 wavelengths covering the coronal temperature range. Photospheric magnetograms are used to identify likely regions of strong coronal magnetic field, and the circular polarization measured by the VLA is used as a tracer of gyroresonance contributions to F10.7. We discuss the conversion of the EUV data to bremsstrahlung radio fluxes via the construction of differential emission measure images, and analyze the relative contributions of the different sources of F10.7 flux. Title: Current and future solar observation using focusing hard X-ray imagers Authors: Glesener, Lindsay; Caspi, Amir; Christe, Steven; Hannah, Iain; Hudson, Hugh S.; Hurford, Gordon J.; Grefenstette, Brian; Krucker, Sam; Marsh, Andrew; Mewaldt, Richard A.; Pivovaroff, Michael; Shih, Albert Y.; Smith, David M.; Vogel, Julia; White, Stephen M. Bibcode: 2014AAS...22412364G Altcode: The efficient processes that accelerate particles in solar flares are not currently understood. Hard X-rays (HXRs) are one of the best diagnostics of flare-accelerated electrons, and therefore of acceleration processes. Past and current solar HXR observers rely on indirect Fourier imaging and thus lack the necessary sensitivity and imaging dynamic range to make detailed studies of faint HXR sources in the solar corona (where particle acceleration is thought to occur). A future generation of solar HXR observers will instead likely rely on direct HXR focusing, which can provide far superior sensitivity and imaging dynamic range.The first wave of focused solar HXR studies is already underway, including sounding rocket and high-altitude balloon payloads, and, in the near future, solar observation by the NuSTAR astrophysics observatory. This poster will (1) summarize the capabilities of current solar HXR instruments, comparing the science that can be done from each platform, and (2) discuss the scientific power of a future, dedicated, spaceborne observatory optimized to observe HXRs from the Sun. Title: The Sources of F10.7 Emission Authors: Schonfeld, Samuel J.; White, Stephen M.; Henney, Carl John; McAteer, James; Arge, Charles Bibcode: 2014AAS...22432323S Altcode: The solar radio flux at a wavelength of 10.7 cm, F10.7, serves as a proxy for the Sun’s ionizing flux striking the Earth and is a heavily used index for space weather studies. In principal both the coronal sources of ionizing flux and strong coronal magnetic fields contribute to F10.7 via different emission mechanisms. Recently the Expanded Very Large Array (EVLA) has added the capability to make high-spatial-resolution images of the Sun at 10.7 cm. In this work we compare a trial F10.7 image from the EVLA with the radio emission predicted to be present from EUV images of the Sun acquired by the AIA telescope on the Solar Dynamics Observatory at 6 wavelengths covering the coronal temperature range. Photospheric magnetograms are used to identify likely regions of strong coronal magnetic field, and the circular polarization measured by the EVLA is used as a tracer of gyroresonance contributions to F10.7. We discuss the conversion of the EUV data to bremsstrahlung radio fluxes via the construction of differential emission measure images, and analyze the relative contributions of the different sources of F10.7 flux. Title: Forecasting Solar EUV/FUV & F10.7 Authors: Henney, Carl J.; Hock, Rachel A.; Schooley, Alicia K.; Toussaint, W. Alex; Arge, C. Nick; White, Stephen M. Bibcode: 2014shin.confE.160H Altcode: A new method is presented here to forecast solar 10.7 cm (2.8 GHz) radio flux, abbreviated F10.7, and selected bands of solar ultraviolet (UV) and extreme UV irradiance, ranging from 0.1 to 175 nm, utilizing advanced predictions of the global solar magnetic field generated by the ADAPT (Air Force Data Assimilation Photospheric Flux Transport) model. Initial results reveal a good correlation between the absolute value of the observed photospheric magnetic field and the observed F10.7 and selected UV bands. In Henney et al (2012), the observed F10.7 signal is found to correlate strongly with strong magnetic field regions. In addition, we find that observed integrated full-disk solar UV signals are strongly correlated with weak field. By evolving solar magnetic maps forward 1 to 7 days with a flux transport model, this new method provides a realistic estimation of the Earth-side solar magnetic field distribution used to forecast F10.7 and EUV/FUV. The ADAPT model used in this work was developed with support by a grant from the AFOSR (Air Force Office of Scientific Research). Title: Characteristics of Sustained >100 MeV γ-ray Emissions Observed by Fermi and their Association with Solar Eruptive Events Authors: Share, Gerald H.; Murphy, Ronald; Tylka, Allan J.; Dennis, Brian R.; Schwartz, Richard A.; Tolbert, Anne K; White, Stephen M. Bibcode: 2014AAS...22411101S Altcode: The Fermi Large Area Telescope (LAT) has detected >20 sustained gamma-ray events >100 MeV lasting up to 20 hours. Three of these events have been discussed by the LAT Collaboration in two papers. Similar high-energy events have been observed earlier and were given the name Long Duration Gamma Ray Flares (LDGRFs; Ryan, 2000). We discuss a comprehensive study of the Fermi events beginning with a list of 98 solar eruptive events (SEEs) from 2008 to 2012 May with broad/fast (>800 km/s) or >100 keV hard X-ray emission or SEPs with >10 MeV proton fluxes above 1 proton flux unit. Our study provides the following characteristics of LAT LDGRFs: of 67 disk SEEs, 41 had broad/fast CMEs and 20 had both broad/fast CMEs and impulsive >100 keV emission; 12 of these 20 were detected by LAT above 100 MeV; no LAT events were detected in 21 events with broad/fast CMEs when hard X-ray emission was <100 keV; no LAT events were detected from the 31 CMEs originating from behind the disk. This suggests that sustained emission appears to require both a broad-fast CME and a flare with impulsive emission >100 keV. From our studies of behind-the-limb SEEs and LAT fluxes vs heliolongitude, we conclude that the protons responsible for the sustained >100 MeV events interact within about 20-30 deg. of the active region, but not necessarily at the footpoints of the flare loops. We also find in a study of all events: >300 MeV proton interactions producing the >100 MeV emission begin from <1 min to tens of min from the peak of the HXR emission; durations of the sustained emission events last from ~30 min to 20 hrs; spectral indices of >300 MeV protons at the Sun range from about -2.5 (2012 May 17 GLE) to steeper than -6 (average about -4.8); the proton spectrum can both soften and harden in time; the numbers of >500 MeV solar protons producing the sustained emission are typically ten-times larger than those in the impulsive flare, but there are exceptions; the numbers of >500 MeV protons producing sustained emission is typically ten-times smaller than the numbers in SEPs and is well correlated, based on a limited number of measurements. This work is funded by the NSF/SHINE and NASA's Fermi/GI programs. Title: Coronal Magnetography of a Simulated Solar Active Region from Microwave Imaging Spectropolarimetry Authors: Wang, Zhitao; Gary, Dale E.; White, Stephen M. Bibcode: 2014AAS...22432345W Altcode: We have simulated the Expanded Owens Valley Solar Array (EOVSA) radio images generated at multiple frequencies from a model solar active region, embedded in a realistic solar disk model, and evaluated the resulting datacube for different spectral analysis schemes to evaluate the potential for realizing one of EOVSA's most important scientific goals — coronal magnetography. In this paper, we focus on modeling the gyroresonance and free-free emission from an on-disk solar active region model with realistic complexities in electron density, temperature and magnetic field distribution. We compare the magnetic field parameters extrapolated from the image datacube along each line of sight after folding through the EOVSA instrumental profile with the original (unfolded) parameters used in the model. We find that even the most easily automated, image-based analysis approach (Level 0) provides reasonable quantitative results, although they are affected by systematic effects due to finite sampling in the Fourier (uv) plane. Title: The Expanded Owens Valley Solar Array (EOVSA) Authors: Gary, Dale E.; Hurford, Gordon J.; Nita, Gelu M.; White, Stephen M.; McTiernan, James; Fleishman, Gregory D. Bibcode: 2014AAS...22412360G Altcode: The Expanded Owens Valley Solar Array (EOVSA) near Big Pine, CA is undergoing commissioning as a solar-dedicated microwave imaging array operating in the frequency range 2.5-18 GHz. The solar science to be addressed focuses on the 3D structure of the solar corona (magnetic field, temperature and density), and on the particle acceleration, transport and heating in solar flares. The project will support the scientific community by providing open data access and software tools for analysis and modeling of the data, to exploit synergies with on-going solar research in other wavelengths. The array consists of a total of 15 antennas, including the two 27-m antennas with He-cooled receivers for sensitive calibration, and thirteen 2.1-m antennas that each view the entire disk of the Sun. The system includes a completely new control system, broadband signal transmission, and high-speed digital signal processing, using new technology developed for the Frequency Agile Solar Radiotelescope (FASR). We present an overview of the instrument, the current status of commissioning activities, and some initial observations to assess performance.This research is supported by NSF grants AST-1312802, and NASA grants NNX11AB49G and NNX10AF27G to New Jersey Institute of Technology. Title: Observing the Sun with the Murchison Widefield Array Authors: Oberoi, D.; Sharma, R.; Bhatnagar, S.; Lonsdale, C. J.; Matthews, L. D.; Cairns, I. H.; Tingay, S. J.; Benkevitch, L.; Donea, A.; White, S. M.; Bernardi, G.; Bowman, J. D.; Briggs, F.; Cappallo, R. J.; Corey, B. E.; Deshpande, A.; Emrich, D.; Gaensler, B. M.; Goeke, R.; Greenhill, L. J.; Hazelton, B. J.; Johnston-Hollitt, M.; Kaplan, D. L.; Kasper, J. C.; Kratzenberg, E.; Lynch, M. J.; McWhirter, S. R.; Mitchell, D. A.; Morales, M. F.; Morgan, E.; Offringa, A. R.; Ord, S. M.; Prabu, T.; Rogers, A. E. E.; Roshi, A.; Salah, J. E.; Udaya Shankar, N.; Srivani, K. S.; Subrahmanyan, R.; Waterson, M.; Wayth, R. B.; Webster, R. L.; Whitney, A. R.; William, A.; Williams, C. L. Bibcode: 2014arXiv1403.6250O Altcode: The Sun has remained a difficult source to image for radio telescopes, especially at the low radio frequencies. Its morphologically complex emission features span a large range of angular scales, emission mechanisms involved and brightness temperatures. In addition, time and frequency synthesis, the key tool used by most radio interferometers to build up information about the source being imaged is not effective for solar imaging, because many of the features of interest are short lived and change dramatically over small fractional bandwidths. Building on the advances in radio frequency technology, digital signal processing and computing, the kind of instruments needed to simultaneously capture the evolution of solar emission in time, frequency, morphology and polarization over a large spectral span with the requisite imaging fidelity, and time and frequency resolution have only recently begun to appear. Of this class of instruments, the Murchison Widefield Array (MWA) is best suited for solar observations. The MWA has now entered a routine observing phase and here we present some early examples from MWA observations. Title: Metrewave observations of the Sun Authors: Oberoi, D.; Sharma, R.; Bhatnagar, S.; Lonsdale, C. J.; Matthews, L. D.; Cairns, I. H.; Tingay, S. J.; Benkevitch, L.; Donea, A.; White, S. M.; Bernardi, G.; Bowman, J. D.; Briggs, F.; Cappallo, R. J.; Corey, B. E.; Deshpande, A.; Emrich, D.; Gaensler, B. M.; Goeke, R.; Greenhill, L. J.; Hazelton, B. J.; Johnston-Hollitt, M.; Kaplan, D. L.; Kasper, J. C.; Kratzenberg, E.; Lynch, M. J.; McWhirter, S. R.; Mitchell, D. A.; Morales, M. F.; Morgan, E.; Ord, S. M.; Prabu, T.; Rogers, A. E. E.; Roshi, A.; Salah, J. E.; Udaya-Shankar, N.; Srivani, K. S.; Subrahmanyan, R.; Waterson, M.; Wayth, R. B.; Webster, R. L.; Whitney, A. R.; Williams, A.; Williams, C. L. Bibcode: 2014ASInC..13...13O Altcode: High fidelity solar imaging at low radio frequencies remains a challenge. Solar emission is characterized by its large angular size, complex and dynamic morphological features spanning a large range of angular sizes, emission mechanisms spanning a wide range in brightness temperatures, and temporal and spectral variability of the emission over large ranges in time and frequency. Capturing the solar emission processes with high fidelity and in sufficient detail hence requires a capability to simultaneously track the emission in time, frequency and morphology over a large bandwidth. Traditional interferometers rely on time and frequency synthesis to produce high fidelity and dynamic range imaging, and are hence intrinsically poorly suited to the task of instantaneous imaging over narrow spectral spans.

With the emergence of a new generation of large-N interferometers, the situation has improved in a very significant manner. Of these instruments, the Murchison Widefield Array (MWA), located at the site chosen for the SKA in Western Australia, is the one most suited for solar observations at low radio frequencies. The MWA has recently commenced routine observing. Here we illustrate the diversity of features seen in solar emission using the MWA data to substantiate its suitability for solar studies. Title: Probing Magnetic Energy Release in a Solar Flare with Radio Dynamic Imaging Spectroscopy Authors: Chen, Bin; Bastian, T. S.; Gary, D. E.; White, S. M. Bibcode: 2014AAS...22311804C Altcode: Solar flares involve sudden release of magnetic energy that is previously stored in the Sun's corona. Yet details of the flare energy release processes are still poorly understood. Solar radio bursts are intense and short-lived radio emissions that occur in solar flares. They are believed to be intimately related to flare energy release processes. However, their potential in diagnosing flare energy release has been greatly limited by the lack of simultaneous spatial information. The upgraded Karl G. Jansky Very Large Array (VLA) provides the first opportunity of radio synthesis imaging along with high spectral and temporal resolution, making the new technique of radio dynamic imaging spectroscopy possible. We report VLA observations of a solar flare event using this new technique, during which a rich variety of radio bursts are recorded. With the help of concurrent data in extreme ultra-violet and X-ray wavelengths, these observations allow us to establish the relation between the bursts and flare energy release, and use them to probe physical properties of the energy release site. Title: The chromosphere above sunspots at millimeter wavelengths Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M. Bibcode: 2014A&A...561A.133L Altcode: 2014arXiv1403.3436L
Aims: The aim of this paper is to demonstrate that millimeter wave data can be used to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy.
Methods: We use observations of the temperature contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm observations from Lindsey & Kopp (1995) and 2 cm observations with the Very Large Array. These are compared with the umbral contrast calculated from various atmospheric models of sunspots.
Results: Current mm and submm observational data suggest that the brightness observed at these wavelengths is low compared to the most widely used sunspot models. These data impose strong constraints on the temperature and density stratifications of the sunspot umbral atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region.
Conclusions: A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K). Better spatial resolution as well as better wavelength coverage are needed for a more complete determination of the chromospheric temperature stratification above sunspot umbrae. Title: Solar Drivers for Space Weather Operations (Invited) Authors: White, S. M. Bibcode: 2013AGUFMSM52C..03W Altcode: Most space weather effects can be tied back to the Sun, and major research efforts are devoted to understanding the physics of the relevant phenomena with a long-term view of predicting their occurrence. This talk will focus on the current state of knowledge regarding the solar drivers of space weather, and in particular the connection between the science and operational needs. Topics covered will include the effects of solar ionizing flux on communications and navigation, radio interference, flare forecasting, the solar wind and the arrival of coronal mass ejections at Earth. Title: The detection of a bright 30 THz impulsive solar burst Authors: White, Stephen M.; Kaufmann, P.; Freeland, S. L.; Marcon, R.; Fernandes, L. T.; Kudaka, A. S.; de Souza, R. V.; Marun, A.; Valio, A.; Raulin, J.; Gimenez de Castro, C. Bibcode: 2013SPD....4440206W Altcode: One of the last unexplored wavelength frontiers for solar flares is in the range of submillimeter to infrared wavelengths. We report the detection of an intense impulsive burst at 30 THz using a new imaging system. The 30 THz emission exhibited remarkable time coincidence with peaks observed at microwave, mm/submm, visible, EUV and hard X-ray wavelengths. The 30 THz burst location matches a weak white-light feature, an intense EUV knot, and a hard X-ray source. The two spatial structures at EUV are not time coincident, and appear to correspond to two successive peaks at 30 THz and submm, the second one without time correspondence with the impulsive component. The coincidence with a white-light feature is consistent with heating below the temperature minimum in the atmosphere. However, there are problems in attributing the heating to accelerated electrons. The peak 30 THz flux is several times larger than the usual microwave peak near 9 GHz, attributed to non-thermal electrons in the corona. The 30 THz emission could be consistent with an optically thick spectrum increasing from low to high frequencies. It might be part of the same spectral component found at sub-THz frequencies whose nature remains mysterious. Further observations at these wavelengths will provide a new window for flare studies. Title: A Bright Impulsive Solar Burst Detected at 30 THz Authors: Kaufmann, P.; White, S. M.; Freeland, S. L.; Marcon, R.; Fernandes, L. O. T.; Kudaka, A. S.; de Souza, R. V.; Aballay, J. L.; Fernandez, G.; Godoy, R.; Marun, A.; Valio, A.; Raulin, J. -P.; Giménez de Castro, C. G. Bibcode: 2013ApJ...768..134K Altcode: 2013arXiv1303.5894K Ground- and space-based observations of solar flares from radio wavelengths to gamma-rays have produced considerable insights but raised several unsolved controversies. The last unexplored wavelength frontier for solar flares is in the range of submillimeter and infrared wavelengths. Here we report the detection of an intense impulsive burst at 30 THz using a new imaging system. The 30 THz emission exhibited remarkable time coincidence with peaks observed at microwave, mm/submm, visible, EUV, and hard X-ray wavelengths. The emission location coincides with a very weak white-light feature, and is consistent with heating below the temperature minimum in the atmosphere. However, there are problems in attributing the heating to accelerated electrons. The peak 30 THz flux is several times larger than the usual microwave peak near 9 GHz, attributed to non-thermal electrons in the corona. The 30 THz emission could be consistent with an optically thick spectrum increasing from low to high frequencies. It might be part of the same spectral component found at sub-THz frequencies whose nature remains mysterious. Further observations at these wavelengths will provide a new window for flare studies. Title: Solar flares at submillimeter wavelengths Authors: Krucker, Säm; Giménez de Castro, C. G.; Hudson, H. S.; Trottet, G.; Bastian, T. S.; Hales, A. S.; Kašparová, J.; Klein, K. -L.; Kretzschmar, M.; Lüthi, T.; Mackinnon, A.; Pohjolainen, S.; White, S. M. Bibcode: 2013A&ARv..21...58K Altcode: We discuss the implications of the first systematic observations of solar flares at submillimeter wavelengths, defined here as observing wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The events observed thus far show that this wave band requires a new understanding of high-energy processes in solar flares. Several events, including observations from two different observatories, show during the impulsive phase of the flare a spectral component with a positive (increasing) slope at the highest observable frequencies (up to 405 GHz). To emphasize the increasing spectra and the possibility that these events could be even more prominent in the THz range, we term this spectral feature a "THz component". Here we review the data and methods, and critically assess the observational evidence for such distinct component(s). This evidence is convincing. We also review the several proposed explanations for these feature(s), which have been reported in three distinct flare phases. These data contain important clues to flare development and particle acceleration as a whole, but many of the theoretical issues remain open. We generally have lacked systematic observations in the millimeter-wave to far-infrared range that are needed to complete our picture of these events, and encourage observations with new facilities. Title: Imaging the Sun with the Murchison Widefield Array Authors: Oberoi, D.; Matthews, L. D.; Cairns, I. H.; Tingay, S. J.; Benkevitch, L.; Donea, A.; White, S. M.; Arcus, W.; Barnes, D.; Bernardi, G.; Bowman, J. D.; Briggs, F.; Burns, S.; Bunton, J. D.; Cappallo, R. J.; Corey, B. E.; Deshpande, A.; deSouza, L.; Emrich, D.; Goeke, R.; Gaensler, B. M.; Greenhill, L. -J.; Hazelton, B. J.; Herne, D.; Johnston-Hollitt, M.; Kaplan, D. L.; Kasper, J. C.; Kincaid, B. B.; Koeing, R.; Kratzenberg, E.; Lonsdale, C. J.; Lynch, M. J.; McWhirter, S. R.; Mitchell, D. A.; Morales, M. F.; Morgan, E.; Ord, S. M.; Pathikulungara, J.; Prabu, T.; Remillard, R. A.; Rogers, A. E. E.; Roshi, A.; Salah, J. E.; Sault, R. J.; Udaya-Shankar, N.; Srivani, K. S.; Stevens, J.; Subrahmanyan, R.; Waterson, M.; Wayth, R. B.; Webster, R. L.; Whitney, A. R.; Williams, A.; Williams, C. L.; Wyithe, J. S. B. Bibcode: 2013ASInC..10..131O Altcode: The Murchison Widefield Array (MWA) is a new generation low-frequency radio (80--300 MHz) array. The MWA design exploits recent advances in digital hardware capabilities and affordability of computational capacity to meet the needs of low-frequency radio astronomy. Solar and coronal imaging and studies of the heliosphere and the ionosphere via their propagation effects on low-frequency radio waves comprise one of the four key science goals of the MWA. Here we present some early solar science results to highlight the exceptional imaging dynamic range and fidelity of the MWA and its high time and frequency resolution, ahead of commencement of the regular observing scheduled for mid 2013. Title: Tracing Electron Beams in the Sun's Corona with Radio Dynamic Imaging Spectroscopy Authors: Chen, Bin; Bastian, T. S.; White, S. M.; Gary, D. E.; Perley, R.; Rupen, M.; Carlson, B. Bibcode: 2013ApJ...763L..21C Altcode: 2012arXiv1211.3058C We report observations of type III radio bursts at decimeter wavelengths (type IIIdm bursts)—signatures of suprathermal electron beams propagating in the low corona—using the new technique of radio dynamic imaging spectroscopy provided by the recently upgraded Karl G. Jansky Very Large Array. For the first time, type IIIdm bursts were imaged with high time and frequency resolution over a broad frequency band, allowing electron beam trajectories in the corona to be deduced. Together with simultaneous hard X-ray and extreme ultraviolet observations, we show that these beams emanate from an energy release site located in the low corona at a height below ~15 Mm, and propagate along a bundle of discrete magnetic loops upward into the corona. Our observations enable direct measurements of the plasma density along the magnetic loops, and allow us to constrain the diameter of these loops to be less than 100 km. These overdense and ultra-thin loops reveal the fundamentally fibrous structure of the Sun's corona. The impulsive nature of the electron beams, their accessibility to different magnetic field lines, and the detailed structure of the magnetic release site revealed by the radio observations indicate that the localized energy release is highly fragmentary in time and space, supporting a bursty reconnection model that involves secondary magnetic structures for magnetic energy release and particle acceleration. Title: Opportunities for Solar Science with NuSTAR Authors: Glesener, Lindsay; Boggs, S. E.; Christensen, F.; Craig, W. W.; Hailey, C. J.; Grefenstette, B.; Harrison, F.; Hudson, H. S.; Hurford, G. J.; Krucker, S.; Marsh, A.; Mewaldt, R. A.; Pivovaroff, M.; Smith, D. M.; Stern, D.; Vogel, J.; White, S. M.; Zhang, W.; NuSTAR Team Bibcode: 2013AAS...22124423G Altcode: While NuSTAR was designed to observe faint cosmic sources in hard X-rays (HXR), its unprecedented sensitivity can also be used to address several outstanding questions in high energy solar physics. Medium- and large-sized solar flares have been well -studied in HXR by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), launched in 2002. These flares are always found in active regions and usually emit nonthermal HXR from accelerated electrons, along with thermal bremsstrahlung as those electrons lose their energy and heat the ambient plasma. To date, no HXR flares outside active regions have been observed, though thermal brightenings in soft X-rays and EUV suggest that small "nanoflares" may occur frequently across the entire solar disk, even at quiet times when no active regions are present. Even a few minutes of NuSTAR solar observations will allow a search for HXR from quiet-Sun nanoflares with better sensitivity than any previous study. These observations will have important implications for the role of flares in supplying the corona with its surprisingly hot temperature (1--2 MK, as compared with the photospheric temperature of 5800 K). NuSTAR will also make the first observations of escaping flare electrons associated with Type III radio emission, can image faint coronal sources in partially occulted flares that are below RHESSI's sensitivity, and, combined with RHESSI data, could study the faint, earliest phase of flares, where direct signatures of particle acceleration are most likely to be observed. Title: Solar Hard X-ray Observations with NuSTAR Authors: Marsh, Andrew; Smith, D. M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Harrison, F. A.; Grefenstette, B. W.; Stern, D. Bibcode: 2012AAS...22052112M Altcode: High-sensitivity imaging of coronal hard X-rays allows detection of freshly accelerated nonthermal electrons at the acceleration site. A few such observations have been made with Yohkoh and RHESSI, but a leap in sensitivity could help pin down the time, place, and manner of reconnection. Around the time of this meeting, the Nuclear Spectroscopic Telescope ARray (NuSTAR), a NASA Small Explorer for high energy astrophysics that uses grazing-incidence optics to focus X-rays up to 80 keV, will be launched. Three weeks will be dedicated to solar observing during the baseline two-year mission. NuSTAR will be 200 times more sensitive than RHESSI in the hard X-ray band. This will allow the following new observations, among others: 1) Extrapolation of the micro/nanoflare distribution by two orders of magnitude down in flux; 2) Search for hard X-rays from network nanoflares (soft X-ray bright points) and evaluation of their role in coronal heating; 3) Discovery of hard X-ray bremsstrahlung from the electron beams driving type III radio bursts, and measurement of their electron spectrum; 4) Hard X-ray studies of polar soft X-ray jets and impulsive solar energetic particle events at the edge of coronal holes; 5) Study of coronal bremsstrahlung from particles accelerated by coronal mass ejections as they are first launched; 6) Study of particles at the coronal reconnection site when flare footpoints and loops are occulted; 7) Search for weak high-temperature coronal plasmas in active regions that are not flaring; and 8) Search for hypothetical axion particles created in the solar core via the hard X-ray signal from their conversion to X-rays in the coronal magnetic field. NuSTAR will also serve as a pathfinder for a future dedicated space mission with enhanced capabilities, such as a satellite version of the FOXSI sounding rocket. Title: Direct Comparison Of A Moreton Wave, EUV Wave And CME Authors: White, Stephen M.; Cliver, E.; Balasubramaniam, K. Bibcode: 2012AAS...22020412W Altcode: The first period of major solar activity in the current cycle, due to AR 11158 in mid February 2011, produced a sequence of solar flares exhibiting both Moreton waves in H-alpha images and "EIT-waves" seen in EUV images. Given the rarity of Moreton waves, this offers an excellent opportunity to compare the properties of the two phenomena with simultaneous observations. We focus on the event of 17:24 UT on February 14, which was well-observed by a number of observatories. We find a strong link between the Moreton wave, the EUV wave and the CME in this event. The Moreton wave has the same speed as the EUV wave, but it lags behind the leading edge of the EUV wave. A vertical signature is seen in the H-alpha Doppler images. STEREO observations of the CME indicate that initially the vertical speed of the disturbance was not as high as the EUV wave speed, and the CME rapidly decelerates on merging with a rising loop system. We interpret the results in light of current models for such disturbances. Title: Forecasting F10.7 with solar magnetic flux transport modeling Authors: Henney, C. J.; Toussaint, W. A.; White, S. M.; Arge, C. N. Bibcode: 2012SpWea..10.2011H Altcode: A new method is presented here to forecast the solar 10.7 cm (2.8 GHz) radio flux, abbreviated F10.7, utilizing advanced predictions of the global solar magnetic field generated by a flux transport model. Using indices derived from the absolute value of the solar magnetic field, we find good correlation between the observed photospheric magnetic activity and the observed F10.7 values. Comparing magnetogram data observed within 6 hours of the F10.7 measurements during the years 1993 through 2010, the Spearman correlation coefficient, rs, for an empirical model of F10.7 is found to be 0.98. In addition, we find little change in the empirical model coefficients and correlations between the first and second 9 year intervals of the 18 year period investigated. By evolving solar magnetic synoptic maps forward 1-7 days, this new method provides a realistic estimation of the Earth-side solar magnetic field distribution used to forecast F10.7. Spearman correlation values of approximately 0.97, 0.95, and 0.93 are found for 1 day, 3 day, and 7 day forecasts, respectively. The method presented here can be expanded to forecast other space weather parameters, e.g., total solar irradiance and extreme ultraviolet flux. In addition, near-term improvements to the F10.7 forecasting method, e.g., including far-side magnetic data with solar magnetic flux transport, are discussed. Title: The Solar Decimetric Spike Burst of 2006 December 6: Possible Evidence for Field-aligned Potential Drops in Post-eruption Loops Authors: Cliver, E. W.; White, S. M.; Balasubramaniam, K. S. Bibcode: 2011ApJ...743..145C Altcode: A 1.4 GHz solar radio burst associated with a 3B/X6 eruptive flare on 2006 December 6 had the highest peak flux density (~106 sfu) of any event yet recorded at this frequency. The decimetric event characteristics during the brightest emission phase (numerous intense, short-lived, narrow-band bursts that overlapped to form a continuous spectrum) suggest electron cyclotron maser (ECM) emission. The peak 1.4 GHz emission did not occur during the flare impulsive phase but rather ~45 minutes later, in association with post-eruption loop activity seen in Hα and by the Hinode EUV Imaging Spectrometer. During the Waves/LASCO era, three other delayed bursts with peak intensities >105 sfu in the 1.0-1.6 GHz (L-band) frequency range have been reported that appear to have characteristics similar to the December 6 burst. In each of these three cases, high-frequency type IV bursts were reported in a range from ~150 to ~1500 MHz. Assuming a common ECM emission mechanism across this frequency range implies a broad span of source heights in the associated post-eruption loop systems. Difficulties with an ECM interpretation for these events include the generation of the lower frequency component of the type IVs and the long-standing problem of escape of the ECM emission from the loops. Magnetic-field-aligned potential drops, analogous to those observed for Earth's auroral kilometric radiation, could plausibly remove both of these objections to ECM emission. Title: Sub-THz and Hα Activity during the Preflare and Main Phases of a GOES Class M2 Event Authors: Kaufmann, Pierre; Marcon, Rogério; Giménez de Castro, C. Guillermo; White, Stephen M.; Raulin, Jean-Pierre; Correia, Emilia; Olavo Fernandes, Luis; de Souza, Rodney V.; Godoy, Rodolfo; Marun, Adolfo; Pereyra, Pablo Bibcode: 2011ApJ...742..106K Altcode: 2011arXiv1110.3013K Radio and optical observations of the evolution of flare-associated phenomena have shown an initial and rapid burst at 0.4 THz only followed subsequently by a localized chromospheric heating producing an Hα brightening with later heating of the whole active region. A major instability occurred several minutes later producing one impulsive burst at microwaves only, associated with an M2.0 GOES X-ray flare that exhibited the main Hα brightening at the same site as the first flash.The possible association between long-enduring time profiles at soft X-rays, microwaves, Hα, and sub-THz wavelengths is discussed. In the decay phase, the Hα movie shows a disrupting magnetic arch structure ejecting dark, presumably chromospheric, material upward. The time sequence of events suggests genuine interdependent and possibly non-thermal instabilities triggering phenomena, with concurrent active region plasma heating and material ejection. Title: The Relationship Between Solar Radio and Hard X-ray Emission Authors: White, S. M.; Benz, A. O.; Christe, S.; Fárník, F.; Kundu, M. R.; Mann, G.; Ning, Z.; Raulin, J. -P.; Silva-Válio, A. V. R.; Saint-Hilaire, P.; Vilmer, N.; Warmuth, A. Bibcode: 2011SSRv..159..225W Altcode: 2011SSRv..tmp..263W; 2011SSRv..tmp..244W; 2011SSRv..tmp..164W; 2011arXiv1109.6629W; 2011SSRv..tmp...88W This review discusses the complementary relationship between radio and hard X-ray observations of the Sun using primarily results from the era of the Reuven Ramaty High Energy Solar Spectroscopic Imager satellite. A primary focus of joint radio and hard X-ray studies of solar flares uses observations of nonthermal gyrosynchrotron emission at radio wavelengths and bremsstrahlung hard X-rays to study the properties of electrons accelerated in the main flare site, since it is well established that these two emissions show very similar temporal behavior. A quantitative prescription is given for comparing the electron energy distributions derived separately from the two wavelength ranges: this is an important application with the potential for measuring the magnetic field strength in the flaring region, and reveals significant differences between the electrons in different energy ranges. Examples of the use of simultaneous data from the two wavelength ranges to derive physical conditions are then discussed, including the case of microflares, and the comparison of images at radio and hard X-ray wavelengths is presented. There have been puzzling results obtained from observations of solar flares at millimeter and submillimeter wavelengths, and the comparison of these results with corresponding hard X-ray data is presented. Finally, the review discusses the association of hard X-ray releases with radio emission at decimeter and meter wavelengths, which is dominated by plasma emission (at lower frequencies) and electron cyclotron maser emission (at higher frequencies), both coherent emission mechanisms that require small numbers of energetic electrons. These comparisons show broad general associations but detailed correspondence remains more elusive. Title: Chromosphere above sunspots as seen at millimeter wavelengths Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen M. Bibcode: 2011IAUS..273..408L Altcode: Millimeter emission is known to be a sensitive diagnostic of temperature and density in the solar chromosphere. In this work we use millimeter wave data to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy. From mm brightness simulations we expect a radio umbra to change its appearance from dark to bright (compared to the Quiet Sun) at a given wavelength in the millimeter spectrum (depending on the exact temperature in the model used). Thereby the millimeter brightness observed above an umbra at several wavelengths imposes strong constraints on temperature and density stratification of the sunspot atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region. Current mm/submm observational data suggest that brightness observed at short wavelengths is unexpectedly low compared to the most widely used sunspot models such as of Maltby et al. (1986). A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K), such as in the models of Severino et al. (1994). However, we are not able to resolve the umbra cleanly with the presently available observations and better resolution as well as better wavelength coverage are needed for accurate diagnostics of umbral brightness at millimeter wavelengths. This adds one more scientific objective for the Atacama Large Millimeter/Submillimeter Array (ALMA). Title: Microwave Depolarization above Sunspots Authors: Lee, Jeongwoo; White, Stephen M. Bibcode: 2011IAUS..273..487L Altcode: Microwave emissions from sunspots are circularly polarized in the sense of rotation (right or left) determined by the polarity (north or south) of coronal magnetic fields. However, they may convert into unpolarized emissions under certain conditions of magnetic field and electron density in the corona, and this phenomenon of depolarization could be used to derive those parameters. We propose another diagnostic use of microwave depolarization based on the fact that an observed depolarization strip actually represents the coronal magnetic polarity inversion line (PIL) at the heights of effective mode coupling, and its location itself carries information on the distribution of magnetic polarity in the corona. To demonstrate this diagnostic utility we generate a set of magnetic field models for a complex active region with the observed line-of-sight magnetic fields but varying current density distribution and compare them with the 4.9 GHz polarization map obtained with the Very Large Array (VLA). The field extrapolation predicts very different locations of the depolarization strip in the corona depending on the amount of electric currents assumed to exist in the photosphere. Such high sensitivity of microwave depolarization to the coronal magnetic field can therefore be useful for validating electric current density maps inferred from vector magnetic fields observed in the photosphere. Title: Solar Hard X-ray Observations with NuSTAR Authors: Smith, David M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Stern, D.; Grefenstette, B. W.; Harrison, F. A. Bibcode: 2011SPD....42.1501S Altcode: 2011BAAS..43S.1501S High-sensitivity imaging of coronal hard X-rays allows detection of freshly accelerated nonthermal electrons at the acceleration site. A few such observations have been made with Yohkoh and RHESSI, but a leap in sensitivity could help pin down the time, place, and manner of reconnection.

In 2012, the Nuclear Spectroscopic Telescope Array (NuSTAR), a NASA Small Explorer for high energy astrophysics that uses grazing-incidence optics to focus X-rays up to 80 keV, will be launched. NuSTAR is capable of solar pointing, and three weeks will be dedicated to solar observing during the baseline two-year mission. NuSTAR will be 200 times more sensitive than RHESSI in the hard X-ray band. This will allow the following new observations, among others:

1) Extrapolation of the micro/nanoflare distribution by two orders of magnitude down in flux

2) Search for hard X-rays from network nanoflares (soft X-ray bright points) and evaluation of their role in coronal heating

3) Discovery of hard X-ray bremsstrahlung from the electron beams driving type III radio bursts, and measurement of their electron spectrum

4) Hard X-ray studies of polar soft X-ray jets and impulsive solar energetic particle events at the edge of coronal holes, and comparison of these events with observations of 3He and other particles in interplanetary space

5) Study of coronal bremsstrahlung from particles accelerated by coronal mass ejections as they are first launched

6) Study of particles at the coronal reconnection site when flare footpoints are occulted; and

7) Search for hypothetical axion particles created in the solar core via the hard X-ray signal from their conversion to X-rays in the coronal magnetic field.

NuSTAR will also serve as a pathfinder for a future dedicated space mission with enhanced capabilities, such as a satellite version of the FOXSI sounding rocket. Title: Simultaneous Observations of Hα Moreton Waves and EUV Waves Authors: White, Stephen M.; Balasubramanian, K. S.; Cliver, E. W. Bibcode: 2011SPD....42.1307W Altcode: 2011BAAS..43S.1307W The first period of major solar activity in the current cycle, due to AR 11158 in mid February 2011, produced a sequence of solar flares exhibiting both Moreton waves in H-alpha images and "EIT-waves" seen in EUV images. Given the rarity of Moreton waves, this offers an excellent opportunity to compare the properties of the two phenomena with simultaneous observations. We analyze several events and compare the speeds and locations of the disturbances using high-cadence H-alpha data from both the ISOON telescope at Sunspot, NM, and the GONG network, together with EUV images in several wavelengths from the SDO/AIA telescope, and interpret the results in light of current models for such disturbances. Title: Impulsive High-Energy Particle Acceleration in theSOL2010-06-12T00:57 M2 X-rayFlare Authors: Share, Gerald H.; Briggs, M. S.; Gruber, D.; Longo, F.; Murphy, R. J.; Omodei, N.; Schwartz, R. A.; White, S. M.; Tylka, A. J.; Fermi LAT Collaboration; Fermi GBM Collaboration Bibcode: 2011SPD....42.1102S Altcode: 2011BAAS..43S.1102S The GOES M2-class solar flare, SOL2010-06-12T00:57, was modest in many respects yet exhibited remarkable acceleration of energetic particles. While both radio and SDO/AIA UV/EUV images indicate a compact flare with foot-point separation of just 10 arc sec, this small region produced an 70 sec burst of hard X-and gamma-ray emission up to at least 200 MeV observed by the Fermi GBM and LAT experiments. The gamma-ray line and >300 keV bremsstrahlung fluences from this flare were about ten times higher than that typically observed from this modest GOES-class of X-ray flare. Analysis of the combined nuclear line and high-energy gamma-ray emissions suggests that the accelerated proton spectrum at the Sun softened significantly above 50 MeV. We compare these observations with measurements of solar energetic protons to determine whether the particle populations at the Sun and in space may have a common origin. The 34 and 80 GHz microwave emissions are very bright and well correlated with the few hundred keV X-ray emission, but with small time lags suggesting mild trapping of electrons in the corona. Title: The Great Decimetric Solar Spike Burst of 2006 December 6: Possible Evidence for Field-aligned Potential Drops in Post-eruption Loops Authors: Cliver, Edward W.; White, S. M.; Balasubramaniam, K. S. Bibcode: 2011SPD....42.2223C Altcode: 2011BAAS..43S.2223C A 1.4 GHz solar radio burst associated with a 3B/X6 eruptive flare on 2006 December 6 had the highest peak flux density ( 106 sfu) of any event yet recorded at this frequency. The decimetric event characteristics during the brightest emission phase (numerous intense, short-lived, narrow-band bursts that overlapped to form a continuous spectrum) suggest electron cyclotron maser (ECM) emission. The peak 1.4 GHz emission did not occur during the flare impulsive phase but rather 45 minutes later, in association with post-eruption loop activity seen in H-alpha and by Hinode EIS. During the Waves/LASCO era, three other delayed bursts with peak intensities >105 sfu in the 1.0-1.6 GHz (L-band) frequency range have been reported and appear to have characteristics similar to the December 6 burst. In each of these three cases type IV bursts were reported in a range from 150 to 1500 MHz. Assuming a common ECM emission mechanism across this frequency range implies a broad span of source heights in the associated post-eruption loop systems. Difficulties with an ECM interpretation for these events include the generation of the lower frequency component of the type IVs and the long-standing problem of escape of the ECM emission from the loops. Magnetic-field-aligned potential drops, analogous to those observed for Earth's auroral kilometric radiation, could plausibly remove both of these objections to ECM emission. Title: The Expanded Owens Valley Solar Array Authors: Gary, Dale E.; Hurford, G. J.; Nita, G. M.; White, S. M.; Tun, S. D.; Fleishman, G. D.; McTiernan, J. M. Bibcode: 2011SPD....42.0102G Altcode: 2011BAAS..43S.0102G The Expanded Owens Valley Solar Array (EOVSA) is now under construction near Big Pine, CA as a solar-dedicated microwave imaging array operating in the frequency range 1-18 GHz. The solar science to be addressed focuses on the 3D structure of the solar corona (magnetic field, temperature and density), on the sudden release of energy and subsequent particle acceleration, transport and heating, and on space weather phenomena. The project will support the scientific community by providing open data access and software tools for analysis of the data, to exploit synergies with on-going solar research in other wavelengths.

