Author name code: canfield ADS astronomy entries on 2022-09-14 author:"Canfield, Richard C." ------------------------------------------------------------------------ Title: It Takes a Village Authors: Canfield, Richard C. Bibcode: 2022SoPh..297...94C Altcode: My parents taught me the value of a good education. My studies at the state universities of Michigan and Colorado and postgraduate studies at the University of Utrecht built on an interest in astronomy that dated back to high school. These institutions enabled me to have a rewarding fifty-year career focused on the physics of the Sun. My work combined research and education at the High Altitude Observatory, the University of Utrecht, the Sacramento Peak Observatory, the University of California San Diego, the University of Hawaii, and Montana State University. My professional interests ranged from spectroscopic diagnostics and radiative transfer, especially of the flaring solar chromosphere, to the helicity of magnetic fields of active regions in the chromosphere, corona, and interplanetary medium, part of what is now called heliophysics and space weather. I am honored to have been recognized for my efforts as a scientific leader, mentor, and teacher. I am lucky to have lived at a time when access to space led the field of solar physics to grow dramatically, including global studies of solar activity, the heliosphere, and space weather. Title: Performance of Major Flare Watches from the Max Millennium Program (2001 - 2010) Authors: Bloomfield, D. S.; Gallagher, P. T.; Marquette, W. H.; Milligan, R. O.; Canfield, R. C. Bibcode: 2016SoPh..291..411B Altcode: 2015arXiv151204518B; 2016SoPh..tmp....1B The physical processes that trigger solar flares are not well understood, and significant debate remains around processes governing particle acceleration, energy partition, and particle and energy transport. Observations at high resolution in energy, time, and space are required in multiple energy ranges over the whole course of many flares to build an understanding of these processes. Obtaining high-quality, co-temporal data from ground- and space- based instruments is crucial to achieving this goal and was the primary motivation for starting the Max Millennium program and Major Flare Watch (MFW) alerts, aimed at coordinating observations of all flares ≥ X1 GOES X-ray classification (including those partially occulted by the limb). We present a review of the performance of MFWs from 1 February 2001 to 31 May 2010, inclusive, which finds that (1) 220 MFWs were issued in 3407 days considered (6.5 % duty cycle), with these occurring in 32 uninterrupted periods that typically last 2 - 8 days; (2) 56% of flares ≥ X1 were caught, occurring in 19 % of MFW days; (3) MFW periods ended at suitable times, but substantial gain could have been achieved in percentage of flares caught if periods had started 24 h earlier; (4) MFWs successfully forecast X-class flares with a true skill statistic (TSS) verification metric score of 0.500, that is comparable to a categorical flare/no-flare interpretation of the NOAA Space Weather Prediction Centre probabilistic forecasts (TSS = 0.488). Title: Relationship between the photospheric Poynting flux and the active region luminosity Authors: Kazachenko, Maria D.; Canfield, Richard C.; Fisher, George H.; Hudson, Hugh S.; Welsch, Brian Bibcode: 2014AAS...22412349K Altcode: How does energy radiated by active regions compare with magnetic energy that propagates lower across the photosphere? This is a fundamental question for energy storage and release in active regions, yet it is presently poorly understood. In this work we quantify and compare both energy terms using SDO observations of the active region (AR) 11520. To quantify the magnetic energy crossing the photosphere, or the Poynting flux, we need to know both the magnetic field vector B and electric field vector E as well. Our current electric field inversion technique, PDFI, combines the Poloidal-Toroidal-Decomposition method with information from Doppler measurements, Fourier local correlation tracking (FLCT) results, and the ideal MHD constraint, to determine the electric field from vector magnetic field and Doppler data. We apply the PDFI method to a sequence of Helioseismic and Magnetic Imager (HMI/SDO) vector magnetogram data, to find the electric-field and hence the Poynting-flux evolution in AR 11520. We find that most of the magnetic energy in this AR is injected in the range of $10^7$ to $10^8$ $ergs/{cm^2 s}$, with the largest fluxes reaching $10^{10}$ $ergs/{cm^2 s}$. Integrating over the active region this yields a total energy of order $10^{28}$ ergs/s. To quantify the active region luminosity, we use EUV Variability Experiment (EVE) and Atmospheric Imaging Assembly (AIA) spectrally resolved observations. We find the active region luminosity of order $10^{28}$ ergs/s. We compare derived magnetic and radiated energy fluxes on different temporal and spatial scales and estimate their uncertainties. We also discuss the roles that potential/non-potential and emerging/shearing terms play in the total magnetic energy budget. Title: Estimating active region luminosity using EVE/SDO observations Authors: Kazachenko, Maria D.; Hudson, H. S.; Fisher, G. H.; Canfield, R. C. Bibcode: 2013SPD....44...44K Altcode: Do solar active regions typically radiate more coronal energy during flares than the quiescent periods between them? This is a fundamental question for storage and release models of flares and active regions, yet it is presently poorly answered by observations. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of the Sun in the "Sun-as-a-point source" mode. It covers a wide range of temperatures and thus allows a detailed study of thermal emissions. Here we present two approaches for computing the active region luminosity, using EVE observations of fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE data in a time-series sense, when only one active region is present on the disk; this allows us to subtract the background due to the quiet sun and get the contribution from the active region alone. In the second approach, we analyze correlations of the radiative signatures with proxy indices (total solar magnetic and Poynting fluxes) during several months of data, when multiple active regions are present on the solar disk. We discuss capabilities of the two approaches, and what we can learn from them.Abstract (2,250 Maximum Characters): Do solar active regions typically radiate more coronal energy during flares than the quiescent periods between them? This is a fundamental question for storage and release models of flares and active regions, yet it is presently poorly answered by observations. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of the Sun in the "Sun-as-a-point source" mode. It covers a wide range of temperatures and thus allows a detailed study of thermal emissions. Here we present two approaches for computing the active region luminosity, using EVE observations of fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE data in a time-series sense, when only one active region is present on the disk; this allows us to subtract the background due to the quiet sun and get the contribution from the active region alone. In the second approach, we analyze correlations of the radiative signatures with proxy indices (total solar magnetic and Poynting fluxes) during several months of data, when multiple active regions are present on the solar disk. We discuss capabilities of the two approaches, and what we can learn from them. Title: Twisting and Writhing with George Ellery Hale Authors: Canfield, Richard C. Bibcode: 2013SPD....4410201C Altcode: Early in his productive career in astronomy, George Ellery Hale developed innovative instrumentation that allowed him to image the magnetically-dominated solar chromosphere. Among the solar phenomena he discovered were sunspot vortices, which he attributed to storms akin to cyclones in our own atmosphere. Much more recently, physicists discovered a quantity that is very well conserved in ideal magnetohydrodynamics: magnetic helicity. Our contemporary understanding of Hale's vortices as a consequence of large-scale twist in sunspot magnetic fields hinges on this conservation. I will review the crucial role that this property plays in the hemispheric and solar cycle dependences of Hales vortices, as well as solar flares and CMEs. Title: Twisting and Writhing with George Ellery Hale Authors: Canfield, Richard C. Bibcode: 2013AAS...22210601C Altcode: Early in his productive career in astronomy, George Ellery Hale developed innovative solar instrumentation that allowed