Author name code: nindos ADS astronomy entries on 2022-09-14 author:"Nindos, Aexander" ------------------------------------------------------------------------ Title: Magnetic helicity and energy of emerging solar active regions and their erruptivity Authors: Liokati, E.; Nindos, A.; Liu, Y. Bibcode: 2022A&A...662A...6L Altcode: 2022arXiv220204353L
Aims: We investigate the role of the accumulation of magnetic helicity and magnetic energy in the generation of coronal mass ejections (CMEs) from emerging solar active regions (ARs).
Methods: Using vector magnetic field data obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we calculated the magnetic helicity and magnetic energy injection rates as well as the resulting accumulated budgets in 52 emerging ARs from the start time of magnetic flux emergence until they reached a heliographic longitude of 45° West (W45).
Results: Seven of the ARs produced CMEs, but 45 did not. In a statistical sense, the eruptive ARs accumulate larger budgets of magnetic helicity and energy than the noneruptive ARs over intervals that start from the flux emergence start time and end (i) at the end of the flux emergence phase and (ii) when the AR produces its first CME or crosses W45, whichever occurs first. We found magnetic helicity and energy thresholds of 9 × 1041 Mx2 and 2 × 1032 erg. When these thresholds were crossed, ARs are likely to erupt. In terms of accumulated magnetic helicity and energy budgets, the segregation of the eruptive from the noneruptive ARs is violated in one case when an AR erupts early in its emergence phase and in six cases in which noneruptive ARs exhibit large magnetic helicity and energy budgets. Decay index calculations may indicate that these ARs did not erupt because the overlying magnetic field provided a stronger or more extended confinement than in eruptive ARs.
Conclusions: Our results indicate that emerging ARs tend to produce CMEs when they accumulate significant budgets of both magnetic helicity and energy. Any study of their eruptive potential should consider magnetic helicity together with magnetic energy. Title: First detection of metric emission from a solar surge Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bouratzis, C.; Hillaris, A. Bibcode: 2022A&A...662A..14A Altcode: 2022arXiv220301043A We report the first detection of metric radio emission from a surge, observed with the Nançay Radioheliograph (NRH), STEREO, and other instruments. The emission was observed during the late phase of the M9 complex event SOL2010-02-012T11:25:00, described in a previous publication. It was associated with a secondary energy release, also observed in STEREO 304 Å images, and there was no detectable soft X-ray emission. The triangulation of the STEREO images allowed for the identification of the surge with NRH sources near the central meridian. The radio emission of the surge occurred in two phases and consisted of two sources, one located near the base of the surge, apparently at or near the site of energy release, and another in the upper part of the surge; these were best visible in the frequency range of 445.0 to about 300 MHz, whereas a spectral component of a different nature was observed at lower frequencies. Sub-second time variations were detected in both sources during both phases, with a 0.2-0.3 s delay of the upper source with respect to the lower, suggesting superluminal velocities. This effect can be explained if the emission of the upper source was due to scattering of radiation from the source at the base of the surge. In addition, the radio emission showed signs of pulsations and spikes. We discuss possible emission mechanisms for the slow time variability component of the lower radio source. Gyrosynchrotron emission reproduced the characteristics of the observed total intensity spectrum at the start of the second phase of the event fairly well, but failed to reproduce the high degree of the observed circular polarization or the spectra at other instances. On the other hand, type IV-like plasma emission from the fundamental could explain the high polarization and the fine structure in the dynamic spectrum; moreover, it gives projected radio source positions on the plane of the sky, as seen from STEREO-A, near the base of the surge. Taking all the properties into consideration, we suggest that type IV-like plasma emission with a low-intensity gyrosynchrotron component is the most plausible mechanism.

Movie associated to Fig. A.2 is available at https://www.aanda.org Title: A first look at the submillimeter Sun with ALMA Authors: Alissandrakis, C. E.; Bastian, T. S.; Nindos, A. Bibcode: 2022A&A...661L...4A Altcode: 2022arXiv220501008A We present the first full-disk solar images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm; 347 GHz). In spite of the low spatial resolution (21″), several interesting results were obtained. During our observation, the sun was practically devoid of active regions. Quiet Sun structures on the disk are similar to those in Atmospheric Imaging Assembly images at 1600 Å and 304 Å, after the latter are smoothed to the ALMA resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm) images; they are also similar to negative Hα images of equivalent resolution. Polar coronal holes, which are clearly seen in the 304 Å band and small Hα filaments, are not detectable at 0.86 mm. We computed the center-to-limb variation of the brightness temperature, Tb, in Band 7, as well as in Bands 6 and 3, which were obtained during the same campaign, and we combined them to a unique curve of Tb(log μ100), where μ100 is the cosine of the heliocentric angle reduced to 100 GHz. Assuming that the absolute calibration of the Band 3 commissioning observations is accurate, we deduced a brightness temperature at the center of the disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated from the recommended values for Bands 3 and 6. More importantly, the Tb(log μ100) curve flattens at large values of μ100, and so does the corresponding Te(log τ100) at large τ100. This is probably an indication that we are approaching the temperature minimum. Title: Multiwavelength observations of a metric type-II event Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.; Hillaris, A. Bibcode: 2021A&A...654A.112A Altcode: 2021arXiv210802855A We have studied a complex metric radio event that originated in a compact flare, observed with the ARTEMIS-JLS radiospectrograph on February 12, 2010. The event was associated with a surge observed at 195 and 304 Å and with a coronal mass ejection observed by instruments on board STEREO A and B near the eastern and western limbs respectively. On the disk the event was observed at ten frequencies by the Nançay Radioheliograph (NRH), in Hα by the Catania observatory, in soft X-rays by GOES SXI and Hinode XRT, and in hard X-rays by RHESSI. We combined these data, together with MDI longitudinal magnetograms, to get as complete a picture of the event as possible. Our emphasis is on two type-II bursts that occurred near respective maxima in the GOES light curves. The first, associated with the main peak of the event, showed an impressive fundamental-harmonic structure, while the emission of the second consisted of three well-separated bands with superposed pulsations. Using positional information for the type-IIs from the NRH and triangulation from STEREO A and B, we found that the type-IIs were associated neither with the surge nor with the disruption of a nearby streamer, but rather with an extreme ultraviolet (EUV) wave probably initiated by the surge. The fundamental-harmonic structure of the first type-II showed a band split corresponding to a magnetic field strength of 18 G, a frequency ratio of 1.95 and a delay of 0.23−0.65 s of the fundamental with respect to the harmonic; moreover it became stationary shortly after its start and then drifted again. The pulsations superposed on the second type-II were broadband and had started before the burst. In addition, we detected another pulsating source, also before the second type-II, polarized in the opposite sense; the pulsations in the two sources were out of phase and hence hardly detectable in the dynamic spectrum. The pulsations had a measurable reverse frequency drift of about 2 s−1. Title: ALMA observations of the variability of the quiet Sun at millimeter wavelengths Authors: Nindos, A.; Patsourakos, S.; Alissandrakis, C. E.; Bastian, T. S. Bibcode: 2021A&A...652A..92N Altcode: 2021arXiv210604220N
Aims: We address the variability of the quiet solar chromosphere at 1.26 mm and 3 mm with a focus on the study of spatially resolved oscillations and transient brightenings, which are small, weak events of energy release. Both phenomena may have a bearing on the heating of the chromosphere.
Methods: We used Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quiet Sun at 1.26 mm and 3 mm. The spatial and temporal resolution of the data were 1 − 2″ and 1 s, respectively. The concatenation of light curves from different scans yielded a frequency resolution in spectral power of 0.5−0.6 mHz. At 1.26 mm, in addition to power spectra of the original data, we degraded the images to the spatial resolution of the 3 mm images and used fields of view that were equal in area for both data sets. The detection of transient brightenings was made after the effect of oscillations was removed.
Results: At both frequencies, we detected p-mode oscillations in the range 3.6−4.4 mHz. The corrections for spatial resolution and field of view at 1.26 mm decreased the root mean square (rms) of the oscillations by a factor of 1.6 and 1.1, respectively. In the corrected data sets, the oscillations at 1.26 mm and 3 mm showed brightness temperature fluctuations of ∼1.7 − 1.8% with respect to the average quiet Sun, corresponding to 137 and 107 K, respectively. We detected 77 transient brightenings at 1.26 mm and 115 at 3 mm. Although their majority occurred in the cell interior, the occurrence rate per unit area of the 1.26 mm events was higher than that of the 3 mm events; this conclusion does not change if we take into account differences in spatial resolution and noise levels. The energy associated with the transient brightenings ranged from 1.8 × 1023 to 1.1 × 1026 erg and from 7.2 × 1023 to 1.7 × 1026 erg for the 1.26 mm and 3 mm events, respectively. The corresponding power-law indices of the energy distribution were 1.64 and 1.73. We also found that ALMA bright network structures corresponded to dark mottles or spicules that can be seen in broadband Hα images from the GONG network.
Conclusions: The fluctuations associated with the p-mode oscillations represent a fraction of 0.55−0.68 of the full power spectrum. Their energy density at 1.26 mm is 3 × 10−2 erg cm−3. The computed low-end energy of the 1.26 mm transient brightenings is among the smallest ever reported, irrespective of the wavelength of the observation. Although the occurrence rate per unit area of the 1.26 mm transient brightenings was higher than that of the 3 mm events, their power per unit area is smaller likely due to the detection of many weak 1.26 mm events. Title: Tracking solar wind flows from rapidly varying viewpoints by the Wide-field Imager for Parker Solar Probe Authors: Nindos, A.; Patsourakos, S.; Vourlidas, A.; Liewer, P. C.; Penteado, P.; Hall, J. R. Bibcode: 2021A&A...650A..30N Altcode: 2020arXiv201013140N
Aims: Our goal is to develop methodologies to seamlessly track transient solar wind flows viewed by coronagraphs or heliospheric imagers from rapidly varying viewpoints.
Methods: We constructed maps of intensity versus time and elongation (J-maps) from Parker Solar Probe (PSP) Wide-field Imager (WISPR) observations during the fourth encounter of PSP. From the J-map, we built an intensity on impact-radius-on-Thomson-surface map (R-map). Finally, we constructed a latitudinal intensity versus time map (Lat-map). Our methodology satisfactorily addresses the challenges associated with the construction of such maps from data taken from rapidly varying viewpoint observations.
Results: Our WISPR J-map exhibits several tracks, corresponding to transient solar wind flows ranging from a coronal mass ejection down to streamer blobs. The latter occurrence rate is about 4-5 per day, which is similar to the occurrence rate in a J-map made from ~1 AU data obtained with the Heliospheric Imager-1 (HI-1) on board the Solar Terrestrial Relations Observatory Ahead spacecraft (STEREO-A). STEREO-A was radially aligned with PSP during the study period. The WISPR J-map tracks correspond to angular speeds of 2.28 ± 0.7°/h (2.49 ± 0.95°/h), for linear (quadratic) time-elongation fittings, and radial speeds of about 150-300 km s−1. The analysis of the Lat-map reveals a bifurcating streamer, which implies that PSP was flying through a slightly folded streamer during perihelion.
Conclusions: We developed a framework to systematically capture and characterize transient solar wind flows from space platforms with rapidly varying vantage points. The methodology can be applied to PSP WISPR observations as well as to upcoming observations from instruments on board the Solar Orbiter mission.

Movie associated to Fig. 8 is available at https://www.aanda.org Title: Relative field line helicity of a large eruptive solar active region Authors: Moraitis, K.; Patsourakos, S.; Nindos, A. Bibcode: 2021A&A...649A.107M Altcode: 2021arXiv210303643M Context. Magnetic helicity is a physical quantity of great importance in the study of astrophysical and natural plasmas. Although a density for helicity cannot be defined, a good proxy for this quantity is field line helicity. The appropriate quantity for use in solar conditions is relative field line helicity (RFLH).
Aims: This work aims to study in detail the behaviour of RFLH, for the first time, in a solar active region (AR).
Methods: The target AR is the large, eruptive AR 11158. In order to compute RFLH and all other quantities of interest, we used a non-linear force-free reconstruction of the AR coronal magnetic field of excellent quality.
Results: We find that the photospheric morphology of RFLH is very different than that of the magnetic field or electrical current, and this morphology is not sensitive to the chosen gauge in the computation of RFLH. The value of helicity experiences a large decrease, that is ∼25% of its pre-flare value, during an X-class flare of the AR; this change is also depicted in the photospheric morphology of RFLH. Moreover, the area of this change coincides with the area that encompasses the flux rope, which is the magnetic structure that later erupted.