The New Jersey Institute of Technology (NJIT) is expanding OVSA from its previous complement of 7 antennas to a total of 15 by adding 8 new antennas, and will reinvest in the existing infrastructure by replacing the existing control systems, signal transmission, and signal processing with modern, far more capable and reliable systems based on new technology developed for the Frequency Agile Solar Radiotelescope (FASR). The project will be completed in time to provide solar-dedicated observations during the upcoming solar maximum in 2013 and beyond. We provide an update on current status and our preparations for exploiting the data through modeling and data analysis tools.

This research is supported by NSF grants AST-0908344, and AGS-0961867 and NASA grant NNX10AF27G to New Jersey Institute of Technology. Title: First Spectroscopic Imaging Observations of the Sun at Low Radio Frequencies with the Murchison Widefield Array Prototype Authors: Oberoi, Divya; Matthews, Lynn D.; Cairns, Iver H.; Emrich, David; Lobzin, Vasili; Lonsdale, Colin J.; Morgan, Edward H.; Prabu, T.; Vedantham, Harish; Wayth, Randall B.; Williams, Andrew; Williams, Christopher; White, Stephen M.; Allen, G.; Arcus, Wayne; Barnes, David; Benkevitch, Leonid; Bernardi, Gianni; Bowman, Judd D.; Briggs, Frank H.; Bunton, John D.; Burns, Steve; Cappallo, Roger C.; Clark, M. A.; Corey, Brian E.; Dawson, M.; DeBoer, David; De Gans, A.; deSouza, Ludi; Derome, Mark; Edgar, R. G.; Elton, T.; Goeke, Robert; Gopalakrishna, M. R.; Greenhill, Lincoln J.; Hazelton, Bryna; Herne, David; Hewitt, Jacqueline N.; Kamini, P. A.; Kaplan, David L.; Kasper, Justin C.; Kennedy, Rachel; Kincaid, Barton B.; Kocz, Jonathan; Koeing, R.; Kowald, Errol; Lynch, Mervyn J.; Madhavi, S.; McWhirter, Stephen R.; Mitchell, Daniel A.; Morales, Miguel F.; Ng, A.; Ord, Stephen M.; Pathikulangara, Joseph; Rogers, Alan E. E.; Roshi, Anish; Salah, Joseph E.; Sault, Robert J.; Schinckel, Antony; Udaya Shankar, N.; Srivani, K. S.; Stevens, Jamie; Subrahmanyan, Ravi; Thakkar, D.; Tingay, Steven J.; Tuthill, J.; Vaccarella, Annino; Waterson, Mark; Webster, Rachel L.; Whitney, Alan R. Bibcode: 2011ApJ...728L..27O Altcode: 2011arXiv1101.0620O We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array, observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9- 201.6 MHz. Though our observing period is characterized as a period of "low" to "medium" activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low-frequency radio interferometers come online over the next few years. Title: On the Origin of the Solar Moreton Wave of 2006 December 6 Authors: Balasubramaniam, K. S.; Cliver, E. W.; Pevtsov, A.; Temmer, M.; Henry, T. W.; Hudson, H. S.; Imada, S.; Ling, A. G.; Moore, R. L.; Muhr, N.; Neidig, D. F.; Petrie, G. J. D.; Veronig, A. M.; Vršnak, B.; White, S. M. Bibcode: 2010ApJ...723..587B Altcode: We analyzed ground- and space-based observations of the eruptive flare (3B/X6.5) and associated Moreton wave (~850 km s-1 ~270° azimuthal span) of 2006 December 6 to determine the wave driver—either flare pressure pulse (blast) or coronal mass ejection (CME). Kinematic analysis favors a CME driver of the wave, despite key gaps in coronal data. The CME scenario has a less constrained/smoother velocity versus time profile than is the case for the flare hypothesis and requires an acceleration rate more in accord with observations. The CME picture is based, in part, on the assumption that a strong and impulsive magnetic field change observed by a GONG magnetograph during the rapid rise phase of the flare corresponds to the main acceleration phase of the CME. The Moreton wave evolution tracks the inferred eruption of an extended coronal arcade, overlying a region of weak magnetic field to the west of the principal flare in NOAA active region 10930. Observations of Hα foot point brightenings, disturbance contours in off-band Hα images, and He I 10830 Å flare ribbons trace the eruption from 18:42 to 18:44 UT as it progressed southwest along the arcade. Hinode EIS observations show strong blueshifts at foot points of this arcade during the post-eruption phase, indicating mass outflow. At 18:45 UT, the Moreton wave exhibited two separate arcs (one off each flank of the tip of the arcade) that merged and coalesced by 18:47 UT to form a single smooth wave front, having its maximum amplitude in the southwest direction. We suggest that the erupting arcade (i.e., CME) expanded laterally to drive a coronal shock responsible for the Moreton wave. We attribute a darkening in Hα from a region underlying the arcade to absorption by faint unresolved post-eruption loops. Title: A Smoking Gun in the Carina Nebula Authors: Hamaguchi, Kenji; Corcoran, Michael F.; Ezoe, Yuichiro; Townsley, Leisa; Broos, Patrick; Gruendl, Robert A.; Vaidya, Kaushar; White, Stephen M.; Strohmayer, Tod; Petre, Rob; Chu, You-Hua Bibcode: 2010AIPC.1248...17H Altcode: Massive stars are born from giant molecular clouds along with many lower mass stars, forming a stellar cluster or association. They dominate the pressure of the interstellar gas through their strong UV radiation, stellar winds and, ultimately, supernova explosions at the end of their life. These processes help the formation of the next generation of stars, but this trigger of star formation is not yet well understood.

The Carina Nebula is one of the youngest, most active sites of massive star formation in our Galaxy. In this nebula, we have discovered a bright X-ray source that has persisted for ~30 years. The soft X-ray spectrum, consistent with a kT~128 eV lackbody with mild extinction, and no counterpart in the optical and infrared wavelengths indicate that it is a 106 year-old neutron star. Current star formation theory does not allow the progenitor of the neutron star and the other massive stars in the Carina Nebula (in particular η Carinae) to be coeval. This result suggests that the Carina Nebula experienced at least two episodes of massive star formation. The neutron star may be responsible for part or all of the diffuse X-ray emission which permeates the Nebula. Title: Complexity Analysis of Impulsive Type IIIs and Type III-L Solar Radio Bursts Authors: Duffin, Robert Thomas; White, Stephen M.; Ray, Paul S. Bibcode: 2010shin.confE..78D Altcode: Type III-L bursts are a sub-class of type III solar radio bursts that tend to occur after the impulsive phase of flares; are longer in duration than individual type IIIs and tend to be low-frequency. Work has been developed on a Type III fitting tool. A Template is chosen from a representative individual Type III burst and fit to individual Type III bursts and components of Complex Type III bursts in order to help analyze and distinguish these bursts. This Type III fitting tool can also be used to fit and distinguish Impulsive Type IIIs and Type III-L bursts and help analyze various characteristics of the components of these bursts such as drift-rate and change in the duration of their intensity-time profiles with frequency. Title: The ADAHELI solar mission: Investigating the structure of Sun's lower atmosphere Authors: Berrilli, F.; Bigazzi, A.; Roselli, L.; Sabatini, P.; Velli, M.; Alimenti, F.; Cavallini, F.; Greco, V.; Moretti, P. F.; Orsini, S.; Romoli, M.; White, S. M.; ADAHELI Team; Ascani, L.; Carbone, V.; Curti, F.; Consolini, G.; Di Mauro, M. P.; Del Moro, D.; Egidi, A.; Ermolli, I.; Giordano, S.; Pastena, M.; Pulcino, V.; Pietropaolo, E.; Romano, P.; Ventura, P.; Cauzzi, G.; Valdettaro, L.; Zuccarello, F.; ADAHELI Team Bibcode: 2010AdSpR..45.1191B Altcode: 2010AdSpR..45.1191A ADAHELI (ADvanced Astronomy for HELIophysics) is a small-class (500 kg) low-budget (50 MEuro) satellite mission for the study of the solar photosphere and the chromosphere and for monitoring solar flare emission. ADAHELI's design has completed its Phase-A feasibility study in December 2008, in the framework of ASI's (Agenzia Spaziale Italiana) 2007 "Small Missions" Program (calling for two missions at 50 MEeuros each, plus the launch budget). ADAHELI's main purpose is to explore Sun's lower atmosphere in the near-infrared, a region so far unexplored by solar observations from space. ADAHELI will carry out observations of the solar photosphere and of the chromosphere at high-temporal rate and high spatial and spectral resolutions. ADAHELI will contribute to the understanding of Space Weather through the study of particle acceleration during flares. A radiometer operating in the millimeter radio band will continuously monitor the solar disk, throughout the spacecraft's life time. ADAHELI's baseline instruments are a 50-cm high-resolution telescope operating in the visible and the near-infrared, and a lightweight full-disk radiometer operating at millimeter wavelengths (90 GHz). The core of the telescope's focal plane suite is the spectral imager based on two Fabry-Perot interferometers, flying for the first time on a solar mission. The instrument will return fast-cadence, full bi-dimensional spectral images at high-resolution, thus improving on current slit-scan, mono-dimensional architectures. Moreover, the possibility of working in polarized light will enable full 3D magnetic field reconstruction on the photosphere and the chromosphere. An optional instrumental package is also being proposed to further extend ADAHELI's scope: a full-disk telescope for helioseismology based on a double Magneto-Optical Filter, a Neutral Particle Analyzer for magnetospheric research, an Extreme Ultraviolet imaging and spectro-radiometry instrument. These options fall outside the prescribed budget. ADAHELI, flying a Sun-Synchronous orbit at 800 km, will perform continuous, long-duration (4-h), daily acquisitions, with the possibility of extending them up to 24 h. ADAHELI's operating life is two years, plus one extension year. Launch would be nominally planned for 2014. Title: Measurements of the Coronal Acceleration Region of a Solar Flare Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Masuda, S.; Lin, R. P. Bibcode: 2010AAS...21630603K Altcode: The most discussed coronal hard X-ray source has been the above-the-loop-top source observed in the Masuda flare. We present new RHESSI hard X-ray observations of a similar event with simultaneous microwave observations by NoRH. These observations clearly establish the non-thermal nature of the above-the-loop-top source and triggered a new interpretation. To account for the extremely bright hard X-ray source in a rather low ambient density plasma, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region. Title: Type III-L Solar Radio Bursts and Their Associations with Solar Energetic Proton Events Authors: Duffin, Robert T.; White, S. M.; Ray, P. S.; Kaiser, M. L. Bibcode: 2010AAS...21632106D Altcode: 2010BAAS...41..912D Type III-L bursts are a sub-class of type III solar radio bursts that tend to occur after the impulsive phase of flares; are longer in duration than individual type IIIs and tend to be low-frequency. There has been a proposal that type III-Ls are connected to solar energetic proton (SEP) events. Most work on this connection has started from samples of SEP events, but if type III-Ls are to be useful for prediction of SEP events, then we need to understand the properties of samples of type III-L bursts. This talk reports preliminary results from such a study. An operating definition based on previous work is used to identify type III-L events amongst M- and X-class flares from 2001; and then associations with other properties of these events are investigated, including association with SEP events. If there is an association with SEP events, one important factor that these bursts allow us to address is the question of whether acceleration takes place at an associated CME, or closer to the flare site well below the CME. Work has been developed on a type III fitting tool. A Template is chosen from a representative individual type III burst and fit to individual type III bursts and components of Complex type III bursts in order to help analyze and distinguish these bursts. This type III fitting tool can also be used to fit and distinguish Impulsive type III and type III-L bursts and help analyze various characteristics of the components of these bursts such as drift-rate and change in the duration of their intensity-time profiles with frequency.

Funding for this research came from the Naval Research Laboratory where basic research in radio astronomy is funded by the Office of Naval Research, and from NASA LWS Grant FRS 526249. Title: Measurements of the Coronal Acceleration Region of a Solar Flare Authors: Krucker, Säm; Hudson, H. S.; Glesener, L.; White, S. M.; Masuda, S.; Wuelser, J. -P.; Lin, R. P. Bibcode: 2010ApJ...714.1108K Altcode: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph (NoRH) are used to investigate coronal hard X-ray and microwave emissions in the partially disk-occulted solar flare of 2007 December 31. The STEREO mission provides EUV images of the flare site at different viewing angles, establishing a two-ribbon flare geometry and occultation heights of the RHESSI and NoRH observations of ~16 Mm and ~25 Mm, respectively. Despite the occultation, intense hard X-ray emission up to ~80 keV occurs during the impulsive phase from a coronal source that is also seen in microwaves. The hard X-ray and microwave source during the impulsive phase is located ~6 Mm above thermal flare loops seen later at the soft X-ray peak time, similar in location to the above-the-loop-top source in the Masuda flare. A single non-thermal electron population with a power-law distribution (with spectral index of ~3.7 from ~16 keV up to the MeV range) radiating in both bremsstrahlung and gyrosynchrotron emission can explain the observed hard X-ray and microwave spectrum, respectively. This clearly establishes the non-thermal nature of the above-the-loop-top source. The large hard X-ray intensity requires a very large number (>5 × 1035 above 16 keV for the derived upper limit of the ambient density of ~8 × 109 cm-3) of suprathermal electrons to be present in this above-the-loop-top source. This is of the same order of magnitude as the number of ambient thermal electrons. We show that collisional losses of these accelerated electrons would heat all ambient electrons to superhot temperatures (tens of keV) within seconds. Hence, the standard scenario, with hard X-rays produced by a beam comprising the tail of a dominant thermal core plasma, does not work. Instead, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region. Title: Observations of the solar chromosphere at millimeter wavelengths Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M. Bibcode: 2010MmSAI..81..592L Altcode: Millimeter wavelengths provide a powerful tool to study the thermal structure of the solar chromosphere and its response to dynamic processes. We present initial studies of chromospheric fine structure and its dynamics obtained from observations of the quiet Sun with BIMA at 3.5 mm with a resolution of 12 arcsec. The two-dimensional millimeter maps of the solar chromosphere reveal brightness features corresponding to supergranular network boundaries and bright points within cells. Significant intensity oscillations with frequencies of 1.5 - 8.0 mHz with a tendency toward shorter-period oscillations in the internetwork and longer periods in network regions are found in the data. However, higher spatial resolution is required for a more detailed study. We discuss the requirements imposed on observations in the millimeter domain that might provide an insight into the fundamental questions of solar physics. We also review the capabilities of the current and future millimeter-wave interferometers, including the CARMA and ALMA arrays. Title: Type III-L Solar Radio Bursts and their Associations with Solar Energetic Proton Events Authors: Duffin, R. T.; White, S. M.; Ray, P. S.; Kaiser, M. L. Bibcode: 2009AGUFMSH33A1475D Altcode: Type III-L bursts are a sub-class of type III solar radio bursts that tend to occur after the impulsive phase of flares; are longer in duration than individual type IIIs and tend to be low-frequency. There has been a proposal that type III-Ls are connected to solar energetic proton (SEP) events. Most work on this connection has started from samples of SEP events, but if type III-Ls are to be useful for prediction of SEP events, then we need to understand the properties of samples of type III-L bursts. This talk reports preliminary results from such a study. An operating definition based on previous work is used to identify type III-L events amongst M- and X-class flares from 2001; and then associations with other properties of these events are investigated, including association with SEP events. If there is an association with SEP events, one important factor that these bursts allow us to address is the question of whether acceleration takes place at an associated CME, or closer to the flare site well below the CME. Title: High-Energy Emission from a Solar Flare in Hard X-rays and Microwaves Authors: Kundu, M. R.; Grechnev, V. V.; White, S. M.; Schmahl, E. J.; Meshalkina, N. S.; Kashapova, L. K. Bibcode: 2009SoPh..260..135K Altcode: 2009arXiv0908.0385K We investigate accelerated electron energy spectra for different sources in a large flare using simultaneous observations obtained with two instruments, the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) at hard X-rays. This flare is one of the few in which emission up to energies exceeding 200 keV can be imaged in hard X-rays. Furthermore, we can investigate the spectra of individual sources up to this energy. We discuss and compare the HXR and microwave spectra and morphology. Although the event overall appears to correspond to the standard scenario with magnetic reconnection under an eruptive filament, several of its features do not seem to be consistent with popular flare models. In particular we find that (1) microwave emissions might be optically thick at high frequencies despite a low peak frequency in the total flux radio spectrum, presumably due to the inhomogeneity of the emitting source; (2) magnetic fields in high-frequency radio sources might be stronger than sometimes assumed; (3) sources spread over a very large volume can show matching evolution in their hard X-ray spectra that may provide a challenge to acceleration models. Our results emphasize the importance of studies of sunspot-associated flares and total flux measurements of radio bursts in the millimeter range. Title: A Smoking Gun in the Carina Nebula Authors: Hamaguchi, Kenji; Corcoran, M. F.; Ezoe, Y.; Townsley, L.; Broos, P.; Gruendl, R.; Vaidya, K.; White, S. M.; Strohmayer, T.; Petre, R.; Chu, Y. -H. Bibcode: 2009cfdd.confE.114H Altcode: Massive stars are born from giant molecular clouds along with many lower mass stars, forming a stellar cluster or association. They dominate the pressure of the interstellar gas through their strong UV radiation, stellar winds and, ultimately, supernova explosions at the end of their life. These processes help the formation of the next generation of stars, but this trigger of star formation is not yet well understood. The Carina Nebula is one of the youngest, most active sites of massive star formation in our Galaxy. In this nebula, we have discovered a bright X-ray source that has persisted for ∼30 years. The soft X-ray spectrum, consistent with a kT ∼128 eV blackbody with mild extinction, and no counterpart in the optical and infrared wavelengths indicate that it is a 106 year-old neutron star. Current star formation theory does not allow the progenitor of the neutron star and the other massive stars in the Carina Nebula (in particular Eta Carinae) to be coeval. This result suggests that the Carina Nebula experienced at least two episodes of massive star formation. The neutron star may be responsible for part or all of the diffuse X-ray emission which permeates the Nebula. Title: Dynamically Driven Evolution of the Interstellar Medium in M51 Authors: Koda, Jin; Scoville, Nick; Sawada, Tsuyoshi; La Vigne, Misty A.; Vogel, Stuart N.; Potts, Ashley E.; Carpenter, John M.; Corder, Stuartt A.; Wright, Melvyn C. H.; White, Stephen M.; Zauderer, B. Ashley; Patience, Jenny; Sargent, Anneila I.; Bock, Douglas C. J.; Hawkins, David; Hodges, Mark; Kemball, Athol; Lamb, James W.; Plambeck, Richard L.; Pound, Marc W.; Scott, Stephen L.; Teuben, Peter; Woody, David P. Bibcode: 2009ApJ...700L.132K Altcode: 2009arXiv0907.1656K Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage. Title: Type III-L Solar Radio Bursts and their Associations with Solar Energetic Proton Events Authors: Duffin, Robert Thomas; White, Stephen M.; Ray, Paul S.; L., Michael Bibcode: 2009shin.confE.164D Altcode: Type III-L bursts are a sub-class of type III solar radio bursts that tend to occur after the impulsive phase of flares; are longer in duration than individual type IIIs and tend to be low-frequency. There has been a proposal that type III-Ls are connected to solar energetic proton (SEP) events. Most work on this connection has started from samples of SEP events, but if type III-Ls are to be useful for prediction of SEP events, then we need to understand the properties of samples of type III-L bursts. This talk reports preliminary results from such a study. An operating definition based on previous work is used to identify type III-L events amongst M- and X-class flares from 2001; and then associations with other properties of these events are investigated, including association with SEP events. If there is an association with SEP events, one important factor that these bursts allow us to address is the question of whether acceleration takes place at an associated CME, or closer to the flare site well below the CME. Title: Type III-L Solar Radio Bursts and their Associations with Solar Energetic Proton Events Authors: Duffin, Robert T.; White, S. M.; Ray, P. S.; Kaiser, M. L. Bibcode: 2009SPD....40.2117D Altcode: Type III-L bursts are a sub-class of type III solar radio bursts that tend to occur after the impulsive phase of flares; are longer in duration than individual type IIIs and tend to be low-frequency. There has been a proposal that type III-Ls are connected to solar energetic proton (SEP) events. Most work on this connection has started from samples of SEP events, but if type III-Ls are to be useful for prediction of SEP events, then we need to understand the properties of samples of type III-L bursts. This talk reports preliminary results from such a study. An operating definition based on previous work is used to identify type III-L events amongst M- and X-class flares from 2001; and then associations with other properties of these events are investigated, including association with SEP events. If there is an association with SEP events, one important factor that these bursts allow us to address is the question of whether acceleration takes place at an associated CME, or closer to the flare site well below the CME.

Research described in this talk will contribute towards the PhD dissertation for author RTD.

Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research. Title: Direct Observations of the Coronal Acceleration Region of a Solar Flare Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Lin, R. P. Bibcode: 2009SPD....40.3601K Altcode: Solar flares essentially convert the intrinsic energy of coronal magnetic field into the kinetic energy of accelerated particles. Hard X-ray emission from flare-accelerated electrons produced by the bremsstrahlung mechanism provides the most direct diagnostics of electron acceleration. The most discussed coronal hard X-ray source has been the above-the-loop-top source observed in the Masuda flare. The poor spectral resolution of these observations, however, made an interpretation ambiguous, and the exact location of the acceleration remained elusive. We present high spatial and spectral resolution RHESSI hard X-ray observations of an above-the-loop-top source with simultaneous microwave observations from NoRH. These observations provide a unambiguous interpretation of above-the-loop-top sources: The above-the-loop-top source itself is the acceleration region, where all electrons within an extended volume (1e27 cm3) are accelerated. The distribution of the accelerated electrons is definitely non-thermal, with a power law distribution extending from 10 keV up to the relativistic range (few MeV). The plasma beta in the acceleration region changes from the pre-flare value of 0.01 to 1, indicating that roughly half of the magnetic energy has been transformed into kinetic energy. Title: High-Energy Emission from a Solar Flare in Hard X-Rays and Microwaves Authors: Kundu, Mukul R.; Grechnev, V. V.; White, S. M.; Schmahl, E. J.; Meshalkina, N. S.; Kashapova, L. K. Bibcode: 2009SPD....40.1920K Altcode: We investigate accelerated electron energy spectra for different sources in a large flare using simultaneous observations obtained with two instruments, the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) at hard X-rays. This flare is one of the few in which emission up to energies exceeding 200 keV can be imaged in hard X-rays. Furthermore, we can investigate the spectra of individual sources up to this energy. We discuss and compare the HXR and microwave spectra and morphology. Although the event overall appears to correspond to the standard scenario with magnetic reconnection under an eruptive filament, several of its features do not seem to be consistent with popular flare models. In particular we find that (1) microwave emissions might be optically thick at high frequencies despite a low peak frequency in the total flux radio spectrum, presumably due to the inhomogeneity of the emitting source; (2) magnetic fields in high-frequency radio sources might be stronger than sometimes assumed; (3) sources spread over a very large volume can show matching evolution in their hard X-ray spectra that may provide a challenge to acceleration models. Our results emphasize the importance of studies of sunspot-associated flares and total flux measurements of radio bursts in the millimeter range. Title: A Smoking Gun in the Carina Nebula Authors: Hamaguchi, Kenji; Corcoran, Michael F.; Ezoe, Yuichiro; Townsley, Leisa; Broos, Patrick; Gruendl, Robert; Vaidya, Kaushar; White, Stephen M.; Strohmayer, Tod; Petre, Rob; Chu, You-Hua Bibcode: 2009ApJ...695L...4H Altcode: 2009arXiv0902.4162H The Carina Nebula is one of the youngest, most active sites of massive star formation in our Galaxy. In this nebula, we have discovered a bright X-ray source that has persisted for ~30 years. The soft X-ray spectrum, consistent with kT ~ 128 eV blackbody radiation with mild extinction, and no counterpart in the near- and mid-infrared wavelengths indicates that it is a ~106 year old neutron star housed in the Carina Nebula. Current star formation theory does not suggest that the progenitors of the neutron star and massive stars in the Carina Nebula, in particular η Car, are coeval. This result suggests that the Carina Nebula experienced at least two major episodes of massive star formation. The neutron star may be responsible for remnants of high-energy activity seen in multiple wavelengths. Title: On the relation between photospheric magnetic field and chromospheric emission in the quiet Sun Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen M. Bibcode: 2009IAUS..259..185L Altcode: In this contribution we present an observational study of the interaction of the photosphere with different chromospheric layers. We study the correlations between emissions at varying temperature from the temperature minimum region (UV continuum at 1600 Å from TRACE) through the low chromosphere (CaII K-line from BBSO) to the middle chromosphere (continuum at 3.5 mm from BIMA) and photospheric magnetic field from MDI/SOHO. For the first time millimeter observational data are included in such analysis.

We report a high degree of correlation between considered emissions formed at different heights in the chromosphere. A power law is found to be a good representation for the relationship between photospheric magnetic field and chromospheric emissions at all considered wavelengths. Our analysis shows that the dependence of chromospheric intensities on magnetic field is different for the network and internetwork regions. In the network a power law provides the best fit with the exponent being close to 0.5-0.6, while almost no dependence of chromospheric intensity on magnetic flux is found for the cell interiors. The obtained results support the idea of different heating mechanisms acting in the network (magnetic) and cell interiors (acoustic). Title: The relationship between chromospheric emissions and magnetic field strength Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M. Bibcode: 2009A&A...497..273L Altcode: 2009arXiv0910.1985L Aims: We analyze observational data from 4 instruments to study the correlations between chromospheric emission, spanning the heights from the temperature minimum region to the middle chromosphere, and photospheric magnetic field.
Methods: The data consist of radio images at 3.5 mm from the Berkeley-Illinois-Maryland Array (BIMA), UV images at 1600 Å from TRACE, Ca II K-line filtergrams from BBSO, and MDI/SOHO longitudinal photospheric magnetograms. For the first time interferometric millimeter data with the highest currently available resolution are included in such an analysis. We determine various parameters of the intensity maps and correlate the intensities with each other and with the magnetic field.
Results: The chromospheric diagnostics studied here show a pronounced similarity in their brightness structures and map out the underlying photospheric magnetic field relatively well. We find a power law to be a good representation of the relationship between photospheric magnetic field and emission from chromospheric diagnostics at all wavelengths. The dependence of chromospheric brightness on magnetic field is found to be different for network and internetwork regions. Title: A Resolved Ring of Debris Dust around the Solar Analog HD 107146 Authors: Corder, Stuartt; Carpenter, John M.; Sargent, Anneila I.; Zauderer, B. Ashley; Wright, Melvyn C. H.; White, Stephen M.; Woody, David P.; Teuben, Peter; Scott, Stephen L.; Pound, Marc W.; Plambeck, Richard L.; Lamb, James W.; Koda, Jin; Hodges, Mark; Hawkins, David; Bock, Douglas C. -J. Bibcode: 2009ApJ...690L..65C Altcode: 2008arXiv0811.2713C We present resolved images of the dust continuum emission from the debris disk around the young (80-200 Myr) solar-type star HD 107146 with CARMA at λ = 1.3 mm and the CSO at λ = 350 μ. Both images show that the dust emission extends over an approximately 10'' diameter region. The high-resolution (3'') CARMA image further reveals that the dust is distributed in a partial ring with significant decrease in a flux inward of 97 AU. Two prominent emission peaks appear within the ring separated by ~140° in the position angle. The morphology of the dust emission is suggestive of dust captured into a mean motion resonance, which would imply the presence of a planet at an orbital radius of ~45-75 AU. Title: Hard X-ray emission from the solar corona Authors: Krucker, S.; Battaglia, M.; Cargill, P. J.; Fletcher, L.; Hudson, H. S.; MacKinnon, A. L.; Masuda, S.; Sui, L.; Tomczak, M.; Veronig, A. L.; Vlahos, L.; White, S. M. Bibcode: 2008A&ARv..16..155K Altcode: 2008A&ARv.tmp....8K This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration. Title: The ADAHELI Solar Mission Authors: Berrilli, F.; Velli, M.; Roselli, L.; Bigazzi, A.; Moretti, P. F.; Romoli, M.; Orsini, S.; Cavallini, F.; Greco, V.; Carbone, V.; Consolini, G.; Di Mauro, M. P.; Ermolli, I.; Pietropaolo, E.; Romano, P.; Ventura, P.; White, S. M.; Zuccarello, F.; Cauzzi, G.; Valdettaro, L. Bibcode: 2008ESPM...12..6.6B Altcode: ADAHELI (Advanced Astronomy for HELIOphysics) is an Italian Space project for the investigation of solar photospheric and chromospheric dynamics, via high-resolution spectro-polarimetric observations in the near-infrared spectral range. The mission has been financed for phase A study in the framework of ASI Italian Space Agency Small Missions Program call of September 2007.

Four fields have been selected to highlight the specific benefits of ADAHELI scientific payload: 1) Photospheric and chromospheric dynamics and structure, 2) Emergence and evolution of solar active regions and solar irradiance, 3) Chromospheric and corona heating and turbulence, 4) Solar flares in the millimeter wavelength region.

The principal science instrument, ISODY, is a 50 cm solar telescope equipped with an innovative Focal Plane Suite composed of a spectro-polarimetric imager, based upon two Fabry-Perot interferometers operating in the NIR regions around 845nm and 1083nm, a broad band imager, and a correlation tracker used as image stabilization system.

Designed Mission Profiles for ADAHELI intend to achieve continuous high-spectral and spatial resolution observations of the Sun for a routine duration of 4 hours with a goal to be extended to 24 hours.

ADAHELI also carries MIOS, a millimeter wavelengths radiometer operating at around 90 GHz for flare detection.

The ADAHELI payload's instrument suite integrates and complements, without overlap, the present major objectives of ESA, NASA and the International Living with a Star program, in particular Solar Dynamics Observatory, PICARD, Solar Orbiter, and the Solar Probe missions.