him to make narrow-band images. Among the solar phenomena he discovered were sunspot vortices, which he attributed to storms akin to cyclones in our own atmosphere. Using the concept of magnetic helicity, physicists and mathematicians describe the topology of magnetic fields, including twisting and writhing. Our contemporary understanding of Hale's vortices as a consequence of large-scale twist in sunspot magnetic fields hinges on a key property of helicity: conservation. I will describe the critical role that this property plays, when applied to twist and writhe, in a fundamental aspect of global solar magnetism: the hemispheric and solar cycle dependences of active region electric currents with respect to magnetic fields. With the advent of unbroken sequences of high-resolution magnetic images, such as those presently available from the Helioseismic and Magnetic Imager on Solar Dynamics Observatory, the flux of magnetic helicity through the photosphere can be observed quantitatively. As magnetic flux tubes buoy up through the convection zone, buffeted and shredded by turbulence, they break up into fragments by repeated random bifurcation. We track these rising flux fragments in the photosphere, and calculate the flux of energy and magnetic helicity there. Using a quantitative model of coronal currents, we also track connections between these fragments to calculate the energy and magnetic helicity stored at topological interfaces that are in some ways analogous to the storage of stress at faults in the Earth's crust. Comparison of these values to solar flares and interplanetary coronal mass ejections implies that this is the primary storage mechanism for energy and magnetic helicity released in those phenomena, and suggests a useful tool for quantitative prediction of geomagnetic storms. Title: Predictions of Energy and Helicity in Four Major Eruptive Solar Flares Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana W.; Qiu, Jiong Bibcode: 2012SoPh..277..165K Altcode: 2011arXiv1104.3593K In order to better understand the solar genesis of interplanetary magnetic clouds (MCs), we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model (Longcope, 1996, Solar Phys.169, 91) and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity, and orientation of the flux-rope poloidal field. We compare model predictions of those quantities to flare and MC observations, and within the estimated uncertainties of the methods used find the following: The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux-rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC's poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancelation, the energy and helicity that we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity. Title: Predictions of Energy and Helicity in Four Major Eruptive Solar Flares Authors: Kazachenko, Maria; Canfield, Richard C.; Longcope, Dana W.; Qiu, Jiong Bibcode: 2011shin.confE...4K Altcode: In order to better understand the solar genesis of interplanetary magnetic clouds (MCs), we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model (Longcope in Solar Phys. 169, 91, 1996) and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity, and orientation of the flux rope poloidal field. We compare model predictions of those quantities to flare and MC observations and within the estimated uncertainties of the methods used find the following. The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux-rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC's poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancelation, the energy and helicity that we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity. Title: Predictions Of Energy And Helicity In Four Major Eruptive Solar Flares Authors: Kazachenko, Maria; Canfield, R. C.; Longcope, D. W.; Qiu, J. Bibcode: 2011SPD....42.2218K Altcode: 2011BAAS..43S.2218K In order to better understand the solar genesis of interplanetary magnetic clouds (MCs) we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity and orientation of the flux rope poloidal field. We compare model predictions of those quantities to flare and MC observations and within the estimated uncertainties of the methods used find the following. The predicted model reconnection fluxes are equal to or lower than the observed reconnection fluxes from the ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC's poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancellation, the energy and helicity we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity. Title: Sunspot Rotation, Flare Energetics, and Flux Rope Helicity: The Halloween Flare on 2003 October 28 Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana W.; Qiu, Jiong Bibcode: 2010ApJ...722.1539K Altcode: We study the X17 eruptive flare on 2003 October 28 using Michelson Doppler Imager observations of photospheric magnetic and velocity fields and TRACE 1600 Å images of the flare in a three-dimensional model of energy buildup and release in NOAA 10486. The most dramatic feature of this active region is the 123° rotation of a large positive sunspot over 46 hr prior to the event. We apply a method for including such rotation in the framework of the minimum current corona model of the buildup of energy and helicity due to the observed motions. We distinguish between helicity and energy stored in the whole active region and that released in the flare itself. We find that while the rotation of a sunspot contributes significantly to the energy and helicity budgets of the whole active region, it makes only a minor contribution to that part of the region that flares. We conclude that in spite of the fast rotation, shearing motions alone store sufficient energy and helicity to account for the flare energetics and interplanetary coronal mass ejection helicity content within their observational uncertainties. Our analysis demonstrates that the relative importance of shearing and rotation in this flare depends critically on their location within the parent active region topology. Title: Sunspot Rotation, Eruptive Flare Energetics And Flux Rope Helicity: Topology Matters. Authors: Kazachenko, Maria; Canfield, R.; Longcope, D.; Qiu, J. Bibcode: 2010AAS...21632003K Altcode: 2010BAAS...41..911K We study the role of rotation in the flare energy and helicity budget of two active regions: NOAA 10486 and NOAA 10759. Using MDI and TRACE observations of photospheric magnetic and velocity fields in those active regions we build a topological model of their three-dimensional coronal magnetic field. In both active regions a fast rotating sunspot is observed. We apply a method for including such rotation in the framework of the minimum current corona model (MCC, Longcope 1996) to the buildup of energy and helicity associated with the X17.2 eruptive flare on 2003 October 28 (the Halloween flare) and M8.0 eruptive flare on 2005 May 13. We find that including the sunspot rotation in the model changes the total flare thermal energy and flux rope helicity by only 10% for the Halloween flare, but by more than 200% for the 2005 May 13 flare. While for the Halloween flare shearing motions alone store sufficient energy and helicity to account for the flare energetics and ICME helicity content within their observational uncertainties, for the 2005 May 13 flare it is the rotation that dominates. We demonstrate that the relative importance of shearing and rotation in those two flares depends critically on their location within the parent active region topology. Title: Sunspot Rotation, Flare Energetics, and Flux Rope Helicity: The Eruptive Flare on 2005 May 13 Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana W.; Qiu, Jiong; Des Jardins, Angela; Nightingale, Richard W. Bibcode: 2009ApJ...704.1146K Altcode: We use the Michelson Doppler Imager and TRACE observations of photospheric magnetic and velocity fields in NOAA 10759 to build a three-dimensional coronal magnetic field model. The most dramatic feature of this active region is the 34° rotation of its leading polarity sunspot over 40 hr. We describe a method for including such rotation in the framework of the Minimum Current Corona model. We apply this method to the buildup of energy and helicity associated with the eruptive flare of 2005 May 13. We find that including the sunspot rotation almost triples the modeled flare energy (1.0 × 1031 erg) and flux rope self-helicity (-7.1 × 1042 Mx2). This makes the results consistent with observations: the energy derived from GOES is 1.0 × 1031 erg, the magnetic cloud helicity from WIND is -5 × 1042 Mx2. Our combined analysis yields the first quantitative picture of the helicity and energy content processed