Conclusions: The use of RFLH can provide important information about the value and location of the magnetic helicity expelled from the solar atmosphere during eruptive events. 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: Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.; Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos, A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2020SSRv..216..131P Altcode: 2020arXiv201010186P A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation). Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Title: When do solar erupting hot magnetic flux ropes form? Authors: Nindos, A.; Patsourakos, S.; Vourlidas, A.; Cheng, X.; Zhang, J. Bibcode: 2020A&A...642A.109N Altcode: 2020arXiv200804380N
Aims: We investigate the formation times of eruptive magnetic flux ropes relative to the onset of solar eruptions, which is important for constraining models of coronal mass ejection (CME) initiation.
Methods: We inspected uninterrupted sequences of 131 Å images that spanned more than eight hours and were obtained by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory to identify the formation times of hot flux ropes that erupted in CMEs from locations close to the limb. The appearance of the flux ropes as well as their evolution toward eruptions were determined using morphological criteria.
Results: Two-thirds (20/30) of the flux ropes were formed well before the onset of the eruption (from 51 min to more than eight hours), and their formation was associated with the occurrence of a confined flare. We also found four events with preexisting hot flux ropes whose formations occurred a matter of minutes (from three to 39) prior to the eruptions without any association with distinct confined flare activity. Six flux ropes were formed once the eruptions were underway. However, in three of them, prominence material could be seen in 131 Å images, which may indicate the presence of preexisting flux ropes that were not hot. The formation patterns of the last three groups of hot flux ropes did not show significant differences. For the whole population of events, the mean and median values of the time difference between the onset of the eruptive flare and the appearance of the hot flux rope were 151 and 98 min, respectively.
Conclusions: Our results provide, on average, indirect support for CME models that involve preexisting flux ropes; on the other hand, for a third of the events, models in which the ejected flux rope is formed during the eruption appear more appropriate.

Movies attached to Figs. 2, 5, 8, and 10 are available at http://www.aanda.org Title: The Solar Orbiter Science Activity Plan. Translating solar and heliospheric physics questions into action Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.; Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.; Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.; Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.; Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio, L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun, A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso, F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.; Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.; Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.; van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi, L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine, D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot, S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham, G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier, K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins, J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis, I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.; Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis, G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.; Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.; Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis, K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien, H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.; Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.; Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines, J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.; Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.; Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.; Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.; Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.; Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula, G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio, A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.; Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann, T.; Young, P. R.; Zhukov, A. N. Bibcode: 2020A&A...642A...3Z Altcode: 2020arXiv200910772Z Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, we introduce Solar Orbiter's SAP through a series of examples and the strategy being followed. Title: Modeling the quiet Sun cell and network emission with ALMA Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T. S.; Patsourakos, S. Bibcode: 2020A&A...640A..57A Altcode: 2020arXiv200609886A Observations of the Sun at millimeter wavelengths with the Atacama Large Millimeter/submillimeter Array (ALMA) offer a unique opportunity to investigate the temperature structure of the solar chromosphere. In this article we expand our previous work on modeling the chromospheric temperature of the quiet Sun, by including measurements of the brightness temperature in the network and cell interiors, from high-resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We also examine the absolute calibration of ALMA full-disk images. We suggest that the brightness temperature at the center of the solar disk in Band 6 is ∼440 K above the value recommended by White et al. (2017, Sol. Phys., 292, 88). In addition, we give improved results for the electron temperature variation of the average quiet Sun with optical depth and the derived spectrum at the center of the disk. We found that the electron temperature in the network is considerably lower than predicted by model F of Fontenla et al. (1993, ApJ, 406, 319) and that of the cell interior considerably higher than predicted by model A. Depending on the network/cell segregation scheme, the electron temperature difference between network and cell at τ = 1 (100 GHz) ranges from ∼660 K to ∼1550 K, compared to ∼3280 K predicted by the models; similarly, the electron temperature, Te ratio ranges from ∼1.10 to 1.24, compared to ∼1.55 of the model prediction. We also found that the network/cell Te(τ) curves diverge as τ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior. Title: Transient brightenings in the quiet Sun detected by ALMA at 3 mm Authors: Nindos, A.; Alissandrakis, C. E.; Patsourakos, S.; Bastian, T. S. Bibcode: 2020A&A...638A..62N Altcode: 2020arXiv200407591N
Aims: We investigate transient brightenings, that is, weak, small-scale episodes of energy release, in the quiet solar chromosphere; these episodes can provide insights into the heating mechanism of the outer layers of the solar atmosphere.
Methods: Using Atacama Large Millimeter/submillimeter Array (ALMA) observations, we performed the first systematic survey for quiet Sun transient brightenings at 3 mm. Our dataset included images of six 87″ × 87″ fields of view of the quiet Sun obtained with angular resolution of a few arcsec at a cadence of 2 s. The transient brightenings were detected as weak enhancements above the average intensity after we removed the effect of the p-mode oscillations. A similar analysis, over the same fields of view, was performed for simultaneous 304 and 1600 Å data obtained with the Atmospheric Imaging Assembly.
Results: We detected 184 3 mm transient brightening events with brightness temperatures from 70 K to more than 500 K above backgrounds of ∼7200 - 7450 K. All events showed light curves with a gradual rise and fall, strongly suggesting a thermal origin. Their mean duration and maximum area were 51.1 s and 12.3 Mm2, respectively, with a weak preference of appearing at network boundaries rather than in cell interiors. Both parameters exhibited power-law behavior with indices of 2.35 and 2.71, respectively. Only a small fraction of ALMA events had either 304 or 1600 Å counterparts but the properties of these events were not significantly different from those of the general population except that they lacked their low-end energy values. The total thermal energies of the ALMA transient brightenings were between 1.5 × 1024 and 9.9 × 1025 erg and their frequency distribution versus energy was a power law with an index of 1.67 ± 0.05. We found that the power per unit area provided by the ALMA events could account for only 1% of the chromospheric radiative losses (10% of the coronal ones).
Conclusions: We were able to detect, for the first time, a significant number of weak 3 mm quiet Sun transient brightenings. However, their energy budget falls short of meeting the requirements for the heating of the upper layers of the solar atmosphere and this conclusion does not change even if we use the least restrictive criteria possible for the detection of transient brightenings. Title: Modeling of the Brightness of the Chromospheric Network Based on ALMA High Resolution Observations of the Quiet Sun Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T.; Patsourakos, S. Bibcode: 2020AAS...23610607A Altcode: ALMA observations of the Sun at mm-λ offer a unique opportunity to investigate the temperature/density structure of the solar chromosphere. In a previous work (Alissandrakis et al 2017, A&A 605, A78) we measured the center-to-limb variation of the brightness temperature, Tb, using low resolution ALMA full-disk observations in Band 3 (3mm) and Band 6 (1.26 mm), together with data at 0.85 mm from Bastian et al. 1993 (ApJ, 415, 364). Combining all data and inverting the solution of the transfer equation we found that the electron temperature, Te, in the range of 0.34 < τ100 < 12, where τ100 is the optical depth at 100 GHz, was ~5% (~300 K) below the one predicted by model C (average quiet sun) of FAL93 (Fontenla, Avrett, & Loeser, 1993, ApJ, 406, 319). Here we expand that work by including measurements of the brightness temperature in the network and cell interiors, from high resolution ALMA images in Bands 3 and 6. We found that the observed Tb in the network is considerably lower than predicted by the FAL93 model F and that of the cell interior considerably higher than predicted by the FAL93 model A. The observed network/cell difference of brightness temperature at the center of the disk, at 100 GHz is about 920 K, compared to ~3250 K predicted by the FAL93 models; similarly, the Tb, ratio is ~1.14, against ~1.51 of the model prediction. After inversion of the observed data, the electron temperature of cell interior at τ100=1 is ~390 K below the average (~600 K above model A) and of the network ~400 K above the average (~1800 K below model A). The implications of these results will be discussed. We will also discuss the question of the normalization of brightness temperature observed by ALMA. Title: Observations of solar chromospheric oscillations at 3 mm with ALMA Authors: Patsourakos, S.; Alissandrakis, C. E.; Nindos, A.; Bastian, T. S. Bibcode: 2020A&A...634A..86P Altcode: 2019arXiv191203480P
Aims: We aim to study spatially resolved chromospheric oscillations of the quiet Sun (QS) in the mm-domain at a resolution of a few arcsec, typically 2.4″ × 4.5″.
Methods: We used Atacama Large millimeter and submillimeter Array (ALMA) time series of interferometric observations of the QS obtained at 3 mm with a 2-s cadence and a spatial resolution of a few arcsec. The observations were performed on March 16, 2017 and seven 80″ × 80″ fields of view (FoV) going from disk center to limb were covered, each one observed for 10 min, therefore limiting the frequency resolution of the power spectra to 1.7 mHz. For each FoV, masks for cell and network were derived, and the averaged power spectral densities (PSDs) for the entire FoV, cell, and network were computed. The resulting power spectra were fit with an analytical function in order to derive the frequency and the root-mean-square (rms) power associated with the peaks. The same analysis, over the same FoVs and for the same intervals, was performed for simultaneous Atmospheric Imaging Assembly (AIA) image sequences in 1600 Å.
Results: Spatially resolved chromospheric oscillations at 3 mm, with frequencies of 4.2 ± 1.7 mHz are observed in the QS, in both cell and network. The coherence length-scale of the oscillations is commensurate with the spatial resolution of our ALMA observations. Brightness-temperature fluctuations in individual pixels could reach up to a few hundred K, while the spatially averaged PSDs yield rms in the range ≈55-75 K, i.e., up to ≈1% of the averaged brightness temperatures and exhibit a moderate increase towards the limb. For AIA 1600 Å, the oscillation frequency is 3.7 ± 1.7 mHz. The relative rms is up to 6% of the background intensity, with a weak increase towards the disk center (cell, average). ALMA 3 mm time-series lag AIA 1600 Å by ≈100 s, which corresponds to a formation-height difference of ≈1200 km, representing a novel determination of this important parameter.
Conclusions: The ALMA oscillations that we detected exhibit higher amplitudes than those derived from previous lower (≈10″) resolution observations at 3.5 mm by the Berkeley-Illinois-Maryland Array. Chromospheric oscillations are, therefore, not fully resolved at the length-scale of the chromospheric network, and possibly not even at the spatial resolution of our ALMA observations. Any study of transient brightenings in the mm-domain should take into account the oscillations. Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic Configuration of Coronal Mass Ejections: Diagnostics, Challenges and Future Observables Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.; Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2019shin.confE.194P Altcode: Our thinking about the pre-eruptive magnetic configuration of Coronal Mass Ejections has been effectively dichotomized into two opposing and often fiercely contested views: namely, sheared magnetic arcades and magnetic flux ropes. Finding a solution to this issue will have important implications for our understanding of CME initiation. We first discuss the very value of embarking into the arcade vs. flux rope dilemma and illustrate the corresponding challenges and difficulties to address it. Next, we are compiling several observational diagnostics of pre-eruptive sheared magnetic arcades stemming from theory/modeling, discuss their merits, and highlight potential ambiguities that could arise in their interpretation. We finally conclude with a discussion of possible new observables, in the frame of upcoming or proposed instrumentation, that could help to circumvent the issues we are currently facing. Title: Solar physics with the Square Kilometre Array Authors: Nindos, A.; Kontar, E. P.; Oberoi, D. Bibcode: 2019AdSpR..63.1404N Altcode: 2018arXiv181004951N The Square Kilometre Array (SKA) will be the largest radio telescope ever built, aiming to provide collecting area larger than 1 km2. The SKA will have two independent instruments, SKA-LOW comprising of dipoles organized as aperture arrays in Australia and SKA-MID comprising of dishes in South Africa. Currently the phase-1 of SKA, referred to as SKA1, is in its late design stage and construction is expected to start in 2020. Both SKA1-LOW (frequency range of 50-350 MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050, 950-1760, and 4600-15,300 MHz, respectively) are important for solar observations. In this paper we present SKA's unique capabilities in terms of spatial, spectral, and temporal resolution, as well as sensitivity and show that they have the potential to provide major new insights in solar physics topics of capital importance including (i) the structure and evolution of the solar corona, (ii) coronal heating, (iii) solar flare dynamics including particle acceleration and transport, (iv) the dynamics and structure of coronal mass ejections, and (v) the solar aspects of space weather. Observations of the Sun jointly with the new generation of ground-based and space-borne instruments promise unprecedented discoveries. Title: First high-resolution look at the quiet Sun with ALMA at 3mm Authors: Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos, S.; De Pontieu, B.; Warren, H.; Ayres, T.; Hudson, H. S.; Shimizu, T.; Vial, J. -C.; Wedemeyer, S.; Yurchyshyn, V. Bibcode: 2018A&A...619L...6N Altcode: 2018arXiv181005223N We present an overview of high-resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun regions were observed at a resolution of up to 2.5″ by 4.5″. We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full-disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by ∼305 K while the intranetwork is less bright by ∼280 K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 Å images, with somewhat larger size. We detect, for the first time, spicular structures, rising up to 15″ above the limb with a width down to the image resolution and brightness temperature of ∼1800 K above the local background. No trace of spicules, either in emission or absorption, is found on the disk. Our results highlight the potential of ALMA for the study of the quiet chromosphere. Title: Center-to-limb observations of the Sun with ALMA . Implications for solar atmospheric models Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian, T. S. Bibcode: 2017A&A...605A..78A Altcode:
Aims: We seek to derive information on the temperature structure of the solar chromosphere and compare these results with existing models.