Proposals for optional instruments are also under evaluation: DIMMI-2h, a double channel MOF based full disk imager operating at 589nm and 770nm, allowing high temporal resolution velocity and magnetic field measurements; EISR a two channel spectrometer operating in the 50-130 nm wavelength range, and NPA, an in-situ Neutral Particle Analyzer to detect Energetic Neutral Atoms (ENA). Science objectives related to optional instruments include: solar high and low-degree p modes oscillations, EUV solar structures and variability, solar gravitational red-shift measurement, detection of ENA originating from the plasma region in the Earth's magnetosphere and undergoing reflection from the Earth's atmosphere. Title: Coronal Hard X-Ray Emission Associated with Radio Type III Bursts Authors: Krucker, Säm; Saint-Hilaire, P.; Christe, S.; White, S. M.; Chavier, A. D.; Bale, S. D.; Lin, R. P. Bibcode: 2008ApJ...681..644K Altcode: We report on a purely coronal hard X-ray source detected in a partially disk-occulted solar flare by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) that is associated with radio type III bursts and a suprathermal electron event detected near 1 AU by the WIND 3-D Plasma and Energetic Particle (3DP) instrument. Several observational characteristics suggest that the coronal hard X-ray source is thin target bremsstrahlung emission from the escaping electrons that produce the radio type III bursts. The hard X-ray emission correlates in time with the radio type III bursts and originates from a radially elongated source in the corona with a length (~65 Mm) similar to typical coronal density scale heights. Furthermore, the difference between the hard X-ray photon spectral index (γ = 4.1 +/- 0.4) and the electron spectral index of the in situ observed event (δin situ = 2.9 +/- 0.3) is around 1, consistent with the thin target interpretation. A further test for the thin target scenario is to compare the number of electrons needed to produce the observed hard X-ray emission with the number of in situ observed electrons. However, the number of escaping electrons derived from the single-spacecraft WIND measurement is in the best case an order of magnitude estimate and could easily underestimate the actual number of escaping electrons. Using the WIND observations, the estimated number of escaping electrons is about an order of magnitude too low. Thus, the thin target interpretation only holds if the WIND measurements are significantly underestimating the actual number of escaping electrons. Future multispacecraft observations with STEREO, Solar Orbiter, and Sentinels will resolve this uncertainty. Title: Positions of Fiber Bursts in the 2006 December 6 Flare Authors: White, S. M.; Liu, Z.; Gary, D. E. Bibcode: 2008AGUSMSP44A..07W Altcode: The large flare of 2006 December 6 produced one of the brightest radio bursts ever recorded. The burst was observed with very rapid sampling by the FASR Subsystem Testbed, operating on 3 antennas of the OVSA array at Owens Valley. The burst contained a range of emission types within the FST band (1.0-1.5 GHz), including continuum and "fiber" bursts. This is the first time that fiber bursts have been observed over a significant frequency range with an interferometer. Data are processed into visibilities using a software correlator with positional information retained. We present an analysis of the fiber bursts relative to the continuum, in order to investigate whether the projected position of the bursts varies with frequency. This information is used to test models for fiber bursts, in particular the model that they represent propagating whistler wave trains. Title: Solar Flare Hard X-Ray Emission from the High Corona Authors: Krucker, Säm; White, S. M.; Lin, R. P. Bibcode: 2007ApJ...669L..49K Altcode: One of the largest solar hard X-ray (HXR) flares and solar energetic particle (SEP) events recorded by the Mars Odyssey mission while orbiting Mars occurred on 2002 October 27 and is related to a very fast (~2300 km s-1) coronal mass ejection (CME). From the Earth, the flare site is 40.4° +/- 3.5° behind the solar limb and only emissions from the high corona at least 1.5 × 105 km radially above the main flare site can be seen. Nevertheless, the Earth-orbiting Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observed HXR emission up to 60 keV with a relatively flat, nonthermal spectrum (γ between 3 and 3.5) that has an onset simultaneous with the main HXR emission observed above 60 keV by the Gamma-Ray Spectrometer (GRS) orbiting Mars. While GRS records several smaller enhancements after the main peak, the high coronal source observed by RHESSI shows a long exponential decay (τ = 135 +/- 5 s) with progressive spectral hardening. The emissions from the high corona originate from an extended source (~1.5 × 105 km in diameter) that expands (390 +/- 70 km s-1) and moves upwards (750 +/- 80 km s-1) in the same direction as the CME. These observations reveal the existence of energetic electrons in the high corona in closed magnetic structures related to the CME that are accelerated at the same time as the main energy release in the flare. Although the number of energetic electrons in the high corona is only a small fraction of the total accelerated electrons, about 10% of all electrons in the high coronal source are nonthermal (>10 keV). Title: Deep Impact's Small Telescope Science Program: Phase II Results Authors: McFadden, Lucy-Ann A.; Kuppusamy, V. A.; Papp, G. M.; Beal, S. E.; Clarke, T.; Skillman, D. R.; White, S. M.; McLaughlin, S. A.; Warner, E. M.; Observers, STSP Bibcode: 2007DPS....39.2704M Altcode: 2007BAAS...39..463M From February, 2000 through August, 2005, the Deep Impact mission's science and education and public outreach team received images of Comet 9P/Tempel 1 from small telescope owners around the world. Deep Impact explored the composition and structure of the nucleus of Comet Tempel 1. The Small Telescope Program (STSP) engaged technically proficient observers to complement and augment the comet's characterization carried out by large telescopes in support of planning and analysis of the mission results. The first phase of the program is reported in McLaughlin et al. 2004. Over 3700 images of Tempel 1 from 64 observers were submitted to the second phase of STSP between December 2004 and August 2005. Telescopes ranging in size from 10 - 80 cm, with CCD cameras and a range of filters including unfiltered, V, R (625-740 nm) and I (750-1000 nm) filters were used. Both calibrated and uncalibrated images were submitted. We report on the cataloging and analysis of this dataset. The field of view and orientation of each image was measured using catalogue overlays generated by the Astrometrica program. The orientation of the images and the star fields are identified, the star fluxes are calculated and the counts are converted to instrumental magnitude. Scaling to apparent photometric magnitude is made from magnitudes available in the USNO-B1.0 star catalog. Photometry of the comet was then determined for a circular aperture of radius of 27,000 km around the comet in each filter. We examine the resulting magnitudes as a function of time before perihelion and calculate Af [?], a measurement of dust activity. Dust activity increased by a factor of 6 between 203 and 62 days before perihelion, then decreased by >2x, 2 days before perihelion. At impact it increased by 3x. This work is supported by NASA grant NNX07AG22G. Title: Radio Frequency Interference Excision Using Spectral-Domain Statistics Authors: Nita, Gelu M.; Gary, Dale E.; Liu, Zhiwei; Hurford, Gordon J.; White, Stephen M. Bibcode: 2007PASP..119..805N Altcode: A radio frequency interference (RFI) excision algorithm based on spectral kurtosis, a spectral variant of time-domain kurtosis, is proposed and implemented in software. The algorithm works by providing a robust estimator for Gaussian noise that, when violated, indicates the presence of non-Gaussian RFI. A theoretical formalism is used that unifies the well-known time-domain kurtosis estimator with past work related to spectral kurtosis, and leads naturally to a single expression encompassing both. The algorithm accumulates the first two powers of M power spectral density (PSD) estimates, obtained via Fourier transform, to form a spectral kurtosis (SK) estimator whose expected statistical variance is used to define an RFI detection threshold. The performance of the algorithm is theoretically evaluated for different time-domain RFI characteristics and signal-to-noise ratios η. The theoretical performance of the algorithm for intermittent RFI (RFI present in R out of M PSD estimates) is evaluated and shown to depend greatly on the duty cycle, d=R/M. The algorithm is most effective for d=1/(4+η), but cannot distinguish RFI from Gaussian noise at any η when d=0.5. The expected efficiency and robustness of the algorithm are tested using data from the newly designed FASR Subsystem Testbed radio interferometer operating at the Owens Valley Solar Array. The ability of the algorithm to discriminate RFI against the temporally and spectrally complex radio emission produced during solar radio bursts is demonstrated. Title: Electron acceleration in a post-flare decimetric continuum source Authors: Subramanian, P.; White, S. M.; Karlický, M.; Sych, R.; Sawant, H. S.; Ananthakrishnan, S. Bibcode: 2007A&A...468.1099S Altcode: 2007astro.ph..3612S Aims:To calculate the power budget for electron acceleration and the efficiency of the plasma emission mechanism in a post-flare decimetric continuum source.
Methods: We have imaged a high brightness temperature (∼ 109 K) post-flare source at 1060 MHz with the Giant Metrewave Radio Telescope (GMRT). We use information from these images and the dynamic spectrum from the Hiraiso spectrograph together with the theoretical method described in Subramanian & Becker (2006, Sol. Phys., 237, 185) to calculate the power input to the electron acceleration process. The method assumes that the electrons are accelerated via a second-order Fermi acceleration mechanism.
Results: We find that the power input to the nonthermal electrons is in the range 3× 1025-1026 erg/s. The efficiency of the overall plasma emission process starting from electron acceleration and culminating in the observed emission could range from 2.87× 10-9 to 2.38 × 10-8. Title: Positional Properties of Solar Decimetric Fiber Bursts Authors: Liu, Zhiwei; Gary, D. E.; Nita, G. M.; White, S. M.; Hurford, G. J. Bibcode: 2007AAS...210.9328L Altcode: 2007BAAS...39..215L The large solar flare of 2006 December 6 was detected by the newly constructed FASR Subsystem Testbed (FST; Liu et al. 2007, PASP, 119), which is operating on three antennas of Owens Valley Solar Array. This record-setting burst produced an especially fine set of fibers bursts--so-called intermediate-drift bursts that drift from high to low frequencies over 6-10s. According to a leading theory, the fibers are generated by packets of whistler waves propagating along a magnetic loop, which coalesce with Langmuir waves to produce escaping electromagnetic radiation in the decimeter band. With this three element interferometer, for the first time fiber burst source locations can be determined. We use the radio information over a 500 MHz band (1.0-1.5 GHz) to determine the trajectories of the bursts. We attempt to define three-dimensional trajectories by combining the two-dimensional radio positions with height of the fiber source determined from a coronal density model. Title: Interferometric Observations of the Record-Breaking Decimetric Solar Radio Event of 2006 December 06 Authors: Gary, Dale E.; White, S. M.; Hurford, G. J.; Nita, G. M.; Liu, Z. Bibcode: 2007AAS...210.9329G Altcode: 2007BAAS...39..216G The radio burst associated with the 2006 Dec 06, X6.5 solar flare attained a record-breaking radio flux density of nearly 1 million solar flux units in the decimetric ( 1 GHz) frequency band, exceeding the previous record by nearly a factor of 10. The event was so strong in the range 1-2 GHz that it caused loss of lock of Global Positioning System (GPS) receivers everywhere in the sunlit hemisphere of Earth. The event was observed with a new instrument operating at the Owens Valley Solar Array (OVSA), the FASR Subsystem Testbed (FST), which features not only unprecedented time and frequency resolution ( 1 microsecond and < 1 MHz, respectively), but also interferomety on three baselines to provide corresponding spatial information on a 10 arcsecond scale. The event produced about 10 million individual decimetric spike bursts produced by the Electron Cyclotron Maser (ECM) mechanism over a period of nearly 1 hour. We discuss the spectral, temporal, and positional characteristics of individual spike bursts, their statistical properties and their relation to the flare observed at other wavelengths. This work is supported by NSF grant AST-0352915 to the New Jersey Institute of Technology. We acknowledge additional support for the Owens Valley Solar Array through NSF grant AST-0607544 and NASA grant NNG06GJ40G. Title: Fiber Bursts in Solar Flares at High Time and Frequency Resolution Authors: White, Stephen M.; Liu, Z.; Gary, D. E.; Nita, G. M.; Hurford, G. J. Bibcode: 2007AAS...210.6806W Altcode: 2007BAAS...39..176W The FASR Subsystem Testbed (FST), operating on three dishes of the Owens Valley Solar Array, detected radio fiber bursts in two large solar flares in 2006. The frequencies of observation correspond to densities of order 10 to the 10 per cc, so the bursts are being emitted low in the corona, presumably near the energy release region. The fiber bursts drift in frequency with time, and appear to be travelling outwards. In one model, fiber bursts can be used to infer the magnetic field in the corona in the emission site. The FST data provide radio spectra of the bursts with a time resolution of milliseconds: using this exceptional data, we present the properties of the bursts and discuss their physical interpretation. Title: A Subsystem Test Bed for the Frequency-Agile Solar Radiotelescope Authors: Liu, Zhiwei; Gary, Dale E.; Nita, Gelu M.; White, Stephen M.; Hurford, Gordon J. Bibcode: 2007PASP..119..303L Altcode: This paper describes the design and operation of the Frequency-Agile Solar Radiotelescope (FASR) Subsystem Testbed (FST) and the first observational results. Three antennas of the Owens Valley Solar Array have been modified so that each sends a 1-9 GHz band of radio frequency to a central location using a broadband analog optical fiber link. A dynamically selected 500 MHz subset of this frequency range is digitized at 1 Gsps (gigasample per second) and recorded to disk. The full-resolution time-domain data thus recorded are then correlated through offline software to provide interferometric phase and amplitude spectra on three baselines. An important feature of this approach is that the data can be reanalyzed multiple times with different digital signal-processing techniques (e.g., different bit-sampling, windowing, and radio frequency interference [RFI] excision methods) to test the effect of different designs. As a prototype of the FASR system, FST provides the opportunity to study the design, calibration, and interference-avoidance requirements of FASR. In addition, FST provides, for the first time, the ability to perform broadband spectroscopy of the Sun with spectrograph-like spectral and temporal resolution, while providing locating ability for simple sources. Initial observations of local RFI, geostationary satellite signals, global positioning system (GPS) satellite signals, and the Sun are presented to illustrate the system performance. Title: High-resolution millimeter-interferometer observations of the solar chromosphere Authors: White, S. M.; Loukitcheva, M.; Solanki, S. K. Bibcode: 2006A&A...456..697W Altcode: The use of millimeter-interferometer data for the study of chromospheric structure and dynamics is tested using 85 GHz observations with the 10-element Berkeley-Illinois-Maryland Array (BIMA). Interferometer data have the advantage over single-dish data that they allow both high spatial resolution and dense temporal sampling simultaneously. However, snapshot imaging of the quiet solar atmosphere with a small number of dishes is challenging. We demonstrate that techniques are available to carry out this task successfully using maximum entropy deconvolution from a default image constructed from the entire observation: one of our results is that the solar chromosphere at millimeter wavelengths exhibits features that are long-lasting and the map of the entire observation is significant provided that atmospheric phase errors do not prevent deconvolution. We compare observations of quiet Sun, active region and coronal hole targets. The interferometer is not sensitive to the disk emission and the positivity constraint of the maximum entropy algorithm used forces the zero level in the images to be at the temperature of the coolest feature in each field. The brightest features in the images are typically 1000-1500 K above the zero level, with a snapshot noise level of order 100 K. We use extensive tests to determine whether oscillation power can be recovered from sequences of snapshot images and show that individual sources can be down to quite weak levels at locations in the image where significant flux is present; oscillation power located in cool regions of the image is not well recovered due to the deconvolution method used and may be redistributed to brighter regions of the millimeter image. We then investigate whether the data do show oscillation power using uninterrupted 30-min scans of the target regions. Intensity oscillations with significant power in the frequency range 1.5-8.0 mHz are found in the quiet-Sun and active region targets. For the quiet-Sun region we compare the oscillation properties of network boundaries and cell interiors (internetwork) in the spatially-resolved time series. In agreement with investigations at other wavelengths, in the millimeter data the power in the network tends to be at periods of 5 min and longer while power in the internetwork is present also at shorter (3-min) periods. Title: The Morphology of Decimetric Emission from Solar Flares: GMRT Observations Authors: Kundu, M. R.; White, S. M.; Garaimov, V. I.; Subramanian, P.; Ananthakrishnan, S.; Janardhan, P. Bibcode: 2006SoPh..236..369K Altcode: 2006SoPh..tmp....7K Observations of a solar flare at 617 MHz with the Giant Meter-wave Radio Telescope (GMRT) are used to study the morphology of flare radio emission at decimetric wavelengths. There has been very little imaging in the 500 - 1000 MHz frequency range, but it is of great interest, since it corresponds to densities at which energy is believed to be released in solar flares. This event has a very distinctive morphology at 617 MHz: the radio emission is clearly resolved by the 30″ beam into arc-shaped sources seeming to lie at the tops of long loops, anchored at one end in the active region in which the flare occurs, with the other end lying some 200 000 km away in a region of quiet solar atmosphere. Microwave images show fairly conventional behaviour for the flare in the active region: it consists of two compact sources overlying regions of opposite magnetic polarity in the photosphere. The decimetric emission is confined to the period leading up to the impulsive phase of the flare, and does not extend over a wide frequency range. This fact suggests a flare mechanism in which the magnetic field at considerable height in the corona is destabilized a few minutes prior to the main energy release lower in the corona. The radio morphology also suggests that the radiating electrons are trapped near the tops of magnetic loops, and therefore may have pitch angles near 90˚. Title: Solar Radio Astronomy with the Long Wavelength Array Authors: Duffin, Robert T.; Erickson, W. C.; Kassim, N. E.; Ray, P. S.; White, S. M. Bibcode: 2006SPD....37.0816D Altcode: 2006BAAS...38..233D The Long Wavelength Array (LWA) will be a ground-based interferometric array of up to 50 stations, with an operating frequency range of 20 to 80 MHz. Each station will have about 256 active dipoles with a maximum station-to-station baseline of 400 km. The long baselines and large area, combined with advances in ionospheric calibration, will allow the LWA to achieve angular resolution and sensitivity more than two orders of magnitude better than previous instruments in this frequency range. The LWA will perform a broad range of studies of the Sun in the metric and decametric regime, where a diverse range of emission mechanisms carry information about processes occurring in the solar atmosphere and in interplanetary space. It will also enable low frequency imaging of solar activity and brightness distributions in support of the upcoming STEREO (Solar Terrestrial Relations Observatory) observations at lower frequencies. Solar emission observed by the LWA will help us to better understand particle acceleration and shocks in the solar atmosphere and the dynamics of coronal mass ejections (CMEs) which travel from the sun's lower atmosphere to Earth. Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research. Title: Microwave and Hard X-ray Imaging Observations of Energetic Electrons in Solar Flares: Event of 2003 June 17 Authors: Kundu, Mukul R.; Schmahl, E. J.; White, S. M. Bibcode: 2006SPD....37.1306K Altcode: 2006BAAS...38..241K We discuss one large flare using simultaneous observations obtained with two instruments--Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, and Reuven Ramaty High Energy Solar Spectroscopic imager (RHESSI) in the context of flare models. As we know energetic electrons accelerated to nonthermal energies are seen via microwave and hard X-ray emission from the solar corona. Imaging observations are necessary to study the energetic electrons in flare events. Nonthermal microwave emission during large solar flares is produced by gyrosynchrotron mechanism which involves coronal magnetic fields of at least a few hundred gauss and electrons of energy of a few hundred keV. Hard X-ray emission, on the other hand, is produced by bremsstrahlung from beamed or trapped electrons of a few to hundreds of keV energy. The two different methods of mapping energetic flare electrons therefore complement each other, and provide good means of testing flare models which have been abundant in the recent literature. The RHESSI HXR and NoRH microwave imaging observations of the flare of 2003 June 17 are used here as an example. The flare in question was of class M6.8, and it was observed in AR 10386, a beta-gamma-delta region, two days after its east-limb passage. Title: FASR Subsystem Testbed Authors: Liu, Zhiwei; Gary, D. E.; Nita, G. M.; White, S. M.; Hurford, G. J. Bibcode: 2006SPD....37.0602L Altcode: 2006BAAS...38..225L The construction of the Frequency Agile Solar Radiotelescope (FASR) Subsystem Testbed (FST) and first results are described. Three antennas of Owens Valley Solar Array (OVSA) are upgraded with the newly designed, state of art technology. The 1-9 GHz RF signal from the feed is transmitted through the fiber optical system to the control room. Then it is downconverted to a 500 MHz single-sideband signal that can be tuned across the 1-9 GHz RF band. The data are sampled with an 8-bit, 1 GHz sampling-rate digitizer, and further saved to the hard disk. The correlated (phase and amplitude) spectra are derived through offline software. As a prototype of the FASR system, FST provides the opportunity to study the design, calibration, and interference-avoidance requirements of FASR. FST provides, for the first time, the ability to perform broadband imaging spectroscopy with high spectral, temporal and moderate spatial resolution. With this three element interferometer, we have the ability to determine the location of simple source structures with very high time resolution (20 ms) and frequency resolution ( <1 MHz) as well as the dynamic spectrum. Initial examples of geostationary satellite, GPS satellite and solar observations are presented. Title: First Solar Results with the FASR Subsystem Testbed Authors: Gary, Dale E.; Hurford, G. J.; Liu, Z.; Nita, G. M.; White, S. M. Bibcode: 2006SPD....37.2502G Altcode: 2006BAAS...38..252G The Frequency Agile Solar Radiotelescope (FASR) Subsystem Testbed (FST) is a new, three-element interferometer, utilizing three antennas of the Owens Valley Solar Array. FST is the first radio instrument capable of measuring solar burst locations with spectrograph-like time and frequency resolution. Operating with 500 MHz instantaneous bandwidth, tunable anywhere in the 1-9 GHz range, FST can measure centroid locations of typical bursts on millisecond timescales and MHz frequency resolution. The system is designed as a prototype for FASR, for several purposes including the study of radio frequency interference (RFI) mitigation techniques, a testbed for the design of FASR's digitial signal processing, and investigating use of satellite signals for calibration. In addition, however, FST is unique in its ability to locate solar bursts on the extremely fine frequency and time scales on which their emission varies.A technical description of the instrument can be found in an accompanying poster (Liu et al.), along with first results on the several topics mentioned above. In this talk we focus on the solar applications of FST. We describe the operation of the system for solar observations, the science goals of the instrument, and some first results of observations of solar bursts. The first recorded burst, obtained on the first day of solar observations, was a group of type III bursts associated with a B5.1 X-ray burst, also observed with RHESSI. Individual type IIIs show downward propagation (reverse-frequency slope), with significant positional differences from one type III to another, while within a type III are seen smaller positional changes as a function of frequency. This suggests smooth trajectories for individual bursts, but widely diverging locations for separate bursts. We describe joint RHESSI/FST observations and findings for those bursts observed so far. This work is supported by NSF grant AST-0352915 to NJIT. Title: Imaging Observations of a Very High Frequency Type II Burst Authors: White, S. M.; Mercier, C.; Bradley, R.; Bastian, T.; Kerdraon, A.; Pick, M. Bibcode: 2006AGUSMSH24A..05W Altcode: A remarkable Type II burst was detected by the high-frequency system of the Green Bank Solar Radio Burst Spectrometer on 2005 November 14. The harmonic branch of the Type II extended up to 800 MHz, making it one of the highest frequency Type II bursts ever detected, but it failed to propagate to heights corresponding to frequencies below 100 MHz. At such high frequencies, it implies the formation of a shock relatively low in the corona. No coronal mass ejection was evident in the LASCO data for this east limb event. It is one of the few Type II bursts to be observable at every frequency of observation of the Nancay Radio Heliograph (164-432 MHz). Here we present analysis of images of the event, including simultaneous imaging of the fundamental and harmonic branches. Title: The FASR Reference Instrument Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Kawakubo, H.; Ruf, C.; White, S. M.; Zurbuchen, T. Bibcode: 2006AGUSMSH33A..08B Altcode: The Frequency Agile Solar Radiotelescope (FASR) is a unique, solar-dedicated radio facility slated for completion by 2012. The instrument will address an extremely broad range of solar and space weather science, including routine measurement of coronal magnetic fields, imaging coronal mass ejections near the solar surface, quantitative diagnostics of energy release and particle acceleration in flares, and the extension of the solar corona into the heliosphere. Although the precise details of the FASR design are still being developed, we present for the first time a complete high-level design referred to as the FASR Reference Instrument. The Reference Instrument meets the science requirements and will serve as the basis for cost estimates for construction and operation of the instrument. This paper gives an overview of the FASR Reference Instrument, describes the science goals and objectives, and gives the flowdown of science goals to engineering specifications. The innovative aspects of the FASR design are highlighted, and a complete, end-to-end description of the instrument is given. The instrument operations plan is also discussed. Title: Flare-Associated Decimeter TypeIV Bursts at High Frequencies Authors: Kundu, M. R.; White, S. M.; Hori, K.; Schmahl, E. J. Bibcode: 2006AGUSMSH24A..06K Altcode: Type IV radio bursts are broadband continuum events associated with solar flares that typically occur after the impulsive phase, can last for tens of minutes and are usually confined to low frequencies, below 200 MHz. However some flares show clear evidence for strong Type IV-like emission that is confined to high frequencies, around 1 GHz. Type IV events require nonthermal electrons and imply the presence of either long-lived energetic electrons trapped in coronal loops, or else ongoing acceleration lasting for tens of minutes following a flare. If, as is widely believed, the emission mechanism of Type IV bursts is plasma emission, then the high- frequency events require electrons at densities of over 3 109 cm-3, far higher densities than are involved in the low frequency Type IV bursts. We present a sample of such high frequency Type IV bursts and analyze their properties and association with other flare phenomena, and discuss their possible sources. Title: Solar Radio Astronomy With the Long Wavelength Array Authors: Duffin, R. T.; Erickson, W. C.; Kassim, N. E.; Ray, P. S.; White, S. M. Bibcode: 2006AGUSMSH33A..05D Altcode: The Long Wavelength Array (LWA) will be a ground-based interferometric array of up to 50 stations, with an operating frequency range of 20 to 80 MHz. Each station will have about 256 active dipoles with a maximum station-to-station baseline of 400 km. The long baselines and large area, combined with advances in ionospheric calibration, will allow the LWA to achieve angular resolution and sensitivity more than two orders of magnitude better than previous instruments in this frequency range. The LWA will perform a broad range of studies of the Sun in the metric and decametric regime, where a diverse range of emission mechanisms carry information about processes occurring in the solar atmosphere and in interplanetary space. It will also enable low frequency imaging of solar activity and brightness distributions in support of the upcoming STEREO (Solar Terrestrial Relations Observatory) observations at lower frequencies. Solar emission observed by the LWA will help us to better understand particle acceleration and shocks in the solar atmosphere and the dynamics of coronal mass ejections (CMEs) which travel from the sun's lower atmosphere to Earth. Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research. Title: Radio Measurements of the Height of Strong Coronal Magnetic Fields Above Sunspots at the Solar Limb Authors: Brosius, Jeffrey W.; White, Stephen M. Bibcode: 2006ApJ...641L..69B Altcode: We measure coronal magnetic field strengths of 1750 G at a height of 8000 km above a large sunspot in NOAA AR 10652 at the west solar limb on 2004 July 29 using coordinated observations with the Very Large Array, the Transition Region and Coronal Explorer, and three instruments (CDS, EIT, MDI) aboard the Solar and Heliospheric Observatory. This observation is the first time that coronal radio brightness temperatures have been analyzed in a 15 GHz solar radio source projected above the limb. Observations at 8 GHz yield coronal magnetic field strengths of 960 G at a height of 12,000 km. The field strength measurements combine to yield a magnetic scale height LB=6900 km. The radio brightness temperature maxima are located away from a sunspot plume that appears bright in EUV line emission formed at temperatures around several ×105 K. We use the density-sensitive emission-line intensity ratio of O IV 625.8 Å/554.5 Å to derive an electron density ne (in units of cm-3) of logne=10.1+/-0.2 at the base of the plume. Title: Preflare Nonthermal Emission Observed in Microwaves and Hard X-Rays Authors: Asai, Ayumi; Nakajima, Hiroshi; Shimojo, Masumi; White, Stephen M.; Hudson, Hugh S.; Lin, Robert P. Bibcode: 2006PASJ...58L...1A Altcode: We present a detailed examination on nonthermal emissions during the preflare phase of the X4.8 flare that occurred on 2002 July 23. The microwave (17GHz and 34GHz) data obtained with the Nobeyama Radioheliograph at Nobeyama Solar Radio Observatory and the hard X-ray data taken with the Reuven Ramaty High Energy Solar Spectroscopic Imager obviously showed nonthermal features in the preflare phase. We also found a faint ejection associated with the flare in the EUV images taken with the Transition Region and Coronal Explorer. We discuss the temporal and spatial features of the nonthermal emissions in the preflare phase, and their relation with the ejection. Title: Microwave and Hard X ray Imaging of Energetic Electrons in Solar Flares: Event of 2003 June 17 Authors: Kundu, M. R.; Schmahl, E. J.; White, S. M. Bibcode: 2006IAUS..233..334K Altcode: We discuss a large flare that was observed simultaneously by RHESSI in hard X-rays and by the Nobeyama Radio Heliograph (NoRH) in microwaves. The imaging observations made both by RHESSI and NoRH show many interesting features which may be relevant for producing realistic flare models. Title: Preflare Nonthermal Emission Observed in Microwaves and Hard X-Rays Authors: Asau, A.; Nakajima, H.; Shimojo, M.; White, S. M.; Hudson, H. S. Bibcode: 2006apri.meet...46A Altcode: No abstract at ADS Title: Preflare Features in Microwaves and in Hard X-Rays Authors: Asai, Ayumi; Nakajima, Hiroshi; Shimojo, Masumi; White, Stephen M. Bibcode: 2006aogs....2...33A Altcode: We present a detailed examination on the nonthermal emissions during the pre-flare phase of the X4.8 flare which occurred on July 23, 2002. The microwave (17 and 34 GHz) data obtained with Nobeyama Radioheliograph (NoRH), at Nobeyama Solar Radio Observatory, National Astronomical Observatory of Japan, and the hard X-ray (HXR) data taken with Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) distinctly showed nonthermal features. We examined the temporal, spatial, and spectroscopic characteristics of the emission sources, and found loop-top sources during the preflare phase both in HXRs and in microwaves. Moreover, we found that the electron spectral index derived from microwave emission closely corresponds to that obtained from the HXR emission. We also discuss the energy release mechanism in the preflare phase. Title: Low-Frequency Solar Radio Bursts from Green Bank Authors: White, S. M.; Bastian, T. S.; Bradley, R.; Parashare, C.; Wye, L. Bibcode: 2005ASPC..345..176W Altcode: A low-frequency spectrometer for the study of solar radio bursts is under development at Green Bank. Since January 2004 an 18-70 MHz system has been operating daily. The system is described and examples of data from the low-frequency system are shown.. Title: Instrument Development for the Green Bank Solar Radio Burst Spectrometer (GB/SRBS) Authors: Bradley, R.; Parashare, C.; White, S. M.; Bastian, T. S. Bibcode: 2005ASPC..345..357B Altcode: Details of two on-going technical development projects for the Green Bank Solar Radio Burst Spectrometer (GB/SRBS) are presented. One is a high dynamic range active balun covering 20-350 MHz. The other is a log periodic array feed for the Green Bank 45-Foot radio telescope covering 300-3000 MHz. Basic design, fabrication, and evaluation information are included for both projects. Title: Radio Measurements of Coronal Magnetic Fields Authors: White, S. M. Bibcode: 2005ESASP.596E..10W Altcode: 2005ccmf.confE..10W No abstract at ADS Title: Multiwavelength Analysis of a Solar Flare on 2002 April 15 Authors: Sui, Linhui; Holman, Gordon D.; White, Stephen M.; Zhang, Jie Bibcode: 2005ApJ...633.1175S Altcode: We carried out a multiwavelength analysis of the solar limb flare on 2002 April 15. The observations all indicate that the flare occurred in an active region with an asymmetric dipole magnetic configuration. The earlier conclusion that magnetic reconnection is occurring in a large-scale current sheet in this flare is further supported by these observations: (1) Several bloblike sources, seen in RHESSI 12-25 keV X-ray images later in the flare, appeared along a line above the flare loops. These indicate the continued presence of the current sheet and are likely to be magnetic islands in the stretched sheet produced by the tearing-mode instability. (2) A cusplike structure is seen in Nobeyama Radioheliograph (NoRH) 34 GHz microwave images around the time of the peak flare emission. We quantitatively demonstrate that the X-ray-emitting thermal plasma seen with RHESSI had a higher temperature than the microwave-emitting plasma seen with NoRH. Since the radio data preferentially see cooler thermal plasma, this result is consistent with the picture in which energy release occurs at progressively greater heights and the hard X-rays see hot new loops while the radio sees older cooling loops. The kinetic energy of the coronal mass ejection (CME) associated with this flare was found to be about 1 order of magnitude less than both the thermal energy in the hot plasma and the nonthermal energy carried by the accelerated electrons in the flare, as deduced from the RHESSI observations. This contrasts with the higher CME kinetic energies typically deduced for large flares. Title: The Radio Cycle of Eta Carinae Authors: White, S. M.; Duncan, R. A.; Chapman, J. M.; Koribalski, B. Bibcode: 2005ASPC..332..126W Altcode: 2005ASPC..332..129W We briefly summarize recent radio observations of the η Car cycle, including the surprising failure to detect a velocity shift from binary motion in the system. Title: RHESSI & Nobeyama Imaging Observations of the X 2.7 Flare of 2003 Nov 3, 00:58 UT Authors: Kundu, M. R.; Schmahl, E. J.; Garaimov, V. I.; White, S. M. Bibcode: 2005AGUSMSP52A..02K Altcode: We describe the RHESSI HXR and Nobeyama (NoRH) microwave imaging observations of an X-class event that was observed during the great solar activity period of Oct 19--Nov 10, 2003. The flare in question of class X 2.7 was observed in AR 0488 (position N08W66) on 2003 Nov 3, 00:58:33 UT, with a large peak at 01:15:25 UT, and ending at 03:08:19 UT. The maximum fluxes at 17 and 34 GHz are respectively 678 and 536 SFU. Besides the peak at 01:15 there are two other peaks at approx 01:20 and 01:32:30 UT. In NoRP data the peak at 01:20 is the most dominant at frequencies 9.40, 3.75, 2.0 GHz. At 1.0 GHz the emission prior to this peak is fluctuating and a peak with fluctuations exists at around 01:05 UT. The third peak at 01:32:30 UT is spiky and it is nonthermal with a turnover frequency between 17 and 34 GHz. One significant property of the HXR emission (below 12 keV) during the period 01:00 - 01:08 is that the preflare emission, in projection, lies close to and above the W-limb, although the associated microwave source is on the disk. There are two preflare sources (RHESSI at 12-25 keV, and NoRH at 17 and 34 Ghz), but it is not clear how they are related. The RHESSI source seems to coincide in position with the EIT flaring source at its apex. The morphological structure seems to suggest that we are dealing with a loop-top HXR source situated at the top of the EIT flaring loop and flaring sources at 17 and 34 GHz, whose morphology suggests a complex flaring arcade structure. Title: Antenna Configurations for the FASR B Array Authors: Tun, S. D.; Gary, D.; White, S. M. Bibcode: 2005AGUSMSP43A..09T Altcode: The proposed Frequency-Agile Solar Radiotelescope (FASR) is to produce high space, time, and frequency resolution full disk solar images. These will allow the reconstructions of the 3-D structure of the Sun's atmosphere from chromosphere to mid-corona. The high resolutions are required to carry out detailed diagnostics of fast-moving phenomena, such as flares and CMEs, as well as of quasi-static structures above active and quiet regions. The full disk images will capture all activity in the corona, even when there are many active regions at solar maximum. One of the tasks necessary to ensure optimal imaging for the array is to determine the best antenna configuration. The current poster presents work done towards this goal. We focus on the FASR-B array, which will cover the 200-3000 MHz range with 60 to 90 antennas of 6-m diameter. We have examined the imaging characteristics for a 3 arm, log-spiral configuration with radius of about 3 km. Multi-frequency observations are simulated for arrays varying in initial antenna spacing, number of antenna elements, and number of turns for each arm. The images reconstructed from the simulated snapshot observations are compared to the original model, convolved with the appropriate beam, to see which configuration yields the most faithful images. We discuss the implications of the findings for the planning of the FASR array configurations. Title: Millimeter-Interferometer Observations of Oscillations in the Solar Chromosphere Authors: White, S. M.; Loukitcheva, M.; Solanki, S. K. Bibcode: 2005AGUSMSH12A..03W Altcode: The use of millimeter-interferometer data for the study of chromospheric structure and dynamics is tested using 85 GHz observations with the 10-element Berkeley-Illinois-Maryland Array (BIMA). Interferometer data have the advantage over single-dish data that they allow both high spatial resolution and dense temporal sampling simultaneously. However, snapshot imaging of the quiet solar atmosphere with a small number of dishes is challenging. We demonstrate that techniques are available to carry out this task successfully. We compare observations of quiet Sun, active region and coronal hole targets using images with 10 arcsec resolution at 15 second cadence. The brightest features in the images are typically 1500 K above the background level, with a snapshot noise level of order 100 K. We use extensive tests to determine whether oscillation power can be recovered from sequences of snapshot images and show that they can down to quite weak levels. Intensity oscillations with significant power in the frequency range 1.5-8.0 mHz are found in the quiet-Sun and active region targets. For the quiet-Sun region we use the spatially-resolved time series to investigate the properties of network boundaries and cell interiors (internetwork) separately. In agreement with investigations at other wavelengths, the millimeter data show that power in the network tends to be at periods of 5 minutes and longer while power in the internetwork is present also at shorter (3-minute) periods. Title: Imaging Type IIIdm Burst Trajectories Authors: Liu, Z.; Gary, D. E.; Nita, G. M.; White, S. M.; Hurford, G. J. Bibcode: 2005AGUSMSP21B..06L Altcode: Type IIIdm radio bursts are important diagnostics of the upward and downward directed beams of nonthermal electrons that originate in the energy release site. Because these bursts have a very high frequency drift rate ( |df/dt|=(0.09±0.03)× f1.35±0.10, Melendez et al., 1999), high time and frequency resolution is a necessary for their observation. Since existing interferometers do not have this combination of spectral and time resolution in the decimetric range, we still do not have detailed information about the location and trajectory of Type IIIdm bursts. We are developing the new FASR Subsystem Testbed (FST) with three existing antennas of Owens Valley Solar Array (OVSA) that will permit imaging with high time and spectral resolution over the 1-9 GHz band. To accomplish this, the received signal is downconverted to 500 MHz bandwidth, then digitized with 1 GHz sampling rate, with digital correlation performed offline. With this three element interferometer, we have the ability to determine the centroid of simple source structures with very high time resolution (10 ms) and frequency resolution (<1 MHz). Such centroids as a function of frequency may outline the trajectories of nonthermal electron beams and so may show the magnetic topology at the energy release site. The FST is expected to be operational in early 2006. This poster presents the FST system configuration and the results of simulations of such trajectories, using a loop model with a simple magnetic geometry, density and temperature profiles given by hydrostatic equilibrium, for different loop heating functions (Aschwanden 2004, Physics of The Solar Corona, Chapter 3). Title: Updated Expressions for Determining Temperatures and Emission Measures from Goes Soft X-Ray Measurements Authors: White, Stephen M.; Thomas, Roger J.; Schwartz, Richard A. Bibcode: 2005SoPh..227..231W Altcode: {We investigate the conversion of the 0.5-4 and 1-8 Å soft X-ray flux measurements made by detectors on the Geostationary Operational Environmental Satellites (GOES) into temperature and emission measures of coronal plasma using modern spectral models and modern understanding of coronal abundances. In particular, the original analysis by Thomas, Starr and Crannell (1985) is updated to take into account the realization that coronal abundances may be quite different from photospheric abundances. An important result of this analysis is that the derived temperatures and emission measures depend strongly on the assumed abundances even at high temperatures where continuum rather than spectral lines dominates the Sun's X-ray spectrum. This occurs because the higher coronal abundances mean that most of the continuum is due to free-bound emission processes, not free-free emission, and thus is abundance-dependent. We find significant differences between modern calculations of the temperature response of the flux measurements and the versions currently in use: for a typical flare, emission measures may be up to a factor of 4 smaller than the current software suggests. Derived temperatures are similar for both photospheric and coronal abundances for cool flares (e.g., 15 MK), but for hot flares (e.g., 35 MK) coronal abundances can lead to significantly (~25%) lower temperatures being derived. Title: Molecular Line Observations of IRAM 04191+1522 Authors: Lee, Chin-Fei; Ho, Paul T. P.; White, Stephen M. Bibcode: 2005ApJ...619..948L Altcode: We have mapped the CO, HCO+, CS, and N2H+ emission around a low-luminosity Class 0 source IRAM 04191+1522 in the Taurus molecular cloud using the Berkeley-Illinois-Maryland Association array. A bipolar molecular outflow is seen in CO, HCO+, and CS emission originating from around the IRAM source, while a flattened envelope is seen in N2H+ emission surrounding the waist of the molecular outflow around the IRAM source. Outflow, rotation, and probably infall are seen around the Class 0 source, indicating a complicated context for star formation in the earliest stages. The N2H+ envelope is a thick clumpy torus with a tenuous outer part and a ringlike dense inner edge. The dense inner edge has a mean radius of 10", or 1400 AU. The outer part of the torus exhibits differential rotation, and the infalling material appears to carry angular momentum inward toward the central source. The region surrounded by the inner edge of the torus may have a solid-body rotation. The envelope may result from the collapse of a rotating, magnetized toroid toward the central source. Outflow motion is seen in the outer part of the inner edge of the torus, probably due to an interaction with the molecular outflow. Two armlike structures are seen extending out from the inner edge in the torus. One of them is clearly seen with the velocity increasing roughly linearly with the distance. The HCO+ emission around the source may trace the central core around the source, showing a velocity structure connecting to that of the outer part of the torus. The molecular outflow is best seen in CO. It is bipolar with both a southwest and a northeast lobe. Two internal structures are seen along the main outflow axis within the lobes: (1) a strong CO emission at ~20" to the northeast of the IRAM source, likely tracing an internal bow shock driven by a collimated, episodic jet and (2) a linear HCO+ structure of ~50" length at low blueshifted velocity to the southwest of the IRAM source, requiring some interaction with a jet to be produced. In addition, a dense condensation is also seen in HCO+ and CS to the south of the IRAM source, likely representing a recent strong outflow interaction. Title: High-Cadence Radio Observations of an EIT Wave Authors: White, S. M.; Thompson, B. J. Bibcode: 2005ApJ...620L..63W Altcode: Sensitive radio observations of the 1997 September 24 EIT wave show its velocity to be 830 km s-1. The wave first appears a short distance from the flare site, and its trajectory projects back to the flare site at the peak of the impulsive phase. The radio spectrum appears to be consistent with optically thin coronal emission rather than chromospheric emission. The observed radio brightness temperatures are consistent with the EIT fluxes if the temperature of the emitting gas is not at the peak formation temperature of the Fe XII 195 Å line or if abundances are closer to photospheric than coronal. An important result is that no deceleration is observed during the 4 minutes that the wave is visible in the radio images: the discrepancy between EIT wave and Hα Moreton wave speeds requires that EIT waves slow substantially as they propagate, if they are the same disturbance. Title: The solar stellar connection Authors: White, S. M. Bibcode: 2004NewAR..48.1319W Altcode: 2004astro.ph..9157W Stars have proven to be surprisingly prolific radio sources and the added sensitivity of the Square Kilometer Array will lead to advances in many directions. This chapter discusses prospects for studying the physics of stellar atmospheres and stellar winds across the HR diagram. Title: Coronal Magnetic Field Measurements Through Gyroresonance Emission Authors: White, Stephen M. Bibcode: 2004ASSL..314...89W Altcode: This article reviews the use of gyroresonance emission at radio wavelengths to measure coronal magnetic fields. The spiralling motion of electrons in the 200-2000 G fields in the solar corona produces sufficient opacity to render the corona optically thick, making it easy to recognize such sources in microwave images from their coronal brightness temperatures. Where gyroresonance sources are present they may be used as sensitive probes of the magnetic field strength above active regions, and this unique capability is one of the strengths of radio observations. Typically a gyroresonance radio source shows the temperature on an optically thick surface of constant magnetic field within the corona. Since each radio frequency corresponds to a different magnetic field strength, the coronal structure can be "peeled away" by using different frequencies. Title: FASR Simulation of Solar Flare Microwave Spectrum Authors: Lee, J.; Gary, D. E.; White, S. M. Bibcode: 2004AAS...204.5414L Altcode: 2004BAAS...36..760L We investigate microwave diagnostics on solar flare electrons at the quality of imaging spectroscopy expected from the proposed Frequency Agile Solar Radiotelescope (FASR). We mainly concern ourselves with influence of the spatial inhomogeneities upon microwave spectral features and realistic magnetic configurations of flare active regions. Solar flare microwave radiations are calculated by using nonthermal electrons distributed along a set of reconnected field lines, and by sampling the resulting gyrosynchrotron intensities at the resolution of the FASR. The simulated observations are then compared with reference spectra of homogeneous source models to find a strategy for inversion of the observed spectrum to the physical parameters of flare electrons.