through a flare in an active region with an obviously rotating sunspot and shows that rotation dominates the energy and helicity budget of this event. Title: Sunspot Rotation, Flare Energetics and Flux Rope Helicity: The Eruptive Flare on 2005 May 13 Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana W.; Qiu, Jiong; DesJardins, Angela; Nightingale, Richard W. Bibcode: 2009shin.confE..53K Altcode: We use MDI and TRACE observations of photospheric magnetic and elocity fields in NOAA 10759 to build a three-dimensional coronal magnetic field model. The most dramatic feature of this active region is the 34 degree rotation of its leading polarity sunspot over 40 hours. We describe a method for including such rotation in the framework of the minimum current corona (MCC) model. We apply this method to the buildup of energy and helicity associated with the eruptive flare of 2005 May 13. We find that including the sunspot rotation almost triples the modeled flare energy (-1.0 ×10^{31}ergs) and flux rope self helicity (-7.1 ×10^{42}, Mx^2). This makes the results consistent with observations: the energy derived from GOES is -1.0×10^{31} ergs, the magnetic cloud helicity from WIND is -5 ×10^{42}, Mx^2. Our combined analysis yields the first quantitative picture of the helicity and energy content processed through a flare in an active region with an obviously rotating sunspot and shows that rotation dominates the energy and helicity budget of this event. Title: Sunspot Rotation, Flare Energetics and Flux Rope Helicity: The Eruptive Flare on 2005 May 13 Authors: Kazachenko, Maria; Canfield, R. C.; Longcope, D. W.; Qiu, J.; DesJardins, A.; Nightingale, R. W. Bibcode: 2009SPD....40.2013K Altcode: We use MDI and TRACE observations of photospheric magnetic and velocity fields in NOAA 10759 to build a three-dimensional coronal magnetic field model. The most dramatic feature of this active region is the 34 degree rotation of its leading polarity sunspot over 40 hours. We describe a method for including such rotation in the framework of braiding and spinning in a magnetic charge topology (MCT) model. We apply this method to the buildup of energy and helicity associated with the eruptive flare of 2005 May 13. We find that adding rotation almost triples the modeled flare energy (-1.0×1031ergs) and flux rope self helicity (-7.1×1042 Mx2). This makes the results consistent with observations: the energy derived from GOES is -1.0×1031ergs, the magnetic cloud helicity from WIND is -5×1042 Mx2. Our combined analysis yields the first quantitative picture of the helicity and energy content processed through a flare in an active region with an obviously rotating sunspot and shows that rotation dominates the energy and helicity budget of this event. Title: Reconnection in Three Dimensions: The Role of Spines in Three Eruptive Flares Authors: Des Jardins, Angela; Canfield, Richard; Longcope, Dana; Fordyce, Crystal; Waitukaitis, Scott Bibcode: 2009ApJ...693.1628D Altcode: 2009ApJ...693.1628J In order to better understand magnetic reconnection and particle acceleration in solar flares, we compare the RHESSI hard X-ray (HXR) footpoint motions of three flares with a detailed study of the corresponding topology given by a Magnetic Charge Topology model. We analyze the relationship between the footpoint motions and topological spine lines and find that the examined footpoint sources move along spine lines. We present a three-dimensional topological model in which this movement can be understood. As reconnection proceeds, flux is transferred between the reconnecting domains, causing the separator to move. The movement of the separator's chromospheric ends, identified with the HXR footpoints, is along those spine lines on which the separator ends. Title: Signatures of Magnetic Stress Prior to Three Solar Flares Observed by RHESSI Authors: des Jardins, Angela; Canfield, Richard; Longcope, Dana; McLinden, Emily; Dillman, Amanda Bibcode: 2009ApJ...693..886D Altcode: 2009ApJ...693..886J We examine the hard X-ray (HXR) footpoint sources of three flares, as observed by RHESSI, in combination with the topology given by the extrapolation of line-of-sight magnetograms into the corona. Assuming the HXR footpoint sources are chromospheric consequences of magnetic reconnection that takes place on separators, we further assume a relationship between the buildup of energy in stressed coronal magnetic fields and the measurement of the change in separator flux per unit length. We find that the value of this quantity is larger for the separators that connect the HXR footpoint sources than the quantity for the separators that do not. Therefore, we conclude that we are able to understand the location of HXR sources observed in flares in terms of a physical and mathematical model of the topology of the active region. Title: Modeling and measuring the flux and helicity ejected by the two-ribbon flare on 2005-05- 13 Authors: Kazachenko, M.; Canfield, R. C.; Longcope, D. W.; Qiu, J. Bibcode: 2008AGUSMSP43A..08K Altcode: In this work we study an eruptive flare event in order to quantitatively understand flux and helicity transfer through reconnection in the associated coronal flux rope formation. Quantitative observational studies of solar flare reconnection have been made by a number of research groups (Poletto and Kopp, 1986, Fletcher and Hudson 2001, Qiu and Yurchyshyn, 2005). However, their work does not allow one to predict, from observations, both the flux and twist that coronal reconnection contributes to interplanetary flux ropes. Our hypothesis is that the flux and helicity associated with eruptive solar flares is created through a sequence of magnetic reconnections. Longcope et al. (2007) introduced a topological method of studying flux rope formation in 3D. In our study we use MDI, SOHO and TRACE data. We apply two methods to derive the reconnection sequence: (1) the flaring sequence from the TRACE observations of ribbon brightening (Qiu et al, 2007) and (2) the reconnection sequence from the topological model of the coronal field based on the Minimum Current Corona model (Longcope, 2001). From the topological model we calculate the toroidal flux and total helicity of the flux rope. We study in detail the M8.0 flare in active region NOAA 10759, 13-May-2005 16:13UT. The total magnetic flux of the active region is 1.7 ·10 22 Mx. From the photospheric magnetogram evolution we determine that 1.9 · 1042 Mx2 of magnetic helicity has been injected into the active region during the 40-hour build-up prior to the flare. From the domain flux evolution we show that reconnection in the model (2) must occur in a specific sequence which would produce a twisted flux rope containing significantly less flux and helicity than the whole active region. The reconnection sequence from the model (2) compares favorably with the one inferred from the observation sequence (1). However, the topological analysis might be sensitive to the beginning of the buildup time, so its choice will be discussed. Title: On the Solar Cycle Variation of the Hemispheric Helicity Rule Authors: Pevtsov, A. A.; Canfield, Richard C.; Sakurai, T.; Hagino, M. Bibcode: 2008ApJ...677..719P Altcode: We study the statistical significance of observed temporal variations of the solar active-region hemispheric helicity rule, as measured by the latitudinal gradient of the best-fit linear force-free-field parameter, dα/dvarphi . Using data from four different vector magnetographs, we compute and compare average annual dα/dvarphi values for these instruments for 19 years from solar cycles 21, 22, and 23. We find that although every instrument shows the "wrong" sign for the hemispheric rule in some years, there is no agreement among the instruments on which years are abnormal. None of the four data sets shows annual values of dα/dvarphi departing from the hemispheric helicity rule by more than 3 σ. We conclude that because the hemispheric helicity rule is a weak tendency with significant scatter, an annual subset of active regions is likely to produce statistically unreliable results. Title: Hinode XRT observations of a long-lasting coronal sigmoid Authors: McKenzie, D. E.; Canfield, R. C. Bibcode: 2008A&A...481L..65M Altcode: Aims:Coronal sigmoids are important sources of eruptions into interplanetary space, and a handful of models have been proposed to explain their characteristic S shape. However, the coronal X-ray images available to date have generally not had sufficient resolution to distinguish between these models. The goal of the present investigation is to determine whether the new observations from Hinode can help us to make such a distinction.