Methods: We measured the center-to-limb variation of the brightness temperature, Tb, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, Te as a function of optical depth, τ.
Results: The ALMA images are very similar to AIA images at 1600 Å. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz, respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is 961.1 ± 2.5'' and 964.1 ± 4.5'' at 239 and 100 GHz, respectively. A slight but statistically significant limb brightening is observed at both frequencies.
Conclusions: The inversion of the center-to-limb curves shows that Te varies linearly with the logarithm of optical depth for 0.34 <τ100 GHz< 12, with a slope dTe/ dlnτ = -608 K. Our results are 5% lower than predicted by the average quiet Sun model C of Fontenla et al. (1993, ApJ. 406, 319), but do not confirm previous reports that the mm-λ solar spectrum is better fitted with models of the cell interior. Title: Evidence for two-loop interaction from IRIS and SDO observations of penumbral brightenings Authors: Alissandrakis, C. E.; Koukras, A.; Patsourakos, S.; Nindos, A. Bibcode: 2017A&A...603A..95A Altcode: 2017arXiv170407344A
Aims: We investigate small scale energy release events which can provide clues on the heating mechanism of the solar corona.
Methods: We analyzed spectral and imaging data from the Interface Region Imaging Spectrograph (IRIS), images from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatoty (SDO), and magnetograms from the Helioseismic and Magnetic Imager (HMI) aboard SDO.
Results: We report observations of small flaring loops in the penumbra of a large sunspot on July 19, 2013. Our main event consisted of a loop spanning 15'', from the umbral-penumbral boundary to an opposite polarity region outside the penumbra. It lasted approximately 10 min with a two minute impulsive peak and was observed in all AIA/SDO channels, while the IRIS slit was located near its penumbral footpoint. Mass motions with an apparent velocity of 100 km s-1 were detected beyond the brightening, starting in the rise phase of the impulsive peak; these were apparently associated with a higher-lying loop. We interpret these motions in terms of two-loop interaction. IRIS spectra in both the C II and Si iv lines showed very extended wings, up to about 400 km s-1, first in the blue (upflows) and subsequently in the red wing. In addition to the strong lines, emission was detected in the weak lines of Cl I, O I and C I, as well as in the Mg II triplet lines. Absorption features in the profiles of the C II doublet, the Si iv doublet and the Mg II h and k lines indicate the existence of material with a lower source function between the brightening and the observer. We attribute this absorption to the higher loop and this adds further credibility to the two-loop interaction hypothesis. Tilts were detected in the absorption spectra, as well as in the spectra of Cl I, O I, and C I lines, possibly indicating rotational motions from the untwisting of magnetic flux tubes.
Conclusions: We conclude that the absorption features in the C II, Si iv and Mg II profiles originate in a higher-lying, descending loop; as this approached the already activated lower-lying loop, their interaction gave rise to the impulsive peak, the very broad line profiles and the mass motions.

Movies associated to Figs. A.1-A.3 are available at http://www.aanda.org Title: Center-to-limb observations of the Sun with ALMA Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian, T. S. Bibcode: 2017arXiv170509008A Altcode: We measured the center-to-limb variation of the brightness temperature, $T_b$, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, $T_e$ as a function of optical depth, $\tau$. The ALMA images are very similar to AIA images at 1600Å. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is $961.1\pm2.5$ arcsec and $964.1\pm4.5$ arcsec at 239 and 100 GHz respectively. A slight but statistically significant limb brightening is observed at both frequencies. The inversion of the center-to-limb curves shows that $T_e$ varies linearly with the logarithm of optical depth for $0.34<\tau_{100\,GHz}<12$, with a slope $d\ln T_e/d\tau=-608$ K. Our results are 5% lower than predicted by the average quiet sun model C of Fontenla et al. (1993), but do not confirm previous reports that the mm-$\lambda$ solar spectrum is better fitted with models of the cell interior. Title: Interplanetary Type IV Bursts Authors: Hillaris, A.; Bouratzis, C.; Nindos, A. Bibcode: 2016SoPh..291.2049H Altcode: 2016arXiv160407677H We study the characteristics of moving type IV radio bursts that extend to hectometric wavelengths (interplanetary type IV or type IVIP bursts) and their relationship with energetic phenomena on the Sun. Our dataset comprises 48 interplanetary type IV bursts observed with the Radio and Plasma Wave Investigation (WAVES) instrument onboard Wind in the 13.825 MHz - 20 kHz frequency range. The dynamic spectra of the Radio Solar Telescope Network (RSTN), the Nançay Decametric Array (DAM), the Appareil de Routine pour le Traitement et l' Enregistrement Magnetique de l' Information Spectral (ARTEMIS-IV), the Culgoora, Hiraso, and the Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN) Radio Spectrographs were used to track the evolution of the events in the low corona. These were supplemented with soft X-ray (SXR) flux-measurements from the Geostationary Operational Environmental Satellite (GOES) and coronal mass ejections (CME) data from the Large Angle and Spectroscopic Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). Positional information of the coronal bursts was obtained by the Nançay Radioheliograph (NRH). We examined the relationship of the type IV events with coronal radio bursts, CMEs, and SXR flares. The majority of the events (45) were characterized as compact, their duration was on average 106 minutes. This type of events was, mostly, associated with M- and X-class flares (40 out of 45) and fast CMEs, 32 of these events had CMEs faster than 1000 kms−1. Furthermore, in 43 compact events the CME was possibly subjected to reduced aerodynamic drag as it was propagating in the wake of a previous CME. A minority (three) of long-lived type IVIP bursts was detected, with durations from 960 minutes to 115 hours. These events are referred to as extended or long duration and appear to replenish their energetic electron content, possibly from electrons escaping from the corresponding coronal type IV bursts. The latter were found to persist on the disk, for tens of hours to days. Prominent among them was the unusual interplanetary type IV burst of 18 - 23 May 2002, which is the longest event in the Wind/WAVES catalog. The three extended events were typically accompanied by a number of flares, of GOES class C in their majority, and of CMEs, many of which were slow and narrow. Title: Multi-viewpoint Observations of a Widely distributed Solar Energetic Particle Event: The Role of EUV Waves and White-light Shock Signatures Authors: Kouloumvakos, A.; Patsourakos, S.; Nindos, A.; Vourlidas, A.; Anastasiadis, A.; Hillaris, A.; Sandberg, I. Bibcode: 2016ApJ...821...31K Altcode: On 2012 March 7, two large eruptive events occurred in the same active region within 1 hr from each other. Each consisted of an X-class flare, a coronal mass ejection (CME), an extreme-ultraviolet (EUV) wave, and a shock wave. The eruptions gave rise to a major solar energetic particle (SEP) event observed at widely separated (∼120°) points in the heliosphere. From multi-viewpoint energetic proton recordings we determine the proton release times at STEREO B and A (STB, STA) and the first Lagrange point (L1) of the Sun-Earth system. Using EUV and white-light data, we determine the evolution of the EUV waves in the low corona and reconstruct the global structure and kinematics of the first CME’s shock, respectively. We compare the energetic proton release time at each spacecraft with the EUV waves’ arrival times at the magnetically connected regions and the timing and location of the CME shock. We find that the first flare/CME is responsible for the SEP event at all three locations. The proton release at STB is consistent with arrival of the EUV wave and CME shock at the STB footpoint. The proton release time at L1 was significantly delayed compared to STB. Three-dimensional modeling of the CME shock shows that the particle release at L1 is consistent with the timing and location of the shock’s western flank. This indicates that at L1 the proton release did not occur in low corona but farther away from the Sun. However, the extent of the CME shock fails to explain the SEP event observed at STA. A transport process or a significantly distorted interplanetary magnetic field may be responsible. 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 Major Geoeffective Solar Eruptions of 2012 March 7: Comprehensive Sun-to-Earth Analysis Authors: Patsourakos, S.; Georgoulis, M. K.; Vourlidas, A.; Nindos, A.; Sarris, T.; Anagnostopoulos, G.; Anastasiadis, A.; Chintzoglou, G.; Daglis, I. A.; Gontikakis, C.; Hatzigeorgiu, N.; Iliopoulos, A. C.; Katsavrias, C.; Kouloumvakos, A.; Moraitis, K.; Nieves-Chinchilla, T.; Pavlos, G.; Sarafopoulos, D.; Syntelis, P.; Tsironis, C.; Tziotziou, K.; Vogiatzis, I. I.; Balasis, G.; Georgiou, M.; Karakatsanis, L. P.; Malandraki, O. E.; Papadimitriou, C.; Odstrčil, D.; Pavlos, E. G.; Podlachikova, O.; Sandberg, I.; Turner, D. L.; Xenakis, M. N.; Sarris, E.; Tsinganos, K.; Vlahos, L. Bibcode: 2016ApJ...817...14P Altcode: During the interval 2012 March 7-11 the geospace experienced a barrage of intense space weather phenomena including the second largest geomagnetic storm of solar cycle 24 so far. Significant ultra-low-frequency wave enhancements and relativistic-electron dropouts in the radiation belts, as well as strong energetic-electron injection events in the magnetosphere were observed. These phenomena were ultimately associated with two ultra-fast (>2000 km s-1) coronal mass ejections (CMEs), linked to two X-class flares launched on early 2012 March 7. Given that both powerful events originated from solar active region NOAA 11429 and their onsets were separated by less than an hour, the analysis of the two events and the determination of solar causes and geospace effects are rather challenging. Using satellite data from a flotilla of solar, heliospheric and magnetospheric missions a synergistic Sun-to-Earth study of diverse observational solar, interplanetary and magnetospheric data sets was performed. It was found that only the second CME was Earth-directed. Using a novel method, we estimated its near-Sun magnetic field at 13 R⊙ to be in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun CME magnetic field are required to match the magnetic fields of the corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream solar-wind conditions, as resulting from the shock associated with the Earth-directed CME, offer a decent description of its kinematics. The magnetospheric compression caused by the arrival at 1 AU of the shock associated with the ICME was a key factor for radiation-belt dynamics. Title: SSALMON - The Solar Simulations for the Atacama Large Millimeter Observatory Network Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu, B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.; Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.; Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier, A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M. Bibcode: 2015AdSpR..56.2679W Altcode: 2015arXiv150205601W The Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) was initiated in 2014 in connection with two ALMA development studies. The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful new tool, which can also observe the Sun at high spatial, temporal, and spectral resolution. The international SSALMONetwork aims at co-ordinating the further development of solar observing modes for ALMA and at promoting scientific opportunities for solar physics with particular focus on numerical simulations, which can provide important constraints for the observing modes and can aid the interpretation of future observations. The radiation detected by ALMA originates mostly in the solar chromosphere - a complex and dynamic layer between the photosphere and corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. Potential targets include active regions, prominences, quiet Sun regions, flares. Here, we give a brief overview over the network and potential science cases for future solar observations with ALMA. Title: A tiny event producing an interplanetary type III burst Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.; Kontogeorgos, A.; Tsitsipis, P. Bibcode: 2015A&A...582A..52A Altcode: 2015arXiv150708423A
Aims: We investigate the conditions under which small-scale energy release events in the low corona gave rise to strong interplanetary (IP) type III bursts.
Methods: We analyzed observations of three tiny events, detected by the Nançay Radio Heliograph (NRH), two of which produced IP type III bursts. We took advantage of the NRH positioning information and of the high cadence of AIA/SDO data to identify the associated extreme-UV (EUV) emissions. We measured positions and time profiles of the metric and EUV sources.