This work is supported by the NSF grant AST-0138317 and NASA grant NAG5-10891 to New Jersey Institute of Technology. Title: Radio Observations of Rapid Acceleration in a Slow Filament Eruption/Fast Coronal Mass Ejection Event Authors: Kundu, M. R.; White, S. M.; Garaimov, V. I.; Manoharan, P. K.; Subramanian, P.; Ananthakrishnan, S.; Janardhan, P. Bibcode: 2004ApJ...607..530K Altcode: We discuss a filament eruption/coronal mass ejection (CME) event associated with a flare of GOES class M2.8 that occurred on 2001 November 17. This event was observed by the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz. NoRH observed the filament during its eruption both as a dark feature against the solar disk and a bright feature above the solar limb. The high cadence of the radio data allows us to follow the motion of the filament at high time resolution to a height of more than half a solar radius. The filament eruption shows a very gradual onset and then a rapid acceleration phase coincident with the launch of a fast halo CME. Soft X-ray and extreme-ultraviolet (EUV) images show heating in a long loop underneath the filament prior to the flare. The NoRH height-time plot of the filament shows a roughly constant gradual acceleration for 1 hr, followed by a very abrupt acceleration coincident with the impulsive phase of the associated flare, and then a phase of constant velocity or much slower acceleration. This pattern is identical to that recently found to occur in the motion of flare-associated CMEs, which also show a sharp acceleration phase closely tied to the impulsive phase of the flare. When the rapid acceleration occurs in this event, the flare site and the filament are separated by ~0.5 Rsolar, making it unlikely that a disturbance propagates from one location to the other. Models in which a disruption of the large-scale coronal magnetic field simultaneously permits the acceleration of the filament and the flare energy release seem to be a better explanation for this event. Title: Nobeyama Radioheliograph and RHESSI Observations of the X1.5 Flare of 2002 April 21 Authors: Kundu, M. R.; Garaimov, V. I.; White, S. M.; Krucker, S. Bibcode: 2004ApJ...600.1052K Altcode: We present an overview of the radio observations of the X1.5 flare of 2002 April 21 and complementary data from other wavelengths. This flare was fairly well observed by the Ramaty High Energy Spectroscopic Imager (RHESSI) spacecraft and fully observed by the Nobeyama Radioheliograph (NoRH) at 17 and 34 GHz. This long-duration event lasted more than 2 hr and featured a beautiful arcade of rising loops on the limb visible at X-ray, EUV, and radio wavelengths. The main flare was preceded by a small event 90 minutes earlier showing a long EUV loop connecting well-separated radio and hard X-ray sources. The main flare itself starts with a compact radio and hard X-ray source at the eastern end of the region that develops into emission close to the solar surface (and well inside the solar limb) over a large region to the northwest. As the flare proceeds, a large set of loops is seen to rise well above the solar limb. Distinct regions of radio emission with very different time behavior can be identified in the radio images, and, in particular, a peculiar nonthermal source seen in radio and hard X-rays low in the corona at the base of the arcade is seen to turn on 30 minutes after the start of the impulsive phase. At about the same time, an extremely intense burst of coherent radio emission is seen from 500 to 2000 MHz; we speculate that this lower-frequency burst is produced by electrons that are accelerated in the nonthermal source at the base of the arcade and injected into the loop system where they radiate plasma emission in the 1010 cm-3 density plasma at the top of the arcade of loops. This event is striking as a demonstration of the many ways in which a flare can produce radio emission, and the combined data at different wavelengths reveal a diversity of energy release and nonthermal acceleration sites. Title: Close Association of an Extreme-Ultraviolet Sunspot Plume with Depressions in the Sunspot Radio Emission Authors: Brosius, Jeffrey W.; White, Stephen M. Bibcode: 2004ApJ...601..546B Altcode: We obtained coordinated observations of the large sunspot in NOAA Region 8539 on 1999 May 9 and 13 with the Very Large Array and three instruments (CDS, EIT, MDI) aboard the Solar and Heliospheric Observatory satellite. The EUV observations reveal a plume in the sunspot umbra on both observing dates. The plume appears brightest in emission lines formed at temperatures between 1.6×105 and 5.0×105 K. Radio emission from the sunspot umbra is dominated by thermal gyroemission from the plume, which accounts for radio brightness temperatures <1×106 K in the umbra on both dates, as well as umbral brightness temperature depressions in the 4.535 and 8.065 GHz observations on May 13. A compact 14.665 GHz source persists near the umbra/penumbra boundary during our observing period, indicating a long-lived, compact flux tube with coronal magnetic field strength of at least 1748 G. It occurs in a portion of the sunspot that appears very dark in EUV emission. Title: Radio Observations of Rapid Acceleration in a Slow Filament Eruption/Fast CME Event Authors: Kundu, M. R.; White, S. M.; Garaimov, V. I.; Manoharan, P. K.; Subramanian, P.; Ananthakrishnan, S.; Janardhan, P. Bibcode: 2003AGUFMSH21A..06K Altcode: We discuss a filament eruption/coronal mass ejection (CME) associated flare event of GOES class M2.8 that occurred on November 17, 2001. This event was observed simultaneously by Nobeyama RadioHeliograph (NoRH) at 17 and 34 GHz and by the Giant Meterwave Radio Telescope (GMRT) in India at 1060 MHz. The flare occurred in active region 9704 at S18E41. NoRH observes the filament during its eruption both as a dark feature against the solar disk and a bright feature above the solar limb. The filament eruption shows a very gradual onset and then a rapid acceleration phase coincident with the launch of a fast halo CME. The flare on the disk in microwaves has a gradual rise, a broad maximum and a gradual decay. At 1060 MHz, its time profile had three impulsive peaks. The NoRH height--time plot of the filament shows a roughly constant gradual acceleration for an hour, followed by a very abrupt acceleration coincident with the impulsive phase of the associated flare. Soft X-ray images show heating in a long loop underneath the filament prior to the flare. The impulsive behavior of 1060 MHz emission combined with high brightness temperatures indicated the existence of nonthermal electrons in loops that clearly are not the same as the loops containing the microwave-emitting electrons. The latter are dominated by thermal bremsstrahlung and agree well with the EUV and soft X-ray loops. This study is consistent with recent findings that the rapid acceleration of flare--associated CMEs is closely tied to the impulsive phase of the flare. A trigger is required to cause the rapid acceleration to occur at the same time as the flare even though the two events are spatially well separated. We speculate that this trigger is provided by some kind of reconnection in the multiple flux systems that exist between the flaring active region and the erupting filament. Title: Coronal Magnetic Fields in a Sunspot Plume at the Limb Authors: Brosius, J. W.; White, S. M. Bibcode: 2003AGUFMSH42B0507B Altcode: We obtained coordinated EUV and radio observations of NOAA Active Region 10139 on 2002 October 14 when the region was on the west limb of the Sun. Observations were obtained with the Very Large Array (VLA) and three instruments aboard the Solar and Heliospheric Observatory (SOHO) satellite, including the Coronal Diagnostic Spectrometer (CDS), the Extreme-ultraviolet Imaging Telescope (EIT), and the Michelson Doppler Imager (MDI). A sunspot plume is clearly seen in EUV emission lines formed at temperatures between about 0.2 and 0.5 MK. Polarized 8 GHz radio emission from the plume suggests 4th harmonic gyroemission (from 760 Gauss fields) above the limb, and 3rd harmonic gyroemission (from 960 Gauss fields) on the disk. Title: Coronal Magnetic Diagnostics With FASR Authors: White, S. M.; Gary, D. E.; Lee, J.; Giordano, G. Bibcode: 2003AGUFMSH42D..04W Altcode: Coronal magnetography is one of the main scientific drivers for the proposed Frequency Agile Solar Radiotelescope (FASR). Radio emission is particularly valuable as a diagnostic of coronal magnetic fields because (a) the emission mechanisms all depend on magnetic field, and (b) typical values of the electron gyroresonance frequency f_B for coronal field strengths lie in the radio domain. The microwave emission from active regions is dominated by thermal gyroresonance emission at low harmonics of f_B and this provides a well-understood diagnostic. Since f_B is proportional to magnetic field strength, there is a simple mapping between frequency and magnetic field. A wide range of coronal magnetic field strengths can be sampled by observing across a wide range of radio frequencies simultaneously, and FASR is designed to do this quickly enough to follow changes in coronal fields. We demonstrate the ability to measure coronal fields with this technique by simulating a FASR observation of a realistic three-dimensional model of an active region and then determining the coronal magnetic field at the base of the corona from the simulated images. Comparison with radio images of gyroresonance emission from active regions is also a valuable tool for assessing extrapolations of surface magnetic field measurements into the corona, and we discuss several applications of this comparison. Gyrosynchrotron radio emission from nonthermal electrons accelerated by solar flares also can reveal the magnetic topology of the flare source and we discuss this briefly. Title: Giant Meterwave Radio Telescope observations of an M2.8 flare: Insights into the initiation of a flare-coronal mass ejection event Authors: Subramanian, Prasad; Ananthakrishnan, S.; Janardhan, P.; Kundu, M. R.; White, S. M.; Garaimov, V. I. Bibcode: 2003SoPh..218..247S Altcode: 2003astro.ph..9661S We present the first observations of a solar flare with the GMRT. An M2.8 flare observed at 1060 MHz with the GMRT on 17 November 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronagraph. Towards the start of the eruption, we find evidence for reconnection above the prominence. Subsequently, we find evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the flare and prominence eruption. Title: Radio and Hard X-Ray Images of High-Energy Electrons in an X-Class Solar Flare Authors: White, S. M.; Krucker, S.; Shibasaki, K.; Yokoyama, T.; Shimojo, M.; Kundu, M. R. Bibcode: 2003ApJ...595L.111W Altcode: We present the first comparison between radio images of high-energy electrons accelerated by a solar flare and images of hard X-rays produced by the same electrons at photon energies above 100 keV. The images indicate that the high-energy X-rays originate at the footpoints of the loops dominating the radio emission. The radio and hard X-ray light curves match each other well and are quantitatively consistent with an origin in a single population of nonthermal electrons with a power-law index of around 4.5-5. The high-frequency radio spectral index suggests a flatter energy spectrum, but this is ruled out by the X-ray spectrum up to 8 MeV. The preflare radio images show a large hot long-lived loop not visible at other wavelengths. Flare radio brightness temperatures exceed 109 K, and the peak in the radio spectrum is as high as 35 GHz: both these two features and the hard X-ray data require very high densities of nonthermal electrons, possibly as high as 1010 cm-3 above 20 keV at the peak of the flare. Title: Radio and RHESSI X-ray Studies of Solar Flares Authors: White, S. M. Bibcode: 2003SPD....34.1403W Altcode: 2003BAAS...35..829W Correlative studies of particle acceleration in solar flares using radio data and RHESSI hard X-ray data will be discussed. In particular, the big flares of 2002 (April 21 and July 23) will be reviewed. The July 23 flare is notable as the first event in which we can compare radio images of nonthermal gyrosynchrotron-emitting electrons with hard X-ray images at photon energies above 100 keV. Quantitatively this event indicates that the same nonthermal electron population is responsible for both the radio and hard X-ray emission. However, both the radio and hard X-ray data imply extreme number densities for the nonthermal electrons: over 1010 cm-3 at energies above 20 keV. The April 21 flare is a much more extended event, and features a peculiar nonthermal radio source in the later stages of the event. Title: Quasi-periodic Pulsations in a Solar Microwave Burst Authors: Grechnev, V. V.; White, S. M.; Kundu, M. R. Bibcode: 2003ApJ...588.1163G Altcode: Quasi-periodic pulsations in solar flares can provide important information on physical conditions in flaring regions. In this paper, we study a microwave burst that showed deep quasi-periodic pulsations. The most dramatic feature of this event has been discussed by Asai and coworkers. In the second of four bursts during the flare, strongly modulated pulsations appear in radio images from the eastern end of a long loop and in hard X-rays from the western end of the loop. We show, in addition, that (1) at least five distinct radio sources with very different time profiles can be identified, including emission from the long loop connecting the modulated radio and X-ray sources; (2) substructure is also present in the radio emission from the eastern end of the long loop during the first burst of the flare, but with timescales shorter than in the second burst; (3) radio modulations are seen at the western end of the loop during the second burst but at a level some 20 times weaker than at the eastern end; (4) these radio modulations at the western end of the loop, like the hard X-ray modulations at the same location, appear to lead the modulations at the eastern end by about 0.5 s, but all have the same period. The period of the modulation can be explained by MHD oscillations of the loop ~120" long connecting the sources: both oscillations that change the magnetic field strength in the loop, such as propagating fast-mode waves, and torsional oscillations that change the direction of the magnetic field in the loop can explain the observed properties of the modulation of the radio emission. An impulsive reconnection episode is a plausible source of oscillating fast-mode waves and is consistent with some other aspects of the event. However, it is difficult to reconcile the strength of the radio modulations at the eastern end of the loop with their delay relative to the emissions at the western end, where the modulation is observed to be much weaker. If the electrons originate at the western end where the main energy release seems to occur, and the modulation is imposed on them there before they propagate 1010 cm to the other end of the loop, any effects due to a spread in electron energies or pitch angles would lead to a spread in propagation times that should smooth out the modulation of the radio emission from the remote source, as should any trapping of electrons in the loop: yet the radio emission from the eastern end of the loop shows much stronger modulation than the radio and hard X-ray emission from the western end of the loop. Title: Nobeyama Radio Heliograph and RHESSI Observations of the X1.5 Flare of April 21, 2002 Authors: Kundu, M. R.; Garaimov, V. I.; White, S. M.; Krucker, S. Bibcode: 2003SPD....34.1812K Altcode: 2003BAAS...35..841K We present radio observations of the X1.5 flare of April 21, 2002, and complementary data from other wavelengths. This flare was fairly well observed by the spacecraft RHESSI and fully observed by the Nobeyama Radio Heliograph at 17 and 34 GHz. This long-duration event lasted more than 2 hours and features a beautful arcade of rising loops on the limb visible at X-ray, EUV and radio wavelengths. The main flare was preceded by a small event 90 minutes earlier showing a long EUV loop connecting well-separated radio and hard X-ray sources. The main flare itself starts with a compact radio and hard X-ray source at the eastern end of the region that is followed by emission close to the solar surface (well inside the solar limb) over a large region to the northwest. As the flare proceeds a large set of loops is seen to rise well above the solar limb. Distinct regions of radio emission with very different time behaviour can be identified in the radio images, and in particular a peculiar nonthermal 17 GHz source low in the corona at the base of the arcade is seen to turn on 30 minutes after the start of the impulsive phase. At about the same time an extremely intense burst of coherent radio emission is seen from 500 to 2000 MHz: we speculate that this lower-frequency burst is located at the top of the arcade of loops. Title: The Frequency Agile Solar Radiotelescope (FASR) Authors: White, S. M.; Gary, D. E.; Bastian, T. S.; Hurford, G. J.; Lanzerotti, L. J. Bibcode: 2003EAEJA....11021W Altcode: The Frequency Agile Solar Radiotelescope (FASR) is a radio interferometer designed to make high spatial resolution images of the Sun across a broad range of radio wavelengths simultaneously, allowing the technique of imaging spectroscopy to be exploited on a routine basis. The telescope will cover the frequency range 0.1-30 GHz using several sets of receiving elements that provide full-disk imaging, with of order 100 antennas at highest frequency range. FASR will be optimized for solar radio phenomena and will be the most powerful and versatile radioheliograph ever built, providing an improvement of orders of magnitude in image quality over existing instruments. FASR recently received the top ranking amongst all small projects considered by the decadal survey of the National Academy of Science Committee on Solar and Space Physics. FASR will probe all phenomena in the solar atmosphere from the mid-chromosphere outwards. In particular, FASR will provide direct measurement of coronal magnetic field strengths, will image the nonthermal solar atmosphere and show directly the locations of electrons accelerated by solar flares, will provide images of coronal mass ejections travelling outwwards through the solar corona, and supply extensive data products for forecasting and synoptic studies. A major emphasis in the project is to make FASR data as widely and easily used as possible, i.e., providing the general user with processed, fully-calibrated high-quality images that do not need particular knowledge of radio astronomy for interpretation. This paper will describe the telescope and its science goals, and summarize its current status. Title: Radio evidence of recent mass ejection from η Carinae Authors: Duncan, R. A.; White, S. M. Bibcode: 2003MNRAS.338..425D Altcode: The luminous blue variable η Carinae is almost certainly a double star: a hot secondary in an eccentric orbit around a massive primary. Radio observations with the Australia Telescope Compact Array at a wavelength of 3 cm, covering the period from 1992 to 2002, suggest that at the time of last periastron, 1998.0, material was tidally lifted from the primary star into its equatorial disc. Title: Radio observations of explosive energy releases on the Sun Authors: Kundu, M. R.; White, S. M. Bibcode: 2003dysu.book..288K Altcode: This chapter is devoted to a discussion of the radio observations of explosive energy releases (normal flares and small-scale energy releases) on the Sun. Radio imaging observations of solar flares and coronal transients and the relationship of radio phenomena with those observed in hard and soft X-rays, and underlying physics are discussed. Title: Recent X-ray Variability of Eta Car Approaching The X-ray Eclipse Authors: Corcoran, M.; Swank, J. H.; Ishibashi, K.; Gull, T.; Humphreys, R.; Damineli, A.; Walborn, N.; Hillier, D. J.; Davidson, K.; White, S. M.; Petre, R.; Pittard, J.; Butt, Y.; Verner, K. Bibcode: 2002AAS...201.4901C Altcode: 2002BAAS...34.1184C We discuss recent X-ray spectral variability of the supermassive star Eta Car in the interval since the last X-ray eclipse in 1998. We concentrate on the interval just prior to the next X-ray eclipse which is expected to occur in June 2003. We compare the X-ray behavior during the 2001-2003 cycle with the previous cycle (1996-1998) and note similarities and differences in the temporal X-ray behavior. We also compare a recent X-ray observation of Eta Car obtained with the Chandra high energy transmission grating in October 2002 with an earlier observation from Nov 2002, and interpret these results in terms of the proposed colliding wind binary model for the star. In addition we discuss planned observations for the upcoming X-ray eclipse. Title: The Solar Atmosphere at Radio Wavelengths Authors: White, S. M. Bibcode: 2002ASPC..277..299W Altcode: 2002sccx.conf..299W No abstract at ADS Title: Investigating the HH 212 Molecular Outflow: the Jet- and Wind-Driven Models Authors: Chapman, N. L.; Mundy, L. G.; Lee, C. -F.; White, S. M. Bibcode: 2002AAS...201.2001C Altcode: 2002BAAS...34.1133C Molecular outflows are a common stage in star formation and over 200 outflows have been discovered to date, but the fundamental driving mechanism for outflows remains uncertain. The current leading theories are the wide-angle wind-driven model (Shu et al. 1991) and the jet-driven model (Raga & Cabrit 1993; Masson & Chernin 1993). Using the BIMA interferometer, we have made observations of the HH 212 molecular outflow in several molecules (CO, HCO+, SiO). This outflow was selected because of its high-degree of bipolar symmetry, the numerous well- defined H2 bow shocks (Zinnecker et al. 1998), and excellent morphological relationship of H2 to CO (Lee et al. 2000). Molecular shock physics describes how molecules behave at different temperatures and velocities. Using this, it is possible to describe the emission characteristics expected in both wind- and jet-driven scenarios. We compare our observations with the models to investigate the driving mechanism of molecular outflows. This research was supported by NASA grant NAG 510611. Title: The Physical Properties of a Flaring Loop Authors: White, S. M.; Kundu, M. R.; Garaimov, V. I.; Yokoyama, T.; Sato, J. Bibcode: 2002ApJ...576..505W Altcode: We use high-resolution radio observations to study the physical parameters of a flaring loop. The loop is visible at radio wavelengths because of gyrosynchrotron emission by nonthermal electrons (energies typically above several hundred keV) accelerated by the flare. We are able to measure the loop thickness and length with a precision on the order of 1". We find that the loop length increases from about 60" initially to about 80" in the decay phase of the event. The loop (averaged along its length) initially is no more than 3" wide. The soft and hard X-ray data obtained with the Soft X-Ray Telescope and Hard X-Ray Telescope on the Yohkoh satellite are consistent with the same loop as observed at radio wavelengths (although the soft X-ray morphology has some small differences early in the event). This event was accompanied by a coronal mass ejection and a coronal dimming visible in Solar and Heliospheric Observatory Extreme-Ultraviolet Imaging Telescope images, so it involved a very large volume of the corona, yet the radio observations clearly indicate that much of the energy release in the low corona was restricted to a region apparently no more than 2000 km across. As the event proceeds, the loop develops a bright feature at the loop top in both the radio and soft X-ray images that cannot be reproduced in gyrosynchrotron loop models in which the electron distribution has relaxed by pitch angle scattering to fill the loop. This prevents us from using the flare properties to measure the magnetic field strength and variation along the loop. The bright loop-top source may require that trapping of electrons take place at the loop top late in the event. Title: New radio instrumentation for the study of sunspots and starspots Authors: White, S. M. Bibcode: 2002AN....323..265W Altcode: Much of the radio emission from the Sun and similar stars depends directly on magnetic fields for its origin. For this reason, radio emission contains important diagnostic information on solar and stellar magnetic fields. This paper reviews radio measurements of coronal magnetic fields and discusses new radio instrumentation that will exploit this technique. Title: Radio Observations of the Onset of an EIT Wave Authors: White, S. M.; Thompson, B. J. Bibcode: 2002AAS...200.2904W Altcode: 2002BAAS...34R.681W We present observations of the early development of an ``EIT wave'' made with the Nobeyama Radio Heliograph at 17 GHz. EIT waves are propagating disturbances generated in conjunction with solar flares. They have most easily been seen to date as emission enhancements in full-disk EUV images taken in spectral lines sensitive to 1-2 million degree material. We demonstrate that they can also be seen in high dynamic range radio observations as well. The high cadence of the radio data allows us to show that the EIT wave is not visible until after the onset of the impulsive phase of the flare. A radio movie of the event will be shown. We discuss the implications of this result for the nature of EIT waves and their relationship with other phenomena such as CMEs. Title: Measuring Coronal Magnetic Fields Authors: White, S. M. Bibcode: 2002AAS...200.4903W Altcode: 2002BAAS...34..721W Coronal magnetic fields are implicated in most active phenomena in the Sun's corona but their measurement is a difficult problem. While a selection of coronal magnetic field lines can be seen in soft X-ray and EUV images, they contain no information on magnetic field strength. Magnetic field measurements in the lower solar atmosphere are a priority for the Solar-B and Solar Dynamics Observatory missions, but they will not measure coronal magnetic fields directly, relying instead on extrapolations of surface field measurements. On the other hand, radio measurements can determine the magnetic field strength in the corona, including regions seen against the solar disk, in a straightforward fashion at least for field strengths larger than a few hundred gauss. This talk will discuss progress in the established technique of measuring active region magnetic fields through their gyroresonance emission at centimeter wavelengths and prospects for vastly improved measurements with the Frequency Agile Solar Radiotelescope. Such measurements can be valuable for establishing the validity of extrapolations and, e.g., resolving the 180 degree ambiguity of vector magnetic field measurements, in addition to their direct diagnostic power. The basis of the method and examples of its application will be discussed, and comparison with other approaches will be made. This work is supported by the NSF and NASA. Title: New Measurements of 3-D Sunspot Coronal Magnetic Fields From Coordinated SOHO EUV and VLA Radio Observations Authors: Brosius, J. W.; White, S. M.; Landi, E.; Cook, J. W.; Newmark, J. S.; Gopalswamy, N.; Lara, A. Bibcode: 2002AAS...200.0307B Altcode: 2002BAAS...34..642B Three-dimensional sunspot coronal magnetograms were derived from coordinated extreme-ultraviolet (EUV) and radio observations of NOAA regions 8108 (N21E18 on 1997 November 18) and 8539 (N20W12 on 1999 May 13). The EUV spectra and images, obtained with the Coronal Diagnostic Spectrometer (CDS) and the Extreme-ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO) satellite, were used to derive the differential emission measure (DEM) and the plasma electron density for each spatial pixel (along each line of sight) within both regions. These were subsequently used to calculate maps of the expected thermal bremsstrahlung brightness temperature at the Very Large Array (VLA) radio observing frequencies of 1.4, 4.9, 8.4, and 15 GHz. The thermal bremsstrahlung maps reproduce neither the structure nor the intensity of the observed maps, and indicate that thermal gyroemission must dominate the observed radio emission. The radio observations were used to constrain the magnetic scale height and the gross temperature structure of the atmosphere. These, along with the DEM, electron density, and observed radio brightness temperature maps, were used to derive the temperature distribution of the coronal magnetic field strength B(T) that reproduced simultaneously the observed right-hand and left-hand circularly polarized emission at the radio observing frequencies for each spatial pixel in the images. Magnetic field strengths corresponding to 3rd harmonic gyroemission at 4.9 GHz (580 Gauss) are found in coronal plasmas at temperatures as high as 3.2 MK, while magnetic field strengths corresponding to 3rd harmonic gyroemission at 15 GHz (1800 Gauss) are found in coronal plasmas at temperatures as high as 1.6 MK. B(T) was ultimately converted to B(h) and compared with extrapolations from photospheric magnetograms. Title: Radio images of four luminous blue variable stars Authors: Duncan, R. A.; White, S. M. Bibcode: 2002MNRAS.330...63D Altcode: We have used the Australia Telescope Compact Array to image four southern luminous blue variable stars: AG Car, He3-519, HR Car and WRA 751, at wavelengths of 3 and 6cm, and resolutions of 1 and 2arcsec respectively. With the partial exception of HR Car, all radio images show an unresolved stellar core surrounded by a large ionized gaseous nebula, and agree well with published Hα and [Nii] optical images. The image of WRA 751 shows a stellar torus or disc. HR Car's radio image is unusual, and seems best explained by the presence of a hot binary companion. Title: Correlation effects in microwave observations of selected RS CVn-like stars Authors: Budding, E.; Lim, J.; Slee, O. B.; White, S. M. Bibcode: 2002NewA....7...35B Altcode: Sets of dual frequency microwave data on selected chromospherically active stars, from the Australia Telescope Compact Array, have been investigated for their auto and cross-correlation effects. Comparison of cross-correlation peak values with theoretical expectation indicates a high degree of real physical connection between the emission at the pairs of frequencies (4.8 and 8.64 GHz) compared. This fact should help constrain models for the emission mechanism. The timescale of observed time-shifts between the emissions at the two frequencies is consistent, in general, with the underlying energization being propagated by magnetohydrodynamic waves in a compact turbulent medium. Title: Double Loop Configuration of a Flaring Region from Microwave, Extreme-Ultraviolet, and X-Ray Imaging Data Authors: Kundu, M. R.; Grechnev, V. V.; Garaimov, V. I.; White, S. M. Bibcode: 2001ApJ...563..389K Altcode: We use extensive multiwavelength data to study a flare belonging to the interacting-loop class of events identified by Hanaoka. The class of flares is identified morphologically from the presence of two well-separated radio sources in 17 GHz images (in this event, 160" apart), with only one source showing soft X-ray emission. This event shows many of the other properties apparently shared by this class of flares: a gradual rise showing many subsidiary peaks in both radio and hard X-ray light curves with a quasi-oscillatory nature, the presence of a bright compact X-ray-emitting loop in the main flare source, a delay of the radio emission from the remote source relative to the main X-ray-emitting source, higher circular polarization in the radio emission of the remote source than in the main source, and stronger photospheric magnetic field in the remote source. The new results of our analysis are that we are able to show, using a sequence of magnetograms, that the magnetic field in the main flare site changes sharply at the time of the flare, and further we argue that the remote site is magnetically connected to the main flare site only up to the time of the main impulsive phase, at which point we believe the magnetic connection to the remote site was broken and further flare manifestations are largely confined to the main flare site. This severing of the magnetic connection between two well-separated active regions may be an intrinsic part of the energy release in this flare. The region around the main flare site also exhibits rotation in the magnetogram in the period leading up to the flare. Radio and hard X-ray oscillations with periods of order 5-10 s are observed in the rise phase of this event. If they are due to transverse oscillations of the flare loop at the Alfvén speed, then the density in the loop is inferred to be of order 1011 cm-3 and to increase with time as expected. Title: The Timing of Radio Emission in Flares and CMEs Authors: White, S. M. Bibcode: 2001AGUFMSH31C..06W Altcode: Radio observations are very sensitive to emission by electrons accelerated in the solar corona. This talk will review evidence for acceleration in flares and coronal mass ejections from the radio perspective, focussing on the timing of radio emission in relation to other high-energy phenomena. Amongst other topics, the relationship of coronal type II radio bursts and coronal mass ejections will be discussed in the light of Zhang et al's LASCO data, and recent results on radio emission from CMEs by Bastian et al will be reviewed. Title: Reconciling Extreme-Ultraviolet and Radio Observations of the Sun's Corona Authors: Zhang, J.; Kundu, M. R.; White, S. M.; Dere, K. P.; Newmark, J. S. Bibcode: 2001ApJ...561..396Z Altcode: The Sun's corona, which is composed of plasma at a temperature of a few millions of degrees, can be best viewed in two electromagnetic domains, one from wavelengths of a few angstroms to hundreds of angstroms (in the soft X-ray and EUV domain), the other from wavelengths of a few centimeters to several tens of centimeters (in the radio domain). In this paper, we present a quantitative comparison of coronal observations made in these two domains with high spatial resolution over the full disk of the Sun. The EUV observations were taken with the EIT (Extreme-Ultraviolet Imaging Telescope) on board SOHO (Solar and Heliospheric Observatory), and the radio observations were taken with the VLA (Very Large Array). The two sets of images show very similar morphologies, indicating that the different wavelengths originate from common solar features. We predict radio fluxes using the temperature and emission measure of the corona calculated from EIT observations, adopting Meyer's table of coronal abundances for the calculations. In each of the seven observations investigated, there always exists a good linear correlation in the pixel-by-pixel correlation plot between the predicted and the observed radio flux for coronal features over a wide range of flux variation. Nevertheless, the predicted radio flux is systematically larger than that observed by a factor of 2.0+/-0.2, on average. We attribute the difference to the underestimation of the abundance of Fe relative to H in the abundances adopted by Meyer. On this basis, we place the absolute Fe abundance in the corona at 7.8×10-5, which has an enrichment factor of 2.4 relative to the accepted photospheric Fe abundance. Title: On the Temporal Relationship between Coronal Mass Ejections and Flares Authors: Zhang, J.; Dere, K. P.; Howard, R. A.; Kundu, M. R.; White, S. M. Bibcode: 2001ApJ...559..452Z Altcode: The temporal relationship between coronal mass ejections (CMEs) and associated solar flares is of great importance to understanding the origin of CMEs, but it has been difficult to study owing to the nature of CME detection. In this paper, we investigate this issue using the Large Angle and Spectrometric Coronagraph and the EUV Imaging Telescope observations combined with GOES soft X-ray observations. We present four well-observed events whose source regions are close to the limb such that we are able to directly measure the CMEs' initial evolution in the low corona (~1-3 Rsolar) without any extrapolation; this height range was not available in previous space-based coronagraph observations. The velocity-time profiles show that kinematic evolution of three of the four CMEs can be described in a three-phase scenario: the initiation phase, impulsive acceleration phase, and propagation phase. The initiation phase is characterized by a slow ascension with a speed less than 80 km s-1 for a period of tens of minutes. The initiation phase always occurs before the onset of the associated flare. Following the initiation phase, the CMEs display an impulsive acceleration phase that coincides very well with the flares' rise phase lasting for a few to tens of minutes. The acceleration of CMEs ceases near the peak time of the soft X-ray flares. The CMEs then undergo a propagation phase, which is characterized by a constant speed or slowly decreasing in speed. The acceleration rates in the impulsive acceleration phase are in the range of 100-500 m s-2. One CME (on 1997 November 6, associated with an X9.4 flare) does not show an initiation phase. It has an extremely large acceleration rate of 7300 m s-2. The possible causes of CME initiation and acceleration in connection with flares are explored. Title: R Aquarii: Evidence for Differential Rotation of the SiO Maser Shell Authors: Hollis, J. M.; Boboltz, D. A.; Pedelty, J. A.; White, S. M.; Forster, J. R. Bibcode: 2001ApJ...559L..37H Altcode: We previously reported Very Large Array and Berkeley-Illinois-Maryland Association (BIMA) array observations that suggested rotation of the SiO maser shell surrounding the long-period variable (LPV) in the R Aquarii binary system. In the present Very Long Baseline Array (VLBA) work, we report high spatial and spectral resolution observations of the v=1, J=1-0, SiO maser line that confirm our previous result and further suggest that the LPV maser shell is undergoing differential rotation. The 8-34 yr range of rotational periods resulting from differential rotation of the maser shell contains the ~18 yr period reported previously. The velocity structure of the VLBA data suggests a rotation symmetry axis oriented at a position angle of ~150°. The differential rotation model can be envisioned as a series of nested thin spherical shells that have a common rotation axis; each thin shell is characterized by its radius, r, with the innermost shell rotating fastest and the outermost shell slowest, in accordance with an equatorial plane velocity law of the form v~(1/rq)1/2. We find that q~1.09 is necessary to approximate the VLBA data, suggesting that the differential rotation is approximately Keplerian. Title: A Multiwavelength Study of Three Solar Flares Authors: Kundu, M. R.; Nindos, A.; White, S. M.; Grechnev, V. V. Bibcode: 2001ApJ...557..880K Altcode: In this paper we seek a self-consistent model for three strong limb flares observed at 17 and 34 GHz by the Nobeyama radioheliograph and also in soft X-rays and hard X-rays by the Yohkoh SXT (Soft X-Ray Telescope) and HXT (Hard X-Ray Telescope) instruments. Additional radio spectral data were provided by the Nobeyama polarimeter. The flare geometry is simple, with one well-defined flaring loop in each event. The 17 and 34 GHz emissions are optically thin gyrosynchrotron radiation from energetic electrons that outlines the flaring loops and peaks close to the loop tops. We infer that the variation of magnetic field along the loops is very small. We try to reproduce the observed radio morphologies and fluxes using a model gyrosynchrotron loop. The results of our modeling rely on the model magnetic field geometry that we choose. Although the exact loop geometry cannot be constrained from a two-dimensional snapshot, we choose for simplicity a line-dipole magnetic field, and the model field lines are circular. The SXT/HXT images are used to provide the physical parameters of the model loops. The high-frequency polarimeter data give the energy spectral index of the radio-emitting electrons. We could not reconcile the observed radio morphologies and fluxes using classic dipole magnetic field models. The best-fit model that uses the same input parameters for both frequencies and partly reconciles the observed 17 and 34 GHz morphologies and fluxes is produced when we invoke a magnetic field with constant strength along the model loop. These model loops have uniform thickness. The derived densities of the radio-emitting electrons are (1-6)×104 cm-3 with energy limits between 60 and 5000 keV. These models are the best fits we can get under the best assumptions we can justify, but they do not in fact match the radio morphologies very well; their problems and limitations are discussed. Title: Double Loop Configuration of a Flaring Region from Microwave, EUV and X-ray Imaging Data Authors: Garaimov, V. I.; Kundu, M. R.; Grechnev, V. V.; White, S. M. Bibcode: 2001AGUSM..SP51A04G Altcode: We present the results of a study of a flare of importance M1.7 that was observed with the Nobeyama Radio Heliograph (NoRH), SOHO/EIT and MDI and Yohkoh/HXT & SXT. The flare ocurred in a complex of two active regions AR~8741 and AR~8739, which were separated by ~ 3 arc min. The main flaring region was AR~8741, which consisted of two compact oppositely polarized components as judged by MDI magnetogram data. 17~GHz and 34~GHz microwave emissions as well as EUV, and hard and soft X-ray emissions from the flare originated primarily from the vicinity of the compact components of opposite polarity in AR~8741. There was some 17~GHz flaring emission from AR~8739. Clearly, we are dealing with a large magnetic loop connecting AR~8741 and AR~8739, as well as a compact loop in AR~8741 which was the main flare site. One of the magnetic components of the compact loop increased in field strength approximately 1 hour prior to the flare onset. MDI magnetograms showed some other changes in the magnetic fields associated with the compact loop. We suggest that the interaction of the compact loop with the large loop was responsible for causing the flare. The radio source structures at 17 and 34 GHz along with EUV, hard and soft X-ray source structures of the flaring region will be presented. Title: The Physical Properties of a Flaring Coronal Loop Authors: White, S. M.; Kundu, M. R.; Garaimov, V. I.; Yokoyama, T. Bibcode: 2001AGUSM..SP42A06W Altcode: We present high resolution radio and X-ray images of emission from nonthermal electrons trapped in a flaring coronal loop and measure various physical properties of the loop, including the loop thickness as a function of position along the loop and intensity profiles along the loop. Gyrosynchrotron modelling provides a complementary method for inferring parameters such as the magnetic field variation along the loop, and we compare the results of the two approaches. Both agree that there is little variation in magnetic field strength along the loop; this purely radio measurement thus supports the same inference derived from measurements of EUV and X-ray loop thicknesses. Title: A Radio Study of the Evolution of Spatial Structure of an Active Region and Flare Productivity Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Raulin, J. -P. Bibcode: 2001ApJS..133..467K Altcode: We present the results of a radio study of the evolution of an active region through its flare productivity. The radio study was carried out with data obtained by the Nobeyama Radio Heliograph at 17 GHz. We chose the active region AR 7515, which appeared at the east limb on 1993 May 23 and then evolved during its passage across the disk. We followed its evolution until June 2. This region produced many small flares. We consider this region to be a typical active region in the sense that it did not produce any large flares, but a large number of weak flares. We investigate the optical and magnetic development of the region and show how this affects the locations of the flaring activity. We discuss a number of events in detail in order to investigate the roles of nonthermal and thermal radio emission in the flares. The nonthermal gyrosynchrotron emission generally occurs in regions of strong magnetic fields, is generally circularly polarized, and often varies rapidly in time. On the other hand, gradual radio components tend to be thermal and only weakly polarized, if at all. An interesting aspect of evolution of the flares in this region is that many of the flares in the early phase of the evolution show strong but brief nonthermal radio emission in the impulsive phase followed by gradual thermal emission, whereas in the last 3 days more gradual events without a strong spike of radio emission in the impulsive phase tend to be seen. Correspondingly, the flare images suggest that the radio sources are more compact during the early phases and more extended in the last half of the period covered. The most dominant component of the preflare region is often not the component that undergoes immediate flaring. Sometimes a number of components in the preflare region participate in the flare process together. We speculate that these component sources are unresolved compact bipolar loops that flare in sequence. Loop-loop interactions occurring at many different sites at the same time seems to be a less plausible explanation of these events. Title: Spatial Structure of Simple Spiky Bursts at Microwave/Millimeter Wavelengths Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T.; Grechnev, V. V. Bibcode: 2001ApJ...547.1090K Altcode: We present the results of a study of spatial structure of sources of microwave and millimeter bursts with simple spiky time profiles at 17 and 34 GHz, similar to those found to be common at 3 mm wavelength. These bursts are of short duration, with fast 2-4 s rise time to peak, followed by a rapid exponential decay. When mapped at high spatial resolution with the Nobeyama Radio Heliograph (NoRH), the radio images show direct evidence that the radio sources are compact bipolar loops: source sizes are less than 5" and three of the five events studied show closely spaced oppositely polarized components in the circular polarization maps. All five events are located directly over magnetic neutral lines in the photosphere. The soft X-ray behavior is not entirely consistent with the Neupert effect in these events, since all five events show a rise in the soft X-ray flux well before any nonthermal electrons are present in the corona and the ratio of peak soft X-ray flux to peak radio 17 GHz flux may vary by many orders of magnitude from one event to the next. The abrupt time profiles of these events and their physical properties are consistent with a single-loop scenario in which magnetic energy release and acceleration of nonthermal electrons are confined to a compact localized region. Title: Spatial Distribution and Temporal Evolution of Coronal Bright Points Authors: Zhang, Jie; Kundu, Mukul R.; White, Stephen M. Bibcode: 2001SoPh..198..347Z Altcode: We present a statistical study of the spatial distribution and temporal evolution of coronal bright points (BPs) by analyzing a continuous set of observations of a quiet-Sun region of size 780'' × 780'' over a period of 55 hours. The main data set consists of observations taken by EIT (the Extreme-ultraviolet Imaging Telescope on board the SOHO spacecraft) in its Fe xii 195 Å channel which is sensitive to coronal plasma of temperature ∼ 1.5 MK; we also use soft X-ray observations by SXT (Soft X-ray Telescope on the Yohkoh spacecraft) which is sensitive to coronal plasma of temperature > 2.5 MK. The flux histogram for all pixels in EIT 195 Å images indicates that BPs have a power law flux distribution extending down to a level of 3σ (σ, root mean square deviation) above the average flux of the quiet Sun, while the bulk quiet Sun has a Gaussian-like flux distribution. Using a 3σ intensity threshold, we find a spatial density of one BP per 90 Mm × 90 Mm area, or equivalently 800 BPs for the entire solar surface at any moment. The average size of a BP is 110 Mm2. About 1.4% of the quiet-Sun area is covered by bright points and the radiation from all BPs is only about 5% of that from the whole quiet Sun. Thus, the atmosphere above quiet-Sun regions is not energetically dominated by BPs. During the 55-hour period of EIT observations, we identify 48 full-life-cycle BPs which can be tracked from their initial appearance to final disappearance. The average lifetime of these BPs is 20 hours, which is much longer than the previously reported 8 hours based on Skylab X-ray observations (Golub et al., 1974). We also see shorter life times and smaller numbers of BPs in the soft X-ray images than in the EIT 195 Å observations, suggesting that the temperature of BPs is generally below 2 MK. Title: The Frequency-Agile Solar Radiotelescope (FASR) (invited) Authors: Gary, D. E.; Bastian, T. S.; White, S. M.; Hurford, G. J. Bibcode: 2001aprs.conf..236G Altcode: No abstract at ADS Title: Observations of Solar Coronal Magnetic Fields Authors: White, S. M. Bibcode: 2001ASPC..248...67W Altcode: 2001mfah.conf...67W No abstract at ADS Title: The Absolute Abundance of Iron in the Solar Corona (CD-ROM Directory: contribs/white) Authors: White, S. M.; Thomas, R. J.; Brosius, J. W.; Kundu, M. R. Bibcode: 2001ASPC..223.1361W Altcode: 2001csss...11.1361W No abstract at ADS Title: Nonthermal Flare Emission from MEV-Energy Electrons at 17, 34, and 86 GHZ Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T. Bibcode: 2000ApJ...545.1084K Altcode: We present analyses of two solar flares observed with high spatial resolution at 86 GHz with the BIMA millimeter-wavelength telescope and at 17 and 34 GHz with the Nobeyama radioheliograph. The flares were observed on 1998 November 24 and 1999 May 1. At millimeter wavelengths these are impulsive events, and therefore they must be produced by MeV-energy electrons. The present study using simultaneous observations of two flares at 86, 34, and 17 GHz provides an excellent opportunity to study high-energy electrons with high spatial resolution observations at three optically thin frequencies. The morphology of millimeter emission can reveal both the properties of the MeV-energy electrons and the nature of the coronal magnetic field lines where they radiate. One of the two events we present is the first clear case of a λ=3 mm source in which both footpoints of a loop are detected. In the second event the polarization image at 17 GHz also suggests a bipolar or looplike morphology. Such morphological observations can be used to constrain the nature of the magnetic field in the solar corona. Title: R Aquarii: Constraints on the Rotational Period of the Long-Period Variable Authors: Hollis, J. M.; Pedelty, J. A.; Forster, J. R.; White, S. M.; Boboltz, D. A.; Alcolea, J. Bibcode: 2000ApJ...543L..81H Altcode: We report Very Large Array (VLA) observations taken in 1996 November and 1998 May of the v=1, J=1-0, SiO maser line and BIMA array observations taken in 1999 December and 2000 February of the v=1, J=2-1, SiO maser line associated with the long-period variable (LPV) in the R Aquarii binary system that suggest rotation of the maser shell. From these interferometric data cubes, we determine that the maser shell rotation axis is approximately northeast-southwest, thus aligning approximately with the direction of the R Aqr jet; the sense of the maser shell rotation is such that northwest is approaching and southeast is receding; the period of rotation is ~17 yr. Alternatively, co-adding 72 time series spectra of the v=1, J=1-0, SiO maser line obtained during the period 1984 July-1990 May with a single-dish antenna, we constructed a composite spectral emission envelope that shows the LSR velocity limits of maser emission over this epoch. From this composite spectral emission envelope and Very Long Baseline Array observations in 1996 February of the v=1, J=1-0, SiO maser line, which show the maximal spatial extent of the maser shell, we obtain a shell rotation period of ~18 yr, which is in excellent agreement with the VLA and BIMA array results and represents the maximum rotation period of the LPV if corotating with the maser shell. On the other hand, we obtain a minimum rotation period for the LPV of ~5 yr if the LPV supplies material to the maser shell under the constraint of conservation of angular momentum. The ~5-18 yr range for the rotational period of the LPV determined here and the ~18 yr rotational period for the hot companion determined by previous investigators suggest that tidal effects at successive periastron passages in the R Aqr binary system are tending to synchronize these stellar rotational periods to the orbital period of ~44 yr. Title: Radio Detection of a Rapid Disturbance Launched by a Solar Flare Authors: Janardhan, P.; White, S. M.; Kundu, M. R. Bibcode: 2000SPD....31.0243J Altcode: 2000BAAS...32Q1290J This paper reports 333 MHz observations of motion associated with a solar flare at a speed of 26000 km s-1. The motion is seen from a radio source which suddenly starts moving during the flare. At its peak the radio source covers a quiet region of dimension 500 arcsec. The disturbance itself does not seem to radiate, but it excites coronal features which continue to radiate after it passes. The inferred velocity is larger than any previously inferred velocity of a disturbance in the solar atmosphere apart from freely-streaming beams of accelerated electrons. The observed motion of the source at a fixed frequency, low polarization and moderate bandwidth are more consistent with the typical properties of moving Type IV radio bursts than with classical coronal--shock--associated Type II bursts. Title: Soft X-Ray and Gyroresonance Emission above Sunspots Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Shibasaki, K.; Gopalswamy, N. Bibcode: 2000ApJS..130..485N Altcode: Using Yohkoh SXT and Nobeyama 17 GHz data, we have studied the soft X-ray and microwave emission above several stable, large sunspots near central meridian passage. Our study confirms the well-known fact that soft X-ray emission is depressed above sunspots. It also shows that the distribution of their soft X-ray intensity is not uniform; usually the darkest pixels are associated with the umbra or the far edges of the leading part of the penumbra while the following part of the penumbra may contain higher intensity pixels associated with brighter loops. For the first time, we present a systematic survey of the temperatures and emission measures of the soft X-ray material above sunspots. Sunspots always contain the lowest temperatures and emission measures in the active regions. The mean umbral temperature is 1.8×106 K, and the mean penumbral temperature is 2.4×106 K. The mean umbral and penumbral emission measures are logEM=26.60 cm-5 and logEM=27.00 cm-5, respectively. The differences between the umbral and penumbral plasma temperatures are physically significant. The higher penumbral values imply that the loops associated with the penumbrae are generally hotter and denser than the loops associated with the umbrae. The highest sunspot temperatures and emission measures are still lower than the average active region parameters but higher than the quiet-Sun plasma parameters. The coronal radiative energy loss rate above the umbrae is 15% higher than the radiative loss rate of the quiet-Sun plasma but a factor of 8.3 lower than the typical active region radiative loss rate. The radio emission comes from the gyroresonance mechanism, and, as expected, it is sensitive to the magnetic field rather than the soft X-ray-emitting plasma. Title: The Radio Nebula around HR Carinae Authors: White, S. M. Bibcode: 2000ApJ...539..851W Altcode: We present a sensitive radio image of the nebula associated with the luminous blue variable star HR Carinae. This nebula is small and difficult to observe optically because of the presence of the bright star. The radio image shows the filaments in the outer regions of the nebula as seen in optical coronagraphic images. The core of the nebula is elongated north-south on the sky. A compact source associated with HR Car is clearly detected at the western edge of the nebula, but the nebula is very asymmetric with respect to the star, lying almost entirely to the east. The inner nebula shows no evidence for the bipolar structure inferred to exist from observations of the outer nebula: the symmetry axes in the inner nebula are 45° away from the bipolar axes. If the compact radio emission at the location of the star is a classical stellar wind source, we estimate a mass-loss rate for ionized gas of 1.8×10-5 Msolar yr-1. The mass in the central core of the nebula is about 0.3 Msolar, while the outer nebula may contain as much as 0.5 Msolar. We believe that a colliding winds explanation of the nebula is unlikely; a symbiotic-like explanation in which ionization of neutral ejecta is provided by a hot companion star may be possible if the companion star is on the far side of the nebula and heavily extincted, but it is not clear whether the properties required of this star are compatible with the fact that it is not detected in a 10 μm image. A B0 V companion without much dust may be consistent with the data. Title: The Absolute Abundance of Iron in the Solar Corona Authors: White, S. M.; Thomas, R. J.; Brosius, J. W.; Kundu, M. R. Bibcode: 2000ApJ...534L.203W Altcode: 2000astro.ph..4007W We present a measurement of the abundance of Fe relative to H in the solar corona using a technique that differs from previous spectroscopic and solar wind measurements. Our method combines EUV line data from the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory with thermal bremsstrahlung radio data from the VLA. The coronal Fe abundance is derived by equating the thermal bremsstrahlung radio emission calculated from the EUV Fe line data to that observed with the VLA, treating the Fe/H abundance as the sole unknown. We apply this technique to a compact cool active region and find Fe/H=1.56×10-4, or about 4 times its value in the solar photosphere. Uncertainties in the CDS radiometric calibration, the VLA intensity measurements, the atomic parameters, and the assumptions made in the spectral analysis yield net uncertainties of ~20%. This result implies that low first ionization potential elements such as Fe are enhanced in the solar corona relative to photospheric values. Title: Microwave/Millimeter Wavelength Bursts with Simple Spiky Time Profiles Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T. Bibcode: 2000SPD....31.0242K Altcode: 2000BAAS...32..818K We report the detection at 17 and 34 GHz of microwave and millimeter bursts which have simple spiky time profiles similar to those found to be common at λ = 3 mm. These bursts are of short duration, with fast 2 - 4 sec rise time to peak, followed by an exponential decay. These bursts can be of any intensity, from 1 sfu to 10's of sfu; they are very strongly polarized (> 50%), and they have similar properties regardless of the nature of the active region in which the bursts originate. The bursts seem to originate in compact sources which are generally unresolved with 15" and 7" resolution of the Nobeyama Radio Heliograph at 17 and 34 GHz respectively. We provide both direct and indirect evidence that these compact sources are low-lying bipolar loops. The direct evidence follows from the physical appearance of the loop as well as from the bipolar nature of the loop. The indirect evidence follows from the offset in position of the footpoint emission in microwaves and hard X-rays, implying a compact asymmetric loop with microwaves originating from the stronger magnetic field foot point and the hard X-rays originating from the weaker field foot point. Title: The Absolute Abundance of Iron in the Solar Corona Authors: White, S. M.; Thomas, R. J.; Brosius, J. W.; Kundu, M. R. Bibcode: 2000SPD....31.1301W Altcode: 2000BAAS...32Q.845W We present a measurement of the abundance of Fe relative to H in the solar corona using a technique which differs from previous spectroscopic and solar wind measurements. Our method combines EUV line data from the CDS spectrometer on SOHO with thermal bremsstrahlung radio data from the VLA. The coronal Fe abundance is derived by equating the thermal bremsstrahlung radio emission calculated from the EUV Fe line data to that observed with the VLA, treating the Fe/H abundance as the sole unknown. We apply this technique to a compact cool active region and find Fe/H = 1.56 x 10-4, or about 4 times its value in the solar photosphere. Uncertainties in the CDS radiometric calibration, the VLA intensity measurements, the atomic parameters, and the assumptions made in the spectral analysis yield net uncertainties of order 20%. This result implies that low first ionization potential elements such as Fe are enhanced in the solar corona relative to photospheric values. Title: Observations and models of a flaring loop. Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E. Bibcode: 2000BAAS...32..818N Altcode: No abstract at ADS Title: Observations and Models of a Flaring Loop Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E. Bibcode: 2000SPD....31.0243N Altcode: Simultaneous images of a flaring loop at two frequencies are used to model the magnetic structure of the loop and the energy distribution of the radiating electrons. The imaging data were obtained with the VLA at 5 and 15 GHz. Additional spectral data were provided by the OVRO Solar Array at several frequencies between 2 GHz and 15 GHz. At 15 GHz, the flare emission was optically thin and came from the footpoints of the flaring loop, while at 5 GHz the loop itself was outlined. Most of the 5 GHz emission was optically thick and its spatial maximum was close to the loop top. A striking feature of the observations is that the 5 GHz emission does not reach down to the 15 GHz footpoints. We compare the observations with calculations of gyrosynchrotron emission from an inhomogeneous magnetic loop in order to determine the conditions in the flaring loop. The best fit to the OVRO fluxes was reached with a model flaring loop with photospheric footpoint magnetic field strength of 870 G. The thickness of the model loop was small compared to its footpoint separation. The energy spectral index of the energetic electrons was 3.7 and their number density was 7.9 x 107 cm-3. The low and high energy cutoffs of the nonthermal electrons were 8 and 210 keV. The 5 GHz emission in this model is at low harmonics (3--7) and harmonic effects are responsible for the weak 5 GHz emission at the footpoints. The absence of electrons above 210 keV is necessary in this model to explain why no emission is observed from the loop top at 15 GHz. That model reproduced well the high frequency part of the OVRO flux spectrum as well as the VLA spatial structure. Thus comparisons between the spatially--resolved observations and models reveal the three-dimensional structure of the loop geometry. Title: LASCO and EIT observations of CMEs associated with flares Authors: Zhang, J.; Dere, K. P.; Howard, R. A.; Kundu, M. R.; White, S. M. Bibcode: 2000SPD....31.0906Z Altcode: 2000BAAS...32..841Z Coronal mass ejections (CME) and flares are two primary causes of adverse space weather. These two solar eruptive phenomenon are often observed to be associated with each other. Yet the relationship between them is not well known. With unprecedented LASCO (Large-Angle and Spectrometric Coronagraph) and EIT (Extreme-ultraviolet Imaging Telescope) observations combined with simultaneous HXT (Hard X-ray Telescope), GOES and other flare observations, we pursue to make a comprehensive study on the spatial, temporal and energetic relationship between CMEs and flares. In order to make accurate measurement of the onset time of CMEs, we primarily select CME events whose source regions are close to the limb and which are well observed by LASCO/C1 from 1.1 to 3 solar radii. Although a flare occurs in a rather small area of active region, the CME's source region often covers much larger longitudinal and latitudinal extension. Some CMEs occur simultaneously with flare (within only a few minutes), however, some CMEs occur well before the flares (more than 30 minutes earlier). Fast CMEs are associated with strong flares. These studies are aimed to understand the initiation process of solar eruptive phenomenon, and to fit a variety of observational aspects into a consistent picture. Title: Observations and Models of a Flaring Loop Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E. Bibcode: 2000ApJ...533.1053N Altcode: Simultaneous images of a flaring loop at two frequencies are used to model the magnetic structure of the loop and the energy distribution of the radiating electrons. The imaging data were obtained with the VLA at 5 and 15 GHz. Additional spectral data were provided by the Owens Valley Radio Observatory (OVRO) solar array at several frequencies between 2 and 15 GHz. At 15 GHz, the flare emission was optically thin and came from the footpoints of the flaring loop, while at 5 GHz the loop itself was outlined. Most of the 5 GHz emission was optically thick, and its spatial maximum was close to the loop top. A striking feature of the observations is that the 5 GHz emission does not reach down to the 15 GHz footpoints. We compare the observations with calculations of gyrosynchrotron emission from an inhomogeneous magnetic loop in order to determine the conditions in the flaring loop. The best fit to the OVRO fluxes was reached with a model flaring loop with photospheric footpoint magnetic field strength of 870 G. The thickness of the model loop was small compared with its footpoint separation. The energy spectral index of the energetic electrons was 3.7, and their number density was 7.9×107 cm-3. The low- and high-energy cutoffs of the nonthermal electrons were 8 and 210 keV, respectively. The 5 GHz emission in this model is at low harmonics (3-7), and harmonic effects are responsible for the weak 5 GHz emission at the footpoints. The absence of electrons above 210 keV is necessary in this model to explain why no emission is observed from the loop top at 15 GHz. That model reproduced well the high-frequency part of the OVRO flux spectrum as well as the VLA spatial structure. Thus, comparisons between the spatially resolved observations and models reveal the three-dimensional structure of the loop geometry. Title: Radio Detection of a Rapid Disturbance Launched by a Solar Flare Authors: White, S. M.; Janardhan, P.; Kundu, M. R. Bibcode: 2000ApJ...533L.167W Altcode: We report the direct observation of motion associated with a solar flare at a speed of 26,000 km s-1. The motion is seen from a radio source at 0.33 GHz, which suddenly starts moving during the flare. At its peak, the radio source covers a quiet region of dimension 500". Emission from any given location is sporadic. The disturbance itself does not seem to radiate, but it excites coronal features that continue to radiate after it passes. The inferred velocity is larger than any previously inferred velocity of a disturbance in the solar atmosphere apart from freely streaming beams of accelerated electrons. The observed motion of the source at a fixed frequency, low polarization, and moderate bandwidth are more consistent with the typical properties of moving type IV radio bursts than with classical coronal shock-associated type II bursts, but any disturbance at such a high velocity must be highly supersonic and should drive a shock. We speculate that the disturbance is associated with the realignment of magnetic fields connecting different portions of an active region. Title: Nonthermal Flare Emission from MeV-Energy Electrons at 17, 34, and 86, GHz Authors: Kundu, M. R.; White, S. M.; Shibasaki, K.; Sakurai, T. Bibcode: 2000ASPC..206..307K Altcode: 2000hesp.conf..307K No abstract at ADS Title: Millimeter-Interferometer Observations of Flares in Conjunction with HESSI Authors: White, S. M.; Kundu, M. R. Bibcode: 2000ASPC..206..335W Altcode: 2000hesp.conf..335W No abstract at ADS Title: Observations and Models of a Flaring Loop Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E. Bibcode: 2000ASPC..206..359N Altcode: 2000hesp.conf..359N No abstract at ADS Title: The Contributions of the VLA to the Study of Radio Stars Authors: White, Stephen M. Bibcode: 2000riss.conf...86W Altcode: No abstract at ADS Title: The Ultracompact HII-region G45.07+0.13: An Ionized Bipolar Outflow? Authors: Lim, J.; White, S. M. Bibcode: 1999sf99.proc..306L Altcode: We present continuum and hydrogen recombination line observations of the ultracompact HII-region G45.07+0.13 at cm-wavelengths with the VLA, as well as molecular line observations of the surrounding neutral gas at 3 mm with the BIMA-array. We show that the bright inner region of G45.07+0.13 has a bipolar structure, which is well aligned with and has a velocity gradient in the same sense as the larger-scale bipolar molecular outflow seen in 13CO. We discuss the possibility that G45.07+0.13 is an ionized bipolar outflow. The bipolar structure may reflect the preferential expansion of an ionized stellar wind or the stellar UV ionization front along the less dense polar direction of the surrounding molecular gas core, partially or nearly entirely evacuated in this direction by the passage of a bipolar outflow. Alternatively, the stellar UV may be lighting up a genuine, ongoing bipolar outflow. Title: Two-Temperature Coronal Models from SOHO/EIT Observations Authors: Zhang, J.; White, S. M.; Kundu, M. R. Bibcode: 1999ApJ...527..977Z Altcode: We present a method for deriving a two-thermal-component approximation to the differential emission measure distribution of plasma in the Sun's corona in the temperature range to which the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) spacecraft is sensitive. EIT takes high-resolution full-disk coronal images in three of its four optimized channels by observing emission lines of highly ionized Fe whose formation temperatures overlap and cover the range from 0.7 to 2.8 MK. It is straightforward to show that the traditional single-temperature models based on the ratio of a pair of EIT images at different wavelengths are not able adequately to represent the plasma contributing to all three wavelength ranges. In this paper, we develop a modified image-ratio method that results in a two-thermal-component model for the plasma producing the coronal emission observed by EIT. The products of this method are two temperature and two emission measure full-disk maps of the Sun's corona, with the full resolution of the EIT telescope, in two temperature regimes: one from 0.8 to 1.6 MK and the other from 1.6 to 2.6 MK. The two-component solutions are tested using a series of model differential emission measures (DEMs) from the CHIANTI atomic database package. This method appears to produce realistic results in all regions of the Sun's atmosphere with the exception of coronal holes, where very cool Si VII/Mg VII lines (<0.7 MK) contribute more to the EIT 284 Å image than the otherwise dominant hot Fe XV lines and result in unrealistically high temperatures for the hot component there. We demonstrate that while the raw EIT images are dominated by the spatial distribution of emission measure in the corona, the temperature maps often emphasize fine structure, which is less visible in the flux images. The emission measure of the hot component is always larger than that of the cool component. On the disk there appears to be a firm lower limit to the integrated column emission measure along any line of sight, including toward coronal holes. There is no overall correlation between temperature and emission measure. Title: Microwave and Extreme Ultraviolet Observations of Solar Polar Regions Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Gary, D. E.; Shibasaki, K.; Dere, K. P. Bibcode: 1999ApJ...527..415N Altcode: The poles of the Sun are brighter than the rest of the quiet Sun's emission in a limited range of radio frequencies from 17 GHz to 87 GHz. We have studied microwave images of the quiet Sun made with the Nobeyama radioheliograph at 17 GHz. They show that the so-called polar-cap brightening consists of two components: a diffuse component of 1500 K excess brightness and patchy compact sources with localized excess brightness of about 3500 K. We test the reality of the compact sources using the maximum entropy method deconvolution. The total flux and the number of compact polar sources as well as the north-south extent of the diffuse polar emission are larger in the pole that is closest to the Earth. We compared the microwave polar emission with nearly simultaneous SOHO EIT images taken in the lines of He II at 304 Å and Fe XII at 195 Å. No one-to-one correlation between the compact radio sources and the bright EUV features was found: most of the radio emission arises between the plumes visible to EIT. The boundaries of the polar-cap brightenings did not match exactly the boundaries of the coronal holes as seen in either the Fe XII 195 Å images or the He II 304 Å images. The temporal variations of the compact microwave sources did not correspond to any significant changes in EUV emission. On the other hand, most He II 304 Å changing features were associated with the diffuse polar microwave emission, which was practically constant. Our data suggest that the origin of the polar brightening is not coronal; it seems that the bulk of the patchy radio emission comes from heights below the 80,000 K layer. Title: A Microwave Study of Coronal and Chromospheric Ejecta Authors: Nindos, A.; Kundu, M. R.; Raulin, J. -P.; Shibasaki, K.; White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M. Bibcode: 1999spro.proc..135N Altcode: We have studied the radio properties of 18 X-ray coronal jets (observed by the Yohkoh SXT) using Nobeyama 17 GHz data. We also searched for chromospheric ejecta (Hα surges) during the time intervals that the X-ray images were available. Microwave emission was associated with the majority of the X-ray jets. The radio emission came from the base or the lower part of the jets. We detected radio emission from almost all jets which showed flare-like activity at their footpoints. The 17 GHz time profiles were gradual and unpolarized, implying that the emission was thermal. When possible, we computed the physical properties of the X-ray-emitting ejected plasma. In one two-sided-loop type jet and one anemone-type jet, the observed microwave fluxes from the lower part of the jets were well above the fluxes predicted from the computed electron temperatures and emission measures of the soft X-ray-emitting material on the basis of thermal free-free emission. We interpreted the large discrepancies in terms of the presence of lower temperature material which cannot be detected by the SXT but produces strong microwave free-free emission. This is the first time that such material is observed in two-sided-loop type jets. Thus our observations confirm the theoretical prediction by Yokoyama and Shibata (1996). We detected no cool material at the base of the jets. We also observed an Hα surge which was not associated with an X-ray jet and showed no signatures on the SXT images but was detected with the Nobeyama Radioheliograph. The emission of the microwave surge-associated source was free-free from the chromospheric plasma. Constraints for the surge density were derived. Title: Millimeter Interferometer Observations of Flares Authors: White, S. M. Bibcode: 1999spro.proc..223W Altcode: This paper addresses the current state of millimeter interferometry of solar flares and the issues which can be addressed by such observations. Millimeter interferometers such as BIMA detect gyrosynchrotron emission from MeV--energy electrons in the impulsive phase of solar flares as well as from hot dense plasma in the thermal decay phase. BIMA now consists of 10 antennas and is capable of snapshot imaging of solar flares with excellent spatial resolution and dynamic range of up to 100. The properties of MeV--energy electrons deduced from such observations vary widely: in some flares they are present for a much shorter time than the lower--energy hard--X--ray producing electrons, while in other flares they are present longer. Examples of both circumstances are given. It is widely observed that the energy distribution determined from the optically--thin radio spectrum is inconsistent with that determined from hard X--ray data: generally this is interpreted to mean that the electron energy distribution is flatter at higher energies, but even this interpretation can run into difficulties. This discrepancy between radio and hard X--ray spectral indices may be revealing something fundamental about electron acceleration in solar flares. Title: Active Region Emissions and Coronal Field Extrapolations Authors: Lee, J.; White, S. M.; Kundu, M. R.; Mikic, Z. Bibcode: 1999spro.proc...65L Altcode: With vector magnetographs set to fly on the Solar--B mission, the extrapolation of photospheric magnetic fields into the corona will be increasingly important. As the techniques of coronal field extrapolations grow more sophisticated, we require a more powerful means to test them and to make full use of the information they contain. Radio data can play an important role in testing extrapolation methods. In this paper, we discuss a new test of coronal field extrapolation using the concept of field line connectivity. The motivating idea is that temperature should be nearly uniform on a given magnetic field line due to the rapid transport of physical quantities along field lines in the corona. Optically--thick gyroresonance emission provides the temperature on a surface of known magnetic field strength in the corona. As a consequence, we may expect that radio intensities observed at different frequencies at points connected by field lines should show a good correlation. This suggests that a test of a magnetic field extrapolation model is whether the field--line connectivity it predicts shows such a correlation. A second application of field--line connectivity is to try to understand the relationship between physical quantities in the photosphere at the footpoints of magnetic field lines and the heating process in the corona on the same field lines. If a particular magnetic quantity, such as shear, plays a role in coronal heating then one expects the coronal extension of field lines passing through peaks in this quantity will show the highest coronal temperatures. This idea can be used to test candidate coronal heating mechanisms. We demonstrate these ideas using the combination of high--resolution VLA observations of a complex active region together with state--of--the--art nonlinear force--free field modeling. Title: Radio Versus EUV/X-Ray Observations of the Solar Atmosphere Authors: White, S. M. Bibcode: 1999SoPh..190..309W Altcode: This paper reviews the contrasting properties of radio and EUV/X-ray observations for the study of the solar atmosphere. The emphasis is placed on explaining the nature of radio observations to an EUV/X-ray audience. Radio emission is produced by mechanisms which are well-understood within classical physics. Bremsstrahlung tends to be dominant at low frequencies, while gyro-resonance emission from strong magnetic fields produces bright sources at higher frequencies. At most radio frequencies the images of the Sun are dominated almost everywhere by bremsstrahlung opacity, which may be optically thick or thin depending on circumstances. Where gyro-resonance sources are present they may be used as sensitive probes of the regions above active regions where magnetic field strengths exceed several hundred gauss, and this unique capability is one of the strengths of radio observations. Typically a gyro-resonance radio source shows the temperature on an optically thick surface of constant magnetic field within the corona. Since each radio frequency corresponds to a different magnetic field strength, the coronal structure can be `peeled away' by using different frequencies. The peculiarities of radio observing techniques are discussed and contrasted with EUV/X-ray techniques. Radio observations are strong at determining temperatures and coronal magnetic field strengths while EUV/X-ray observations better sense densities and reveal coronal magnetic field lines: in this way the two wavelength domains are nicely complementary. Title: Multiple Components in the Millimeter Emission of a Solar Flare Authors: Raulin, J. -P.; White, S. M.; Kundu, M. R.; Silva, A. V. R.; Shibasaki, K. Bibcode: 1999ApJ...522..547R Altcode: We analyze a small flare using imaging data at millimeter, microwave, and soft X-ray wavelengths and microwave and hard X-ray spectral observations. The remarkable aspect of this flare is evidence for the presence of MeV-energy electrons, which are responsible for the nonthermal millimeter emission, at a time when no hard X-rays from lower energy electrons are detected. This occurs during a smoothly varying phase, which is seen at radio wavelengths to last several minutes and is the brightest phase at millimeter wavelengths but is undetected in hard X-rays: it follows a brief spike of emission at flare onset, which has the more usual properties of impulsive events and features nonthermal microwave, millimeter, and hard X-ray emission. We interpretthe phase that is brightest at millimeter wavelengths as being due to efficient trapping of a relatively small number of nonthermal electrons, whereas during the hard X-ray emission, trapping is much less efficient, and the decay time is much shorter at all energies, which leads to a larger ratio of hard X-ray flux to radio flux. As in many previous events studied at millimeter wavelengths, there is a discrepancy between the electron energy spectral indices inferred from the milllimeter and hard X-ray data during the impulsive phase when both are detected: again it appears that the energy spectrum at 1 MeV must be significantly flatter than at several hundred keV and below. However, there are problems in reconciling quantitatively the energy spectra for the hard X-ray-emitting and radio-emitting components: based on the most plausible parameters, the radio-emitting electrons should produce most of the hard X-rays. One solution to this contradiction is to invoke a coronal magnetic field stronger than seems likely based on the photospheric magnetic field. Title: A Microwave Study of Coronal Ejecta Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.; White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M. Bibcode: 1999ApJ...520..391K Altcode: Using Nobeyama 17 GHz data, we have studied the radio properties of 19 coronal jets identified in Yohkoh soft X-ray imaging telescope (SXT) X-ray observations. The radio data provide information on the physical conditions in the jets, which complements the data from the X-ray surveys. Microwave emission was associated with the majority of the X-ray jets in our sample. The radio emission typically came from the base or the base and lower part of the jets. We detected radio emission from almost all jets that showed flarelike activity at their bases. The jets that were not associated with radio emission did not show any significant increase in X-ray emission at their bases. The strongest radio emission came from two of the largest jets in our sample. Our data show a general correlation between the X-ray jet fluxes and the associated radio fluxes. The 17 GHz time profiles were gradual and unpolarized, implying that the emission was thermal. In a two-sided-loop jet (1992 July 22 event) and one anemone-type jet (1993 February 9 event), the observed microwave fluxes from the lower part of the jets were well above the fluxes calculated from the computed physical parameters of the soft X-ray-emitting material on the basis of thermal free-free emission. We interpret the large discrepancies in terms of the presence of lower temperature material, which cannot be detected by the SXT (the SXT is most sensitive to hot plasma above 2×106 K), but which produces strong microwave free-free emission. This is the first time that such material has been observed in two-sided-loop-type jets. We also observed motion of a jet-associated microwave source with a velocity of 55 km s-1. The microwave motion occurred after the appearance of the X-ray jet. There is clear evidence that the microwave emission of that source was associated with the jet and not with the associated small flare. Title: Microwave and Extreme Ultraviolet Observations of Solar Polar Regions Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Gary, D. E.; Shibasaki, K.; Dere, K. P. Bibcode: 1999AAS...194.3207N Altcode: 1999BAAS...31..871N The radio emission of solar poles is brighter than the rest of the quiet Sun's emission in a limited range of frequencies from 17 GHz to 87 GHz. We have studied microwave images of the quiet Sun made with the Nobeyama Radioheliograph at 17 GHz. They show that the so-called polar-cap brightening consists of two components: a diffuse component of 1500 K excess brightness, and patchy compact sources with localized excess brightness of about 3500 K. The total flux and the number of compact polar sources as well as the North-South extent of the diffuse polar emission are larger in the pole which is closest to the Earth. We compared the microwave polar emission with nearly simultaneous SoHO EIT images taken in the lines of He ii at 304 Angstroms and Fe xii at 195 Angstroms. No one-to-one correlation between the compact radio sources and the bright EUV features was found: most of the radio emission arises between the plumes visible to EIT. The boundaries of the polar-cap brightenings did not match exactly the boundaries of the coronal holes as seen in the Fe xii 195 Angstroms images. The temporal variations of the compact microwave sources did not correspond to any significant changes in EUV emission. On the other hand, most He ii 304 Angstroms changing features were associated with the diffuse polar microwave emission which was practically constant. Our data suggest that the origin of the polar brightening is not coronal; it seems that the bulk of the patchy radio emission comes from heights below the 80000 K layer. Title: FASR - A Frequency-Agile Solar Radiotelescope Authors: Hurford, G. J.; Gary, D. E.; Bastian, T. S.; White, S. M. Bibcode: 1999AAS...194.7603H Altcode: 1999BAAS...31..956H The Frequency-Agile Solar Radiotelescope (FASR) is a multi-frequency imaging array designed specifically for imaging spectroscopy of the Sun. Using < 100 antennas, it will combine high-quality/high spatial resolution imaging (1" resolution at 20 GHz) with spectroscopy (dnu /nu 0.01-0.03) and high time resolution (<1 s) across two decades in frequency from 0.3-30 GHz. In so doing, it will produce a continuous, dynamic, three-dimensional picture of the solar atmosphere from the chromosphere through the mid-corona. These capabilities represent a quantum leap beyond existing solar radio instruments, yet are well within reach of emerging technologies. The range of science that can be addressed by such an instrument is as broad as solar physics itself. Virtually every solar feature from within a few hundred km of the visible surface of the Sun to high in the solar corona can be studied in detail with the unique diagnostics available in the radio regime. Particular diagnostics include measuring the properties of both thermal and nonthermal electrons accelerated in solar flares from the largest events to the tiniest microflares/nanoflares, measuring coronal magnetic field strengths in active regions and elsewhere (coronal magnetography), and mapping kinetic electron temperatures throughout the chromosphere and corona. In addition, FASR's far-reaching exploration of the Sun in the radio regime gives the instrument tremendous potential for new discoveries beyond those that we can now anticipate. FASR is expected to be one of the major new ground-based solar instruments of the next decade, and can be operational by 2006, well before the decade is out. It will play a major role in supporting NASA space missions with the unique diagnostics and perspective provided by high-resolution radio imaging/spectroscopy. Title: A Microwave Study of Coronal Ejecta Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.; White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M. Bibcode: 1999AAS...194.1704K Altcode: 1999BAAS...31..853K Using Nobeyama 17 GHz data, we have studied the radio properties of 19 coronal jets identified in Yohkoh SXT X-ray observations. The radio data provide information on the physical conditions in the jets which complements the data from the X-ray surveys. Microwave emission was associated with the majority of the X-ray jets in our sample. The radio emission typically came from the base or the base and lower part of the jets. We detected radio emission from almost all jets which showed flare-like activity at their bases. The jets which were not associated with radio emission did not show any significant increase in X-ray emission at their bases. The strongest radio emission came from two of the largest jets in our sample. Our data show a general correlation between the X-ray jet fluxes and the associated radio fluxes. The 17 GHz time profiles were gradual and unpolarized, implying that the emission was thermal. In a two-sided-loop jet (July 22, 1992 event) and one anemone-type jet (February 9, 1993 event), the observed microwave fluxes from the lower part of the jets were well above the fluxes calculated from the computed physical parameters of the soft X-ray-emitting material on the basis of thermal free-free emission. We interpret the large discrepancies in terms of the presence of lower temperature material which cannot be detected by the SXT (the SXT is most sensitive to hot plasma above 2 x 10(6) K) but which produces strong microwave free-free emission. This is the first time that such material has been observed in two-sided-loop type jets. We also observed motion of a jet-associated microwave source with a velocity of 55 km/sec. The microwave motion occurred after the appearance of the X-ray jet. There is clear evidence that the microwave emission of that source was associated with the jet and not with the associated small flare. Title: Millimeter Imaging Observations of High Energy Electrons in Solar Flares Authors: White, S. M.; Kundu, M. R. Bibcode: 1999AAS...194.8009W Altcode: 1999BAAS...31..966W The 10-element BIMA array will be used in to image solar flares at millimeter wavelengths during campaign periods in the upcoming solar maximum. Since millimeter emission in the impulsive phase of flares comes from electrons with energies typically in excess of 1 MeV, these observations complement observations of lower energy electrons at other wavelengths. Recent BIMA flare data will be presented and plans for the near future will be discussed. Title: A Study of Microwave-selected Coronal Transient Brightenings Authors: Nindos, A.; Kundu, M. R.; White, S. M. Bibcode: 1999ApJ...513..983N Altcode: We present the results of a search for radio-selected transient brightenings (TBs) in the solar atmosphere as a complement to the more common X-ray-selected surveys. The Sun was generally quiet during the observations, making these data sensitive to weak TBs both in and outside active regions. Five small impulsive events were identified in a set of VLA observations at 4.5, 1.5, and 0.33 GHz and compared with soft X-ray images from Yohkoh and EUV images from SOHO/EIT. Four of the events were located at the edges of an active region, but one was located 100" away in a quiet region of the atmosphere. Possible emission mechanisms for these brightenings are investigated. The time profiles of the radio TBs show impulsive peaks, while the corresponding soft X-ray profiles are gradual. The impulsive radio peaks were up to 35% polarized. Our data favor an interpretation in terms of gyrosynchrotron radiation from mildly relativistic electrons. A small number of nonthermal electrons with spectral index 3 can explain the observed properties of the TBs. Thus, nonthermal TBs can be found away from active regions. Two of the microwave TBs also show evidence for type III radio emission at 327 MHz. Title: Continuum and Spectral Observations of η Carinae at Wavelengths of 3 & 6 Centimeters Authors: Duncan, R. A.; White, S. M.; Reynolds, J. E.; Lim, J. Bibcode: 1999ASPC..179...54D Altcode: 1999ecm..conf...54D No abstract at ADS Title: A Test for Coronal Magnetic Field Extrapolations Authors: Lee, Jeongwoo; White, Stephen M.; Kundu, Mukul R.; Mikić , Zoran; McClymont, A. N. Bibcode: 1999ApJ...510..413L Altcode: As models for the physical properties of the corona above solar active regions grow more sophisticated, we will require better means for testing them. In this paper we discuss and apply such a test to a magnetic field model for an active region. This test is based on the expectation that the temperatures at different points on a given magnetic field line should be well correlated because of the rapid transport of heat along field lines in the corona. We use radio observations of an active region to measure the temperatures on field lines as they cross two isogauss surfaces (at 430 and 750 G) in the corona. The field lines and isogauss surfaces are derived from a coronal magnetic field model obtained via a nonlinear force-free field extrapolation of a photospheric vector magnetogram; for comparison, we also investigate a potential-field extrapolation of the same magnetogram. In a region in which strongly sheared fields are present, the nonlinear force-free field model does indeed show a good correlation between the temperatures in the two surfaces at points on the same field line, while the potential-field model does not. This diagnostic acts both as a test of the magnetic field model as well as of the interpretation of the radio data, and we show how this test can also aid in understanding the radio data. Title: Images of Gradual Millimeter Emission and Multi-Wavelength Observations of the 17 august 1994 Solar Flare Authors: Silva, Adriana V. R.; Lin, R. P.; de Pater, Imke; White, Stephen M.; Shibasaki, K.; Nakajima, H. Bibcode: 1998SoPh..183..389S Altcode: We present a comprehensive analysis of the 17 August 1994 flare, the first flare imaged at millimeter (86 GHz) wavelengths. The temporal evolution of this flare displays a prominent impulsive peak shortly after 01:02 UT, observed in hard X-rays and at microwave frequencies, followed by a gradual decay phase. The gradual phase was also detected at 86 GHz. Soft X-ray images show a compact emitting region (≲20''), which is resolved into two sources: a footpoint and a loop top source. Nonthermal emissions at microwave and hard X-ray wavelengths are analyzed and the accelerated electron spectrum is calculated. This energy spectrum derived from the microwave and hard X-ray observations suggests that these emissions were created by the same electron population. The millimeter emission during the gradual phase is thermal bremsstrahlung originating mostly from the top of the flaring loop. The soft X-rays and the millimeter flux density from the footpoint source are only consistent with the presence of a multi-temperature plasma at the footpoint. Title: The Composition of Centaur 5145 Pholus Authors: Cruikshank, D. P.; Roush, T. L.; Bartholomew, M. J.; Geballe, T. R.; Pendleton, Y. J.; White, S. M.; Bell, J. F.; Davies, J. K.; Owen, T. C.; de Bergh, C.; Tholen, D. J.; Bernstein, M. P.; Brown, R. H.; Tryka, K. A.; Dalle Ore, C. M. Bibcode: 1998Icar..135..389C Altcode: We present a new spectrum of the Centaur object 5145 Pholus between 1.15 and 2.4 μm. We model this, and the previously published (0.4- to 1.0-μm) spectrum, using Hapke scattering theory. Seen in absorption are the 2.04-μm band of H2O ice and a strong band at 2.27 μm, interpreted as frozen methanol and/or a photolytic product of methanol having small molecular weight. The presence of small molecules is indicative of a chemically primitive surface, since heating and other processes remove the light hydrocarbons in favor of macromolecular carbon of the kind found in carbonaceous meteorites. The unusually red slope of Pholus' spectrum is matched by fine grains of a refractory organic solid (tholin), as found previously by M. Hoffmannet al. (1993,J. Geophys. Res.98, 7403-7407) and P. D. Wilsonet al.(1994,Icarus107, 288-303). Olivine (which we model with Fo 82) also appears to be present on Pholus. We present a five-component model for the composite spectrum of all spectroscopic and photometric data available for 5145 Pholus and conclude that this is a primitive object which has not yet been substantially processed by solar heat. The properties of Pholus are those of the nucleus of a large comet that has never been active. Title: The Height Structure of the Solar Atmosphere from the Extreme-Ultraviolet Perspective Authors: Zhang, Jie; White, S. M.; Kundu, M. R. Bibcode: 1998ApJ...504L.127Z Altcode: 1998astro.ph..7175Z We investigate the structure of the solar chromosphere and transition region using full Sun images obtained with the Extreme-Ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory spacecraft. The limb seen in the EIT coronal images (taken in lines of Fe IX/X at 171 Å, Fe XII at 195 Å, and Fe XV at 284 Å) is an absorption limb predicted by models to occur at the top of the chromosphere where the density of neutral hydrogen becomes significant (~1010 cm-3). The transition-region limb seen in He II λ304 images is an emission limb. We find that (1) the limb is higher at the poles than at the equator both in the coronal images (by 1300+/-650 km) and the 304 Å images (by 3500+/-1200 km), and (2) the 304 Å limb is significantly higher than the limb in the coronal images. The height difference is 3100+/-1200 km at the equator and 6600+/-1200 km at the poles. We suggest that the elevation of the 304 Å limb above the limb in the coronal images may be due to the upper surface of the chromosphere being bumpy, possibly because of the presence of spicules. The polar extension is consistent with a reduced heat input to the chromosphere in the polar coronal holes compared with the quiet-Sun atmosphere at the equator. Title: Centaur 5145 Pholus As a Comet Nucleus Authors: Cruikshank, D. P.; Roush, T. L.; Bartholomew, M. J.; Pendleton, Y. J.; White, S. M.; Bernstein, M. P.; Dalle Ore, C. M.; Khare, B. N.; Geballe, T. R.; Davies, J. K.; Owen, T. C.; Tholen, D. J.; de Bergh, C.; Bell, J. F., III; Brown, R. H.; Tryka, K. A. Bibcode: 1998DPS....30.4201C Altcode: 1998BAAS...30.1094C New spectra and models of the surface of Centaur 5145 Pholus suggest that this object is the equivalent of a giant comet nucleus that is not, and may have never been, active. We have used published spectra and our own data, plus the radiometrically estimated diameter of 190 km, to compile the reflectance spectrum from 0.4 to 2.4 mu m. The photovisual spectrum slopes steeply upward toward the red, a characteristic matched by some refractory organic solids. The spectrum shows two absorption bands of H2O ice (1.5 and 2.0 mu m), and a third clear absorption near 2.27 mu m, at the position of two absorption bands in solid CH3OH and other solid hydrocarbons (e.g., (CH2)6N4 = hexamethylenetetramine). We computed Hapke scattering models with the ices of H2O and CH3OH (although we do not claim a firm identification of CH3OH), plus the refractory organic solid known as Titan tholin (B. N. Khare et al. 1984, Icarus 60, 127), plus amorphous carbon to adjust the albedo. An additional component is required to fit the spectrum at 1-1.4 mu m; olivine (Fo 82) provides exactly the absorption needed. In this two-terrain model, 38.5 % of Pholus is covered with an intimate mixture of 55 % olivine (grain size 20 mu m), 15 % Titan tholin (1 mu m), 15 % H2O ice (10 mu m), and 15 % CH3OH ice (10 mu m); the second terrain material, amorphous carbon, covers 61.5 % of the surface. The spectrally active components in our model are the principal constituents of a comet nucleus. We suggest that if Pholus were to approach the Sun and begin sublimating, it would show the compositional characteristics of a comet. This work is in Press in Icarus. Title: Broadband microwave imaging spectroscopy with a solar-dedicated array Authors: Bastian, Tim S.; Gary, D. E.; White, S. M.; Hurford, Gordon J. Bibcode: 1998SPIE.3357..609B Altcode: For many years, ground-based radio observations of the Sun have proceeded into two directions: (1) high resolution imaging at a few discrete wavelengths; (2) spectroscopy with limited or no spatial resolution at centimeter, decimeter, and meter wavelengths. Full exploitation of the radio spectrum to measure coronal magnetic fields in both quiescent active regions and flares, to probe the thermal structure of the solar atmosphere, and to study energy release and particle energization in transient events, requires a solar-dedicated, frequency-agile solar radiotelescope, capable of high-time, - spatial, and -spectral resolution imaging spectroscopy. In this paper we summarize the science program and instrument requirements for such a telescope, and present a strawman interferometric array composed of many (greater than 40), small (2 m) antenna elements, each equipped with a frequency- agile receiver operating over the range 1 - 26.5 GHz. Title: Coronal Currents, Magnetic Fields, and Heating in a Solar Active Region Authors: Lee, Jeongwoo; McClymont, A. N.; Mikić, Zoran; White, Stephen M.; Kundu, Mukul R. Bibcode: 1998ApJ...501..853L Altcode: We compare microwave images of a solar active region with state-of-the-art fully nonlinear force-free extrapolations of the photospheric fields in order to study the link between coronal currents and heating of the corona. This extrapolation fully takes into account the nonuniform distribution of electric currents observed in the photosphere and its role in the coronal magnetic structure. We carry out the comparison for AR 6615, a complex region observed with the VLA on 1991 May 7. Under the assumption that the microwave emission is dominated by optically thick gyroresonance radiation, we may use the radio images to infer the temperature of the corona at different heights and locations. This is then compared with heating models based on the observed current distribution. We are able to reproduce the radio images remarkably well with a model in which temperature is structured along magnetic field lines, depends on the current on the field line, and increases with height in a manner similar to that inferred from static heated loop models. This result implies a direct link between electric currents and coronal heating. Title: Microwave Mode Coupling Above Active Regions as a Coronal Density Diagnostic Authors: Lee, Jeongwoo; White, Stephen M.; Kundu, M. R.; Mikić, Zoran; McClymont, A. N. Bibcode: 1998SoPh..180..193L Altcode: It is well recognized that the phenomenon of depolarization (the conversion of polarized radio emission into unpolarized emission) of microwaves over solar active regions can be used to infer the coronal electron density once the coronal magnetic field is known. In this paper we explore this technique using an active region for which we have excellent radio data showing depolarization at two frequencies, and for which we have an excellent magnetic field model which has been tested against observations. We show that this technique for obtaining coronal densities is very sensitive to a number of factors. When Cohen's (1960) theory where depolarization is due to magnetic field rotation alone is used, the result is particularly sensitive to the location of the surface on which the magnetic field is orthogonal to the line of sight. Depending on whether we take into account the presence of electric currents in the photosphere or not, their extrapolation into the corona can result in very different heights being deduced for the location of the depolarization strip, and this changes the density which is then deduced from the depolarization condition. Such extreme sensitivity to the magnetic field model requires that field extrapolations be able to accurately predict the polarity of magnetic fields up to coronal heights as high as ∼ 105 km in order to exploit depolarization as a density diagnostic. Title: Large convection cells as the source of Betelgeuse's extended atmosphere Authors: Lim, Jeremy; Carilli, Chris L.; White, Stephen M.; Beasley, Anthony J.; Marson, Ralph G. Bibcode: 1998Natur.392..575L Altcode: Supergiant stars such as Betelgeuse have very extended atmospheres, the properties of which are poorly understood. Alfvén waves, acoustic waves,, and radial pulsations have all been suggested as likely mechanisms for elevating these atmospheres and driving the massive outflows of gas seen in these stars: such mechanisms would heat the atmosphere from below, and there are indeed observations showing that Betelgeuse's extended atmosphere is hotter than the underlying photosphere,. Here we report radio observations of Betelgeuse that reveal the temperature structure of the extended atmosphere from two to seven times the photospheric radius. Close to the star, we find that the atmosphere has an irregular structure, and a temperature (3,450 +/- 850K) consistent with the photospheric temperature but much lower than that of gas in the same region probed by optical and ultraviolet observations. This cooler gas decreases steadily in temperature with radius, reaching 1,370 +/- 330K by seven stellar radii. The cool gas coexists with the hot chromospheric gas, but must be much more abundant as it dominates the radio emission. Our results suggest that a few inhomogeneously distributed large convective cells (which are widely believed to be present in such stars) are responsible for lifting the cooler photospheric gas into the atmosphere; radiation pressure on dust grains that condense from this gas may then drive Betelgeuse's outflow. Title: The Emergence of a Solar Active Region Authors: White, S. M.; Lee, J.; Kundu, M. R.; SOHO/MDI Team Bibcode: 1998ASPC..155..130W Altcode: 1998sasp.conf..130W No abstract at ADS Title: Toward a Frequency-Agile Solar Radiotelescope Authors: Bastian, T. S.; Gary, D. E.; White, S. M.; Hurford, G. J. Bibcode: 1998ASPC..140..563B Altcode: 1998ssp..conf..563B No abstract at ADS Title: The Solar Atmosphere Above a Sunspot Authors: Zlotnik, E. Ya.; White, S. M.; Kundu, M. R. Bibcode: 1998ASPC..155..135Z Altcode: 1998sasp.conf..135Z No abstract at ADS Title: A Search for Hard X-ray Emission from Active Stars Using CGRO/BATSE Authors: White, S. M.; Harmon, B. A.; Lim, J.; Kundu, M. R. Bibcode: 1998ASPC..154.1192W Altcode: 1998csss...10.1192W We report the results of a search for > 20 keV photons from active stars using CGRO/BATSE Earth-occultation observations. Twelve of the "usual suspects" together with 12 "placebo" locations have been analyzed using the BATSE software for occultation analysis developed at NASA/MSFC. There are four detections at the nominal 5sigma level, and eight at the 3sigma level. However the strongest detection (that of AB Dor) shows clear evidence for contamination from the nearby strong source LMC X-4. 18 of the 24 fields yield positive fluxes, indicating a clear bias in the results, and possibly indicating the presence of weak background hard X-ray sources detectable by BATSE in long-term studies. Title: Flares on AB Doradus Observed with ASCA Authors: Ortolani, A.; Pallavicini, R.; Maggio, A.; Reale, F.; White, S. M. Bibcode: 1998ASPC..154.1532O Altcode: 1998csss...10.1532O ASCA observations of the young rapidly rotating star AB Doradus are analysed with special emphasis on its flaring behaviour. Multitemperature model fits with either fixed or variable abundances are used. The results for the quiescent emission are compared with those obtained previously by Mewe et al. (1996) confirming the very low coronal metallicity (nearly one tenth solar) of this star in spite of its measured solar photospheric metallicity. The results for the flares are compared with recent models based on full hydrodynamic calculations (Reale et al. 1997) and realistic values for the loop length and the heating duration are derived. It is shown that the flare is essentially a high temperature phenomenon with little effect on the low-temperature component of the quiescent star. Title: The Radio Outburst of Eta Carinae Authors: White, S. M.; Duncan, R. A.; Lim, J.; Drake, S. A. Bibcode: 1997AAS...191.3404W Altcode: 1997BAAS...29.1261W The massive star Eta Carinae has been undergoing a radio outburst since 1993 which has made it one of the brightest stellar radio sources in the sky. The cause of the outburst is apparently ionization of previously neutral gas within several arcseconds (several hundredths of a parsec) of the star. The nature of the ionizing radiation is not known, but the coincidence of the radio flux increase with the X-ray outburst reported by Corcoran et al is clearly important. In this paper we report on continued monitoring of the radio outburst as the star approaches the shell phase of the 5.5-year cycle proposed by Damineli. At this stage the radio flux behaviour appears to be consistent with the presence of such a cycle. Title: The Measurement of Solar Active Region Properties with EUV Spectra and Spectroheliograms from SERTS Authors: Brosius, J. W.; Davila, J. M.; Thomas, R. J.; White, S. M. Bibcode: 1997AAS...191.7315B Altcode: 1997BAAS...29.1323B The Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS) was successfully flown on six different occasions, and results from several of those flights are presented here. For the flight of 1995 May 15, SERTS included a multilayer coated toroidal diffraction grating which enhanced the throughput above that of a standard gold coated grating for wavelengths between about 170 and 220 Angstroms, with a peak response around 192 Angstroms. Emission lines in this wavelength range are seen in second order. First order lines between about 235 and 335 Angstroms are also detected. A total of nearly 140 lines are identifiable in the combined first and second order wavebands. These include lines from several ionization stages of Ca, Mg, Ni, S, and Si, as well as lines from at least nine ionization stages of Fe (IX -- XVII). Many of the lines are useful for calibration verification, plasma diagnostics, or both. Results from analyses of the high spectral resolution (30 m Angstroms in second order, and 55 m Angstroms in first), spatially resolved (4.4 arcsec spatial resolution) active region spectra are presented. For the flight of 1993 August 17, SERTS included a multilayer coated grating which enhanced the instrumental sensitivity within the first order waveband. For this flight we also obtained coordinated Very Large Array (VLA) radio observations at 20 and 6 cm wavelengths. Because the radio emission is sensitive to the coronal magnetic field while the EUV emission is not, we were able to derive solar coronal magnetograms from the combined SERTS and VLA observations. (This work was supported by NASA grants NASW-96006 and NASW-4933.) Title: Evolution of the radio outburst from the supermassive star eta Carinae from 1992 to 1996 Authors: Duncan, R. A.; White, S. M.; Lim, J. Bibcode: 1997MNRAS.290..680D Altcode: An earlier paper, describing observations at a wavelength of 3 cm with the Australia Telescope Compact Array between 1992 June and 1994 May, showed the supermassive star eta Carinae to be in the throes of a major outburst. This paper, based on observations at wavelengths of both 3 and 6 cm, describes the evolution of the outburst since 1994 May. From 1994 May to 1995 December, eta Carinae's image continued to expand, and its total flux density to increase. These dramatic changes were caused, we believe, by an outburst of UV luminosity, and consequent ionization of pre-existing circumstellar gas clouds. Since 1995 December, eta Carinae's total flux density has fallen. The large-scale structure of the radio image closely resembles Hα optical images of the eta Carinae `Homunculus' nebula. In addition, the central area shows fine-scale structure, principally comprising a secondary source about 1.2 arcsec to the north-west of the star, which we believe to be a dense gas cloud, and a ridge of length 4 arcsec centred on the optical position of the star, which we believe to be an equatorial disc or torus. These structures are seen also in 3-cm (H91alpha) recombination-line emission. Title: Coronal Magnetography of a Solar Active Region Using Coordinated SERTS and VLA Observations Authors: Brosius, Jeffrey W.; Davila, Joseph M.; Thomas, Roger J.; White, Stephen M. Bibcode: 1997ApJ...488..488B Altcode: We observed NOAA region 7563 simultaneously with Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrogaph (SERTS) and with the Very Large Array (VLA) on 1993 August 17. SERTS obtained spectra in the 280-420 Å wavelength range, and images in the lines of Mg IX λ368.1, Fe XV λ284.1, and Fe XVI λ335.4. The VLA obtained microwave images at 20 and 6 cm wavelengths. The microwave emission depends upon the coronal temperature, density, column emission measure, and magnetic field; therefore, the coronal magnetic field can be derived when all of these other quantities are measured. Here we demonstrate this approach by using the SERTS data to derive all the relevant plasma parameters and then fitting the radio observations to a magnetic field model in order to determine the magnetic field structure.