Methods: We present the first observations of a persistent coronal sigmoid obtained with the Hinode X-Ray Telescope (XRT). The excellent angular resolution of XRT (1 arcsec per pixel) and the sigmoid's location near disk center combined to provide an unprecedented view of the formation and eruption of this phenomenon. We compared the observed morphology with expectations inferred from two popular models of sigmoid formation, the bald-patch separatrix surface model and the kinking flux rope model.
Results: The images during the pre-eruptive phase show that the overall S shape of the sigmoid comprises two separate J-shaped bundles of many loops. The straight sections of the two J patterns lie anti-parallel to one another in the middle of the S, on opposite sides of the magnetic polarity inversion line. The images during the eruptive phase reveal that, before any soft X-ray flaring begins, a diffuse linear structure almost as long as the sigmoid lifts off from the middle of the S. It shows slight clockwise rotation. The X-ray flare begins with the appearance of a sheared arcade of short loops, in the area centered between the two J-shaped patterns of the sigmoid.
Conclusions: Taken together, the observational findings provide strong support for the bald-patch separatrix surface model for this sigmoid. Title: Twisted solar active region magnetic fields as drivers of space weather: Observational and theoretical investigations Authors: Nandy, Dibyendu; Mackay, Duncan H.; Canfield, Richard C.; Martens, P. C. H. Bibcode: 2008JASTP..70..605N Altcode: The properties and dynamics of magnetic fields on the Sun's photosphere and outer layers--notably those within solar active regions--govern the eruptive activity of the Sun. These photospheric magnetic fields also act as the evolving lower boundary of the Sun-Earth coupled system. Quantifying the physical attributes of these magnetic fields and exploring the mechanisms underlying their influence on the near-Earth space environment are of vital importance for forecasting and mitigating adverse space weather effects. In this context, we discuss here a novel technique for measuring twist in the magnetic field lines of solar active regions that does not invoke the force-free field assumption. Twist in solar active regions can play an important role in flaring activity and the initiation of CMEs via the kink instability mechanism; we outline a procedure for determining this solar active region eruptive potential. We also discuss how twist in active region magnetic fields can be used as inputs in simulations of the coronal and heliospheric fields; specifically, we explore through simulations, the formation, evolution and ejection of magnetic flux ropes that originate in twisted magnetic structures. The results and ideas presented here are relevant for exploring the role of twisted solar active region magnetic fields and flux ropes as drivers of space weather in the Sun-Earth system. Title: Signatures of Magnetic Stress Prior to Three Solar Flares Observed by RHESSI Authors: Des Jardins, A. C.; Canfield, R. C.; Longcope, D. W. Bibcode: 2007AGUFMSH13A1098D Altcode: In order to better understand the location and evolution of magnetic reconnection, which is thought to be the energy release mechanism in solar flares, we combine the analysis of hard X-ray (HXR) sources observed by RHESSI with a three-dimensional, quantitative magnetic charge topology (MCT) model. We examine the location of reconnection by assuming a relationship between the build-up of energy in stressed coronal magnetic fields and the measurement of the change in separator flux per unit length. We find that the value of this quantity is larger on the separators that connect the HXR footpoint sources than the value on the separators that do not. Therefore, we conclude the MCT model gives useful insight into the relationship between sites of HXR emission and the topology of flare productive active regions. Title: Yohkoh SXT Full-Resolution Observations of Sigmoids: Structure, Formation, and Eruption Authors: Canfield, Richard C.; Kazachenko, Maria D.; Acton, Loren W.; Mackay, D. H.; Son, Ji; Freeman, Tanya L. Bibcode: 2007ApJ...671L..81C Altcode: We study the structure of 107 bright sigmoids using full-resolution (2.5" pixels) images from the Yohkoh Soft X-Ray Telescope (SXT) obtained between 1991 December and 2001 December. We find that none of these sigmoids are made of single loops of S or inverse-S shape; all comprise a pattern of multiple loops. We also find that all S-shaped sigmoids are made of right-bearing loops and all inverse-S-shaped sigmoids of left-bearing loops, without exception. We co-align the SXT images with Kitt Peak magnetograms to determine the magnetic field directions in each sigmoid. We use a potential-field source surface model to determine the direction of the overlying magnetic field. We find that sigmoids for which the relative orientation of these two fields has a parallel component outnumber antiparallel ones by more than an order of magnitude. We find that the number of sigmoids per active region varies with the solar cycle in a manner that is consistent with this finding. Finally, those few sigmoids that are antiparallel erupt roughly twice as often as those that are parallel. We briefly discuss the implications of these results in terms of formation and eruption mechanisms of flux tubes and sigmoids. Title: Solar Active Region Flux Fragmentation, Subphotospheric Flows, and Flaring Authors: Canfield, Richard C.; Russell, Alexander J. B. Bibcode: 2007ApJ...662L..39C Altcode: We explore the properties of the fragmentation of magnetic flux in solar active regions. We apply gradient-based tessellation to magnetograms of 59 active regions to identify flux fragments. First, we find that the distribution function of flux fragments in these regions is highly consistent with lognormal form, which is the most direct evidence yet obtained that repeated random bifurcation dominates fragmentation and coalescence in all active regions. Second, we apply nonparametric statistical methods to the variance of the lognormal distribution of fragment flux, the flare X-ray energy output of the active regions, and kinetic helicity measurements in the upper convection zone to show that there is no significant statistical relationship between the amount of fragmentation of an active region's flux at photospheric levels and the amplitude of either its average kinetic helicity density in the upper convection zone or its X-ray flare energy output. Title: Hinode/XRT Observations of a Coronal Sigmoid Authors: McKenzie, David E.; Canfield, R. C. Bibcode: 2007AAS...210.7208M Altcode: 2007BAAS...39Q.180M We present the first observations of an X-ray sigmoid made with the Hinode X-Ray Telescope, co-aligned with those of TRACE and SoHO/MDI. XRT's extraordinary angular resolution (1 arcsec/pixel) and the sigmoid's location near disk center combined to provide an unprecedented view of the formation and eruption of this phenomenon. XRT observed the sigmoid over several days, with cadences as high as 1 image per 10--30 seconds. The first motions associated with eruption of the sigmoid started at 0600UT on 12-Feb-2007; the first brightening of the ensuing X-ray arcade was seen at 0740UT.