Results: We found that the EUV events that produced IP type III bursts were located near a coronal hole boundary, while the one that did not was located in a closed magnetic field region. In all three cases tiny flaring loops were involved, without any associated mass eruption. In the best observed case, the radio emission at the highest frequency (435 MHz) was displaced by ~55'' with respect to the small flaring loop. The metric type III emission shows a complex structure in space and in time, indicative of multiple electron beams, despite the low intensity of the events. From the combined analysis of dynamic spectra and NRH images, we derived the electron beam velocity as well as the height, ambient plasma temperature, and density at the level of formation of the 160 MHz emission. From the analysis of the differential emission measure derived from the AIA images, we found that the first evidence of energy release was at the footpoints, and this was followed by the development of flaring loops and subsequent cooling.
Conclusions: Even small energy release events can accelerate enough electrons to give rise to powerful IP type III bursts. The proximity of the electron acceleration site to open magnetic field lines facilitates the escape of the electrons into the interplanetary space. The offset between the site of energy release and the metric type III location warrants further investigation.

The movie is available in electronic form at http://www.aanda.org Title: Properties of solar energetic particle events inferred from their associated radio emission Authors: Kouloumvakos, A.; Nindos, A.; Valtonen, E.; Alissandrakis, C. E.; Malandraki, O.; Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Hillaris, A. Bibcode: 2015A&A...580A..80K Altcode: 2015arXiv150703776K
Aims: We study selected properties of solar energetic particle (SEP) events as inferred from their associated radio emissions.
Methods: We used a catalogue of 115 SEP events, which consists of entries of proton intensity enhancements at one AU, with complete coverage over solar cycle 23 based on high-energy (~68 MeV) protons from SOHO/ERNE. We also calculated the proton release time at the Sun using velocity dispersion analysis (VDA). After an initial rejection of cases with unrealistic VDA path lengths, we assembled composite radio spectra for the remaining events using data from ground-based and space-borne radio spectrographs. We registered the associated radio emissions for every event, and we divided the events in groups according to their associated radio emissions. In cases of type III-associated events, we extended our study to the timings between the type III radio emission, the proton release, and the electron release as inferred from VDA based on Wind/3DP 20-646 keV data.
Results: The proton release was found to be most often accompanied by both type III and II radio bursts, but a good association percentage was also registered in cases accompanied by type IIIs only. The worst association was found for the cases only associated with type II. In the type III-associated cases, we usually found systematic delays of both the proton and electron release times as inferred by the particles' VDAs, with respect to the start of the associated type III burst. The comparison of the proton and electron release times revealed that, in more than half of the cases, the protons and electrons were simultaneously released within the statistical uncertainty of our analysis. For the cases with type II radio association, we found that the distribution of the proton release heights had a maximum at ~2.5 R⊙. Most (69%) of the flares associated with our SEP events were located in the western hemisphere, with a peak within the well-connected region of 50°-60° western longitude.
Conclusions: Both flare- and shock-related particle release processes are observed in major proton events at >50 MeV. Typically, the protons are released after the start of the associated type III bursts and simultaneously or before the release of energetic electrons. Our study indicates that a clear-cut distinction between flare-related and CME-related SEP events is difficult to establish.

Appendices are available in electronic form at http://www.aanda.org Title: How Common Are Hot Magnetic Flux Ropes in the Low Solar Corona? A Statistical Study of EUV Observations Authors: Nindos, A.; Patsourakos, S.; Vourlidas, A.; Tagikas, C. Bibcode: 2015ApJ...808..117N Altcode: 2015arXiv150703766N We use data at 131, 171, and 304 Å from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory to search for hot flux ropes in 141 M-class and X-class solar flares that occurred at solar longitudes equal to or larger than 50°. Half of the flares were associated with coronal mass ejections. The goal of our survey is to assess the frequency of hot flux ropes in large flares irrespective of their formation time relative to the onset of eruptions. The flux ropes were identified in 131 Å images using morphological criteria and their high temperatures were confirmed by their absence in the cooler 171 and 304 Å passbands. We found hot flux ropes in 45 of our events (32% of the flares); 11 of them were associated with confined flares while the remaining 34 were associated with eruptive flares. Therefore almost half (49%) of the eruptive events involved a hot flux rope configuration. The use of supplementary Hinode X-Ray Telescope data indicates that these percentages should be considered as lower limits of the actual rates of occurrence of hot flux ropes in large flares. Title: Release timescales of solar energetic particles in the low corona Authors: Agueda, N.; Klein, K. -L.; Vilmer, N.; Rodríguez-Gasén, R.; Malandraki, O. E.; Papaioannou, A.; Subirà, M.; Sanahuja, B.; Valtonen, E.; Dröge, W.; Nindos, A.; Heber, B.; Braune, S.; Usoskin, I. G.; Heynderickx, D.; Talew, E.; Vainio, R. Bibcode: 2014A&A...570A...5A Altcode:
Aims: We present a systematic study of the timing and duration of the release processes of near-relativistic (NR; >50 keV) electrons in the low corona.
Methods: We analyze seven well-observed events using in situ measurements by both the ACE and Wind spacecraft and context electromagnetic observations in soft X-rays, radio, hard X-rays and white light. We make use of velocity dispersion analysis to estimate the release time of the first arriving electrons and compare with the results obtained by using a simulation-based approach, taking interplanetary transport effects into account to unfold the NR electron release time history from in situ measurements.
Results: The NR electrons observed in interplanetary space appear to be released during either short (<30 min) or long (>2 h) periods. The observation of NR electron events showing beamed pitch-angle distributions (PADs) during several hours is the clearest observational signature of sustained release in the corona. On the other hand, the in situ observation of PADs isotropizing in less than a couple of hours is a clear signature of a prompt release of electrons in the low corona. Short release episodes appear to originate in solar flares, in coincidence with the timing of the observed type III radio bursts. Magnetic connectivity plays an important role. Only type III radio bursts reaching the local plasma line measured at 1 AU are found to be related with an associated release episode in the low corona. Other type III bursts may also have a release of NR electrons associated with them, but these electrons do not reach L1. Long release episodes appear associated with signatures of long acceleration processes in the low corona (long decay of the soft X-ray emission, type IV radio bursts, and time-extended microwave emission). Type II radio bursts are reported for most of the events and do not provide a clear discrimination between short and long release timescales. Title: SEPServer catalogues of solar energetic particle events at 1 AU based on STEREO recordings: 2007-2012 Authors: Papaioannou, A.; Malandraki, O. E.; Dresing, N.; Heber, B.; Klein, K. -L.; Vainio, R.; Rodríguez-Gasén, R.; Klassen, A.; Nindos, A.; Heynderickx, D.; Mewaldt, R. A.; Gómez-Herrero, R.; Vilmer, N.; Kouloumvakos, A.; Tziotziou, K.; Tsiropoula, G. Bibcode: 2014A&A...569A..96P Altcode: The Solar Terrestrial Relations Observatory (STEREO) recordings provide an unprecedented opportunity to study the evolution of solar energetic particle (SEP) events from different observation points in the heliosphere, allowing one to identify the effects of the properties of the interplanetary magnetic field (IMF) and solar wind structures on the interplanetary transport and acceleration of SEPs. Two catalogues based on STEREO recordings, have been compiled as a part of the SEPServer project, a three-year collaborative effort of eleven European partners funded under the Seventh Framework Programme of the European Union (FP7/SPACE). In particular, two instruments on board STEREO have been used to identify all SEP events observed within the descending phase of solar cycle 23 and the rising phase of solar cycle 24 from 2007 to 2012, namely: the Low Energy Telescope (LET) and the Solar Electron Proton Telescope (SEPT). A scan of STEREO/LET protons within the energy range 6-10 MeV has been performed for each of the two STEREO spacecraft. We have tracked all enhancements that have been observed above the background level of this particular channel and cross-checked with available lists of interplanetary coronal mass ejections (ICMEs), stream interaction regions (SIRs), and shocks, as well as with the reported events in literature. Furthermore, parallel scanning of the STEREO near relativistic electrons has been performed in order to pinpoint the presence (or absence) of an electron event in the energy range of 55-85 keV, for all of the aforementioned proton events included in our lists. We provide the onset and peak time as well as the peak value of all events for both protons and electrons, the relevant solar associations in terms of electromagnetic emissions, soft and hard X-rays (SXRs and HXRs). Finally, a subset of events with clear recordings at both STEREO spacecraft is presented together with the parent solar events of these multispacecraft SEP events. Title: Parametric study of drag force acting on interplanetary CME Authors: Podladchikova, O.; Patsourakos, S.; Nindos, A. Bibcode: 2013hell.confR..22P Altcode: The interaction of an interplanetary coronal mass ejection (ICME) with the solar wind leads to an equalisation of the ICME and solar wind velocities at 1 AU. The forces acting on ICMEs have been evaluated so far in terms of an empirical drag coefficient C_D ~ 1 that describes the aerodynamic drag experienced by a typical ICME due to its interaction with the ambient solar wind. The consideration of viscous drag coefficients due to proton-magnetic kink encounters is more realistic for solar wind turbulence. We compare aerodynamic and viscous drag description and their impact on ICME propagation in solar wind. We also consider the impact of ICME distortions to their kinematics as they propagate in the inner heliosphere. Title: The spatial relationship between coronal mass ejections and solar flares Authors: Nikou, E.; Nindos, A.; Patsourakos, S. Bibcode: 2013hell.conf...21N Altcode: Using 19 well-observed eruptions that gave both coronal mass ejections (CMEs) and flares, we quantified the spatial relationship between pairs of CMEs and associated flares. The flare and CME source locations were identified using images obtained at 174 A by the SWAP instrument aboard PROBA 2 satellite. The SWAP data are suitable for this study because flare emission does not saturate much. To reduce saturation even more, our database did not contain any M-class or X-class flare events. We selected eruptions that occurred close to disk center, as viewed from Earth, whereas they appeared as limb events in images obtained by the EUV Imagers (EUVI) aboard the SECCHI/STEREO spacecraft. The centroids of the CME-associated EUV dimmings in the SWAP images were used as proxies for the CME source locations. For each event, we compared the location of the flare brightenings with the location of the dimmings' centroid at the time of CME initiation which was determined from the EUVI data. In six cases the CME location was cospatial with flare brightenings while in the remaining cases the distance between each pair of flare-CME locations varied from 4 to 191 arcsecs with a median value of 71 arcsecs. Furthermore, we investigated the CME source locations with respect to the underlying magnetic field structures. Title: Sun-to-Earth Analysis of a Major Geoeffective Solar Eruption within the Framework of the Authors: Patsourakos, S.; Vlahos, L.; Georgoulis, M.; Tziotziou, K.; Nindos, A.; Podladchikova, O.; Vourlidas, A.; Anastasiadis, A.; Sandberg, I.; Tsinganos, K.; Daglis, I.; Hillaris, A.; Preka-Papadema, P.; Sarris, M.; Sarris, T. Bibcode: 2013hell.conf...10P Altcode: Transient expulsions of gigantic clouds of solar coronal plasma into the