We used the method of Monsignori-Fossi & Landini and the coronal elemental abundances of Feldman et al. to derive the differential emission measure (DEM) curve for region 7563 from numerous EUV emission lines in spatially averaged SERTS spectra. A similar curve was estimated for each point (i.e., each pixel or each spatial location) in the two-dimensional region by scaling the average DEM curve with corresponding pixel intensities in the Mg IX, Fe XV, and Fe XVI images. We integrated each such DEM over narrow temperature ranges to obtain the column emission measure (CEM) as a function of temperature, CEM(T). We also obtained electron density measurements from EUV line intensity ratios in the spatially averaged spectrum for several ionization stages of iron. These were used to derive a functional relation between density and temperature, ne(T).

We derived the temperature dependence of the coronal magnetic field [B(T)] at each point in the two-dimensional region by incorporating CEM(T) and ne(T) into expressions for the thermal bremsstrahlung and the gyroresonance opacities, and varying B(T) so as to minimize the difference between the calculated and the observed microwave intensities. The resulting calculated 20 and 6 cm microwave intensity images reproduce the observed images very well. We found that thermal bremsstrahlung alone is not sufficient to produce the observed microwave intensities: gyroemission is required. Further, contrary to several earlier studies, we found no evidence for cool, absorbing plasma in the solar corona above the active region. The coronal magnetic fields derived with our method typically exceed the coronal fields extrapolated with a simple potential model, suggesting the presence of coronal electric currents. However, in the diminutive sunspot which dominates the 6 cm emission this difference is relatively small, suggesting that the sunspot magnetic field itself is nearly potential. Although we cannot firmly establish the uniqueness of our solution in this particular case, the method is quite powerful and should be repeated with other similar data sets. Variations in the coronal elemental abundances could affect the determination of the microwave emission mechanism(s), introduce evidence for the presence of cool coronal plasma, and alter the strengths of the derived coronal magnetic fields. Title: First Images of Impulsive Millimeter Emission and Spectral Analysis of the 1994 August 18 Solar Flare Authors: Silva, Adriana V. R.; Gary, Dale E.; White, Stephen M.; Lin, R. P.; de Pater, Imke Bibcode: 1997SoPh..175..157S Altcode: We present here the first images of impulsive millimeter emission of a flare. The flare on 1994 August 18 was simultaneously observed at millimeter (86 GHz), microwave (1-18 GHz), and soft and hard X-ray wavelengths. Images of millimeter, soft and hard X-ray emission show the same compact ( 8'') source. Both the impulsive and the gradual phases are studied in order to determine the emission mechanisms. During the impulsive phase, the radio spectrum was obtained by combining the millimeter with simultaneous microwave emission. Fitting the nonthermal radio spectra as gyrosynchrotron radiation from a homogeneous source model with constant magnetic field yields the physical properties of the flaring source, that is, total number of electrons, power-law index of the electron energy distribution, and the nonthermal source size. These results are compared to those obtained from the hard X-ray spectra. The energy distribution of the energetic electrons inferred from the hard X-ray and radio spectra is found to follow a double power-law with slope ∼6-8 below ∼50 keV and ∼3-4 above those energies. The temporal evolution of the electron energy spectrum and its implication for the acceleration mechanism are discussed. Comparison of millimeter and soft X-ray emissions during the gradual phase implies that the millimeter emission is free-free radiation from the same hot soft X-ray emitting plasma, and further suggests that the flare source contains multiple temperatures. Title: Radio Observations of Gyroresonance Emission from Coronal Magnetic Fields Authors: White, S. M.; Kundu, M. R. Bibcode: 1997SoPh..174...31W Altcode: We review the basic characteristics of thermal gyroresonance (also known as cyclotron) emission from solar active regions, and show how radio observations combined with our understanding of the basic mechanism can reveal much of the magnetic and thermal structure of the corona over active regions. Title: Signatures of Coronal Currents in Microwave Images Authors: Lee, Jeongwoo; White, Stephen M.; Gopalswamy, N.; Kundu, M. R. Bibcode: 1997SoPh..174..175L Altcode: Microwave emission from solar active regions at frequencies above 4 GHz is dominated by gyroresonance opacity in strong coronal magnetic fields, which allows us to use radio observations to measure coronal magnetic field strengths. In this paper we demonstrate one powerful consequence of this fact: the ability to identify coronal currents from their signatures in microwave images. Specifically, we compare potential-field (i.e., current-free) extrapolations of photospheric magnetic fields with microwave images and are able to identify regions where the potential extrapolation fails to predict the magnetic field strength required to explain the microwave images. Comparison with photospheric vector magnetic field observations indicates that the location inferred for coronal currents agrees with that implied by the presence of vertical currents in the photosphere. The location, over a neutral line exhibiting strong shear, is also apparently associated with strong heating. Title: High-Resolution Imagery of BD + 30degree3639 and Its Dusty Halo Authors: Harrington, J. Patrick; Lame, Nancy Joanne; White, Stephen M.; Borkowski, Kazimierz J. Bibcode: 1997AJ....113.2147H Altcode: The planetary nebula BD +30\arcdeg3639 has been imaged by the Hubble Space Telescope through nine narrow-band filters. A high resolution radio map has been obtained with the VLA. Comparison of Hβ , Hα and radio images allow us to derive the point-to-point variations in both the extinction and the ratio of total to selective absorption. The extinction shows (1) a gradient across the nebula, and (2) a large dust clump in the northeast and many small ( ~ 0farcs2 ) dust features. The ratio of total to selective absorption for this nebula is close to the normal ISM value, and can be modeled by a power law distribution of amorphous carbon grains. It is found that the optical images show a substantial halo due to light scattered by dust in the neutral envelope surrounding the ionized shell. Monte Carlo models indicate that such scattering could result from a dust halo with an optical depth of unity and an albedo of ~ 0.4. Such a high albedo is not consistent with pure carbon dust models. The halo images in [O I]lambda 6300 and [S II]lambda 6717,31 show anomalous structures not seen in the other lines, which seem to be in emission. The strongest [O I] feature is coincident with halo emission previously observed in the 1-0 S(1) transition of H_2. The [O I] and [S II] emission may indicate shock heating of the halo. Title: Radio Observations of the Emergence of a Solar Active Region Authors: White, S. M.; Lee, J.; Kundu, M. R. Bibcode: 1997SPD....28.0603W Altcode: 1997BAAS...29..910W On July 6 1996 a solar active region abruptly began to emerge through the solar photosphere. This region was eventually to produce the first X-class flare since 1993. We happened to be observing the region with the VLA during its emergence, and present a preliminary report on the observations in this paper. The radio data are the only means for observing the magnetic fields of the emerging region in the corona, and can thus be used to test models for the structure of emerging magnetic fields. In this case, we do not see any evidence for strong coronal fields during the initial emergence, indicating that the field lines diverge rapidly above the photosphere as expected from simple theory. We follow the emergence of flux over several days and discuss the implications of these observations for theories of flux emergence. Title: Currents, Magnetic Fields and Heating in a Solar Active Region Authors: Lee, J.; McClymont, A. N.; Mikic, Z.; White, S. M.; Kundu, M. R. Bibcode: 1997SPD....28.1602L Altcode: 1997BAAS...29R.920L We have compared high-quality microwave images of the radio emission from the corona above an active region with state-of-the-art nonlinear force-free extrapolations of the photospheric magnetic field. The radio images, which are dominated by the opacity provided by the coronal magnetic fields, show excess magnetic field in locations consistent with the expected location of coronal currents. We test the hypothesis that the degree of heating on a given coronal magnetic flux tube is related to the current flowing through it by comparing model radio brightness distributions at different frequencies with the actual observations. In the model we assume that temperature is distributed along the field lines according to quasi-static loop models, and that there is effectively no diffusion across the field lines. This coronal heating model is able to reproduce the radio brightness distributions remarkably well. Title: Coronal Magnetography of a Solar Active Region Using Coordinated SERTS and VLA Observations Authors: Brosius, J. W.; Davila, J. M.; Thomas, R. J.; White, S. M. Bibcode: 1997SPD....28.0135B Altcode: 1997BAAS...29..885B We observed NOAA region 7563 simultaneously with Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS) and with the Very Large Array (VLA) on 1993 August 17. SERTS obtained spectra in the 280 to 420 Angstroms wavelength range, and images in the lines of Mg IX lambda 368.1, Fe XV lambda 284.1, and Fe XVI lambda 335.4. The VLA obtained microwave images at 20 and 6 cm wavelengths. The microwave emission depends upon the coronal temperature, density, column emission measure, and magnetic field; therefore, the coronal magnetic field can be derived when all of these other quantities are measured. Here we demonstrate this approach by using the SERTS data to derive all the relevant plasma parameters and then fitting the radio observations to a magnetic field model in order to determine the magnetic field structure. We derived the temperature dependence of the coronal magnetic field (B(T)) at each point (i.e., each pixel or each spatial location) in the two dimensional region by incorporating the corresponding column emission measure (CEM(T)) and electron density (n_e(T)) into expressions for the thermal bremsstrahlung and gyroresonance opacities, and varying B(T) so as to minimize the difference between the calculated and the observed microwave intensities. The resulting calculated 20 and 6 cm microwave intensity images reproduce the observed images very well. Thermal bremsstrahlung emission alone is not sufficient to produce the observed microwave intensities: gyroemission is required. Further, contrary to several earlier studies, we found no evidence for cool, absorbing plasma in the solar corona above the active region. The coronal magnetic fields derived with our method typically exceed the coronal fields extrapolated with a simple potential model, suggesting the presence of coronal electric currents. However, in the diminutive sunspot which dominates the 6 cm emission this difference is relatively small, suggesting that the sunspot magnetic field itself is nearly potential. (This work was supported by NASA grant NASW-4933.) Title: Sun Authors: White, Stephen M. Bibcode: 1997msma.conf...19W Altcode: No abstract at ADS Title: The Eclipsing Radio Emission of the Precataclysmic Binary V471 Tau Authors: Lim, J.; White, S. M.; Cully, S. L. Bibcode: 1997tcca.conf..370L Altcode: No abstract at ADS Title: The Radio Outbursts from Eta Carinae Authors: White, S. M.; Duncan, R. A.; Lim, J.; Drake, S. A. Bibcode: 1997ASPC..120..282W Altcode: 1997lbv..conf..282W No abstract at ADS Title: Abundances and Flares in the ASCA Observation of the Young k0 Star AB Doradus Authors: White, S. M.; Pallavicini, R.; Mewe, R.; Kaastra, J. S.; Lim, J. Bibcode: 1997xisc.conf..573W Altcode: No abstract at ADS Title: Simultaneous EUVE & ASCA observations of AB Doradus: temperature structure and abundances of the quiescent corona. Authors: Mewe, R.; Kaastra, J. S.; White, S. M.; Pallavicini, R. Bibcode: 1996A&A...315..170M Altcode: We have analyzed EUV and X-ray spectra of the non-flaring state of AB Dor, obtained from simultaneous observations with EUVE and ASCA. The coronal temperature structure and the coronal elemental abundances have been derived by jointly fitting the spectra, including lines and continuum. We have applied a multi-temperature fitting method and we have found that an optically thin plasma model with solar abundances (relative to hydrogen) does not yield a good fit. A 4-T fit allowing the abundances of the more important elements (Fe, S, Si, Mg, Ne, and O) plus four other less important abundances (N, Ar, Ca, and Ni) to vary improves the fit markedly. This results in values of the first 6 abundances relative to solar photospheric values that are significantly (a factor 2-3) below solar except for Ne, which is solar, while most of the remaining four abundances are consistent both with solar and reduced abundances. In the course of the 4-T fitting we have determined a best-fit value for the interstellar hydrogen column density of N_H_=(2.0+/-0.5)10^18^cm^-2^. We have applied four differential emission measure (DEM) distribution analysis techniques, viz.: (i) regularization method, (ii) polynomial method, (iii) clean algorithm, and (iv) genetic algorithm. The four different methods all yield a qualitatively similar DEM, showing two maxima in the temperature intervals 5-8MK and 20-30MK. Our analysis demonstrates the great value of simultaneous ASCA and EUVE observations in determining the fundamental parameters of stellar coronae. Title: Comprehensive Multiwavelength Observations of the 1992 January 7 Solar Flare Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.; de Pater, Imke; Gary, Dale E.; McTiernan, James M.; Hudson, Hugh S.; Doyle, J. Gerry; Hagyard, Mona J.; Kundu, Mukul R. Bibcode: 1996ApJS..106..621S Altcode: Observations of a solar flare that occurred at 2022 UT on 1992 January 7, during the 1991 December/1992 January Max `91 campaign, are presented. This flare was observed simultaneously in Hα, radio (at microwave and millimeter wavelengths), and soft and hard X-rays (by the Yohkoh spacecraft) with high spatial and moderate spectral resolution. A comparison of magneto grams before and after the flare shows evidence of the emergence of new magnetic flux of opposite polarity at the flare site. Although this flare was only of moderate size (GOES classification C8.9 and Hα importance SF), it exhibited several distinct bursts and at least 10 spatially distinct hard/soft X-ray sources. Cospatial Hα brightenings suggest that most of the X-ray sources are located at footpoints of magnetic loops. Two of the hard X-ray sources have no Hα counterparts and are therefore believed to be located at loop tops. The flare consisted of three bursts of particle acceleration followed by a purely thermal phase. High spectral resolution Ca XIX line profiles indicate upflows shortly after the second acceleration phase. Analysis of the microwave/hard X-ray/soft X-ray emission from individual sources provides information on the radio emission mechanisms, the energetic electron population, the magnetic field strength, and the plasma density. These parameters were estimated for the two microwave sources observed during the third acceleration burst; these sources were simultaneously detected in soft X-rays, and one of the sources is also seen in hard X-ray maps. Although the microwave emission is consistent with the gyro synchrotron mechanism, the millimeter emission, which peaks during the thermal phase when all nonthermal activity has ceased, is likely due to thermal bremsstrahlung from the soft X-ray emitting hot plasma. The energy lost to collisions by the energetic (>15 keV) electrons and the energy contained in the thermal plasma are calculated for each source. The energy injected by the nonthermal electrons from all sources is estimated to be 1030 ergs. Only the soft X-ray sources with gradual time profiles seem to show the Neupert effect. Title: Limits to Mass Outflows from Late-Type Dwarf Stars Authors: Lim, Jeremy; White, Stephen M. Bibcode: 1996ApJ...462L..91L Altcode: We show that the mass-loss rates of active late-type dwarf stars must be significantly lower than recent estimates of up to ~5 x 10-10 Msolar yr-1, 4 orders of magnitude higher than that of the Sun. First, we present aperture-synthesis observations at 3.5 mm of the dMe flare stars YZ CMi and AD Leo, during which neither star was detected at an upper limit of 10 mJy. Although compatible with the tentative detection of YZ CMi at 1.1 mm reported by Mullan and coworkers if the millimeter emission originates from a ~104 K, 300 km s-1 wind with M dot ~ 5 x 10-10 Msolar yr-1, we show that such a wind would completely absorb the observed radiation from coronal radio flares originating from close to the stellar surface. From this contradiction, we show that the mass-loss rate of any ~104 K wind with solar-wind--like velocities of 300--600 km s-1 must be less than ~10-13 Msolar yr-1, more than 3 orders of magnitude below that inferred by Mullan et al. The corresponding upper limit to a wind at a solar-wind--like temperature of ~106 K is M dot ~ 10-12 Msolar yr-1, an order of magnitude below the lower limit predicted theoretically by Badalyan & Livshits. Our arguments apply to all classes of stars that display coronal radio flares, implying that the mass-loss rate of active late-type dwarf stars from any ~104 or ~106 K winds with solar-wind--like velocities can be no more than 1 or 2 orders of magnitude, respectively, higher than the solar mass-loss rate of ~3 x 10-14 Msolar yr-1. We show that coronal mass ejections also are unlikely to explain the reported millimeter emission from dMe flare stars, and that the time-averaged mass-loss rate from such events can be no higher than in the case of a steady, spherically symmetric stellar wind. Title: Millimeter Observations of a B5.6 Flare Authors: Raulin, J. -P.; White, S. M.; Kundu, M. R.; Silva, A. V. R.; Shibasaki, K. Bibcode: 1996AAS...188.4503R Altcode: 1996BAAS...28..894R We present millimeter imaging observations of a B5.6 flare which occured on 1994 August 16 in the active region AR 7765. The BIMA interferometer and the Nobeyama radioheliograph observed both the impulsive phase and the thermal phase of the flare emission. The 3.5 mm maps obtained with BIMA allowed us to determine the location of the radio source and its properties at different phases of the flare evolution. In X-ray wavelengths the impulsive phase was detected by the first two channels of BATSE (25-50 keV, 50-100 keV); although YOHKOH/SXT did not observe the impulsive phase, it was possible to image the post-flare loop in soft X-rays. We compare the images of the flare at different wavelengths and discuss the relevance of millimeter emission in the context of flare models. Title: Large Scale Features of the Radio Sun Authors: White, S. M.; Gary, D. E.; Kundu, M. R. Bibcode: 1996AAS...188.7907W Altcode: 1996BAAS...28Q.956W We present full-disk images of the Sun at 5 GHz made by observing 26 different fields with the Very Large Array and combining them using mosaicking techniques. The resulting image combines sensitivity to large-scale structures with good resolution. Full-disk images at 0.33, 1.4 and 17 GHz, a high-resolution magnetogram and a soft X-ray image are compared with the 5 GHz image to investigate the physical properties of large-scale features. Title: The Radio Properties of the dMe Flare Star Proxima Centauri Authors: Lim, Jeremy; White, Stephen M.; Slee, O. B. Bibcode: 1996ApJ...460..976L Altcode: No abstract at ADS Title: The Eclipsing Radio Emission of the Precataclysmic Binary V471 Tauri Authors: Lim, Jeremy; White, Stephen M.; Cully, Scott L. Bibcode: 1996ApJ...461.1009L Altcode: We present strong evidence confirming the presence of eclipses in the centimeter radio emission of the eclipsing binary V471 Tau, comprising a K2 dwarf and a white dwarf. In observations spanning two complete orbital periods, we detected one eclipse per orbit: in all, we observed one near-complete radio eclipse, the ingress phase of two other radio eclipses, and the egress phase of yet another radio eclipse. The minimum of the observed near-complete radio eclipse is centered at the orbital phase φ = 0 when the white dwarf is eclipsed and directly behind the K dwarf, and it has a full width of Δφ ≍ 0.3; by comparison, the optical eclipse of the white dwarf occupies only Δφ = 0.066. Inside eclipse, the total flux density of V471 Tau falls to a level ∼20% of that outside eclipse, implying that a large fraction of the radio emission originates from the region between the two stars. Outside eclipse, the radio emission varies slowly and follows, in large part, the same phase dependence over the two observed orbits (separated by one orbit). This suggests that much of the modulation observed outside eclipse may be due to an apparent change in the observed radiation pattern of the source with orbital revolution, rather than intrinsic variability in the radio emission process. From the data, we place constraints on the physical parameters of both the occulter and the occulted radio source; we find that the radio source is most probably radiating by nonthermal gyrosynchrotron emission. We favor a model where the radio-emitting electrons are accelerated by the interaction (collision) between the magnetospheres of the K dwarf and the white dwarf. This region of interaction is likely to be located very close to the surface of the white dwarf, leading naturally to a picture where the radio emission originates from large magnetic structures associated with the K dwarf. Such a model can qualitatively explain many of the features observed in the radio light curve. The proposed magnetic structures may provide the means by which mass is transferred from the K dwarf to the white dwarf, accounting partly or wholly for the inferred accretion of the white dwarf. Title: A model of the atmosphere above a sunspot from radio observations Authors: Zlotnik, E. Ya.; Kundu, M. R.; White, S. M. Bibcode: 1996R&QE...39..255Z Altcode: The results of VLA observations of an unusual source of microwave radiation, associated with the sunspot NOAA 7789 on October, 15, 1994, are presented. The fine structure of the source, which is a ring structure in intensity and polarization at frequencies 4.5 and 8.0 GHz, is discussed. It is shown that the features observed can be explained by a thermal cyclotron mechanism if the magnetic field is approximated by a vertical dipole buried under the photosphere, but the spatial distributions of kinetic temperature and electron density in the atmosphere above the sunspot differ considerably from the standard model. A two-dimensional source model (the dependences of the parameters on the height and distance from the center of the sunspot), which fits the observations at the above frequencies, is evolved. The principal physical result is that the data observed are explainable by the presence of an unexpectedly dense cool plasma in the atmosphere over the center of the umbra. Title: First Images of a Solar Flare at Millimeter Wavelengths Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.; de Pater, Imke; Shibasaki, K.; Hudson, Hugh S.; Kundu, Mukul R. Bibcode: 1996ApJ...458L..49S Altcode: We present the first high spatial resolution images of a solar flare at millimeter wavelengths. On 1994 August 17, a GOES soft X-ray class M1 flare was observed by the Berkeley-Illinois-Maryland Array at 86 GHz by the Nobeyama 17 GHz array and by the Yohkoh spacecraft. The flare displayed both a prominent impulsive phase in microwaves and a gradual phase that lasted over 30 minutes. The millimeter data were taken only during the gradual phase. The millimeter images show a source with a size of ~8", a peak brightness temperature of ~106 K, and maximum optical depth of 0.09. At both X-ray and radio wavelengths, the emitting region appeared to be compact (<~20"). In soft X-ray, the images are resolved into two sources: one located at a footpoint and the other at the top of the flaring loop. The millimeter emission is consistent with the predicted free-free flux from an isothermal temperature (~14 MK) loop-top source, a multitemperature footpoint source with a hot (~22 MK), and a cold (~12 MK) component. Most (80%) of the millimeter flux density originates from the top of the magnetic loop, and the footpoint contribution is only 20%. Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1996ASPC...93..430B Altcode: 1996ress.conf..430B No abstract at ADS Title: The X-ray spectra of flares from AB Doradus Authors: White, S. M.; Pallavicini, R.; Lim, J. Bibcode: 1996ASPC..109..299W Altcode: 1996csss....9..299W No abstract at ADS Title: The Eclipsing Radio Emission of the Precataclysrnic Binary V471 Tau Authors: Lim, J.; White, S. M.; Cully, S. L. Bibcode: 1996ASPC...93..327L Altcode: 1996ress.conf..327L No abstract at ADS Title: Radio emission from cool stars Authors: White, S. M. Bibcode: 1996ASPC..109...21W Altcode: 1996csss....9...21W No abstract at ADS Title: Millimeter Continuum Observations of Stars Authors: Pallavicini, Roberto; White, S. M. Bibcode: 1996slma.conf..268P Altcode: We review the scientific advances to be expected from millimeter continuum observations of stars by a Large Southern Array (LSA). We discuss briefly several topics including millimeter observations of the Sun, winds of hot stars, circumstellar disks of pre-main sequence stars, circumstellar shells and mass loss in cool giants, symbiotic stars, and non-thermal emission in active binaries, flare stars and accretion powered X-ray sources. We show that even in the more limited area of continuum observations, the proposed LSA will allow major advances in virtually all fields of stellar astronomy. Title: First Radio Detection of Solar-Type Stars in an Open Cluster: The Pleiades Authors: Lim, J.; White, S. M. Bibcode: 1996ASPC...93..455L Altcode: 1996ress.conf..455L No abstract at ADS Title: The Radio Outburst of eta Carinae Authors: White, S. M.; Duncan, R. A.; Lim, J.; Drake, S. A.; Kundu, M. R. Bibcode: 1996ASPC...93...59W Altcode: 1996ress.conf...59W No abstract at ADS Title: Radio emission from solar-type stars in the Pleiades Authors: Lim, J.; White, S. M. Bibcode: 1996ASPC..109..369L Altcode: 1996csss....9..369L No abstract at ADS Title: The corona of AB Dor: temperatures and abundances determined with ASCA and EUVE Authors: Mewe, R.; Kaastra, J. S.; White, S. M.; Pallavicini, R. Bibcode: 1996ASPC..109..273M Altcode: 1996csss....9..273M No abstract at ADS Title: A Search for Rotational Modulation in the EUV Emission from AB Doradus Authors: White, S. M.; Lim, J.; Rucinski, S. M.; Roberts, C.; Kilkenny, D.; Ryan, S. G.; Prado, P.; Kundu, M. R. Bibcode: 1996aeu..conf..165W Altcode: 1996IAUCo.152..165W No abstract at ADS Title: Spectroscopic EUVE Observations of the Active Star AB Doradus Authors: Rucinski, Slavek M.; Mewe, Rolf; Kaastra, Jelle S.; Vilhu, Osmi; White, Stephen M. Bibcode: 1996aeu..conf..159R Altcode: 1996IAUCo.152..159R No abstract at ADS Title: First Images of a Solar Flare at Millimeter Wavelengths Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.; de Pater, Imke; Shibasaku, K.; Hudson, Hugh S.; Kundu, Mukul R. Bibcode: 1996ADIL...AS...01S Altcode: We present the first high-spatial-resolution images of a solar flare at millimeter wavelengths. On 1994 August 17, a GOES soft X--ray class M1 flare was observed by the Berkeley--Illinois--Maryland Array (BIMA) at 86 GHz, by the Nobeyama 17 GHz array, and by the Yohkoh spacecraft. The flare displayed both a prominent impulsive phase in microwaves and a gradual phase which lasted over 30 minutes. The millimeter data were taken only during the gradual phase. The millimeter images show a source with a size of $\sim$8\arcsec, a peak brightness temperature of ~ 10^6 K, and maximum optical depth of 0.09. At both X--ray and radio wavelengths the emitting region appeared to be compact (< 20 arcseconds). In soft X--ray the images are resolved into two sources: one located at a footpoint and the other at the top of the flaring loop. The millimeter emission is consistent with the predicted free-free flux from an isothermal temperature (~ 14 MK) looptop source and a multi--temperature footpoint source with a hot (~ 22 MK) and a cold (~ 12 MK) component. Most (80%) of the millimeter flux density originates from the top of the magnetic loop, and the footpoint contribution is only 20%. Title: EUVE and VLA Observations of the Eclipsing Pre-Cataclysmic Variable V471 Tauri Authors: Cully, S. L.; Dupuis, J.; Rodriguez-Bell, T.; Basri, G.; Siegmund, O. H. W.; Lim, J.; White, S. M. Bibcode: 1996aeu..conf..349C Altcode: 1996IAUCo.152..349C No abstract at ADS Title: Microwave and Hard X-Ray Observations of Footpoint Emission from Solar Flares Authors: Kundu, M. R.; Nitta, N.; White, S. M.; Shibasaki, K.; Enome, S.; Sakao, T.; Kosugi, T.; Sakurai, T. Bibcode: 1995ApJ...454..522K Altcode: We investigate radio and X-ray imaging data for two solar flares in order to test the idea that asymmetric precipitation of nonthermal electrons at the two ends of a magnetic loop is consistent with the magnetic mirroring explanation. The events we present were observed in 1993 May by the HXT and SXT X-ray telescopes on the Yohkoh spacecraft and by the Nobeyama 17 GHz radioheliograph. The hard X-ray images in one case show two well-separated sources; the radio images indicate circularly polarized, nonthermal radio emission with opposite polarities from these two sources, indicating oppositely directed fields and consistent with a single-loop model. In the second event there are several sources in the HXT images which appear to be connected by soft X-ray loops. The strongest hard X-ray source has unpolarized radio emission, whereas the strongest radio emission lies over strong magnetic fields and is polarized. In both events the strongest radio emission is highly polarized and not coincident with the strongest hard X-ray emission. This is consistent with asymmetric loops in which the bulk of the precipitation (and hence the X-ray emission) occurs at the weaker field footpoint. Title: Radio Detection of Late-Type (G--K) Dwarf Stars in the Pleiades Authors: Lim, Jeremy; White, Stephen M. Bibcode: 1995ApJ...453..207L Altcode: We report deep 3.6 cm radio observations of a small sample of the most rapidly rotating G-K dwarf stars in the Pleiades. Of the four ultrafast rotators (UFRs) observed, three were detected. The G8 dwarf H II 1136, the fastest rotating G dwarf known in the Pleiades, displayed a flare that rose to a peak flux density of ∼1 mJy (peak radio luminosity of ∼2 × 1016 ergs Hz-1 s-1) in less than ∼1 hr, stayed at approximately this level for ∼2 hr, and then decayed apparently exponentially with an e-folding time of ∼1.4 hr. Following the flare, H II 1136 was detected in what may be its quasi steady state with a flux density of 0.16±0.02 mJy. The K2 dwarf H II 1883, the fastest rotating K dwarf in the Pleiades, was detected on two separate occasions, both times as an apparently steady source. The stellar flux density, however, appeared to change from 0.10±0.02 mJy to 0.05±0.01 mJy in the two observations separated by about 3 months. By contrast, the K0 dwarf H II 625 displayed slowly varying emission with an average flux density of 0.16±0.02 mJy. For all the stars detected, the average radio luminosity of their quasi-steady (perhaps quiescent) emission is 1-3 × 1015 ergs Hz-1 s-1. For the single undetected star, the K2 dwarf H II 3163, we placed an upper limit (5 σ) of 0.12 mJy on its flux density or 2 × 1015 ergs Hz-1 s-1 on its luminosity.