The images during the pre-eruptive phase, which ends with the onset of large-scale motions, show:<ul><li>The overall S shape of the sigmoid is not defined by any single X-ray loop. Rather, many individual loops collectively comprise an S-shaped pattern.<li>The S shape is comprised of two separate J shapes, whose straight sections lie anti-parallel to one another in the middle of the S, on opposite sides of the magnetic polarity inversion line. <li>During the several-day span of the XRT observation, the S shape gradually becomes better defined.</ul>The images during the eruptive phase show:<ul><li>Approximately 100 minutes before any soft X-ray flaring begins, a diffuse linear structure, almost as long as the sigmoid, lifts off from the middle of the S. It shows slight clockwise rotation.<li>The X-ray flare begins with the appearance of a sheared arcade of short loops, in the area centered between the two J-shaped patterns of the sigmoid. <li>Within 16 hours of the start of the flare, no S shape remains.</ul>Taken together, these features provide strong support for the Bald-Patch Separatrix Surface model of sigmoids put forth by Titov & Demoulin (1999). Title: The REU Program in Solar Physics at Montana State University Authors: Martens, Petrus C.; Canfield, R. C.; McKenzie, D. M. Bibcode: 2007AAS...210.8012M Altcode: 2007BAAS...39Q.189M The Solar Physics group at Montana State University has organized an annual summer REU program in Solar Physics, Astronomy, and Space Physics since 1999, with NSF funding since 2003. The number of students applying and being admitted to the program has increased every year, and we have been very successful in attracting female participants. A great majority of our REU alumni have chosen career paths in the sciences, and, according to their testimonies, our REU program has played a significant role in their decisions. From the start our REU program has had an important international component through a close collaboration with the University of St. Andrews in Scotland.

In our poster we will describe the goals, organization, scientific contents, international aspects, and results, and present statistics on applications, participants, gender balance, and diversity. Title: A New Technique For Measuring The Twist Of Photospheric Active Regions Without Recourse To The Force-Free-Field Equation: Reconfirming The Hemispheric Helicity Trend Authors: Nandy, Dibyendu; Calhoun, A.; Windschitl, J.; Canfield, R. C.; Linton, M. G. Bibcode: 2007AAS...210.2402N Altcode: 2007BAAS...39..128N The twist component of magnetic helicity in solar active regions is known to be an important indicator of sub-photospheric flux tube dynamics and solar eruptive activity. Traditionally, estimates of the parameter alpha -- appearing in the force-free-field equation -- has been used to infer the twist of photospheric active regions. However, the photosphere is not force-free and this has lead to recent concerns on the validity of using the alpha parameter for determining photospheric active region twist. We have devised a new flux-tube-fitting technique for determining the twist of active regions without recourse to the force-free-field equation. This method assumes that the underlying active region flux system is cylindrically symmetric and uniformly twisted. By using this new technique, on a statistically compelling number of photospheric active region vector magnetograms, we re-confirm the hemispheric helicity rule independent of the force-free-field assumption. This research has been supported in parts by a NASA Living With a Star grant NNG05GE47G. A.C. and J.W. were supported by a NSF Research Experience for Undergraduates grant ATM-0243923 to Montana State University. M.G.L. acknowledges support from NASA and the Office of Naval Research. Title: Relating RHESSI Footpoints to Reconnection: the Importance of Spines and Separators Authors: Des Jardins, A.; Canfield, R.; Longcope, D.; McLinden, E.; Fordyce, C.; Waitukaitis, S. Bibcode: 2006AGUFMSH23A0349D Altcode: In order to improve the understanding of both flare initiation and evolution, we take advantage of powerful new topological methods and the high spatial resolution of RHESSI to examine where magnetic reconnection takes place in flare-producing solar active regions. We use the MPOLE (http://solar.physics.montana.edu/dana/mpole/) software to extrapolate the observed line of sight photospheric magnetic field into the corona. MPOLE is a suite of IDL programs that implements the Minimum Current Corona Model (Longcope 1996). Recently it has been improved to use a hierarchy of topological features (Beveridge 2006). The coronal extrapolation gives the location of topological features such as poles, nulls, separatricies, separators, and spine lines. For several flares well observed by RHESSI and MDI, we examine the locations of flare HXR emission in the context of these topological features. Two noteworthy relationships are found. First, when footpoints move, they move along spine lines. Second, when separators significantly change over the course of a flare, only those associated with the flare footpoints do so. In this poster, we present observations supporting the relationship between spine lines and footpoint tracks, demonstrate the importance of separator analysis in the study of flares, explore uncertainties in the MPOLE and RHESSI analyses, and survey possible interpretations of the reported results. This work is supported by NASA. Title: Magnetic Topology of the 29 October 2003 X10 flare Authors: Des Jardins, Angela C.; Canfield, R.; Longcope, D. Bibcode: 2006SPD....37.1311D Altcode: 2006BAAS...38..242D In order to improve the understanding of both flare initiation and evolution, we take advantage of powerful new topological methods and the high spatial resolution of RHESSI to examine where magnetic reconnection takes place in flare-producing solar active regions. Up to this time, such studies have been carried out on a very small number of active regions. According to present ideas, reconnection is expected to occur at either a separatrix or separator topological feature. We use the powerful X10 flare on 29 October 2003 (peak: 20:49 UT, location: (80'', 275'')) as a test of the ability to interpret the topological location of reconnection. The 29 October 2003 flare was well observed by RHESSI and MDI, occurred near the sun's central meridian, and thus is thus a prime candidate for a study on the topological location of magnetic reconnection. In this flare study, we use the MPOLE (http://solar.physics.montana.edu/dana/mpole/) software to extrapolate from the photospheric magnetic field, as observed by MDI, to a coronal field. MPOLE is a suite of IDL programs implementing the Minimum Current Corona Model (Longcope 1996) and currently includes a new method that uses a hierarchy of topological features (Beveridge 2006). The extrapolation gives the location of topological features such as poles, nulls, separatricies, separators, and spine lines. We examine the flare emission observed by RHESSI in the context of these topological features. In the case of the 29 October 2003 flare, we find a relationship between the spine lines and the temporal evolution of the HXR flare footpoints. In this poster, we present observations supporting the relationship, explore uncertainties in the consistency between MPOLE and RHESSI data, and survey possible results.This work is supported by NASA. Title: Evolution of magnetic fields and energetics of flares in active region 8210 Authors: Régnier, S.; Canfield, R. C. Bibcode: 2006A&A...451..319R Altcode: To better understand eruptive events in the solar corona, we combine sequences of multi-wavelength observations and modelling of the coronal magnetic field of NOAA AR 8210, a highly flare-productive active region. From