interplanetary space in the form of Coronal Mass Ejections (CMEs) and sudden, intense flashes of electromagnetic radiation, solar flares, are well-established drivers of the variable Space Weather. Given the innate, intricate links and connections between the solar drivers and their geomagnetic effects, synergistic efforts assembling all pieces of the puzzle along the Sun-Earth line are required to advance our understanding of the physics of Space Weather. This is precisely the focal point of the Hellenic National Space Weather Research Network (HNSWRN) under the THALIS Programme. Within the HNSWRN framework, we present here the first results from a coordinated multi-instrument case study of a major solar eruption (X5.4 and X1.3 flares associated with two ultra-fast (>2000 km/s) CMEs) which were launched early on 7 March 2012 and triggered an intense geomagnetic storm (min Dst =-147 nT) approximately two days afterwards. Several elements of the associated phenomena, such as the flare and CME, EUV wave, WL shock, proton and electron event, interplanetary type II radio burst, ICME and magnetic cloud and their spatiotemporal relationships and connections are studied all way from Sun to Earth. To this end, we make use of satellite data from a flotilla of solar, heliospheric and magnetospheric missions and monitors (e.g., SDO, STEREO, WIND, ACE, Herschel, Planck and INTEGRAL). We also present our first steps toward formulating a cohesive physical scenario to explain the string of the observables and to assess the various physical mechanisms than enabled and gave rise to the significant geoeffectiveness of the eruption. Title: Magnetic helicity ejections and coronal activity Authors: Nindos, A. Bibcode: 2013IAUS..294..519N Altcode: Magnetic helicity quantifies the degree of linkage and/or twistedness in the magnetic field. It is probably the only physical quantity which is approximately conserved even in resistive MHD. This makes it an ideal tool for the exploration of the physics of solar eruptions. In this article, I discuss the sources of magnetic helicity injected into active regions and I point out that coronal mass ejections (CMEs) are probably necessary to remove at least part of the excess helicity produced in the Sun. I also discuss the importance of magnetic helicity in the overall coronal evolution that may lead to eruptions. Title: Radio Signatures of Solar Energetic Particles During the 23^rd Solar Cycle Authors: Miteva, R.; Klein, K. -L.; Samwel, S. W.; Nindos, A.; Kouloumvakos, A.; Reid, H. Bibcode: 2013CEAB...37..541M Altcode: 2014arXiv1402.6442M We present the association rates between solar energetic particles (SEPs) and the radio emission signatures in the corona and IP space during the entire solar cycle 23. We selected SEPs associated with X and M-class flares from the visible solar hemisphere. All SEP events are also accompanied by coronal mass ejections. Here, we focus on the correlation between the SEP events and the appearance of radio type II, III and IV bursts on dynamic spectra. For this we used the available radio data from ground-based stations and the Wind/WAVES spacecraft. The associations are presented separately for SEP events accompanying activity in the eastern and western solar hemisphere. We find the highest association rate of SEP events to be with type III bursts, followed by types II and IV. Whereas for types III and IV no longitudinal dependence is noticed, these is a tendency for a higher SEP-association rate with type II bursts in the eastern hemisphere. A comparison with reports from previous studies is briefly discussed. Title: Scientific Analysis within SEPServer - New Perspectives in Solar Energetic Particle Research: The Case Study of the 13 July 2005 Event Authors: Malandraki, O. E.; Agueda, N.; Papaioannou, A.; Klein, K. -L.; Valtonen, E.; Heber, B.; Dröge, W.; Aurass, H.; Nindos, A.; Vilmer, N.; Sanahuja, B.; Kouloumvakos, A.; Braune, S.; Preka-Papadema, P.; Tziotziou, K.; Hamadache, C.; Kiener, J.; Tatischeff, V.; Riihonen, E.; Kartavykh, Y.; Rodríguez-Gasén, R.; Vainio, R. Bibcode: 2012SoPh..281..333M Altcode: 2012SoPh..tmp..287M; 2012SoPh..tmp..267M Solar energetic particle (SEP) events are a key ingredient of solar-terrestrial physics both for fundamental research and space weather applications. Multi-satellite observations are an important and incompletely exploited tool for studying the acceleration and the coronal and interplanetary propagation of the particles. While STEREO uses for this diagnostic two identical sets of instrumentation, there are many earlier observations carried out with different spacecraft. It is the aim of the SEPServer project to make these data and analysis tools available to a broad user community. The consortium will carry out data-driven analysis and simulation-based data analysis capable of deconvolving the effects of interplanetary transport and solar injection from SEP observations, and will compare the results with the electromagnetic signatures. The tools and results will be provided on the web server of the project in order to facilitate further analysis by the research community. This paper describes the data products and analysis strategies with one specific event, the case study of 13 July 2005. The release time of protons and electrons are derived using data-driven and simulation-based analyses, and compared with hard X-ray and radio signatures. The interconnection of the experimental and the simulation-based results are discussed in detail. Title: Scientific analysis within SEPServer: the 13 July 2005 SEP event case study Authors: Malandraki, O. E.; Valtonen, E.; Agueda, N.; Papaioannou, A.; Klein, K. -L.; Heber, B.; Droege, W.; Aurass, H.; Nindos, A.; Vilmer, N.; Sanahuja, B.; Kouloumvakos, A.; Braune, S.; Preka-Papadema, P.; Tziotziou, K.; Hamadache, C.; Kiener, J.; Tatischeff, V.; Kartavykh, J.; Vainio, R. Bibcode: 2012EGUGA..14.9001M Altcode: SEPServer is set out to make the first database of particle and corresponding EM observations of solar energetic particle (SEP) events over roughly three solar cycles. It will also provide users with results from the scientific analysis of multiple datasets using different observational and simulation based methods. Therefore, SEPServer will lead to new perspectives of scientific analysis and will serve as a new asset valuable for SEP and Space Weather research. In this contribution, the event of 13 July 2005 has been used as a case study, which is a proxy for the overall information that the SEPServer will include and at the same time it reveals the capabilities offered to the future users of SEPServer. The analysis of the 13 July 2005 event - focusing on the data driven analysis, i.e., onset and release time determination from SOHO/ERNE, SOHO/EPHIN and ACE/EPAM together with pitch angle distributions from ACE/EPAM, simulations based on WIND/3DP and ACE/EPAM electrons as well as direct comparison of the observed SEP fluxes with the associated electromagnetic emissions - is performed. The physical interpretation and the interconnection of the experimental and the simulation based results are discussed in detail. The 13 July 2005 case study exemplifies the future usage of SEPServer, which will provide a comprehensive and up to date SEP analysis service. Acknowledgements: The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement No 262773 (SEPServer). Title: On the Role of the Background Overlying Magnetic Field in Solar Eruptions Authors: Nindos, A.; Patsourakos, S.; Wiegelmann, T. Bibcode: 2012ApJ...748L...6N Altcode: The primary constraining force that inhibits global solar eruptions is provided by the overlying background magnetic field. Using magnetic field data from both the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory and the spectropolarimeter of the Solar Optical Telescope aboard Hinode, we study the long-term evolution of the background field in active region AR11158 that produced three major coronal mass ejections (CMEs). The CME formation heights were determined using EUV data. We calculated the decay index -(z/B)(∂B/∂z) of the magnetic field B (i.e., how fast the field decreases with height, z) related to each event from the time of the active region emergence until well after the CMEs. At the heights of CME formation, the decay indices were 1.1-2.1. Prior to two of the events, there were extended periods (of more than 23 hr) where the related decay indices at heights above the CME formation heights either decreased (up to -15%) or exhibited small changes. The decay index related to the third event increased (up to 118%) at heights above 20 Mm within an interval that started 64 hr prior to the CME. The magnetic free energy and the accumulated helicity into the corona contributed the most to the eruptions by their increase throughout the flux emergence phase (by factors of more than five and more than two orders of magnitude, respectively). Our results indicate that the initiation of eruptions does not depend critically on the temporal evolution of the variation of the background field with height. Title: Preface Authors: Marqué, C.; Nindos, A.; van Driel-Gesztelyi, L.; Mandrini, C. H. Bibcode: 2012esrs.book....1M Altcode: No abstract at ADS Title: Radio Emission Associated with Solar Energetic Particle Events Authors: Kouloumvakos, A.; Nindos, A.; Preka-Papadema, P.; Hillaris, A.; Caroubalos, C.; Moussas, X.; Alissandrakis, C.; Tsitsipis, P.; Kontogeorgos, A. Bibcode: 2012hell.confR..12K Altcode: The corona probed at meter and decimeter wavelengths is a crucial region for the acceleration and propagation of solar energetic particles (SEPs), and radio diagnostics in this plasma plays a major role in assessing the origin of SEP events. Using data from the ARTEMIS IV solar radio spectro-graph, we report the properties of the radio emission associated with several major SEP events. The association of the radio emission with the related flares and CMEs is also investigated. Title: Multi-wavelength Observations of a Metric Type-II Event Authors: Alissandrakis, C.; Nindos, A.; Patsourakos, S.; Hillaris, Al.; Artemis Group Bibcode: 2012hell.conf....6A Altcode: We have studied a complex metric radio event,observed with the ARTEMIS radiospectrogarph on February 12, 2010. The event was associated with a surge observed at 195 and 304 A and with a coronal mass ejection observed by STEREO A and B instruments near the East wnd West limbs respectively. On the disk the event was observed at 10 frequencies by the Nancay Radioheliograph (NRH), in H-alpha by the Catania observatory and in soft x-rays by GOES SXI. We combined these data, together with MDI longitudinal magnetic field, to get as complete a picture of the event as possible. Our emphasis is on two type-II bursts that occured near respective maxima in the GOES light curves. The first, associated with the main peak of the event, showed a clear foundamental-harmonic structure, while the emission of the second consisted of three well-separated bands. Using positional infornation for the type-IIs from the NRH we explore their possible association with the surge, the coronal front and the CME. We also studied fine structured and foundamental harmonic structure in the metric dynamic spectrum. Title: Preface Authors: Marqué, C.; Nindos, A.; van Driel-Gesztelyi, L.; Mandrini, C. H. Bibcode: 2011SoPh..273..307M Altcode: 2011SoPh..tmp..385M No abstract at ADS Title: On the relationship of shock waves to flares and coronal mass ejections Authors: Nindos, A.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P. Bibcode: 2011A&A...531A..31N Altcode: 2011arXiv1105.1268N Context. Metric type II bursts are the most direct diagnostic of shock waves in the solar corona.
Aims: There are two main competing views about the origin of coronal shocks: that they originate in either blast waves ignited by the pressure pulse of a flare or piston-driven shocks due to coronal mass ejections (CMEs). We studied three well-observed type II bursts in an attempt to place tighter constraints on their origins.
Methods: The type II bursts were observed by the ARTEMIS radio spectrograph and imaged by the Nançay Radioheliograph (NRH) at least at two frequencies. To take advantage of projection effects, we selected events that occurred away from disk center.
Results: In all events, both flares and CMEs were observed. In the first event, the speed of the shock was about 4200 km s-1, while the speed of the CME was about 850 km s-1. This discrepancy ruled out the CME as the primary shock driver. The CME may have played a role in the ignition of another shock that occurred just after the high speed one. A CME driver was excluded from the second event as well because the CMEs that appeared in the coronagraph data were not synchronized with the type II burst. In the third event, the kinematics of the CME which was determined by combining EUV and white light data was broadly consistent with the kinematics of the type II burst, and, therefore, the shock was probably CME-driven.