Our results represent the first detection of late-type dwarf stars in the Pleiades and indeed of any apparently single stars in an open cluster. It demonstrates that solar-type stars recently descended to the zero-age main sequence can be copious radio emitters. Both in their quasi-steady and flaring states, the radio luminosities of the Pleiades stars are similar to those of equally rapidly rotating but relatively nearby late-type dwarf stars belonging to the Local Association, which provides further support for the idea that such stars are physical counterparts of UFRs in the Pleiades. When averaged over the stellar surface, the surface radio luminosity of the Pleiades stars is comparable to that of the most active T Tauri stars, a trend recently noted for the surface soft X-ray emission of these two classes of stars. This may suggest that the magnetic dynamo of rapidly rotating late-type stars operates at a saturated level as these stars descend to the main sequence. Title: The Radio Properties of Solar Active Region Soft X-Ray Transient Brightenings Authors: White, S. M.; Kundu, M. R.; Shimizu, T.; Shibasaki, K.; Enome, S. Bibcode: 1995ApJ...450..435W Altcode: We present the results of a search for radio emission from active-region transient brightenings identified in Yohkoh soft X-ray telescope observations of active region AR 7260. We present detailed observations of four events in which 17 GHz radio emission is clearly detected in observations by the Nobeyama radioheliograph. The time profiles of the 17 GHz data are very similar to those of the soft X-ray fluxes, and the 17 GHz flux is very close to that expected from plasma with the temperature and emission measure derived for the soft X-ray emitting material from filter ratios. No impulsive nonthermal radio emission was detected from any of the four events, although each was at least GOES class B 1 in soft X-rays. Weak hard X-rays may have been detected by GRO/BATSE from the strongest of the events, but not from two others. These negative results leave open the possibility that there is a difference between active region transient brightenings and solar flares, in that the former do not convert a significant amount of the released energy into accelerated electrons. However, confirmation of this hypothesis will require a larger sample of events. Title: Spectroscopic EUVE Observations of the Active Star AB Doradus Authors: Rucinski, Slavek M.; Mewe, Rolf; Kaastra, Jelle S.; Vilhu, Osmi; White, Stephen M. Bibcode: 1995ApJ...449..900R Altcode: We present observations of the pre-main-sequence, rapidly rotating (0.515 day) late-type star, AB Doradus (HD 36705), made by the Extreme Ultraviolet Explorer (EUVE) satellite. A spectrum from 80 to 700 Å with a resolution Δλ ≍ 0.5-2 Å was accumulated between 1993 November 4-11, with an effective exposure time of about 40 hours. No obvious EUV flares were detected during the observation. The data constrain the coronal temperature structure between several 104 K up to roughly 2 × 107 K through a differential emission measure analysis using the optically thin MEKA plasma model. The resulting differential emission measure (DEM) distribution shows: (1) dominant emission from plasma between about 2 × 106 and 2 × 107 K, which may show a substructure with two components around 3 × 106 and 107 K; (2) very little emission from plasma between 105 and 2 × 106 K; and (3) emission from plasma below about 105 K. If solar photospheric abundances are assumed, then the formal DEM solution also requires the presence of a strong high-temperature component (above about 3 × 107 K) in order to explain the strong continuum emission below about 150 Å. We believe that this component of the solution is not physical: it is not present in the solution if we assume lower iron abundance, or if there is significant resonance scattering in some of the stronger (mainly iron) spectral lines with subsequent photon absorption in the lower, dense atmosphere. Finally, the DEM analysis gives a best-fit value for the interstellar hydrogen column density of NH = (2.4±0.5) × 1018 cm-2. Title: Observations of Continuum and - Emission from Eta Carinae at a Wavelength of 3 CM Authors: Duncan, R. A.; White, S. M.; Lim, J.; Nelson, G. J.; Drake, S. A.; Kundu, M. R. Bibcode: 1995RMxAC...2...23D Altcode: No abstract at ADS Title: Composition of 5145 Pholus Authors: Cruikshank, D. P.; Roush, T.; Bartholomew, M. J.; Geballe, T. R.; Davies, J. K.; Brown, R. H.; White, S. M.; Tryka, K. A.; Owen, T. C.; de Bergh, C.; Moroz, L. V.; Pendleton, Y. J.; Bell, J. F., III Bibcode: 1995DPS....27.0106C Altcode: 1995BAAS...27.1056C No abstract at ADS Title: VLA Observations of the Effects of the SL-9 impact on the Synchrotron Emission from the Jovian Magnetosphere at 3 and 6 cms Authors: Kundu, A.; Wang, J. C.; Grossman, A. W.; White, S. M.; Muhleman, D. O.; Gurwell, M. A. Bibcode: 1995DPS....27.2615K Altcode: 1995BAAS...27.1129K No abstract at ADS Title: Circular Polarization in the Radio Emission of RS Canum Venaticorum Binaries Authors: White, Stephen M.; Franciosini, Elena Bibcode: 1995ApJ...444..342W Altcode: A puzzling feature of the polarization of the radio emission of RS CVn binary systems is the observed reversal in the sense of polarization between high and low frequencies. This reversal has been assumed to be a property of the quiescent emission of these systems, which is widely agreed to be gyrosynchrotron emission. However, the reversal has proved difficult to explain in terms of plausible gyrosynchrotron models, and we demonstrate that its location with respect to the spectral peak is not consistent with such models. Instead we propose that a form of coherent radio emission is common at low frequencies in these systems and present several examples in which such a component is clearly present. This coherent component is highly circularly polarized in the sense opposite to that of the higher frequency gyrosynchrotron emission. At low freqeuncies the gyrosynchrotron component is also present and may be stronger than the coherent component, but is probably weakly polarized so that the polarization of the integrated emission is dominated by the coherent component. Based on the sense of polarization of the coherent component, we suggest that it is probably a form of plasma emission. It can show both rapid variability as well as slower modulation similar to that of the gyrosynchrotron component and has a bandwidth of at least 80 MHz. We also point out that the observed fact that the degree of polarization of the quiescent emission at high frequencies tends to increase with frequency is also inconsistent with gyrosynchrotron models. Title: The Magnetic Field Over an Isolated Symmetric Sunspot Authors: White, S. M.; Kundu, M. R.; Zlotnik, E. Ya.; Zheleznyakov, V. V.; Nitta, N. Bibcode: 1995SPD....26..718W Altcode: 1995BAAS...27..970W No abstract at ADS Title: A Solar Radio Telescope for the Future: Science Summary from the SRT Workshop Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1995SPD....26..801G Altcode: 1995BAAS...27..971G No abstract at ADS Title: Intense Radio Outburst from the Supermassive Star eta Carinae Authors: Duncan, R. A.; White, S. M.; Lim, J.; Nelson, G. J.; Drake, S. A.; Kundu, M. R. Bibcode: 1995ApJ...441L..73D Altcode: On five occasions between 1992 June 29 and 1994 May 3, we have used the Australia Telescope Compact Array (ATCA) to image Eta Carinae at a wavelength of 3 cm and a resolution of 1 arcsec. These observations have revealed remarkable activity. Since 1992 June, the total flux density has increase from 0.8 to 2.2 Jy, and the original single compact source has grown to a complex of sources spread over an area of about 16 sq arcsec. Strong hydrogen recombination-line spectral emission has appeared at the site of the strongest of these new sources. This recombination emission has the largest spectral width ever observed from a star, +/- 250 km/s, and reveals gas with turbulent velocities as great as 250 km/s approaching us at an average velocity of about 200 km/s. We believe that this radio outburst has been caused by a more than threefold increase of ultraviolet luminosity, and consequent ionization of previously neutral gas clouds. Title: RE J1255+266: detection of an extremely bright EUV transient. Authors: Dahlem, M.; Kreysing, H. -C.; White, S. M.; Engels, D.; Condon, J. J.; Harmon, B. A.; Zhang, S. N.; Kouveliotou, C.; Paciesas, W. S.; Voges, W. Bibcode: 1995A&A...295L..13D Altcode: During a pointed ROSAT observation in the direction of the Coma cluster of galaxies an exceptionally bright EUV source, RE J1255+266, was detected serendipitously. The source is located close to the Galactic North pole, at b_II_~=89deg. Its observed EUV flux (62-110eV) at the time of the detection was of order 7x10-9ergs/s/cm2, making RE J1255+266 temporarily one of the brightest EUV sources on the sky. The EUV flare of RE J1255+266 has a light curve with a decay time of about 0.86days. With respect to earlier non-detections, the source brightened by a factor of ~7000. Such a behaviour has not been observed before. Thus, it is unclear what type of source RE J1255+266 might be. Up to now no positive identification with any known source could be obtained. Emission at the position of the source was previously only detected in the 1987 Green Bank radio continuum survey. Simultaneous observations with CGRO/BATSE resulted in non-detections of the source in the 8-50keV energy range. Title: Spatially resolved Millimeter (86 GHz), Microwave (17 GHz) and X-Ray Observations of the 17 August 94 flare Authors: Silva, A. V. R.; Lin, R. P.; de Pater, I.; White, S. M.; Kundu, M. R. Bibcode: 1995SPD....26..804S Altcode: 1995BAAS...27..972S No abstract at ADS Title: A Solar Radio Telescope for the Future: Strawman Concept from the SRT Workshop Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.; Klimchuk, J. A.; Petrosian, V.; White, S. M. Bibcode: 1995SPD....26..802H Altcode: 1995BAAS...27..971H No abstract at ADS Title: Characteristics of Two Simple Microwave Bursts Authors: Kundu, M. R.; White, S. M.; Nitta, N.; Shibasaki, K.; Enome, S. Bibcode: 1995LNP...444...75K Altcode: 1995cmer.conf...75K We present simultaneous microwave and X-ray data for two microwave bursts with simple impulsive time profiles. The 17 GHz images show compact sources, and in the one case for which we have simultaneous soft and hard X-ray images, they also show compact sources coincident with the radio source. One of the bursts is barely detected in soft X-rays, yet has a moderate 17 GHz flux,. Title: Coherent Radio Bursts from RS CVn Binaries Authors: Franciosini, Elena; White, Stephen M. Bibcode: 1995LNP...454...40F Altcode: 1995IAUCo.151...40F; 1995flfl.conf...40F No abstract at ADS Title: ASCA X-ray Spectra of Quiescent and Flaring Emission from AB Doradus Authors: White, S. M.; Pallavicini, R.; Lim, J. Bibcode: 1995LNP...454..168W Altcode: 1995IAUCo.151..168W; 1995flfl.conf..168W No abstract at ADS Title: Nobeyama Radio Observatory report, no. 361: The radio properties of solar active region soft x-ray transient brightenings Authors: White, S. M.; Kundu, M. R.; Shimizu, T.; Shibasaki, K.; Enome, S. Bibcode: 1995STIN...9529098W Altcode: We present the results of a search for radio emission from active-region transient brightenings identified in Yohkoh Soft X-ray Telescope observations of active region AR 7260. We present detailed observations of four events in which 17 GHz radio emission is clearly detected in observations by the Nobeyama Radioheliograph. The time profiles of the 17 GHz data are very similar to those of the soft X-ray fluxes, and the 17 GHz flux is very close to that expected from plasma with the temperature and emission measure derived for the soft X-ray-emitting material from filter ratios. No impulsive nonthermal radio emission was detected from any of the 4 events, although each was at least GOES class B1 in soft X-rays. Weak hard X-rays may have been detected by GRO/BATSE from the strongest of the events, but not from two others. These negative results leave open the possibility that there is a difference between active region transient brightenings and solar flares, in that the former do not convert a significant amount of the released energy into accelerated electrons. However, confirmation of this hypothesis will require a larger sample of events. Title: Polarization Features of Solar Radio Emission and Possible Existence of Current Sheets in Active Regions Authors: Gopalswamy, N.; Zheleznyakov, V. V.; White, S. M.; Kundu, M. R. Bibcode: 1994SoPh..155..339G Altcode: We show that it is possible to account for the polarization features of solar radio emission provided the linear mode coupling theory is properly applied and the presence of current sheets in the corona is taken into account. We present a schematic model, including a current sheet that can explain the polarization features of both the low frequency slowly varying component and the bipolar noise storm radiation; the two radiations face similar propagation conditions through a current sheet and hence display similar polarization behavior. We discuss the applications of the linear mode coupling theory to the following types of solar radio emission: the slowly varying component, the microwave radio bursts, metric type U bursts, and bipolar noise storms. Title: Energetic electron populations in solar flares Authors: White, Stephen M. Bibcode: 1994AIPC..294..199W Altcode: 1994hesp.conf..199W Millimeter-interferometer observations of flares are used to study the MeV-energy electrons accelerated in solar flares. The focus of this study is a remarkable similarity found in the time profiles of emission associated with the impulsive onset of a flare. In a large fraction of flares, the impulsive phase emission at millimeter wavelengths consists of a rapid rise (~5 seconds) linear in time to a sharp peak, followed by an exponential decay with a decay constant of order 15 seconds. The onset of millimeter emission may be delayed by several seconds with respect to the onset of hard X-rays. The implications of this homologous property are discussed briefly. Title: Metric Type III Bursts from Flaring X-ray Bright Points Authors: Kundu, M. R.; Strong, K. T.; Pick, M.; Harvey, K. T.; Kane, S. R.; White, S. M.; Hudson, H. S. Bibcode: 1994kofu.symp..343K Altcode: X-ray bright points (XBP's) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP ``flares'' and normal solar flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by searching for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh/SXT data. Unequivocal evidence for type-III-like radio bursts, usually attributed to beams of nonthermal electrons on open field lines, is found. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles. Title: Directivity of the Radio Emission from the K1 Dwarf Star AB Doradus Authors: Lim, Jeremy; White, Stephen M.; Nelson, Graam J.; Benz, Arnold O. Bibcode: 1994ApJ...430..332L Altcode: We present measurements of the spectrum and polarization of the flaring radio emission from the K1 dwarf star AB Doradus, together with previously reported single frequency measurements (with no polarization information) on 3 other days. On all 4 days spanning a 6 month period, the emission was strong and, when folded with the stellar rotation period, showed similar time variations with two prominant peaks at phase 0.35 and 0.75. These peaks coincide in longitude with two large starspots identified from the stellar optical light curve and have half-powe widths as small as 0.1 rotations and no larger than 0.2 rotations. The modulated emission shows no measurable circular polarization, and its two peaks have different turnover frequencies. Title: Multiwavelength Observations of a Solar Flare Authors: White, S. M.; Silva, A.; de Pater, I.; Lin, R. P.; Gary, D. E.; Hudson, H. S.; Doyle, J. G.; Hagyard, M. J.; Kundu, M. R. Bibcode: 1994kofu.symp..203W Altcode: No abstract at ADS Title: The Radio Source around eta Carinae Authors: White, S. M.; Duncan, R. A.; Lim, J.; Nelson, G. J.; Drake, S. A.; Kundu, M. R. Bibcode: 1994ApJ...429..380W Altcode: We present high spatial resolution radio observations of the peculiar southern star Eta Carinae, made with the Australian Telescope. The images, at 8 and 9 GHz with a resolution of 1.0 arcsec show a source of dimension 10 arcsec and total flux of 0.7 Jy dominated by a strong central peak. The radio emission is unpolarized and offers no support to models which invoke degenerate stars or more exotic objects within the core of Eta Car. In these data we find no evidence for more than one energy source in the core with arcsecond separations as some infrared observations have suggested. Several levels of structure are evident in the radio image, which shows symmetry on the larger scales. Conventional formulae for stellar wind radio sources give a mass loss rate of order 3 x 10-4 Solar Mass/yr based on the radio flux in the central peak, which yields a wind momentum flux of order 20% of the momentum flux available from the star's radiation field. The radio emission at these frequencies is consistent with thermal emission from gas flowing away from a 'luminous blue variable' star (LBV) Eta Car is probably the brightest thermal stellar wind radio source in the sky. Title: Microwave Imaging of Jupiter's Troposphere During Impact with Comet P/Shoemaker-Levy 9 Authors: Grossman, A. W.; White, S. M.; Muhleman, D. O.; Gurwell, M. A. Bibcode: 1994DPS....26.0314G Altcode: 1994BAAS...26.1587G No abstract at ADS Title: Nonthermal Processes in Flaring X-Ray--bright Points Authors: Kundu, M. R.; Strong, K. T.; Pick, M.; White, S. M.; Hudson, H. S.; Harvey, K. L.; Kane, S. R. Bibcode: 1994ApJ...427L..59K Altcode: X-ray-bright point (XBPs) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP 'flares' and normal solar flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by searching for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh soft x-ray telescope (SXT) data. Unequivocal evidence for type III-like radio bursts, usually attributed to beams of nonthermal electrons on open field lines, is found. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles. Title: Millimeter, Microwave, Hard X-Ray, and Soft X-Ray Observations of Energetic Electron Populations in Solar Flares Authors: Kundu, M. R.; White, S. M.; Gopalswamy, N.; Lim, J. Bibcode: 1994ApJS...90..599K Altcode: 1994IAUCo.142..599K We present comparisons of multiwavelength data for a number of solar flares observed during the major campaign of 1991 June. The different wavelengths are diagnostics of energetic electrons in different energy ranges: soft X-rays are produced by electrons with energies typically below 10 keV, hard X-rays by electrons with energies in the range 10-200 keV, microwaves by electrons in the range 100 keV-1 MeV, and millimeter-wavelength emission by electrons with energies of 0.5 MeV and above. The flares in the 1991 June active period were remarkable in two ways: all have very high turnover frequencies in their microwave spectra, and very soft hard X-ray spectra. The sensitivity of the microwave and millimeter data permit us to study the more energetic (greater than 0.3 MeV) electrons even in small flares, where their high-energy bremsstrahlung is too weak for present detectors. The millimeter data show delays in the onset of emission with respect to the emissions associated with lower energy electrons and differences in time profiles, energy spectral indices incompatible with those implied by the hard X-ray data, and a range of variability of the peak flux in the impulsive phase when compared with the peak hard X-ray flux which is two orders of magnitude larger than the corresponding variability in the peak microwave flux. All these results suggest that the hard X-ray-emitting electrons and those at higher energies which produce millimeter emission must be regarded as separate populations. This has implications for the well-known 'number problem' found previously when comparing the numbers of non thermal electrons required to produce the hard X-ray and radio emissions. Title: Radio Constraints on Coronal Models for dMe Stars Authors: White, S. M.; Lim, J.; Kundu, M. R. Bibcode: 1994ApJ...422..293W Altcode: Radio data are used to test coronal models for dMe stars. Specifically, we show that photospheric magnetic field observations imply that the low corona of a dMe star should be saturated by magnetic fields with an average strength in excess of 1 kG. In such fields the hot component of the corona detected in X-ray observations (temperature of order 2 x 107 K) would be optically thick at least up to 15 GHz due to thermal gyroresonance opacity. The resulting emission would easily be detectable by radio observations and should have a radio spectrum rising in the microwave range. We have carried out observations to test this prediction, and in the majority of cases find that the observed fluxes at 15 GHz are too low to be consistent with the assumptions. In the few cases where the stars were detected at 15 GHz, the evidence indicates that the observed emission is nonthermal. These results imply that the hot component of the X-ray-emitting plasma in the corona is not coincident with the strong magnetic fields in the lower corona. Because the hot plasma must still be confined by closed magnetic field lines, it is likely to be restricted to heights of the order of a stellar radius above the photosphere. The results seem to imply a different genesis for the two components of the X-ray-emitting corona of flare stars: the hot component may be cooling flare plasma, while the cooler component (temperature of order 3 x 106 K) is associated with a more conventional coronal heating mechanism. Title: Observations of the 1991 Eclipse at 3.5 MM Wavelength Authors: White, S. M.; Kundu, M. R. Bibcode: 1994IAUS..154..167W Altcode: No abstract at ADS Title: Interferometry of Solar Flares at 3-mm Wavelength Authors: Kundu, M. R.; White, S. M.; Gopalswamy, N.; Lim, J. Bibcode: 1994IAUS..154..131K Altcode: No abstract at ADS Title: The Radio Spectra of Weak-Lined T Tauri Stars Authors: White, S. M.; Pallavicini, R.; Lim, J. Bibcode: 1994ASPC...64..501W Altcode: 1994csss....8..501W No abstract at ADS Title: A Search for Radio Emission in the Alpha Persei Cluster. Authors: White, S. M.; Prosser, C. F.; Schmitt, J. H. M. M. Bibcode: 1994ASPC...64..504W Altcode: 1994csss....8..504W No abstract at ADS Title: The Coronae of DM Stars Authors: White, Stephen M. Bibcode: 1994euve.prop...80W Altcode: Optical spectroscopy recently revealed that one of the brightest unidentified sources in the EUVE catalogs, EUVEJ2056-17.1, is an active dM0e flare star located at a distance of ~50 pc. A large flare with energy in excess of 10E35 erg was observed during the EUVE observations in the Deep Survey Lex/B band (60-200 A). This is the most energetic flare event observed on a star of this spectral class. EUVEJ2056-17.1 has a strong Li I 6707.8 line in the spectrum. The unsually high abundance of Li and the high activity of this source favors an interpretation of Li production by spallation reactions during very energetic flares. We request 150 ksec of EUVE time on this source. The requested observing time will provide valuable insights on the energy budget of flares from low mass stars and will allow us to investigate further the Li production by spallation. Title: A Search for gamma-Ray Emission from Active Stars Authors: White, S. M.; Harmon, B. A.; Lim, J.; Kundu, M. R. Bibcode: 1994ASPC...64..498W Altcode: 1994csss....8..498W No abstract at ADS Title: The Radio Continuum from Classical T Tauri Stars Authors: White, S. M.; Mundy, L. G.; Grossman, A. W. Bibcode: 1993AAS...183.3204W Altcode: 1993BAAS...25.1341W We have carried out a survey of radio continuum emission from the classical T Tauri stars in the Taurus and Ophiuchus star-forming clouds not previously observed with the Very Large Array. The detection rate is surprisingly low. We combine our results with those of other surveys in order to analyze the correlation of radio continuum luminosity with other properties of T Tauri stars, and discuss the nature of the radio continuum and its relationship to the winds of these stars. Title: Interferometric observations of solar flares at 3 mm wavelength Authors: Kundu, M. R.; White, S. M.; Gopalswamy, N.; Lim, J. Bibcode: 1993AdSpR..13i.289K Altcode: 1993AdSpR..13..289K We report on the observations of a number of flares at a wavelength of 3.5 mm during the 1991 June solar campaign. Many flares, including small ones, show an impulsive phase at milllimeter wavelengths which indicates the presence of MeV electrons, and the millimeter observations are far more sensitive to such electrons than are current γ-ray detectors. However, these energetic electrons do not always show a good correlation with the lower-energy electrons which produce hard X-rays below 100 keV. The production efficiency of MeV electrons seems to vary considerably from flare to flare. An extended phase similar to the soft X-ray behaviour is also seen at millimeter wavelengths, which we attribute to dense hot material radiating thermal bremsstrahlung. In the impulsive onset the millimeter emission seems to be consistently delayed with respect to the hard X-rays. Title: Simultaneous Observations of Solar Plage with the Solar Extreme Ultraviolet Rocket Telescope and Spectrograph (SERTS), the VLA, and the Kitt Peak Magnetograph Authors: Brosius, Jeffrey W.; Davila, Joseph M.; Thompson, William T.; Thomas, Roger J.; Holman, Gordon D.; Gopalswamy, N.; White, Stephen M.; Kundu, Mukul R.; Jones, Harrison P. Bibcode: 1993ApJ...411..410B Altcode: We obtained simultaneous images of solar plage on 1991, May 7 with SERTS, the VLA,4 and the NASA/National Solar Observatory spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Using intensity ratios of Fe XVI to Fe XV emission lines, we find that the coronal plasma temperature is (2.3-2.9) x 10 exp 6 K throughout the region. The column emission measure ranges from 2.5 x 10 exp 27 to l.3 x 10 exp 28 cm exp -5. The calculated structure and intensity of the 20 cm wavelength thermal bremsstrahlung emission from the hot plasma observed by SERTS is quite similar to the observed structure and intensity of the 20 cm microwave emission observed by the VLA. Using the Meyer (1991, 1992) revised coronal iron abundance, we find no evidence either for cool absorbing plasma or for contributions from thermal gyroemission. Using the observed microwave polarization and the SERTS plasma parameters, we calculate a map of the coronal longitudinal magnetic field. The resulting values, about 30-60 G, are comparable to extrapolated values of the potential field at heights of 5000 and 10,000 km. Title: Meter-Wave Radio Emission from Flaring X-ray Bright Points Authors: Kundu, M. R.; Strong, K. T.; Pick, M.; Kane, S. R.; Harvey, K.; White, S. M. Bibcode: 1993BAAS...25.1180K Altcode: No abstract at ADS Title: A Study of the Solar Active Regions Using Simultaneous VLA and Yohkoh Soft X-ray Imaging: CoMStOC `92 Authors: Gopalswamy, N.; White, S. M.; Kundu, M. R.; Lemen, J. R.; Strong, K. T.; Schmelz, J. T. Bibcode: 1993BAAS...25R1213G Altcode: No abstract at ADS Title: Hard X-ray and Radio Spectra for Solar Flares from AR 6659 Authors: White, S. M.; Murphy, R.; Schwartz, R. A.; Kundu, M. R.; Gopalswamy, N.; Lim, J. Bibcode: 1993BAAS...25Q1222W Altcode: No abstract at ADS Title: Millimeter, Microwave and X-Ray Morphology and Spectra of the 07Jan92 Flare Authors: Silva, A. V.; Lin, R. P.; de Pater, I.; White, S. M.; Kundu, M. R.; Gary, D. E.; Hudson, H. S. Bibcode: 1993BAAS...25Q1223S Altcode: No abstract at ADS Title: Solar Coronal Plasma and Magnetic Field Diagnostics Using SERTS and Coordinated VLA Observations Authors: Brosius, J. W.; Davila, J. M.; Thompson, W. T.; Thomas, R. J.; Holman, G. D.; Gopalswamy, N.; White, S. M.; Kundu, M. R.; Jones, H. P. Bibcode: 1993BAAS...25.1224B Altcode: No abstract at ADS Title: A Multiwavelength Portrait of a Solar Active Region Authors: White, S. M.; Kundu, M. R.; Gopalswamy, N. Bibcode: 1993BAAS...25.1183W Altcode: No abstract at ADS Title: The Radio Source Around eta Carinae Authors: White, S. M.; Duncan, R. A.; Lim, J.; Nelson, G. J.; Drake, S. A.; Kundu, M. R. Bibcode: 1993AAS...182.3906W Altcode: 1993BAAS...25..858W The first high--spatial--resolution (1{('') }) radio image of the source surrounding the famous massive southern star eta Carinae has been obtained with the Australia Telescope. The image shows a number of interesting features: a strong central peak; ridges of emission close to the peak and extending away from it in the directions of putative ``jets'' seen in the HST image of the region, and also other ridges in the directions of the lobes of the Homunculus; a box--like extended feature of dimension 7{('') } \ times 5{('') }, with its major axis orthogonal to the major axis of the Homunculus; and two fainter lobes extending to 5{('') } \ from the star in the directions of both lobes of the Homunculus. The radio image bears a strong resemblance to the high--resolution infra--red images of the region around the star. No radio emission associated with the more extended X-ray--emitting nebula is detected. The current rate of mass loss from the star is estimated, and physical conditions within the nebula are discussed. Title: A 1.5 GHz Radio Survey of the Hyades Open Stellar Cluster Authors: White, S. M.; Jackson, P. D.; Kundu, M. R. Bibcode: 1993AJ....105..563W Altcode: Results of a radio survey of the Hyades open stellar cluster carried out with the VLA at 1.5 GHz are reported. Seventeen fields containing over 150 cataloged stars were mapped down to a limiting sensitivity ranging from 0.3 mJy at the centers of the fields to 0.9 mJy at a distance of 20 arcmin from field centers. Two stars were detected as radio sources: the evolved spectroscopic binary V471 Tau, consisting of a white dwarf and a red dwarf; and the apparently premain-sequence G+K star spectroscopic binary HD 27130. The failure to detect any single stars as radio sources is generally consistent with the age-rotation-activity paradigm, according to which stellar activity is due to magnetic fields produced by dynamo action in rapidly rotating stars and should decrease with age as a star spins down due to magnetic braking. It is concluded that the Hyades M dwarf population is not more active at radio wavelengths than the nearby flare star population, or else the number of flare stars in the Hyades is much less than presently assumed. Title: The Coronae of dM Stars Authors: White, Stephen M. Bibcode: 1993euve.prop...93W Altcode: We propose to use the excellent discrimination of EUVE in the temperature range around 10^(6.7-7.0) K to study the temperature/emission measure distribution in the coronae of the inactive dM2 star GJ 411 and the apparently weakly active dM2e star BD -21 1074. The aims are to determine the emission measure distribution in M dwarf coronae over a wide range in activity levels, to test whether there is a component in the corona of GJ 411 at 10^6.8 K as reported, and thereby indirectly to test an explanation for the presence of the hot 10^7.2 K) component in the coronae of active flare stars as due to stability points in the radiative cooling curve. We request 160 ksec for GJ 411 and 60 ksec for BD -21 1074. Title: 3D reconstruction methods of coronal structures by radio observations. Authors: Aschwanden, M. J.; White, S. M.; Bastian, T. S. Bibcode: 1992ESASP.348..217A Altcode: 1992cscl.work..217A The ability to carry out a three-dimensional reconstruction of structures in the solar corona would represent a major advance in our study of the physical properties in active regions and in flares. The authors describe several new methods which allow a geometric reconstruction of quasi-stationary coronal structures (e.g. active region loops) or dynamic structures (e.g. flaring loops): (1) steroscopy of multi-day imaging observations by the VLA. (2) Tomography of optically thin emission (in radio or soft X-rays). (3) Multi-frequency band imaging by the VLA. (4) Tracing of magnetic field lines by propagating electron beams. Title: Solar Observations with a Millimeter Wavelength Array Authors: White, S. M.; Kundu, M. R. Bibcode: 1992SoPh..141..347W Altcode: Rapid developments in the techniques of interferometry at millimeter wavelengths now permit the use of telescope arrays similar to the Very Large Array at microwave wavelengths. These new arrays represent improvements of orders of magnitude in the spatial resolution and sensitivity of millimeter observations of the Sun, and will allow us to map the solar chromosphere at high spatial resolution and to study solar radio burst sources at millimeter wavelengths with high spatial and temporal resolution. Here we discuss the emission mechanisms at millimeter wavelengths and the phenomena which we expect will be the focus of such studies. We show that the flare observations study the most energetic electrons produced in solar flares, and can be used to constrain models for electron acceleration. We discuss the advantages and disadvantages of millimeter interferometry, and in particular focus on the use of and techniques for arrays of small numbers of telescopes. Title: The High-Frequency Characteristics of Solar Radio Bursts Authors: Lim, J.; White, S. M.; Kundu, M. R.; Gary, D. E. Bibcode: 1992SoPh..140..343L Altcode: We compare the millimeter, microwave, and soft X-ray emission from a number of solar flares in order to determine the properties of the high-frequency radio emission of flares. The millimeter observations use a sensitive interferometer at 86 GHz which offers much better sensitivity and spatial resolution than most previous high-frequency observations. We find a number of important results for these flares: (i) the 86 GHz emission onset appears often to be delayed with respect to the microwave onset; (ii) even in large flares the millimeter-wavelength emission can arise in sources of only a few arc sec dimension; (iii) the millimeter emission in the impulsive phase does not correlate with the soft X-ray emission, and thus is unlikely to contain any significant thermal bremsstrahlung component; and (iv) the electron energy distributions implied by the millimeter observations are much flatter (spectral indices of 2.5 to 3.6) than is usual for microwave or hard X-ray observations. Title: Radio flares and magnetic fields on weak-line T Tauri stars. Authors: White, S. M.; Pallavicini, R.; Kundu, M. R. Bibcode: 1992A&A...259..149W Altcode: We report the first detection of circular polarization in the radio emission of two weak-line T Tauri stars. This is direct confirmation of the presence of magnetic fields in the coronae of these stars. The degree of polarization at 5 GHz is small, consistent with previous observations which did not find measurable polarization. We have also observed a radio flare on one of the two stars. The rising spectrum of the radio emission together with the low degree of polarization are strong evidence that radio outbursts on this class of stars are due to nonthermal gyrosynchrotron emission which is optically thick in the microwave range. One of the two stars shows no evidence for circumstellar material, and thus can be classified as 'naked'. However the other apparently has a dust disk, and the evidence of flaring on this star indicates that the mechanism involved does not require an empty circumstellar environment. Title: Analysis of EUV, Microwave, and Magnetic Field Observations of a Solar Active Region Authors: Brosius, J. W.; Davila, J. M.; Jones, H. P.; Thompson, W. T.; White, S. M.; Gopalswamy, N.; Kundu, M. R. Bibcode: 1992AAS...180.4002B Altcode: 1992BAAS...24R.792B No abstract at ADS Title: Simultaneous Hard X-ray, Soft X-ray, Millimeter and Microwave Observations of a Solar Flare Authors: White, S. M.; Kundu, M. R.; Lim, J.; Gopalswamy, N. Bibcode: 1992AAS...180.4504W Altcode: 1992BAAS...24..802W We present non-imaging data across a wide range of wavelengths for a solar flare which occurred on 1991 June 13. This flare is of interest because it shows a spike in hard X-rays at the beginning of the event which had a relatively hard X-ray spectrum, and was followed by a much softer impulsive phase. We present the BATSE and OSSE observations (from the Gamma Ray Observatory): the former have good time resolution, while the latter provide well-resolved spectral information. These are contrasted with the GOES soft-X-ray data on the hot thermal component in the corona, and radio observations up to 86 GHz which are sensitive to both the nonthermal and thermal components of the flare. The 86 GHz data from the BIMA millimeter interferometer show a spike in the impulsive phase coincident with the hard X-ray spike above 100 keV, as well as a long-duration thermal phase which appears to be consistent with an origin in the same material seen by GOES. We discuss the implications of the observations for particle acceleration in this flare. Title: Millimeter Observations of Solar Flares Authors: Silva, A. V.; Lin, R. P.; de Pater, I.; White, S. M.; Kundu, M. R. Bibcode: 1992AAS...180.4510S Altcode: 1992BAAS...24..803S No abstract at ADS Title: 2-cm Observations of Quiescent and Flaring Emission from a Solar Active Region Authors: Lim, J.; White, S. M.; Kundu, M. R. Bibcode: 1992AAS...180.1102L Altcode: 1992BAAS...24..746L We present 2-cm images of a solar active region comprising a large leading spot and several smaller trailing spots, observed with the VLA on 1989 Dec 14 and 16. On both days, slowly-varying quiescent emission and impulsive flaring emission were detected. Comparisons of the radio images with white-light pictures and magnetograms obtained on Dec 13 and 17 revealed that the quiescent emission originated almost entirely from above the penumbra of large spots, whereas the flaring emission originated from regions of complex magnetic field topology near but not necessarily above magnetic neutral lines. The most intense source of quiescent emission had a relatively low brightness-temperature ( ~ 10(5) K), was highly circularly polarized (ranging from about 30\ trailing edge (i.e., limbward side) of the large leading spot. We discuss models for this emission, constrained by the low brightness temperature but high polarization. The weaker flare of Dec 14 showed a single resolved source, and was weakly circularly-polarized. The stronger flare of Dec 16 comprised a strong source with multiple peaks and several weaker compact sources, all with low degrees of circular polarization. Snapshot images of this flare showed an elongated feature moving rapidly away from the main region of emission and joining with the compact sources. We discuss likely mechanisms for this feature, and emission process(es) responsible for the flares. Title: The Structure of the Corona of dMe Flare Stars Authors: White, S. M.; Lim, J.; Kundu, M. R. Bibcode: 1992AAS...180.6005W Altcode: 1992BAAS...24..826W There is now considerable information on both the surface magnetic fields of dMe flare stars and on the thermal (X-ray emitting) populations in their coronae. We show that the simplest picture of the corona of a dMe flare star based on the measured photospheric magnetic field strengths, the measured filling factors of these fields, and the X-ray emission from their coronae, are inconsistent with radio observations of the coronae. We discuss possible resolutions of this dilemma. Title: A 5GHz radio survey of selected POST T Tauri and naked T Tauri stars. Authors: White, S. M.; Pallavicini, R.; Kundu, M. R. Bibcode: 1992A&A...257..557W Altcode: Results of a radio survey of weak-lined T Tauri (variably classified as either post T Tauri or naked T Tauri) stars at 5 GHz are reported. Thirty-two targets, chosen on the basis of known high-activity levels or youth indications, were observed, and 15, including eight previously unknown radio sources, were detected. It is suggested that most weak-lined T Tauris with high activity levels, e.g., as indicated by a high X-ray flux, are likely to be detectable radio sources at some time, but the radio emission goes through high and low phases. For the subsample of 14 stars observed in Taurus-Auriga which were thoroughly studied at IR, optical, and X-ray wavelengths, a clear association is found between radio activity and youth. Title: Observations on Mode Coupling in the Solar Corona and Bipolar Noise Storms Authors: White, S. M.; Thejappa, G.; Kundu, M. R. Bibcode: 1992SoPh..138..163W Altcode: We review high-spatial-resolution observations of the Sun which reflect on the role of mode coupling in the solar corona, and present a number of new observations. We show that typically polarization inversion is seen at 5 GHz in active region sources near the solar limb, but not at 1.5 GHz. Although this is apparently in contradiction to the simplest form of mode coupling theory, in fact it remains consistent with current models for the active region emission. Microwave bursts show no strong evidence for polarization inversion. We discuss bipolar noise storm continuum emission in some detail, utilizing recent VLA observations at 327 MHz. We show that bipolar sources are common at 327 MHz. Further, the trailing component of the bipole is frequently stronger than the leading component, in apparent conflict with the `leading-spot' hypothesis. The observations indicate that at 327 MHz mode coupling is apparently strong at all mode-coupling layers in the solar corona. The 327 MHz observations require a much weaker magnetic field strength in the solar corona to explain this result than did earlier lower-frequency observations: maximum fields are 0.2 G. This is a much weaker field than is consistent with current coronal models. Title: High Dynamic Range Multifrequency Radio Observations of a Solar Active Region Authors: White, S. M.; Kundu, M. R.; Gopalswamy, N. Bibcode: 1992ApJS...78..599W Altcode: High-dynamic-range multifrequency radio observations of a solar active region are presented. The evolution of the region is followed at 5 GHz as it rotates from the limb to disk center, and when it is at disk center, observations at 0.33, 1.5, 5, 8.4, and 15 GHz are used to analyze the distribution of density and magnetic field within the active region. A dynamic range of up to 1500 (at 8.4 GHz) was achieved because these data were well suited to the self-calibration technique. The signatures of both optically thick gyroresonance emission are unambiguously identified, and magnetic fields and optically thin thermal free-free emission are outlined. Images are compared at 5 and 8.4 GHz in order to identify regions in the trailing part of the active region where optically thin four-harmonic gyroresonance emission is contributing to the observed brightness temperatures at 5 GHz, indicating the presence of 450 G fields. Title: Millimeter and hard X-ray/γ-ray observations of solar flares during the June 91 GRO campaign. Authors: Kundu, M. R.; White, S. M.; Gopalswamy, N.; Lim, J. Bibcode: 1992NASCP3137..502K Altcode: 1992como.work..502K We have carried out high-spatial-resolution millimeter observations of solar flares using the Berkeley-Illinois-Maryland Array (BIMA). At the present time, BIMA consists of only three elements, which is not adequate for mapping highly variable solar phenomena, but is excellent for studies of the temporal structure of flares at millimeter wavelengths at several different spatial scales. We present BIMA observations made during the Gamma Ray Observatories (GRO)/Solar Max 1991 campaign in Jun. 1991 when solar activity was unusually high. Our observations covered the period 8-9 Jun. 1991; this period overlapped the period 4-15 Jun. when the Compton Telescope made the Sun a target of opportunity because of the high level of solar activity. Title: A 5 GHz Survey of Weak-Lined T Tauri Stars Authors: White, S. M.; Pallavicini, R.; Kundu, M. R. Bibcode: 1992ASPC...26..334W Altcode: 1992csss....7..334W No abstract at ADS Title: Multifrequency Observations of a Remarkable Solar Radio Burst Authors: White, S. M.; Kundu, M. R.; Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Kucera, T.; Bieging, J. H. Bibcode: 1992ApJ...384..656W Altcode: Observations of an impulsive solar-radio burst from three observatories are presented. The striking observational aspects of this flare are that the time profile was identical throughout at 8.6, 15, and 86 GHz, that the spectrum was apparently flat from 15 to 86 GHz, and that there was a sharp cutoff in the spectrum between 5.0 and 8.6 GHz. The simplest interpretation of the cutoff, namely as a plasma frequency effect, leads to the conclusion that there was exceptionally high-density material in the solar corona (of about 5 x 10 exp 11/cu cm). Very Large Array images at 15 GHz show a single-loop structure which brightened uniformly and showed little change in size during the whole impulsive phase. The flat spectrum is consistent with optically thin thermal bremsstrahlung emission, but the lack of observed soft X-ray emission and other properties of the flare cannot easily be accommodated by this mechanism. The possibility is explored that the emission is optically thick due to thermal absorption of nonthermal gyrosynchrotron emission, or optically thin gyrosynchrotron emission absorbed by high-density material intervening along the line of sight. Both of these explanations also face difficulties. Title: Radio observations of weak-lined T Tauri stars. Authors: White, S. M.; Pallavicini, R.; Kundu, M. R. Bibcode: 1992MmSAI..63..751W Altcode: We report the results of a search for radio-continuum emission from weak-lined T Tauri stars selected on the basis of a range of criteria. A correlation is found with strong X-ray emission and with youth. All the stars in the survey older than about 20 million years were not detected as radio sources. A flare was seen on one of the survey targets, with a rise time of several hours. Circular polarization was also seen in two of the targets, providing the first direct confirmation of the presence of magnetic fields on these stars. Title: Radio observations of solar and stellar coronae Authors: Kundu, M. R.; White, S. M. Bibcode: 1992MmSAI..63..715K Altcode: Recent results of radio observations of the solar and stellar coronae are reviewed. Attention is given to the results obtained on quiet-sun fine structures; the active region observations, with particular consideration given to the soft X-ray and radio comparisons of observations made during the Coronal Magnetic Structures Observing Campaign and the strength of the magnetic fields in the corona; the radio observations of solar flares; and the radio observations of stellar coronae of the RS Canum Venaticorum binaries, M dwarf stars, pre-main-sequence stars, and chemically-peculiar B stars. Also discussed are current issues in stellar radiophysics and perspectives in solar radio physics. Title: HST Observations of the Flare Star AU MIC Authors: Robinson, R. D.; Shore, S. N.; Carpenter, K. G.; Woodgate, B. E.; Maran, S. P.; Brandt, J. C.; Kundu, M. R.; White, S. M.; Linsky, J. L.; Walter, F. M. Bibcode: 1992ASPC...26...31R Altcode: 1992csss....7...31R No abstract at ADS Title: Large-Scale Features of the Sun at 20 Centimeter Wavelength Authors: Gopalswamy, N.; White, S. M.; Kundu, M. R. Bibcode: 1991ApJ...379..366G Altcode: Results are reported from an experimental study of the characteristics of large-scale coronal structures such as active regions, plages, filaments, and coronal holes using data obtained with the VLA at 1.5 GHz during the period September 11-17, 1988. The radio data were supplemented with He 10830- A, H-alpha, and Calcium-K spectroheliograms. A statistical analysis of some of the characteristics of the active regions is performed. Most of the active region sources were found to be about 100 arcsec in size, with bridges between regions common; lower brightness temperature regions showed a higher degree of polarization in general. The maximum polarization was found at the edge of active regions but well within the associated plages. The degree of polarization from bright active regions was small (not more than 20 percent), in agreement with previous results. Evidence was found for compression of preexisting flux by the emerging flux from a new region, which took place in the apparent absence of magnetic reconnection. Title: Simultaneous EUV, Microwave, and Magnetic Field Observations of Solar Active Regions Authors: Brosius, J. W.; Davila, J. M.; Thompson, W. T.; Gopalswamy, N.; White, S. M.; Jones, H. P.; Metcalf, T. R. Bibcode: 1991BAAS...23.1388B Altcode: No abstract at ADS Title: VLA Observations of Interacting Flaring Loops Authors: Kundu, M. R.; White, S. M.; McConnell, D. M. Bibcode: 1991SoPh..134..315K Altcode: We present 4.9 GHz observations of an impulsive radio burst observed at the Very Large Array on 1981 May 16. The flare occurred in a complex active region containing several spots. The radio burst lay at the edge of an active-region microwave source, close to a neutral line. The compact burst showed morphological evidence for the presence of two loops in the rise phase, with the subsequent burst peak lying between these loops. This suggests that interaction between the loops played some role in the initiation of the flare. The flare spectrum is consistent with thermal gyrosynchrotron emission. The main microwave peak was displaced from the nearest Hα kernels by about 10″, but there is strong evidence for post-flare loops coincident with the Hα kernels during the later stages of the event. Title: Coronal Magnetic Structures Observing Campaign. I. Simultaneous Microwave and Soft X-Ray Observations of Active Regions at the Solar Limb Authors: Nitta, N.; White, S. M.; Kundu, M. R.; Gopalswamy, N.; Holman, G. D.; Brosius, J. W.; Schmelz, J. T.; Saba, J. L. R.; Strong, K. T. Bibcode: 1991ApJ...374..374N Altcode: Using simultaneous microwave and soft X-ray measurements made with the Very Large Array (VLA) at 6 and 20 cm and the X-ray Polychromator (XRP) aboard the Solar Maximum Mission (SMM), we have studied two active regions near the solar limb. These observations were taken as part of the Coronal Magnetic Structures Observing Campaign (CoMStOC), a collaboration designed to study the magnetic field in the solar corona. The images in soft X-rays and at 20 cm wavelength are similar: both show peaks above the active regions and extended bridge of emission 200,000 km long connecting the two regions. The brightness temperature of the 20 cm emission is lower than that predicted from the X-ray emitting material, however; it can be attributed to free-free emission in cooler (<106 K) plasma not visible to XRP, with an optical depth ∼1. The 6 cm emission is concentrated at lower altitudes and in a ∼160,000 km long bundle of loops in the northern active region. Comparison of the 6 cm map with the potential magnetic field lines computed from photospheric magnetic fields (measured 2 days earlier) indicates that the 6 cm emission is associated with fields of less than ∼200 G. Such fields would be too weak to attribute the observed 6 cm emission to gyroresonance radiation. Analysis of the 6 cm loop bundle indicates that it is strongly asymmetric, with the magnetic field in the northern leg ∼2 times stronger than in the southern leg; the 6 cm emission most likely arises from a combination of hot ( ≥ 2 × 106 K) and cool plasmas, while the 20 cm emission becomes optically thick in the cooler (∼9 × 103 K) plasma. We estimate an Alfvén speed ∼7000 km s-1 and ratio of electron gyrofrequency to plasma frequency ∼1.0 in the northern leg of the 6 cm loop. Title: A Search for Solar Oscillations in the Solar Chromosphere Using Millimeter Interferometric Observations Authors: Kundu, M. R.; White, S. M. Bibcode: 1991BAAS...23R1033K Altcode: No abstract at ADS Title: On the Reconciliation of Simultaneous Microwave Imaging and Hard X-Ray Observations of a Solar Flare Authors: Nitta, N.; White, S. M.; Schmahl, E. J.; Kundu, M. R. Bibcode: 1991SoPh..132..125N Altcode: We have compared microwave imaging data for a small flare with simultaneous hard X-ray spectral observations. The X-ray data suggest that the power-law index δ of the energy distribution of the radiating electrons is 5.3 (thick-target) which differs significantly from the estimate (δ = 1.4) from a homogeneous optically-thin gyrosynchrotron model which fits the radio observations well. In order to reconcile these results, we explore a number of options. We investigate a double power-law energy spectrum for the energetic electrons in the flare, as assumed by other authors: the power law is steep at low energies and much flatter at the higher energies which produce the bulk of the microwaves. The deduced break energy is about 230 keV if we tentatively ignore the X-ray emission from the radio-emitting electrons: however, the emission of soft photons by the flat tail strongly contributes to the observed hard X-ray range and would flatten the spectrum there. A thin-target model for the X-ray emission is also inconsistent with radio data. An inhomogeneous gyrosynchrotron model with a number of free parameters and containing an electron distribution given by the thick-target X-ray model could be made to fit the radio data. Title: VLA Observations of Radio Filaments Authors: Gopalswamy, N.; White, S. M.; Kundu, M. R. Bibcode: 1991BAAS...23.1045G Altcode: No abstract at ADS Title: Multifrequency Observations of a Remarkable Solar Radio Burst Authors: White, S. M.; Kundu, M. R.; Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Kucera, T.; Bieging, J. H. Bibcode: 1991BAAS...23.1043W Altcode: No abstract at ADS Title: Millimeter Imaging of Energetic Electrons in Solar Flares in Conjunction with GRO Experiments Authors: Kundu, M. R.; White, S. M. Bibcode: 1991BAAS...23Q1073K Altcode: No abstract at ADS Title: Millimeter Interferometric Observations of Solar Flares During the SOLAR-A Mission Authors: Kundu, M. R.; White, S. M. Bibcode: 1991LNP...387..338K Altcode: 1991fpsa.conf..338K We present the results of the first high-spatial-resolution interferometric observations of solar flares at millimeter wavelengths, carried out with the Berkeley-Illinois-Maryland Array (BIMA). The observations represent an improvement of an order of magnitude in both sensitivity and spatial resolution compared with previous solar observations at these wavelengths. Most of the flares occurring within the field of view during the observations have been detected by BIMA, including both very impulsive and longer-duration events. It appears that millimeter burst sources are not much smaller than microwave sources. If the emission in the flash phase is predominantly due to gyrosynchrotron emission, we can rule out thermal gyrosynchrotron models for the radio emission because the flux at millimeter wavelengths is too high. During the Solar-A mission we plan to obtain both imaging data as well as dedicated patrol observations of flare time profiles at millimeter wavelengths. Title: Strong Magnetic Fields and Inhomogeneity in the Solar Corona Authors: White, S. M.; Kundu, M. R.; Gopalswamy, N. Bibcode: 1991ApJ...366L..43W Altcode: It is shown that fields of 1800 G can exist in the corona based on observations of gyroresonance emission at 15 GHz at coronal temperatures. The strong fields occur in a small source radiating in the extraordinary (x) mode over the penumbra of a large symmetric sunspot. The optically-thin ordinary mode emission from the region shows a nearby peak at only 36,000 K which may be due to a sunspot plume, and a hole over the umbra consistent with the expected low-density material there. The x-mode source is highly asymmetric, despite the apparent symmetry of the sunspot, and its appearance and location imply that the strongest magnetic fields in the corona are localized in a compact flux tube. Title: High spatial resolution observations of solar flares at 3.3 mm wavelength Authors: Kundu, M. R.; White, S. M.; Welch, W. J.; Bieging, J. H. Bibcode: 1991AdSpR..11e..91K Altcode: 1991AdSpR..11...91K We present the first high-spatial-resolution interferometric observations of solar flares at millimeter wavelengths, carried out with the Berkeley-Illinois-Maryland Array (BIMA). The observations represent an improvement of an order of magnitude in both sensitivity and spatial resolution compared with previous solar observations at these wavelengths. Most of the flares occurring within the field of view during the observations were detected by BIMA, including both very impulsive and longer-duration events. It appears that millimeter burst sources are not much smaller than microwave sources. If the emission in the flash phase is predominantly due to gyrosynchrotron emission, we can rule out thermal gyrosynchrotron models for the radio emission because the flux at millimeter wavelengths is too high. During the Flares 22 campaign we will collect both imaging data as well as dedicated patrol observations of flare time profiles at millimeter wavelengths. Title: Millimeter Solar Observing with BIMA Authors: Kundu, M. R.; White, S. M.; Welch, W. J. Bibcode: 1991max..conf...11K Altcode: No abstract at ADS Title: First High Spatial Resolution Interferometric Observations of Solar Flares at Millimeter Wavelengths Authors: Kundu, M. R.; White, S. M.; Gopalswamy, N.; Bieging, J. H.; Hurford, G. J. Bibcode: 1990ApJ...358L..69K Altcode: The first high spatial resolution interferometric observations of solar flares at millimeter wavelengths, carried out with the Berkeley-Illinois-Maryland Array are presented. The observations were made at 3.3 mm wavelength during the very active periods of March 1989, using one or three baselines with fringe spacings of 2-5 arcsec. The observations represent an improvement of an order of magnitude in both sensitivity and spatial resolution compared with previous solar observations at these wavelengths. It appears that millimeter burst sources are not much smaller than microwave sources. The most intense bursts imply brightness temperatures of over 10 to the 6th K and are due to nonthermal gyrosynchrotron emission or possibly thermal free-free emission. If the emission in the flash phase is predominantly due to gyrosynchrotron emission, thermal gyrosynchrotron models can be ruled out for the radio emission because the flux at millimeter wavelengths is too high. Title: CoMStOCI: Physical Properties of an Active Region Loop Observed at the Solar Limb Authors: Holman, G. D.; Brosius, J. W.; Nitta, N.; White, S. M.; Kundu, M. R.; Gopalswamy, N.; Schmelz, J. T.; Saba, J. L. R.; Strong, K. T. Bibcode: 1990BAAS...22..899H Altcode: No abstract at ADS Title: Evolution of Active Regions at 20 cm Authors: Gopalswamy, N.; White, S. M.; Kundu, M. R. Bibcode: 1990BAAS...22..795G Altcode: No abstract at ADS Title: High-Dynamic-Range Multifrequency Radio Observations of a Solar Active Region Authors: White, S. M.; Kundu, M. R.; Gopalswamy, N. Bibcode: 1990BAAS...22R.794W Altcode: No abstract at ADS Title: Millimeter-Interferometer Observations of Solar Flares Authors: Kundu, M. R.; White, S. M.; Gopalswamy, N.; Bieging, J. H.; Hurford, G. J. Bibcode: 1990BAAS...22..823K Altcode: No abstract at ADS Title: Magnetic field reconnection in solar and stellar flares Authors: Kundu, M. R.; White, S. M. Bibcode: 1990AdSpR..10i..85K Altcode: 1990AdSpR..10...85K Radio observations are sensitive to energetic electrons, and thus are an important tool for studying the properties of solar flares. High spatial resolution multifrequency observations at centimeter wavelengths can provide important information on the region of energy release during flares and, therefore, on their triggering mechanisms. The changes in polarization and its structure during the preflare and impulsive phase have led to inferences about magnetic field topology in the flaring region /1/. We have learnt that evolving magnetic field structures often trigger the flare /2/. In this paper, we discuss evidence for magnetic reconnection in both solar and stellar flares derived from radio observations. We present two cases of solar flares well-observed by the VLA which show interacting magnetic structures leading to a flare. Both cases are strongly suggestive of reconnection. In the second part of the paper we discuss more indirect evidence based on flares in the coronae of red dwarf stars. Title: Millimeter and Microwave Activity of the Sun Authors: Kundu, M. R.; White, S. M. Bibcode: 1990IAUS..142..457K Altcode: Preliminary results of high-spatial-resolution millimeter observations of recent solar flares carried out with the Berkeley-Illinois-Maryland Array are presented. Findings obtained from multifrequency observations using the VLA are reported. The time profiles of several bursts during March 1989 are presented. Most of the bursts were impulsive and short-lived, and a number of longer-duration bursts were observed as well. All the bursts were fairly weak, with none exceeding 1 sfu of correlated amplitude. A number had rise times less than the available resolution of 10 sec. VLA observations in September 1988 showed that there is exceptionally good correspondence between the 1.5-GHz radio sources and the regions which are dark in He 10830 images and bright in Ca images. Title: The Radio Spectra of Rs-Canum Stars Authors: White, S. M.; Kundu, M. R.; Uchida, Y.; Nitta, N. Bibcode: 1990ASPC....9..239W Altcode: 1990csss....6..239W The results of observations of several systems are presented for the 0.327-90 GHz range to study the radio coronae and outbursts associated with the RS CVn stars, as well as their visibility at millimeter wavelengths. The radio spectra are observed with the VLA and the Nobeyama 45-m millimeter-wavelength telescope. The three stars observed with the Nobeyama and VLA telescopes are UX Ari, AR Lac, and HR 5110, and the frequencies and conditions of the nonsimultaneous observations are set forth. Observations of the RS CVn systems at mm wavelengths is possible, and it is theorized that important information regarding flares and energy release can be derived from these data. A simple model is presented for the radio-coronae source related to outbursts of up to 5 GHz, and the onset of outbursts are predicted to be accompanied by low frequency type-II plasma emission. Title: The Sun at the Vla's Metric and Decimetric Wavelengths Authors: White, S. M.; Kundu, M. R.; Gopalswamy, N.; Schmahl, E. J. Bibcode: 1990IAUS..142..523W Altcode: Preliminary results of solar observations at 0.333 and 1.5 GHz made with the VLA during the September 11-17, 1988 period are presented. Generally, there are few structural changes in the active region sources from one day to the next, suggesting that structural evolution is relatively slow. Contour maps at 1.5 GHz are presented for each of the four days. Two noise storms were present at 0.33 GHz all week and were highly polarized. Title: A VLA Survey of Nearby Flare Stars Authors: White, Stephen M.; Jackson, Peter D.; Kundu, Mukul R. Bibcode: 1989ApJS...71..895W Altcode: Results from a VLA survey of nearby flare stars are combined with those of other surveys to compare the incidence of radio detection of late-type dwarf stars with other stellar parameters. About 40 percent of known nearby flare stars were detected in the survey. It is found that there is a deficiency of radio detections for M dwarf stars later than dM5.5, in agreement with the falloff in X-ray luminosity of these stars. Evidence is found for a link between radio activity and rotation, because most of the detected sources are stars which are believed to be young disk stars and are still rapid rotators. Title: Simple non-thermal models for the quiescent radio emission of dMe flare stars. Authors: White, S. M.; Kundu, M. R.; Jackson, P. D. Bibcode: 1989A&A...225..112W Altcode: Simple nonthermal gyrosynchrotron models were applied to the quiescent emission of M dwarf stars. Evidence is presented which suggests that these stars have a number of smaller active regions contributing to the observed flux. The evidence includes an absence of Zeeman polarization in optical observations and a lack of polarization in quiescent emission. Title: The First Interferometric Observations with Arc-Second Resolution of Solar Radio Bursts at Millimeter Wavelengths Authors: Kundu, M. R.; White, S. M.; Gopalswamy, N.; Bieging, J. H. Bibcode: 1989BAAS...21..861K Altcode: No abstract at ADS Title: VLA Observations of a Small Impulsive Flare Authors: White, S. M.; Kundu, M. R.; McConnell, D. Bibcode: 1989BAAS...21R.834W Altcode: No abstract at ADS Title: Interpretation of Multiwavelength Observations of Solar Active Regions Obtained During CoMStOC Authors: Brosius, J. W.; Holman, G. D.; Nitta, N.; White, S. M.; Kundu, M. R.; Gopalswamy, N.; Schmelz, J. T.; Saba, J. R. L.; Willson, R. Bibcode: 1989BAAS...21..838B Altcode: No abstract at ADS Title: The Sun at the VLA's Meter and Decimeter Wavelengths Authors: White, S. M.; Gopalswamy, N.; Kundu, M. R. Bibcode: 1989BAAS...21..861W Altcode: No abstract at ADS Title: On the Source Conditions for Herringbone Structure in Type-II Solar Radio Bursts Authors: Cane, H. V.; White, S. M. Bibcode: 1989SoPh..120..137C Altcode: We investigate the correlation of the occurrence of the herringbone phenomenon in type II solar radio bursts with various flare properties. We show that herringbone is strongly correlated with the intensity of the type II burst: whereas about 21% of all type II bursts show herringbone, about 60% of the most intense bursts contain herringbone. This fact can explain most of the correlations between herringbone and other properties such as intense type III bursts, type IV emission, and high type II starting frequencies. We also show that when this is taken into account, there is no need to postulate two classes of type II burst in order to explain why there appears to be a difference in herringbone occurrence between the set of type II bursts associated with the leading edges of coronal mass ejections, and those not so associated. We argue that the data are consistent with the idea that all coronal type II bursts are due to blast waves from flares. Title: Simultaneous Multi-Frequency Imaging Observations of Solar Microwave Bursts Authors: Kundu, M. R.; White, S. M.; Schmahl, E. J. Bibcode: 1989SoPh..121..153K Altcode: 1989IAUCo.104..153K We review the results of simultaneous two-frequency imaging observations of solar microwave bursts with the Very Large Array. Simultaneous 2 and 6 cm observations have been made of bursts which are optically thin at both frequencies, or optically thick at the lower frequency. In the latter case the source structure may differ at the two frequencies, but the two sources usually seem to be related. However, this is not always true of simultaneous 6 and 20 cm observations. The results have implications for the analysis of non-imaging radio data of solar and stellar flares. Title: Quiscent and flaring radio emission from the flare stars AD Leonis, EQ Pegasi, UV Ceti, Wolf 630, YY Geminorum and YZ Canis Minoris. Authors: Jackson, P. D.; Kundu, M. R.; White, S. M. Bibcode: 1989A&A...210..284J Altcode: Observations of the flare stars AD Leo, EQ Peg, UV Cet, Wolf 630, YY Gem, and YZ CMi at 6 and 20 cm wavelengths are presented. These observations display how a sample of the more active radio flare stars behave over periods of several hours. An extensive time analysis at five minute resolution, to investigate the typical variability of these stars, is presented together with a ten-second-resolution time analysis for periods of enhanced activity. Highlights are the enhanced 6-cm activity from both components of the UV Ceti system, quiescent emission from UV Ceti at 20 cm (which has a higher flux than the quiescent emission at 6 cm), the possible detection of polarized quiescent emission, a 20-cm flare from AD Leo in which the polarization changes sign during the rise phase of the flare, and a relatively high level of 6-cm emission from AD Leo. Title: Non-thermal electrons and stellar radio emission. Authors: White, S. M.; Kundu, M. R. Bibcode: 1989sasf.confP..37W Altcode: 1989IAUCo.104P..37W Radio emission from dMe flare stars has both a flaring and a quiescent component. When one compares stellar radio emission with the Sun, however, one finds that the apparent brightness temperature of the quiescent component often exceeds the temperature of non-thermal solar radio flares, and so it is likely that stellar quiescent emission also comes from non-thermal electrons. Here the authors briefly review the observations of quiescent emission, argue that the emitting regions are small, show that such small regions can still account for the observed fluxes, and discuss the source of electrons. Title: Three dimensional structures of coronal streamers, holes and CME plasmoids from multifrequency imaging observations Authors: Kundu, M. R.; Schmahl, E. J.; Gopalswamy, N.; White, S. M. Bibcode: 1989AdSpR...9d..41K Altcode: 1989AdSpR...9R..41K Throughout the quiet Sun years 1982-1987, the Clark Lake Radioheliograph routinely mapped the solar corona on a daily basis at frequencies from 30 to 100 MHz. The Clark Lake maps show a variety of features which we have analyzed quantitatively, providing information about the three dimensional nature of large scale structures of the solar corona. Title: First interferometric observations with arc-sec. resolution of solar radio bursts at millimeter wavelengths Authors: Kundu, Mukul R.; White, S. M.; Gopalswamy, N.; Bieging, J. H. Bibcode: 1989dots.work..119K Altcode: The Berkeley-Maryland-Illinois Array (BIMA) is briefly described in the context of solar observations. Specific areas of research that could be performed using BIMA during the Solar Maximum Mission (SMM) in 1991 are outlined. Some preliminary results of flare observations during March 1989 are presented. Title: Millimeter wavelength observations of solar flares for Max 1991 Authors: Kundu, M. R.; Gopalswamy, N.; Nitta, N.; Schmahl, E. J.; White, S. M.; Welch, W. J. Bibcode: 1988fnsm.work..107K Altcode: The Hat Creek millimeter-wave interferometer (to be known as the Berkeley-Illinois-Maryland Array, BIMA) is being upgraded. The improved array will become available during the coming solar maximum, and will have guaranteed time for solar observing. The Hat Creek millimeter-wave interferometer is described along with the improvements. The scientific objectives are briefly discussed. Title: Co-ordinated VLA and EXOSAT observations of the flrae stars UV Ceti, EQ Pegasi, YZ Canis Minoris and AD Leonis. Authors: Kundu, M. R.; Pallavicini, R.; White, S. M.; Jackson, P. D. Bibcode: 1988A&A...195..159K Altcode: The authors have observed four flare stars (UV Cet, EQ Peg, YZ CMi and AD Leo) simultaneously with the VLA and the EXOSAT satellite over continuous periods of 7 - 10 h. This is the first time that flare stars were observed simultaneously in X-rays and at microwave frequencies with high sensitivity instrumentation. All stars were detected both at the quiescent level and during flares. Although considerable activity in both X-rays and in the radio was found, there was little correlation between the two wavelength domains. The auhors discuss the significance of these observations for coronal activity in red dwarf flare stars. Title: Millimeter Wavelength Observations of Solar Flares for Max'91 Authors: Kundu, M. R.; Gopalswamy, N.; Nitta, N.; Schmahl, E. J.; White, S. M. Bibcode: 1988BAAS...20..746K Altcode: No abstract at ADS Title: A VLA Survey of dM Stars Covering a Broad Range in X-ray Fluxes Authors: Kundu, M. R.; White, S. M.; Agrawal, P. C. Bibcode: 1988BAAS...20..696K Altcode: No abstract at ADS Title: Simultaneous 2 and 6 Centimeter Wavelength Observations of a Solar Flare Using the VLA Authors: Kundu, M. R.; Velusamy, T.; White, S. M. Bibcode: 1987ApJ...321..593K Altcode: VLA observations of a solar active region and a flare are discussed. The event was observed at wavelengths of 2 and 6 cm simultaneously. Radio maps prior to the flare delineate the most important magnetic structures in the region. Interaction between these structures apparently led to preheating of plasma above the active region some 30 minutes prior to the flare. The 2 and 6 cm flare positions were coincident, and the time profiles of the burst at the two wavelengths were almost identical, implying that the same population of electrons was responsible for emission at the two wavelengths. Emission was probably nonthermal gyrosynchrotron radiation, and the physical conditions in the burst source are derived using this assumption. Title: Dynamic Spectrum of a Radio Flare on UV Ceti Authors: Jackson, Peter D.; Kundu, Mukul R.; White, Stephen M. Bibcode: 1987ApJ...316L..85J Altcode: The dMe flare star UV Ceti (L726-8B) was observed at four frequencies simultaneously in the 1385-1652 MHz band using the Very Large Array. A flare lasting 10 minutes was observed with 6.67 s time resolution 'Dynamic spectrum'-type images in the Stokes parameters I and V show considerable complexity in the frequency-time domain; some features show a positive frequency drift with time, while others are more complex, involving both positive and negative frequency drifts. The positive drift features would be consistent with disturbances traveling downward in the star's corona. Title: Very-large-array observations of a complex gradual solar burst at 6 CM wavelength Authors: Kundu, M. R.; McConnell, D.; White, S. M.; Shevgaonkar, R. K. Bibcode: 1987A&A...176..131K Altcode: The authors present a 6 cm VLA observation of a burst from a solar active region. The burst shows a normal impulsive rise-and-fall superimposed on a gradual component. The peak brightness temperature of the burst remains relatively constant, and the flux variations are due to changes in the source structure. It appears as though an arcade of loops is gradually heated or filled with hot plasma. The burst is weakly polarized but unipolar; the authors investigate the possible reasons for this structure. Depolarization of optically thin emission due to twisted magnetic fields within the source can produce significant depolarization, but is probably not active in this event. Instead, it is suggested that the flare emission is optically thick and that hot overlying material preferentially absorbs one polarization. Title: Microwave Observations of Red Dwarf Flare Stars Authors: Kundu, M. R.; Jackson, P. D.; White, S. M. Bibcode: 1987LNP...291..100K Altcode: 1987LNP87.291..100K; 1987csss....5..100K No abstract at ADS Title: A VLA Survey of dMe Flare Stars Authors: Jackson, P. D.; Kundu, M. R.; White, S. M. Bibcode: 1987LNP...291..103J Altcode: 1987csss....5..103J; 1987LNP87.291..103J A survey is underway, which uses the VLA at wavelengths of 6 and 20 cm, to search for radio emission from all dMe flare star. within about 10 parsecs of the Sun, and which have not yet been widely observed at radio wavelengths. At 20 cm, two bands centered at 1415 and 1515 MHz are observed and at 6 cm, two bands centered at 4535 and 4985 MHz are observed. Results so far, based on a total observing time of about one hour each during July 1986 have yielded detections from 9 of the 27 stellar systems in the program. Title: Microwave Observations of Flare Stars UV Ceti, AT Microscopii, and AU Microscopii Authors: Kundu, M. R.; Jackson, P. D.; White, S. M.; Melozzi, M. Bibcode: 1987ApJ...312..822K Altcode: The results of observations of three red dwarf flare star systems, UV Ceti, AT Mic, and AU Mic, made in February and March of 1985, are reported. Flaring was detected from all three systems, and quiescent emission from UV Cet and AU Mic. Models for the quiescent microwave-emitting corona of UV Cet are discussed. The gravitational scale height in current models is similar to or larger than the height of the corona, which is a striking difference from the case of the solar corona and confirms that magnetic structures are required to confine the radio-emitting corona. The role of precipitation into the chromosphere of the energetic particles in such a corona is explored, and it is shown that for plausible parameters it may be the dominant energy loss mechanism. Title: Narrow-Band Radio Flares from Red Dwarf Stars Authors: White, Stephen M.; Kundu, Mukul R.; Jackson, Peter D. Bibcode: 1986ApJ...311..814W Altcode: VLA observations of narrow-band behavior in 20 cm flares from two red dwarf stars, L726 - 8A and AD Leo, are reported. The flare on L726 - 8A was observed at 1415 and 1515 MHz; the flux and the evolution differed significantly at the two frequencies. The flare on AD Leo lasted for 2 hr at 1415 MHz but did not appear at 1515 MHz. The AD Leo flare appears to rule out a source drifting through the stellar corona and is unlikely to be due to plasma emission. In the cyclotron maser model the narrow-band behavior reflects the range of magnetic fields present within the source. The apparent constancy of this field for 2 hr is difficult to understand if magnetic reconnection is the source of energy for the flare. The consistent polarization exhibited by red dwarf flares at 20 cm may be related to stellar activity cycles, and changes in this polarization will permit measuring the length of these cycles. Title: Microwave Observations of Red Dwarf Flare Stars Authors: Kundu, M. R.; Jackson, P. D.; White, S. M. Bibcode: 1986BAAS...18..984K Altcode: No abstract at ADS Title: Solar Microbursts at Meter-Dekameter Wavelengths Authors: Kundu, M. R.; Gergely, T. E.; Szabo, A.; Loiacono, R.; White, S. M. Bibcode: 1986ApJ...308..436K Altcode: Microbursts are low-brightness-temperature bursts observed by the Clark Lake radioheliograph. The bursts occur several times per hour during quiet-sun periods and are seen at the observing frequencies from 30 to 70 MHz. They are stationary at a given frequency, have short rise times and durations of 2-10 s. Here, observations of the bursts are presented and interpretated in terms of plasma emission. The burst properties suggest that they are weak type III bursts. The observations imply that energy releases on the sun continue to be impulsive, with nonthermal electron distributions, for small releases of energy. The relation of the bursts to type III bursts and hard X-ray bursts is discussed. Title: A VLA Survey of dMe Flare Stars Authors: Jackson, P. D.; Kundu, M. R.; White, S. M. Bibcode: 1986BAAS...18..913J Altcode: No abstract at ADS Title: Particle Propagation Effects on Wave Growth in a Solar Flux Tube Authors: White, S. M.; Melrose, D. B.; Dulk, G. A. Bibcode: 1986ApJ...308..424W Altcode: The evolution of a distribution of electrons is followed after they are injected impulsively at the top of a coronal magnetic loop, with the objective of studying the plasma instabilities which result. At early times the downgoing electrons have beamlike distributions and amplify electrostatic waves via the Cerenkov resonance; the anomalous Doppler resonance is found to be less important. Slightly later, while the electrons are still predominantly downgoing, they are unstable to cyclotron maser generation of z-mode waves with omega(p) much less than Omega, or to second harmonic x-mode waves. The energetics of these instabilities, including saturation effects and heating of the ambient plasma, are discussed. It is suggested that coalescence of two z-mode waves generated by cyclotron maser emission of the downgoing electrons may produce the observed microwave spike bursts. Title: Clark Lake microbursts: on a lower limit to type III burst brightnesstemperatures. Authors: White, S. M.; Kundu, M. R.; Szabo, A. Bibcode: 1986SoPh..107..135W Altcode: 1987SoPh..107..135W Further observations of solar microbursts by the Clark Lake radioheliograph are reported. The microbursts have properties consistent with weak type III bursts, with the implication that type III's can have brightness temperatures as low as 106 K. We explore the importance of this result. A single model to explain the stronger type III bursts and the weaker microbursts is sought. We show that none of the models for stabilizing the strongest type III electron streams can explain the observed microbursts: these models have threshold levels of Langmuir waves which imply emission (due to spontaneous scattering off ions) with brightness temperatures in excess of those observed. It appears that either some vital physics is still missing from models for type III bursts, or that microbursts should have properties significantly different from those of type III bursts. In the latter case further observations should allow important tests of type III models. Title: Microwave observations of red dwarf flare stars Authors: Kundu, M. R.; White, S. M.; Jackson, P. D. Bibcode: 1986AdSpR...6h.117K Altcode: 1986AdSpR...6..117K We discuss some recent observations of red dwarf flare stars. When observed over periods of about 8 hours, each of 4 flare star systems displayed at least one major flare at 20 cm. Quiescent emission at 6 cm was seen from UV Ceti and EQ Peg A, but flares were much less frequent at 6 cm than at 20 cm. We also summarize earlier observations of quiescent emission from UV Ceti. Observations of highly polarized flares with brightness temperatures in excess of 1010 K appear to be common on red dwarf stars. We have also found narrowband flares which strengthen the argument that a coheren emission mechanism is involved in these flares. One of those narrowband flares allows us to place severe constraints on conditions in the flare source, and if the flare is cyclotron maser emission it seems unlikely that magnetic reconnection is involved in the flare. Title: Particle propagation, wave growth and energy dissipation in a flaring flux tube Authors: White, S. M.; Melrose, D. B.; Dulk, G. A. Bibcode: 1986AdSpR...6f.163W Altcode: 1986AdSpR...6..163W We investigate wave amplification by downgoing particles in a common flare model. The flare is assumed to occur at the top of a coronal magnetic flux loop, and results in the heating of plasma in the flaring region. The hot electrons propagate down the legs of the flux tube towards increasing magnetic field. It is simple to demonstrate that the velocity distributions which result in this model are unstable to both beam instabilities and cyclotron maser action. We present an explanation for the propagation effects on the distribution, and explore the properties of the resulting amplified waves. We concentrate on cyclotron maser action, which has properties (emission in the z mode below the local gyrofrequency) quite different from maser action by other distributions considered in the context of solar flares. The z mode waves will be damped in the coronal plasma surrounding the flaring flux tube, and lead to heating there. This process may be important in the overall energy budget of the flare. We compare the downgoing maser with the loss cone maser, which is more likely to produce observable bursts. Title: Microwave Observations of Flare Stars UV Ceti, AT Mic, and AU Mic Authors: Kundu, M. R.; Jackson, P. D.; Melozzi, M.; White, S. M. Bibcode: 1986LNP...254..284K Altcode: 1986csss....4..284K No abstract at ADS Title: Microbursts observed at Clark Lake Authors: White, S. M.; Kundu, M. R.; Gergely, T. E. Bibcode: 1986AdSpR...6f.285W Altcode: 1986AdSpR...6Q.285W Microbursts are low brightness temperature burst observed by the Clark Lake radioheliograph. The bursts are seen at the observing frequencies from 30 MHz to 70 MHz, are stationary at a given frequency, and have short rise times and durations of 2 - 10 seconds. We present observations of the bursts, and show that their properties imply that they are weak type III bursts. However, we also show that none of the theories invoked to explain strong type III bursts can accomodate such weak bursts. We expect that important differences in the properties of microbursts and the stronger type III's will be found, and will be important in understanding the propagation of electron streams in the corona. Title: Thermal Effects in the Ultrarelativistic Two-Stream Instability Authors: White, S. M. Bibcode: 1985Ap&SS.116..173W Altcode: The dispersion relation for longitudinal waves in a one-dimensional ultrarelativistic plasma is calculated. Analytical and numerical results for the growth rate and frequency of the two-stream instability are presented as a function of the energy spread in the denser stream when the dilute stream is cold. The case of energy spreads in both beams is investigated numerically: it is found that relatively small energy spreads in both streams can lead to suppression of the instability. Title: On damping of auroral z mode waves Authors: White, S. M. Bibcode: 1985JGR....90.7471W Altcode: This paper discusses the constraints on auroral z mode waves implied by damping. Damping of the magnetoinic z mode in a thermal plasma with ωp <<Q is calculated and shown to be strong near the gyrofrequency. Analytic formulae for the optical depth of the corresponding absorption layer are presented. Ray tracing calculations are used to investigate the effect of refraction during the propagation of a z mode ray: refraction is found to be particularly important in the vicinity of the gyrofrequency layer and tends to cause the wave vector of the ray to change so that it is strongly damped. The results are applied to models for the generation of auroral z mode waves. A point of particular interest concerns the possibility of explaining observed emissions whose bandwidths straddle the gyrofrequency by an emission mechanism which amplifies waves either only above or only below the gyrofrequency. For a dipolar magnetic field and radial plasma density gradient it is found that a z mode ray in the auroral zone is unlikely to be able to pass through the gyrofrequency layer without significant damping occurring. Title: Damping of the MagnetoIonic Z Mode Authors: White, S. M.; Melrose, D. B.; Dulk, G. A. Bibcode: 1985ASSL..116...47W Altcode: 1985rst..conf...47W The authors consider two effects here: damping of the z-mode waves as they propagate through the ambient coronal plasma, and coalescence of z-mode waves to produce second-harmonic radiation. Title: Electron cyclotron masers during solar flares Authors: White, S. M.; Melrose, D. B.; Dulk, G. A. Bibcode: 1983PASA....5..188W Altcode: 1983PASAu...5..188W It has been suggested that solar microwave spike bursts are due to electron cyclotron maser action. The objective of the study reported here is to explore a simple model for the dynamics of the electrons producing the maser emission and to use the model to investigate the conditions under which the maser turns on. It is shown that there are two phases in which maser action is likely to occur following a solar flare. In the later phase, loss cones in upgoing particles develop and are unstable to wave growth; the resulting maser produces radiation at low harmonics. Maser action is also likely to occur in an earlier phase when the particles are predominantly downgoing. The resulting maser emits radiation which should be reflected or reabsorbed at a level where its frequency equals the critical frequency of the appropriate mode. The radiation propagates nearly perpendicular to the field lines so that heating over a large area is possible. The implications of the results obtained are briefly examined. Title: Coherent gyromagnetic emission Authors: White, S. M.; Melrose, D. B. Bibcode: 1982PASA....4..362W Altcode: 1982PASAu...4..362W The properties of interference radiation in coherent gyrotron emission are examined. The idea of interference is analytically explained, and the kinematic requirements for it to occur are identified. The possibility of absorption or maser action is excluded. The dependence of the interference energy on the particle velocities is shown, and the current associated with each particle is presented. These describe the incoherent radiation of each particle. The remaining term is a cross-product between the two particles on the radiation. The results suggest that taking account of the dispersion of particles in bunches emitting coherent curvature radiation permits the expectation that the spectral properties and angular characteristics of the resulting radiation will differ from the predictions of incoherent curvature emission theory. Title: Precipitation from an asymmetric magnetic flux tube Authors: Melrose, D. B.; White, S. M. Bibcode: 1981JGR....86.2183M Altcode: A simple model for precipitation of trapped particles from an asymmetric magnetic flux tube is formulated and analytic solutions are found in the strong diffusion and weak diffusion limits. The loss cones α<α1 and α>π-α2 (α = pitch angle) are assumed unequal (α1<α2), the pitch angle diffusion coefficient D(α) is assumed asymmetric (D(α)≠D(π-α)), and the particles are assumed to be injected at an angle αs≠π/2. The steady state solution in the strong diffusion limit implies that the ratio of the precipitation rates at the two feet is equal to the ratio of the solid angles filled by the loss cones, R1/R2 = α21/α22. An explicit result for R1 is obtained in the weak diffusion limit. For α2-α1>>αD1, where αD1 is roughly the typical angle a particle is deflected in one transit across the flux tube from foot 2 to foot 1, R1 is smaller than R2 by a factor of order exp[-(α2-α1)/αD1]. The effect of asymmetric diffusion, which is to favor precipitation at foot 1 for D(α1)>D(α2) and at foot 2 for D(α2)>D(α1), is described in terms of a factor F(d) of an asymmetry parameter d given by equations (35) and (24), respectively. Applications to the solar corona, to Jupiter's radio emission and to the terrestrial magnetosphere are discussed briefly. Title: Amplified Cerenkov emission of auroral hiss: Limitations implied by quasi-linear theory Authors: Melrose, D. B.; White, S. M. Bibcode: 1980JGR....85.3442M Altcode: Auroral hiss is attributed to amplified Cerenkov emission of nearly resonant whistlers. such emission occurs only when the pricipitating electron flux corresponds to a one-dimensional distribution with a peak in parallel energy. Only particles below this peak contribute to the amplified emission. The back reaction of the hiss on the electrons is treated by using quasi-linear theory. The primary effect is to reduce the positive gradient in energy below the peak. This tends to suppress the generation of the waves. A relation between the expected intensity of the hiss and the properties of the stream is derived. The observational data are consistent with limitation of the intensity by quasi-linear relation. Title: The gyrosynchrotron emission from quasi-thermal electrons and applications to solar flares Authors: Dulk, G. A.; Melrose, D. B.; White, S. M. Bibcode: 1979ApJ...234.1137D Altcode: We present theoretical results on the gyrosynchrotron radiation from electrons with a Maxwellian energy distribution. We review the analytical expressions for the gyromagnetic absorption coefficient and find two which cover the range of interest for microwave emission from solar flares, i.e., frequencies ω to ∼100Ωe and temperatures Te to ∼109 K. Numerical calculations are used to check the analytic expressions and to derive simplified empirical formulae which relate the observable characteristics of the radiation to the temperature and magnetic field in the source.