the photosphere to the corona, the observations give us information about the motion of magnetic elements (photospheric magnetograms), the location of flares (e.g., Hα, EUV or soft X-ray brightenings), and the type of events (Hα blueshift events). Assuming that the evolution of the coronal magnetic field above an active region can be described by successive equilibria, we follow in time the magnetic changes of the 3D nonlinear force-free (nlff) fields reconstructed from a time series of photospheric vector magnetograms. We apply this method to AR 8210 observed on May 1, 1998 between 17:00 UT and 21:40 UT. We identify two types of horizontal photospheric motions that can drive an eruption: a clockwise rotation of the sunspot, and a fast motion of an emerging polarity. The reconstructed nlff coronal fields give us a scenario of the confined flares observed in AR 8210: the slow sunspot rotation enables the occurence of flare by a reconnection process close to a separatrix surface whereas the fast motion is associated with small-scale reconnections but no detectable flaring activity. We also study the injection rates of magnetic energy, Poynting flux and relative magnetic helicity through the photosphere and into the corona. The injection of magnetic energy by transverse photospheric motions is found to be correlated with the storage of energy in the corona and then the release by flaring activity. The magnetic helicity derived from the magnetic field and the vector potential of the nlff configuration is computed in the coronal volume. The magnetic helicity evolution shows that AR 8210 is dominated by the mutual helicity between the closed and potential fields and not by the self helicity of the closed field which characterizes the twist of confined flux bundles. We conclude that for AR 8210 the complex topology is a more important factor than the twist in the eruption process. Title: REU program in Solar Physics at Montana State University Authors: Martens, P. C.; Canfield, R. C.; McKenzie, D. M. Bibcode: 2005AGUFMED31A1202M Altcode: I will present an overview of the REU program in Solar Physics and Space Weather that has existed since 1999 at Montana State University, since 2003 with NSF support. I will briefly describe the goals, organization, scientific contents and results, and present statistics on applications, participants, gender balance, and diversity. This will be concluded by an overview of our plans for the future, Title: Evidence of Small Scale Reconnection in a Moving Feature Authors: Régnier, S.; Canfield, R. C. Bibcode: 2005ESASP.600E..94R Altcode: 2005dysu.confE..94R; 2005ESPM...11...94R No abstract at ADS Title: Describing Coronal Magnetic Fields by Successive Force-Free Equilibia Authors: Régnier, S.; Canfield, R. C. Bibcode: 2005ESASP.596E..13R Altcode: 2005ccmf.confE..13R No abstract at ADS Title: Self and mutual magnetic helicities in coronal magnetic configurations Authors: Régnier, S.; Amari, T.; Canfield, R. C. Bibcode: 2005A&A...442..345R Altcode: Together with the magnetic energy, the magnetic helicity is an important quantity used to describe the nature of a magnetic field configuration. In the following, we propose a new technique to evaluate various components of the total magnetic helicity in the corona for an equilibrium reconstructed magnetic field. The most meaningful value of helicity is the total relative magnetic helicity which describes the linkage of the field lines even if the volume of interest is not bounded by a magnetic surface. In addition if the magnetic field can be decomposed into the sum of a closed field and a reference field (following , Berger 1999 in Magnetic Helicity in Space and Laboratory Plasmas, ed. M. R. Brown, R. C. Canfield, & A. A. Pevtsov, 1), we can introduce three other helicity components: the self helicity of the closed field, the mutual helicity between the closed field and the reference field, and the vacuum helicity (self helicity of the reference field). To understand the meaning of those quantities, we derive them from the potential field (reference) and the force-free field computed with the same boundary conditions for three different cases: a single twisted flux tube derived from the extended Gold-Hoyle solutions, a simple magnetic configuration with three balanced sources and a constant distribution of the force-free parameter, and the AR 8210 magnetic field observed from 17:13 UT to 21:16 UT on May 1, 1998. We analyse the meaning of the self and mutual helicities: the self and mutual helicities correspond to the twist and writhe of confined flux bundles, and the crossing of field lines in the magnetic configuration respectively. The main result is that the magnetic configuration of AR 8210 is dominated by the mutual helicity and not by the self helicity (twist and writhe). Our results also show that although not gauge invariant the vacuum helicity is sensitive to the topological complexity of the reference field. Title: Spatial Relationship between Twist in Active Region Magnetic Fields and Solar Flares Authors: Hahn, Michael; Gaard, Stacy; Jibben, Patricia; Canfield, Richard C.; Nandy, Dibyendu Bibcode: 2005ApJ...629.1135H Altcode: Twisted magnetic field lines in solar active regions constitute stressed flux systems, the reconnection of which can release the stored (excess) magnetic energy in the form of solar flares. Using co-registered photospheric vector magnetograms and chromospheric Hα images for 29 flares, we explore the spatial relationship between these flares and the magnetic topology of the active regions in which they occur. We find two dominant trends. First, flares are preferentially initiated in subregions that have a high gradient in twist. Second, flare initiation occurs close to chirality inversion lines (which separate regions with twist of opposite handedness). Our results demonstrate that magnetic helicity, as manifested in the twist parameter, plays an important role in magnetic reconnection and solar flaring activity. Title: Self and Mutual Helicities in Coronal Magnetic Configurations Authors: Regnier, S.; Canfield, R. C. Bibcode: 2005AGUSMSP33A..04R Altcode: Together with the magnetic energy, the magnetic helicity is an important quantity used to describe the nature of a magnetic field configuration. The most meaningful value of helicity is the relative magnetic helicity (relative to a reference field) which describes the linkage of the field lines even if the volume of interest is not bounded by a magnetic surface. In addition if the magnetic field can be decomposed into the sum of a closed field and a reference field (following Berger 1999), we can introduce three other helicity values: the self helicity of the closed field, the mutual helicity between the closed field and the reference field, and the vacuum helicity (self helicity of the reference field). To understand the meaning of those quantities, we derive them from the potential field (reference) and the nonlinear force-free field computed with the same boundary conditions for three different cases: (i) a single twisted flux tube derived from the extended Gold-Hoyle solutions, (ii) a simple magnetic configuration with three balanced sources and a constant distribution of the force-free parameter, and (iii) the AR 8210 magnetic field observed at 19:40 UT on May 1, 1998. The self and mutual helicities corresponds to the twist and writhe of confined flux bundles, and the crossing of field lines in the magnetic configuration respectively. The vacuum helicity is interpreted as a measure of the topological complexity of the field. Title: The Relationship Between Active Region Twist & Solar Flaring Activity Authors: Nandy, D.; Hahn, M.; Gaard, S.; Jibben, P.; Canfield, R. C. Bibcode: 2005AGUSMSP23B..06N Altcode: Twisted magnetic field lines in solar active