Conclusions: Our study demonstrates the diversity of conditions that may lead to the generation of coronal shocks. Title: Magnetic helicity of solar active regions Authors: Nindos, A. Bibcode: 2009IAUS..257..133N Altcode: Magnetic helicity is a quantity that describes the linkage and twistedness/shear in the magnetic field. It has the unique feature that it is probably the only physical quantity which is approximately conserved even in resistive MHD. This makes magnetic helicity an ideal tool for the exploration of the physics of eruptive events. The concept of magnetic helicity can be used to monitor the whole history of a CME event from the emergence of twisted magnetic flux from the convective zone to the eruption and propagation of the CME into interplanetary space. In this article, I discuss the sources of the magnetic helicity injected into active regions and the role of magnetic helicity in the initiation of solar eruptions. Title: Radio Emission of Flares and Coronal Mass Ejections. Invited Review Authors: Nindos, A.; Aurass, H.; Klein, K. -L.; Trottet, G. Bibcode: 2008SoPh..253....3N Altcode: We review recent progress on our understanding of radio emission from solar flares and coronal mass ejections (CMEs) with emphasis on those aspects of the subject that help us address questions about energy release and its properties, the configuration of flare - CME source regions, coronal shocks, particle acceleration and transport, and the origin of solar energetic particle (SEP) events. Radio emission from electron beams can provide information about the electron acceleration process, the location of injection of electrons in the corona, and the properties of the ambient coronal structures. Mildly relativistic electrons gyrating in the magnetic fields of flaring loops produce radio emission via the gyrosynchrotron mechanism, which provides constraints on the magnetic field and the properties of energetic electrons. CME detection at radio wavelengths tracks the eruption from its early phase and reveals the participation of a multitude of loops of widely differing scale. Both flares and CMEs can ignite shock waves and radio observations offer the most robust tool to study them. The incorporation of radio data into the study of SEP events reveals that a clear-cut distinction between flare-related and CME-related SEP events is difficult to establish. Title: A Statistical Study of Microwave Flare Morphologies Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E. Bibcode: 2008SoPh..253...79T Altcode: 2008SoPh..tmp..166T This study has been motivated by the detection of a small number of optically thin microwave bursts with maximum emission near the loop top, which is contrary to the prediction of isotropic gyrosynchrotron models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution images at 17 and 34 GHz, we study the morphology at the radio peak of 104 flares that occurred relatively close to the limb. Using data from the Nobeyama Polarimeter we were able to determine whether the 17- and 34-GHz emissions came from optically thin or thick sources. We identified single-loop events, taking into account supplementary information from EUV and soft X-ray (SXR) images. We found optically thin emission from the top of the loop in 36% of single-loop events. In agreement with standard models, in this sample 46% and 18% of the events showed optically thin emission from the footpoints and optically thick emission from the entire loop, respectively. The derived percentage of events with gyrosynchrotron emission from isotropic populations of energetic electrons is possibly an upper limit. This point is illustrated by the analysis of an optically thin event that shows footpoint emission during the rise phase and loop-top emission during the decay phase. A model that takes into account both anisotropies in the distribution function of nonthermal electrons and time evolution can reproduce the observed transition from footpoint to loop-top morphology, if electrons with pitch-angle anisotropy are injected near one of the footpoints. Title: Studying The Magnetic Origins Of Solar Eruptions Using "Solar Orbiter" Authors: Nindos, A. Bibcode: 2007ESASP.641E..20N Altcode: Recent studies have provided observational support for the paradigm that solar eruptions result from the inter- play between magnetic reconnection and the approximate magnetic helicity conservation in the corona: confined flares are regarded as reconnection events while CMEs are the valves through which the Sun gets rid of excess helicity. However, several aspects of the paradigm need clarification and possibly modification. The instruments onboard the "Solar Orbiter" mission will provide a unique opportunity for such work. In this article, I propose how to exploit the capabilities of the "Solar Orbiter" remote sensing instruments in order to shed more light on the processes leading to flares and CMEs. Title: Flare Physics with the Nobeyama Radioheliograph and RHESSIs Authors: Nindos, A. Bibcode: 2006spnr.conf...39N Altcode: In this article I review the present status of knowledge in areas of special interest for further study using solar flare observations at microwaves from the Nobeyama Radioheliograph and hard X-ray (HXR) observations from RHESSI. The most direct tracers of the electrons accelerated in a flare are the microwave and HXR emissions they produce. The combined microwave and hard X-ray data provide powerful diagnostics of the physical conditions in flaring regions and particularly the magnetic field configuration of the flare and the properties of the energetic electrons. I point out that in order to tackle better the fundamental problem of particle acceleration in solar flares, detailed comparison of the spatial and spectral data with models is needed. Title: Magnetic Helicity and Coronal Mass Ejections Authors: Nindos, A. Bibcode: 2006GMS...165...59N Altcode: Magnetic helicity is a quantity that descibes the chiral properties of magnetic structures. It has the unique feature that it is probably the only physical quantity which is approximately conserved even in resistive MHD. This makes magnetic helicity an ideal tool for the exploration of the physics of coronal mass ejections (CMEs). CMEs carry away from the Sun twisted magnetic fields and the concept of helicity can be used to monitor the whole history of a CME event from the emergence of twisted magnetic flux from the convective zone to the eruption and propagation of the CME into interplanetary space. I discuss the sources of the helicity shed by CMEs and the role of magnetic helicity in the initiation of CMEs. Title: A Statistical Study of Microwave Flare Morphologies Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E.; Shibasaki, K. Bibcode: 2006AIPC..848..248T Altcode: Using Nobeyama Radioheliograph (NoRH) high spatial resolution images at 17 and 34 GHz, we study the morphology of several flare events that occured relatively close to the limb. Our study has been motivated by the recent detection of a small number of optically thin flares whose maximum emission peaks close to the loop top. These events may show significant anisotropy of the density and pitch angle distribution of the nonthermal electrons. However, we do not know how often they occur. Our sample consists of 104 flare events. Using data from the Nobeyama Polarimeter we were able to determine whether the 17 and 34 GHz emissions are optically thin or thick. Almost half of our events appear unresolved in the NoRH images. Among the resolved events, special attention is paid to those with circular polarization (V) showing two distinct sources. Such V sources are considered proxies to the flaring loop footpoints if the total intensity (I) morphology is consistent with a single flaring loop configuration. A small number of events, as expected at such high frequencies, are optically thick with I maximum between the loop footpoints. The number of optically thin events showing I maxima co-spatial with the V sources is approximately the same as the number of those optically thin resolved events showing emission peak between the V sources. Our future plans concerning the study of our database are also outlined. Title: Ten Years of the Solar Radiospectrograph ARTEMIS-IV Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis, J.; Moussas, X.; Tsitsipis, P.; Kontogeorgos, A.; Petoussis, V.; Bouratzis, C.; Bougeret, J. -L.; Dumas, G.; Nindos, A. Bibcode: 2006AIPC..848..864C Altcode: 2010arXiv1009.4150C The Solar Radiospectrograph of the University of Athens (ARTEMIS-IV) is in operation at the Thermopylae Satellite Communication Station since 1996. The observations extend from the base of the Solar Corona (650 MHz) to about 2 Solar Radii (20 MHz) with time resolution 1/10-1/100 sec. The instruments recordings, being in the form of dynamic spectra, measure radio flux as a function of height in the corona; our observations are combined with spatial data from the Nancay Radioheliograph whenever the need for 3D positional information arises. The ARTEMIS-IV contribution in the study of solar radio bursts is two fold- Firstly, in investigating new spectral characteristics since its high sampling rate facilitates the study of fine structures in radio events. On the other hand it is used in studying the association of solar bursts with interplanetary phenomena because of its extended frequency range which is, furthermore, complementary to the range of the WIND/WAVES receivers and the observations may be readily combined. This reports serves as a brief account of this operation. Joint observations with STEREO/WAVES and LOFAR low frequency receivers are envisaged in the future. Title: The Role of Magnetic Helicity in Solar Eruptions Authors: Nindos, A. Bibcode: 2006AIPC..848...64N Altcode: Magnetic helicity is a quantity that descibes the chiral properties of magnetic structures. It has the unique feature that it is probably the only physical quantity which is approximately conserved even in resistive MHD. This makes magnetic helicity an ideal tool for the exploration of the physics of coronal mass ejections (CMEs). CMEs carry away from the Sun twisted magnetic fields and the concept of helicity can be used to monitor the whole history of a CME event from the emergence of twisted magnetic flux from the convective zone to the eruption and propagation of the CME into interplanetary space. I discuss the sources of the helicity shed by CMEs and the role of magnetic helicity in the initiation of CMEs. Title: What is the spatial distribution of magnetic helicity injected in a solar active region? Authors: Pariat, E.; Nindos, A.; Démoulin, P.; Berger, M. A. Bibcode: 2006A&A...452..623P Altcode: Context: .Magnetic helicity is suspected to play a key role in solar phenomena such as flares and coronal mass ejections. Several investigations have recently computed the photospheric flux of magnetic helicity in active regions. The derived spatial maps of the helicity flux density, called G_A, have an intrinsic mixed-sign patchy distribution.
Aims: . Pariat et al. (2005) recently showed that GA is only a proxy of the helicity flux density, which tends to create spurious polarities. They proposed a better proxy, Gθ. We investigate here the implications of this new approach on observed active regions.
Methods: . The magnetic data are from MDI/SoHO instrument and the photospheric velocities are computed by local correlation tracking. Maps and temporal evolution of GA and Gθ are compared using the same data set for 5 active regions.
Results: . Unlike the usual GA maps, most of our Gθ maps show almost unipolar spatial structures because the nondominant helicity flux densities are significantly suppressed. In a few cases, the Gθ maps still contain spurious bipolar signals. With further modelling we infer that the real helicity flux density is again unipolar. On time-scales larger than their transient temporal variations, the time evolution of the total helicity fluxes derived from GA and Gθ show small differences. However, unlike G_A, with Gθ the time evolution of the total flux is determined primarily by the predominant-signed flux while the nondominant-signed flux is roughly stable and probably mostly due to noise.
Conclusions: .Our results strongly support the conclusion that the spatial distribution of helicity injected into active regions is much more coherent than previously thought: on the active region scale the sign of the injected helicity is predominantly uniform. These results have implications for the generation of the magnetic field (dynamo) and for the physics of both flares and coronal mass ejections. Title: Multiwavelength Analysis of an Active Region Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th. G.; Alissandrakis, C. E.; Nindos, A.; Vial, J. -C.; Tsiropoula, G. Bibcode: 2006SoPh..233...57G Altcode: We study active region NOAA 8541, observed with instruments on board SOHO, as well as with TRACE. The data set mainly covers the transition region and the low corona. In selected loops studied with SUMER on SOHO, the VIII 770 Å line is systematically redshifted. In order to estimate the plasma velocity, we combine the Doppler shifts with proper motions (TRACE) along these loops. In the case of an ejection, apparently caused by the emergence of a parasitic polarity, proper motions and Doppler shifts give consistent results for the velocity. A cooler loop, observed in the same active region with CDS, shows a unidirectional motion reminiscent of a siphon flow. The derived electron temperature and density along a large steady loop confirm that it cannot be described by hydrostatic models. Title: When does the first CME from an emerging active region occur? Authors: Nindos, A. Bibcode: 2006cosp...36.1270N Altcode: 2006cosp.meet.1270N Recently we have found that in a statistical sense the preflare value of the coronal magnetic helicity of active regions producing CME-associated big flares is larger than the value of magnetic helicity of those producing big flares that do not have associated CMEs The obvious question arising from the above result is whether CME initiation is controlled by the amount of the magnetic helicity stored in the corona This problem can be investigated by studying whether the active region itself that forms after a bipole emergence erupts or not and what magnetic helicity has to do with the eruption To this end we have studied several active regions that emerge on the visible side of the solar disk For each active region its magnetic helicity content is monitored from its emergence until it reaches a heliographic longitude of 45-50 degrees or until the occurrence of the first CME associated with it whichever happens first The main statistical result of our study suggests that when CMEs occur the coronal magnetic helicity is larger than the coronal magnetic helicity of those active regions that do not produce any CMEs This result is independent of the strength of CME-associated flares As a byproduct of this study we have devoped a new technique that is able to recover a significant part of the magnetic helicity injected into the corona which is not detectable using conventional techniques The new method has been tested against synthetic data and it will be briefly presented Title: How to derive the real pattern of magnetic helicity injection in an active region? Authors: Pariat, E.; Nindos, A.; Démoulin, P.; Berger, M. Bibcode: 2006cosp...36..851P Altcode: 2006cosp.meet..851P Magnetic helicity a topological quantity which measures the twist the writhe and the shear of a magnetic field has recently appeared as a key quantity to understand some mechanisms of the solar activity such as Coronal Mass Ejections and flare onset It is thus becoming of major importance to be able to compute magnetic helicity in active regions Looking at the pattern of the photospheric injection of magnetic helicity may provide new useful pieces of information to understand the basic properties of solar activity If several helicity flux density maps