We apply the results to the sources of impulsive microwave and hard X-ray bursts from solar flares. For an isothermal source the theory predicts a microwave spectrum where the flux density rises as f2 at low frequencies, maximizes as some frequency fpeak, and falls very rapidly thereafter; this shape fits the observed spectra qualitatively. The optical depth τ of the source varies rapidly with f, with τ = 1 at f ≍ fpeak. For Te ≳ 108 K we derive the relation fpeak ∝ Te0.7B, which allows a direct estimate of the magnetic field B in the impulsive burst source if the temperature is known-for instance, from hard X-ray observations. For the impulsive burst of 1972 May 18, reported by Hoyng and Stevens, we find that the microwave and hard X-ray data are well fitted by a model source with Te ≍ 2.3 × 108 K, B ≍ 370 gauss, ne 2 × 109 cm-3, and scale length L ≍ 8600 km. Title: Effect of asymmetry on a trap model for solar hard X-ray bursts Authors: Melrose, D. B.; White, S. M. Bibcode: 1979PASA....3..369M Altcode: 1979PASAu...3..369M The model proposed by Melrose and White (1979) for the precipitation of trapped energetic particles in an asymmetric flux tube is examined. It is shown that scattering due to Coulomb interactions should be weak for the electrons which emit hard X-rays in a trap model for solar hard X-ray bursts. The X-ray emission would then consist of thin target emission from the trap and thick target emission from the weaker foot of the flux tube, with no emission from the other (stronger-field) foot. The lifetime of the hard X-ray burst provides a measure of the scattering time and electron density in the trap. Title: A gyro-synchrotron maser in the solar corona? Authors: Melrose, D. B.; White, S. M. Bibcode: 1978PASA....3..231M Altcode: 1978PASAu...3..231M It is found that a gyrosynchrotron maser is a possible interpretation of observed high brightness temperatures (10 to the 10th K) in some moving Type IV bursts. However, unless considerably higher brightness temperatures at lower frequencies are found, it appears that nonamplified emission from electrons with energies (about 1 MeV) higher than considered in the past is a more plausible interpretation (Stewart et al., 1978). If the brightness temperature at lower frequencies is found to be much higher (e.g., 10 to the 12th K at 20 MHz), this would provide evidence in favor of the maser mechanism.