regions constitute stressed flux systems -- the reconnection of which can release the stored (excess) energy in the form of solar flares. The explosive release of energy through such flares, beyond contributing to the heating of the solar corona, can sometimes affect the near-Earth Space environment and trigger geomagnetic storms. Here we explore the relationship between solar flares and the pre-flare magnetic topology (characterized by the twist α in the magnetic fields lines) of the active regions in which the flares originate. We have discovered that flares are preferentially initiated in sub-regions that have an high gradient in twist and lie close to chirality inversion lines (which separate regions with twist of opposite handedness). Our results imply that the topology of magnetic field lines -- as characterized by the twist parameter α -- plays an important role in magnetic reconnection and flaring events. Title: On a Cyclic Variation of the Hemispheric Helicity Rule Authors: Pevtsov, A. A.; Hagyard, M. J.; Blehm, Z.; Smith, J. E.; Canfield, R. C.; Sakurai, T.; Hagino, M. Bibcode: 2005HiA....13..140P Altcode: We report the result of a study of magnetic helicity in solar active regions during 1980-2000 (cycles 21-23). Using the vector magnetograms from four different instruments (Haleakala Stokes Polarimeter Marshall Space Flight Center Mitaka Solar Flare Telescope and Okayama Observatory Solar Telescope) we calculated the force-free parameter alpha as in Pevtsov et al. (1995). We use alpha as the proxy for current helicity. For each instrument we computed a gradient dalpha/dL as the linear fit of alpha versus latitude L using annual subsets of data. The hemispheric helicity rule (negative/positive helicity in northern/southern hemisphere) can be expressed in terms of this gradient as dalpha/dL < 0. We find that each instrument exhibits change in sign of this gradient for some years which implies that the hemispheric helicity rule may not hold in some phases of a solar cycle (see Hagino and Sakurai 2002). However we do not see consistency between different instruments in regards to years disobeying the rule. The disagreement may be due to difference in observations and/or insufficient number of magnetograms in some years. We conclude that the present data sets do not allow to make statistically significant inference about possible cyclic variation of the hemispheric helicity rule. Title: Helicity of Magnetic Clouds and Their Associated Active Regions Authors: Leamon, Robert J.; Canfield, Richard C.; Jones, Sarah L.; Lambkin, Keith; Lundberg, Brian J.; Pevtsov, Alexei A. Bibcode: 2005HiA....13..132L Altcode: Magnetic clouds are closely associated with Coronal Mass Ejections (CMEs). Most CMEs are associated with active regions. What is the relationship between the topology of these clouds and the associated active region? For our purposes magnetic clouds can be modeled adequately by a cylindrical force-free magnetic configuration (Lepping 1990). We have modeled the magnetic field topology of 14 magnetic clouds and their associated active regions to determine values of the force-free field parameter for both as well as total currents and fluxes. We find that the number of turns of the magnetic field in the full length of the cloud is typically an order of magnitude greater than the same quantity in the associated active region. This finding compels us to reject models of flux rope formation that do not invoke magnetic reconnection and helicity conservation. Title: Of Tilt and Twist Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca A.; Howard, Robert F.; Pevtsov, Alexei A. Bibcode: 2005HiA....13..135H Altcode: Using Haleakala Stokes Polarimeter active-region vector magnetograms and Mt. Wilson Observatory full-disk longitudinal magnetograms we measure both the overall twist (using the force-free-field parameter alpha) and tilt of 368 active regions. This dataset clearly shows two well-known phenomena Joy's law and the hemispheric helicity rule as well as a lesser-known twist-tilt relationship which is the point of this work. Those regions that closely follow Joy's law show no twist-tilt relationship as expected if the twist originates from convective buffeting of initially untwisted and unwrithed flux tubes within the convection zone through the Sigma effect. Those regions that strongly depart from Joy's law show significantly larger than average twist and a very strong twist-tilt relationship. These properties suggest that the twist-tilt relationship in these regions is due to kinking of flux tubes that are initially highly twisted but not strongly writhed perhaps as a result of dynamo action. Title: How is Magnetic Energy Stored and Released? Authors: Régnier, S.; Canfield, R. C. Bibcode: 2004ESASP.575..255R Altcode: 2004soho...15..255R No abstract at ADS Title: Magnetic Field Topology of Sigmoids Authors: Son, J. H.; Canfield, R. C.; Acton, L. W. Bibcode: 2004AAS...205.1004S Altcode: 2004BAAS...36R1349S Sigmoids are studied due to their eruptive nature, which affects the Earth and the space atmosphere. The shape of the sigmoid (S-shaped or inverse S-shaped) is an indicator of eruption. The origin of this shape has been the topic of many research papers. One such paper by Fan and Gibson, The Emergence of a Twisted Magnetic Flux Tube Into a Preexisting Coronal Arcade, appeared in 2003. Fan and Gibson argue that a sigmoid with left-handed twist has left-handed writhe, which gives the sigmoid its S-shape and right-handed twist the inverse S-shape. Our study determined that there is no correlation between a sigmoid's handedness and shape as claimed in the paper by Fan and Gibson. Doing a statistical study observing the topology of the sigmoid using the data from Yohkoh Soft X-ray Telescope, we classified each sigmoid by its shape, twist, and magnetic field lines. We found that 23% of our data was right-handed and S-shaped, 33% was left-handed and S-shaped, 22% was right-handed and inverse S-shaped, and 22% was left-handed and inverse-S shaped. Thus, we found no systematic relationship between the handedness and shape of the sigmoid -- in disagreement with Fan and Gibson. Title: On the Tilt and Twist of Solar Active Regions Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca A.; Nandy, Dibyendu; Howard, Robert F.; Pevtsov, Alexei A. Bibcode: 2004ApJ...611.1149H Altcode: Tilt and twist are two measurable characteristics of solar active regions that can give us information about subsurface physical processes associated with the creation and subsequent evolution of magnetic flux tubes inside the Sun. Using Mees Solar Observatory active region vector magnetograms and Mount Wilson Observatory full-disk longitudinal magnetograms, we measure the magnetic twist and tilt angles of 368 bipolar active regions. In addition to two well-known phenomena, Joy's law and the hemispheric helicity rule, this data set also shows a lesser known twist-tilt relationship, which is the focus of this study. We find that those regions that closely follow Joy's law do not show any twist-tilt dependence. The dispersion in tilt angles and the dispersion in twist are also found to be uncorrelated with each other. Both of these results are predicted consequences of convective buffeting of initially untwisted and unwrithed flux tubes through the Σ-effect. However, we find that regions that strongly depart from Joy's law show significantly larger than average twist and very strong twist-tilt dependence-suggesting that the twist-tilt relationship in these regions is due to the kinking of flux tubes that are initially highly twisted, but not strongly writhed. This implies that some mechanism other than the Σ-effect (e.g., the solar dynamo itself or the process of buoyancy instability and flux tube formation) is responsible for imparting the