were published no one yet wondered if helicity flux density is a correct physical quantity Unfortunately the classical helicity flux density do induce spurious signal fake polarities which mask the real injection of helicity To map the real helicity injection the knowledge of the complete connectivity of the field lines is fundamental Even without the connectivity improved helicity flux density maps can be derived which present strong differences with the previous incorrect maps This leads to a complete new way of understanding the dynamics of the active region in the frame of the magnetic helicity study Title: Complex Solar Events Observed with the Artemis-Iv Radio-Spectrograph in October/november 2003 Authors: Alissandrakis, C. E.; Nindos, A.; Hilaris, A.; Caroubalos, C.; Artemis Team Bibcode: 2005ESASP.600E.106A Altcode: 2005dysu.confE.106A; 2005ESPM...11..106A No abstract at ADS Title: Coronal Magnetography of Solar Active Region 8365 with the SSRT and NoRH Radio Heliographs Authors: Ryabov, B. I.; Maksimov, V. P.; Lesovoi, S. V.; Shibasaki, K.; Nindos, A.; Pevtsov, A. Bibcode: 2005SoPh..226..223R Altcode: Microwave maps of solar active region NOAA 8365 are used to derive the coronal magnetograms of this region. The technique is based on the fact that the circular polarization of a radio source is modified when microwaves pass through the coronal magnetic field transverse to the line of sight. The observations were taken with the Siberian Solar Radio Telescope (SSRT) on October 21 - 23 and with the Nobeyama Radio Heliograph (NoRH) on October 22 - 24, 1998. The known theory of wave mode coupling in quasi-transverse (QT) region is employed to evaluate the coronal magnetograms in the range of 10 - 30 G at the wavelength 5.2 cm and 50 - 110 G at 1.76 cm, taking the product of electron density and the scale of coronal field divergence to be constant of 1018 cm−2. The height of the QT-region is estimated from the force-free field extrapolations as 6.2 × 109 cm for the 20 G and 2.3 × 109 cm for 85 G levels. We find that on large spatial scale, the coronal magnetograms derived from the radio observations show similarity with the magnetic fields extrapolated from the photosphere. Title: The Association of Big Flares and Coronal Mass Ejections: What is the Role of Magnetic Helicity? Authors: Nindos, A.; Andrews, M. D. Bibcode: 2005IAUS..226..194N Altcode: In a recent study Andrews found that approximately 40% of M-class flares between 1996 and 1999, classified according to GOES X-ray flux, are not associated with Coronal Mass Ejections (CMEs). Using 133 events from his dataset for which suitable photospheric magnetograms and coronal images were available, we studied the pre-flare coronal helicity of the active regions that produced big flares. The coronal magnetic field of 78 active regions was modeled under the "constant α" linear force-free field assumption. We find that in a statistical sense the pre-flare value of α and coronal helicity of the active regions producing big flares that do not have associated CMEs is smaller than the coronal helicity of those producing CME-associated big flares. A further argument supporting this conclusion is that for the active regions whose coronal magnetic field deviates from the force-free model, the change of the coronal sign of α within an active region is twice more likely to occur when the active region is about to produce a confined flare than a CME-associated flare. Our study indicates that the amount of the stored pre-flare coronal helicity may determine whether a big flare will be eruptive or confined. Title: The Association of Big Flares and Coronal Mass Ejections: What Is the Role of Magnetic Helicity? Authors: Nindos, A.; Andrews, M. D. Bibcode: 2004ApJ...616L.175N Altcode: Recently, M. D. Andrews found that approximately 40% of M-class flares between 1996 and 1999, classified according to GOES X-ray flux, are not associated with coronal mass ejections (CMEs). Using 133 events from his data set for which suitable photospheric magnetograms and coronal images were available, we studied the preflare coronal helicity of the active regions that produced big flares. The coronal magnetic field of 78 active regions was modeled under the ``constant α'' linear force-free field assumption. We find that in a statistical sense the preflare value of α and coronal helicity of the active regions producing big flares that do not have associated CMEs is smaller than the coronal helicity of those producing CME-associated big flares. A further argument supporting this conclusion is that for the active regions whose coronal magnetic field deviates from the force-free model, the change of the coronal sign of α within an active region is twice as likely to occur when the active region is about to produce a confined flare than a CME-associated flare. Our study indicates that the amount of the stored preflare coronal helicity may determine whether a big flare will be eruptive or confined. Title: The configuration of simple short-duration solar microwave bursts Authors: Kundu, M. R.; Nindos, A.; Grechnev, V. V. Bibcode: 2004A&A...420..351K Altcode: Using data from the Nobeyama Radioheliograph (NoRH) we study the source configuration of four simple short-duration 17 and 34 GHz bursts which have also been observed partially by the Yohkoh Soft X-ray Telescope (SXT). Two events are consistent with a single flaring loop configuration. In one of them the flaring loop is resolved in the SXT images. We derive a self-consistent model for this event by comparing the radio observations with gyrosynchrotron model loop calculations. Our best-fit model is able to reproduce both the observed flaring loop shape as well as the fluxes and structures of the radio emission at the peak of the event. The flaring loop is relatively small having a footpoint separation of 16 arcsec and maximum height of 7.7 arcsec. The variation of the magnetic field along the loop is small (800 G at the footpoints and 665 G at loop top) and the loop is filled with electrons with energies up to 10 MeV. The other two bursts show two radio sources; one source being cospatial with a compact bright soft X-ray loop associated with a patch of parasitic magnetic polarity whose photospheric magnetic flux increases before the flares while the other source is not prominent at any other wavelength range. The two sources are connected with diffuse loop-like soft X-ray emission. We infer that these bursts originate from the interaction of two sets of loops with different sizes. Therefore the simple short duration microwave bursts we studied do not always appear in the same configuration. Contrary to previous results not all of them appear as single-loop events. It is possible that some events are caused by two interacting loops. Title: The Association of Big Flares and CMEs: What is the Role of Magnetic Helicity? Authors: Nindos, A.; Andrews, M. D. Bibcode: 2004AAS...204.2703N Altcode: 2004BAAS...36..693N In a recent study Andrews (2003) found that approximately 40% of the GOES soft X-ray M-class flares observed during the years 1996-1999 are not associated with CMEs. In this paper, we investigate the role of magnetic helicity in the association of big flares and CMEs. We compute the coronal relative magnetic helicity of all active regions which were located within ±50 degr from the central meridian and produced M-class flares during the years 1996-1999. The computations use SOHO/MDI magnetograms taken close to the initiation of each event as boundary conditions for "constant alpha" linear force-free field magnetic extrapolations. The extrapolated field lines are fitted with the corresponding active region's coronal loops observed with EIT at 195 A. Then the value of alpha giving the best fit between the models and the coronal observations is adopted for the computation of the coronal magnetic helicity. Within the uncertainties involved in our computations, we find that the coronal magnetic helicity of about 65-70% of the active regions which gave M-class flares without CMEs is smaller than the coronal magnetic helicity of all active regions which gave M-class flares associated with CMEs. Title: Coronal Radio Magnetography of Solar Active Region 8365 Authors: Ryabov, B. I.; Nindos, A.; Shibasaki, K.; Maksimov, V. P.; Lesovoi, S. V.; Pevtsov, A. A. Bibcode: 2003AGUFMSH42B0508R Altcode: Using the microwave radio observations with Siberian Solar Radio Telescope (SSRT) and Nobeyama Radio Heliograph (NoRH) we derived 2D coronal magnetograms of solar active region NOAA 8365. The circular polarization (CP) of radio source is modified, when the radiation passes through the overlying magnetic field transverse to the line-of-sight. This change in CP was used to study the properties of coronal fields. We employed the known theory of wave-mode coupling in quasi-transverse (QT) region to evaluate the distribution of the field strength at the level of transformation of 5.2 cm radiation (SSRT, field strength ∼ 10-30 G) and 1.76 cm (NoRH, ∼ 50-110 G). The magnetic field strength was derived under the assumption N~Lα = 1018 cm-2, where N is electron density and Lα is the scale of coronal field divergence along line-of-sight. The height of QT-region (HQTR) was estimated using force free field extrapolations, HQRT = 6.3 x 109 cm (2.3 x 109 cm) for 20 G (85 G). We then compared the coronal radio magnetograms and the force free field extrapolation of photospheric magnetic field, and we found close similarity between them on large spatial scale. Title: The Magnetic Helicity Budget of Solar Active Regions and Coronal Mass Ejections Authors: Nindos, A.; Zhang, J.; Zhang, H. Bibcode: 2003ApJ...594.1033N Altcode: We compute the magnetic helicity injected by transient photospheric horizontal flows in six solar active regions associated with halo coronal mass ejections (CMEs) that produced major geomagnetic storms and magnetic clouds (MCs) at 1 AU. The velocities are computed using the local correlation tracking (LCT) method. Our computations cover time intervals of 110-150 hr, and in four active regions the accumulated helicities due to transient flows are factors of 8-12 larger than the accumulated helicities due to differential rotation. As was first pointed out by Démoulin and Berger, we suggest that the helicity computed with the LCT method yields not only the helicity injected from shearing motions but also the helicity coming from flux emergence. We compare the computed helicities injected into the corona with the helicities carried away by the CMEs using the MC helicity computations as proxies to the CME helicities. If we assume that the length of the MC flux tubes is l=2 AU, then the total helicities injected into the corona are a factor of 2.9-4 lower than the total CME helicities. If we use the values of l determined by the condition for the initiation of the kink instability in the coronal flux rope or l=0.5 AU then the total CME helicities and the total helicities injected into the corona are broadly consistent. Our study, at least partially, clears up some of the discrepancies in the helicity budget of active regions because the discrepancies appearing in our paper are much smaller than the ones reported in previous studies. However, they point out the uncertainties in the MC/CME helicity calculations and also the limitations of the LCT method, which underestimates the computed helicities. Title: Analysis of polarized microwave emission of Flare-Productive Active Region 9415 Authors: Bogod, V. M.; Gelfreikh, G. B.; Drago, F. Ch.; Maximov, V. P.; Nindos, A.; Kaltman, T. I.; Ryabov, B. I.; Tokhchukova, S. Kh. Bibcode: 2003astro.ph..9444B Altcode: The results of the microwave observations of the Sun made with the RATAN-600 have shown the existence of many types of spectral peculiarities in polarized emission of active regions, which produce powerful flares. These phenomena happen at microwaves and reflect inhomogeneous structure of magnetic field in magnetospheres of flaring active regions in wide range of heights above the photosphere. In this presentation we demonstrate an analysis of the AR 9415 during all the period of its passage across the solar disk. Results of the study point out to existence of different scenarios of circular polarization variations in the radio wave band. Here, we separated the phenomenon of the cyclotron emission passage through the quasi-transverse magnetic field (QT-region) and several effects connected with flare activity of active region. New observational data are presented and compared with the data of several observatories: SSRT, NoRH, MDI SOHO, GOES and MEES. The preliminary interpretation of the phenomena are given. Title: The origin of CME helicity Authors: Nindos, A.; Zhang, H. Bibcode: 2002ESASP.506...99N Altcode: 2002svco.conf...99N; 2002ESPM...10...99N We investigate whether the bulk of magnetic helicity carried away from the Sun by CMEs comes from helicity injected to the corona by shearing motions or by emerging magnetic flux. We use photospheric magnetic field observations of active region NOAA 9165 (AR 9165) which is an ideal candidate for such study because (1) it is the site of both new flux emergence and intense horizontal shearing flows; (2) it shows rapid development and rapid decay and for a few days it is the site of violent activity; (3) the horizontal motions occur when it is close to disk center, thus minimizing the errors involved in the relevant computations; (4) observations of a magnetic cloud associated with one of the CMEs linked to the active region are available. The computed helicity change due to horizontal shearing motions is probably the largest ever reported; it amounts to about the total helicity that the active region's differential rotation would have injected within 3 solar rotations. But the CMEs linked to the active regions remove at least a factor of 4-64 more helicity than the helicity injected by horizontal shearing motions. Consequently the main source of the helicity carried away by the CMEs is the new magnetic flux that emerges twisted from the convective zone. Title: Study of coronal loops using TRACE and SOHO Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th.; Nindos, A.; Alissandrakis, C.; Tsiropoula, G.; Vial, J. -C. Bibcode: 2002ESASP.505..417G Altcode: 2002solm.conf..417G; 2002IAUCo.188..417G We analysed coronal loops observed in many spectral lines from TRACE and SUMER, at the active region NOAA 8541, on May 15, 1999. For the loops, which are identified in a number of wavelengths, we try to combine the good temporal and spatial resolution of TRACE with the spectral information obtained by the SUMER rasters, in order to determine some physical parameters such as temperature, flow velocity and electron density. The morphology of the magnetic field of the loops is also approximated by a force-free extrapolation of the photospheric magnetic field, measured with MDI, and is compared to the loop morphology. Title: Photospheric Motions and Coronal Mass Ejection Productivity Authors: Nindos, A.; Zhang, H. Bibcode: 2002ApJ...573L.133N Altcode: Shearing motions have been frequently used in MHD simulations of coronal mass ejection (CME) initiation but have hardly been reported from observations of CME-producing regions. In this Letter, we investigate whether the bulk of magnetic helicity carried away from the Sun by CMEs comes from helicity injected to the corona by such motions or by emerging magnetic flux. We use photospheric magnetic field observations of NOAA Active Region 9165, which is an ideal candidate for such study because (1) it is the site of both new flux emergence and intense horizontal shearing flows; (2) it shows rapid development and rapid decay, and for a few days it is the site of violent activity; (3) the horizontal motions occur when it is close to disk center, thus minimizing the errors involved in the relevant computations; and (4) observations of a magnetic cloud associated with one of the CMEs linked to the active region are available. The computed helicity change due to horizontal shearing motions is probably the largest ever reported; it amounts to about the total helicity that the active region's differential rotation would have injected within three solar rotations. But the CMEs linked to the active region remove at least a factor of 4-64 more helicity than the helicity injected by horizontal shearing motions. Consequently, the main source of the helicity carried away by the CMEs is the new magnetic flux that emerges twisted from the convective zone. Our study implies that shearing motions, even when they are strong, have little effect in the process of buildup of magnetic free energy that leads to the initiation of CMEs. Title: Photospheric Motions and CME Productivity Authors: Nindos, A.; Zhang, H. Bibcode: 2002AGUSMSH22D..05N Altcode: We investigate whether the bulk of magnetic helicity carried away from the Sun by CMEs comes from photospheric horizontal shear motions or from emerging magnetic flux. Our study is focused on active region NOAA 9165 which is an ideal candidate for such study because (1) both new flux emergence and intense horizontal flows are observed during its short lifetime; (2) it shows rapid development and rapid decay and for a few days it is the site of violent activity; (3) the horizontal motions occur when it is close to disk center, thus minimizing the errors involved in the relevant computations; (4) observations of a magnetic cloud associated with one of the CMEs linked to the active region are available. The computed helicity change due to horizontal shear motions is probably the largest ever reported; it amounts to about the total helicity injected by the active region's differential rotation within more than 2 solar rotations. But the CMEs linked to the active region remove a factor of 8--64 more helicity than the helicity injected by horizontal shear motions. Consequently the main source of the helicity carried away by the CMEs is the new magnetic flux that emerges twisted from the convective zone. Our study implies that shear motions, even when they are strong, have little effect in the process of buildup of magnetic free energy that leads to the initiation of CMEs. Title: Spatially resolved microwave oscillations above a sunspot Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Bogod, V. M.; Gontikakis, C. Bibcode: 2002A&A...386..658N Altcode: Using high quality VLA observations, we detected for the first time spatially resolved oscillations in the microwave total intensity (I) and circular polarization (V) emission of a sunspot-associated gyroresonance (g-r) source. Oscillations were detected at 8.5 and 5 GHz during several time intervals of our 10-hour-long dataset. The oscillations are intermittent: they start suddenly and are damped somehow more gradually. Despite their transient nature when they are observed they show significant positional, amplitude and phase stability. The spatial distribution of intensity variations is patchy and the location of the patches of strong oscillatory power is not the same at both frequencies. The strongest oscillations are associated with a small region where the 8.5 GHz emission comes from the second harmonic of the gyrofrequency while distinct peaks of weaker oscillatory power appear close to the outer boundaries of the 8.5 and 5 GHz g-r sources, where the emissions come from the third harmonic of the gyrofrequency. Overall, the 5 GHz oscillations are weaker than the 8.5 GHz oscillations (the rms amplitudes of the I oscillations are 1.3-2.5 x 104 K and 0.2-1.5 x 105 K, respectively). At both frequencies the oscillations have periods in the three-minute range: the power spectra show two prominent peaks at 6.25-6.45 mHz and 4.49-5.47 mHz. Our models show that the microwave oscillations are caused by variations of the location of the third and/or second harmonic surfaces with respect to the base of the chromosphere-corona transition region (TR), i.e. either the magnetic field strength or/and the height of the base of the TR oscillates. The best-fit model to the observed microwave oscillations can be derived from photospheric magnetic field strength oscillations with an rms amplitude of 40 G or oscillations of the height of the base of the TR with an rms amplitude of 25 km. Furthermore, small variations of the orientation of the magnetic field vector yield radio oscillations consistent with the observed oscillations. Tables 1-3 are only available in electronic form at http://www.edpsciences.org Title: Photospheric Motions and CME Productivity Authors: Nindos, A.; Zhang, H. Bibcode: 2002AAS...200.2003N Altcode: 2002BAAS...34..673N Shearing motions have been frequently used in MHD simulations of CME initiation but have hardly been reported from observations of CME--producing regions. In this paper we investigate whether the bulk of magnetic helicity carried away from the Sun by CMEs comes from helicity injected to the corona by such motions or by emerging magnetic flux. We use photospheric magnetic field observations of active region NOAA 9165 which is an ideal candidate for such study because (1) it is the site of both new flux emergence and intense horizontal shearing flows; (2) it shows rapid development and rapid decay and for a few days it is the site of violent activity; (3) the horizontal motions occur when it is close to disk center, thus minimizing the errors involved in the relevant computations; (4) observations of a magnetic cloud associated with one of the CMEs linked to the active region are available. The computed helicity change due to horizontal shearing motions is probably the largest ever reported; it amounts to about the total helicity that the active region's differential rotation would have injected within 3 solar rotations. But the CMEs linked to the active region remove at least a factor of 4--64 more helicity than the helicity injected by horizontal shearing motions. Consequently the main source of the helicity carried away by the CMEs is the new magnetic flux that emerges twisted from the convective zone. Our study implies that shearing motions, even when they are strong, have little effect in the process of buildup of magnetic free energy that leads to the initiation of CMEs. Title: ARTEMIS IV Radio Observations of the 14 July 2000 Large Solar Event Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Nindos, A.; Tsitsipis, P.; Moussas, X.; Bougeret, J. -L.; Bouratzis, K.; Dumas, G.; Kanellakis, G.; Kontogeorgos, A.; Maroulis, D.; Patavalis, N.; Perche, C.; Polygiannakis, J.; Preka-Papadema, P. Bibcode: 2001SoPh..204..165C Altcode: 2010arXiv1009.3654C In this report we present a complex metric burst, associated with the 14 July 2000 major solar event, recorded by the ARTEMIS-IV radio spectrograph at Thermopylae. Additional space-borne and Earth-bound observational data are used, in order to identify and analyze the diverse, yet associated, processes during this event. The emission at metric wavelengths consisted of broad-band continua including a moving and a stationary type IV, impulsive bursts and pulsating structures. The principal release of energetic electrons in the corona was 15-20 min after the start of the flare, in a period when the flare emission spread rapidly eastwards and a hard X-ray peak occurred. Backward extrapolation of the CME also puts its origin in the same time interval, however, the uncertainty of the extrapolation does not allow us to associate the CME with any particular radio or X-ray signature. Finally, we present high time and spectral resolution observations of pulsations and fiber bursts, together with a preliminary statistical analysis. Title: Metric Radio Emission Associated with X-Ray Plasmoid Ejections Authors: Kundu, M. R.; Nindos, A.; Vilmer, N.; Klein, K. -L.; Shibata, K.; Ohyama, M. Bibcode: 2001ApJ...559..443K Altcode: In this paper we report the first detection of metric/decimetric radio emission associated with two soft X-ray plasmoid ejecta events that occurred during two limb flares observed by the Yohkoh SXT. In the first event a loop started to rise slowly (~10 km s-1) before the beginning of the hard X-ray impulsive phase of the flare. At about the onset of the impulsive flare, there was acceleration of the ejecta, resulting in a speed of 130 km s-1 and finally to ~200 km s-1. The associated radio emission was observed with the Nançay radioheliograph (NRH) in the frequency range of 230-450 MHz. It was an unpolarized continuum that lasted 8-10 minutes. The 410 MHz source was located close to the height where the plasmoid was last identified in the SXT images. In the second event an eruption resulted in the expansion of a large-scale, looplike feature and the development of two plasmoid ejecta which moved in different directions. The speed of the ejecta was 60-100 km s-1. In this event, the associated radio emission was a long-lasting (about 2 hr) continuum observed from 450 to 164 MHz. The onset of the low-frequency emission was delayed with respect to the onset of the high-frequency emission. In both cases the radio sources were located above the soft X-ray ejecta in the general direction of the prolongation of the ejecta movement. In both cases the radio emission comes from nonthermal electrons which are accelerated in close relationship with the propagation of the X-ray plasmoid: as the plasmoid reaches higher altitudes, it interacts with increasingly more extended magnetic field lines and new coronal sites of production of nonthermal electrons are created. 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: 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: 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: 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: 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: 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: 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: 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: 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: The relation of CaII K line features to magnetic field intensity Authors: Nindos, A.; Zirin, H. Bibcode: 1997BAAS...29.1119N Altcode: No abstract at ADS Title: A Model for Active Region Emission at Centimeter Wavelengths Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Kundu, M. R.; Dere, K. P.; Korzhavin, A. N.; Bogod, V. M. Bibcode: 1996SoPh..166...55N Altcode: We present multi-frequency observations and model computations of the microwave emission of a solar active region. The radio observations were obtained with the RATAN-600 at several wavelengths between 0.8 and 31.6 cm and with the VLA at 6 and 20 cm. The active region was also observed in the EUV O Iv lines by the HRTS instrument aboard the Space Shuttle Spacelab-2 mission. These lines are formed in the chromosphere-corona transition region and their intensity ratio is sensitive to pressure. Photospheric magnetograms provided both the longitudinal and the transverse component of the magnetic field. The microwave observations were checked against model computations taking into account both the free-free and the gyro-resonance emission mechanisms and using the pressure data from the O IV lines. The magnetic field was computed through constant-α force-free extrapolations of the longitudinal photospheric field. We computed both the flux from 2 to 20 cm and the spatial structure of the microwave emission at 6 and 20 cm. The comparison of the computed and observed flux spectra allowed us to estimate the magnetic field strength at the base of the transition region and in the low corona, as well as the values of the conductive flux and the height of the base of the transition region. The model maps at 6 cm and 20 cm showed that α was not constant above the active region; the same conclusion was reached on the basis of the photospheric observations. The use of pressure measurements allowed us to identify microwave structures which were determined by pressure enhancements. At 6 cm the computations confirmed the fact that the magnetic field is the principal factor that determines the structure of sunspot-associated sources and showed that the effect of pressure variations was small. Pressure variations were more important at 20 cm, where the peak of the emission was associated with the sunspot and a diffuse component was associated with the plage which had an average pressure higher by a factor of 1.54 than the sunspot. Title: Two-Dimensional Mapping of the Sun with the RATAN-600 Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Borovik, V. N.; Korzhavin, A. N.; Bogod, V. M. Bibcode: 1996SoPh..165...41N Altcode: We present two-dimensional solar maps at 2.7, 3.2, 4, and 8.2 cm computed from one-dimensional observations with the RATAN-600, using Earth rotation aperture synthesis techniques. Before the calculation of maps, the position of each scan was corrected with respect to the center of the solar disk and the scans were calibrated. The circular polarization scans were corrected for polarization cross-talk between the I and V channels. Subsequently, the quiet-Sun background emission was subtracted. After all corrections, a dirty map was computed by combining the scans at different position angles. The last step of the processing was an attempt to free the dirty map of the sidelobes, using the standard CLEAN procedure. The resolution of the clean maps at 2.7 cm was 0.5' by 6'. Both active regions which were present on the solar disk were mapped. We studied the flux spectra of different types of sources: one was associated with a sunspot, the second was located over the neutral line of an active region, and the other was associated with the plage. The emission mechanism of the former was attributed to the gyroresonance process, while the short wavelength emission of the others was attributed to the free-free process. For the sunspot-associated source we estimated the magnetic field strengths at the base of the transition region and in the low corona. Title: Evidence for Ordinary Mode Emission from Microwave Bursts Authors: Alissandrakis, C. E.; Nindos, A.; Kundu, M. R. Bibcode: 1993SoPh..147..343A Altcode: We analyze high-resolution, one-dimensional observations of simple microwave bursts, obtained at 4.9 GHz with the Westerbork Synthesis Radio Telescope in 1980, together with Hα photographs of the associated flares from the Observatories of Athens and Meudon. In most cases the polarization structure can be interpreted in terms of extraordinary mode emission, taking into account the polarity of the underlying magnetic field and propagation effects, which may lead to inversion of the sense of polarization in the limbward part of the flaring loop. We found evidence for ordinary mode emission in two classes of events. In one class theo-mode comes from regions overlying strong magnetic field, which we interpret in terms of thermal gyroresonance absorption of the extraordinary mode at the third harmonic of the gyrofrequency. In the other class the entire burst emits in theo-mode, which may be attributed to high gyrosynchrotron optical depth. Title: Spectral observations of active region sources with RATAN-600 and WSRT Authors: Alissandrakis, C. E.; Gel'Frejkh, G. B.; Borovik, V. N.; Korzhavin, A. N.; Bogod, V. M.; Nindos, A.; Kundu, M. R. Bibcode: 1993A&A...270..509A Altcode: We present spectral observations of neutral line and sunspot associated sources obtained with the RATAN-600 radio telescope and the WSRT in the wavelength range of 2 to 6 cm. Sources associated with large sunspots have flat spectra, while neutral line sources have very steep spectra. In the case of a large spot we estimated the magnetic field to be at least 2700 G at the base of the transition region and 1800 G in the low corona. We consider possible interpretations of the radio emission above the neutral lines. Gyroresonance emission at the fourth harmonic is inadequate, whereas emission from a small population of nonthermal electrons (total number 10 exp 30 to 10 exp 31) with a delta = 3 power law distribution seems to be sufficient.