initial twist (at the base of the solar convection zone) to the flux tubes that subsequently become kink-unstable. Title: Twist Propagation in Hα Surges Authors: Jibben, Patricia; Canfield, Richard C. Bibcode: 2004ApJ...610.1129J Altcode: We have studied Mees Solar Observatory Hα spectroheliogram and Doppler velocity movies of surges in 11 active regions. We used these movies to observe the surges' rotating motion, direction of propagation, and the implied handedness of that motion. We were able to coregister movies containing 47 surges with Haleakala Stokes Polarimeter vector magnetograms. We could hence determine the direction of twist stored in the magnetic field at the point of origin of each surge. We found (with a 99% confidence) that the direction of observed spin of these surges is consistent with the relaxation of the stored twist in the magnetic field. Magnetic reconnection of twisted flux tubes with their less twisted surroundings can account for the production and rotating motion of these surges. Title: Photospheric and Coronal Currents in Solar Active Regions Authors: Burnette, Andrew B.; Canfield, Richard C.; Pevtsov, Alexei A. Bibcode: 2004ApJ...606..565B Altcode: Using photospheric line-of-sight magnetograms from the National Solar Observatory (NSO) Kitt Peak and coronal X-ray images from the Yohkoh Soft X-Ray Telescope (SXT), we have determined the value of the constant α of the linear force-free field model (∇XB=αB) that gives the best visual fit to the overall coronal X-ray structure (αc) of 34 flare-productive active regions of relatively simple bipolar morphology. Vector magnetograms for 24 of these active regions are available from the Haleakala Stokes Polarimeter at Mees Solar Observatory. For each of them, we determine the single best-fit value of α in the photosphere (αp) by three quite different methods and show that these methods give statistically consistent values. By combining this data set with that of the NSO and SXT, we are able to compare for the first time quantitatively and statistically the observed values of α in the photosphere and corona of these regions. We find that the distribution of αp and αc values is fully consistent with the hypothesis that the overall twist density of the magnetic fields of active regions, as measured by the linear force-free field parameter α, is the same in the photosphere and the corona. We therefore conclude that the electric currents that create the nonpotential structure of such solar coronal active regions are of subphotospheric origin and pass without significant modification through the photosphere. Title: Helicity of magnetic clouds and their associated active regions Authors: Leamon, Robert J.; Canfield, Richard C.; Jones, Sarah L.; Lambkin, Keith; Lundberg, Brian J.; Pevtsov, Alexei A. Bibcode: 2004JGRA..109.5106L Altcode: In this work we relate the magnetic and topological parameters of twelve interplanetary magnetic clouds to associated solar active regions. We use a cylindrically symmetric constant-α force-free model to derive field line twist, total current, and total magnetic flux from in situ observations of magnetic clouds. We compare these properties with those of the associated solar active regions, which we infer from solar vector magnetograms. Our comparison of fluxes and currents reveals: (1) the total flux ratios ΦMC/ΦAR tend to be of order unity, (2) the total current ratios IMC/IAR are orders of magnitude smaller, and (3) there is a statistically significant proportionality between them. Our key findings in comparing total twists αL are that (1) the values of (αL)MC are typically an order of magnitude greater than those of (αL)AR and (2) there is no statistically significant sign or amplitude relationship between them. These findings compel us to believe that magnetic clouds associated with active region eruptions are formed by magnetic reconnection between these regions and their larger-scale surroundings, rather than simple eruption of preexisting structures in the corona or chromosphere. Title: Magnetic energy and magnetic helicity budget in AR 8210: What are the sources of flaring activity? Authors: Régnier, S.; Canfield, R. C. Bibcode: 2004IAUS..223..297R Altcode: 2005IAUS..223..297R The active region 8210 (AR 8210) observed on May 1, 1998 is the site of numerous flares. By studying a time series (from 17:00 UT to 21:40 UT) of reconstructed coronal magnetic configurations, we give a scenario of the eruptive phenomena in AR 8210 involving the rotation of the sunspot and the complex topology in a reconnection process. We also study the time evolution of the energy and of the helicity budget. We compute the free energy in the corona, the magnetic energy rate due to transverse photospheric motions. We conclude that the photospheric motions are related to flaring activities and occur before the injections of energy into the corona. In terms of magnetic helicity, we are able to derived the relative magnetic helicity (relative to the potential field) and the helicity of the potential field configuration. The vacuum helicity (or helicity of the potential field) is constant during the time period and characterizes the complex topology. For this time period, the relative magnetic helicity does not show obvious changes related to the flaring activity. Title: Detection of a Taylor-like Plasma Relaxation Process in the Sun and its Implication for Coronal Heating Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.; Longcope, Dana W. Bibcode: 2004IAUS..223..473N Altcode: 2005IAUS..223..473N The relaxation dynamics of a magnetized plasma system is a subject of fundamental importance in MHD - with applications ranging from laboratory plasma devices like the Toroidal Field Pinch and Spheromaks to astrophysical plasmas, stellar flaring activity and coronal heating. Taylor in 1974 proposed that the magnetic field in a plasma (of small but finite resistivity) relaxes to a minimum energy state, subject to the constraint that its total magnetic helicity is conserved (Woltjer 1958), such that the final magnetic field configuration is a constant alpha (linear) force-free field - where alpha is a quantity describing the twist in magnetic field lines. However, a clear signature of this mechanism in astrophysical plasmas remained undetected. Here we report observational detection of a relaxation process, similar to what Taylor (1974, 1986) envisaged, in the magnetic fields of flare-productive solar active regions. The implications of this result for magnetic reconnection and the coronal heating problem are discussed. Title: The Active Region 8210: Observations, Coronal Magnetic Fields and Energetics Authors: Regnier, S.; Canfield, R. C. Bibcode: 2003AGUFMSH22A0174R Altcode: The active region 8210 observed on May 1, 1998 is the site of several flares (3 C-class flares from 17:00 UT to 22:00 UT and one M-class flare at 22:40 UT). In this study, we analyse the causes and consequences of these flares inside the coronal magnetic configurations: sunspot rotation, moving features, cancellation of flux. The time evolution of AR 8210 from observations (magnetograms, Hα , EUV, Soft X-rays) and the nonlinear force-free reconstructed magnetic fields reveals that the sites of activity are related to the topological skeleton (null points, separatrix surfaces, separators): field lines crossing a separatrix surface reconnect in an other connectivity domain and produce an Hα brightening at the footpoints. We also determine the free magnetic energy budget (difference between nonlinear and potential field magnetic energy) and the relative magnetic helicity. The photospheric increase (or decrease) of magnetic flux is related to the evolution of the magnetic energy in the corona above the active region as well as to the occurence of flares. The main photospheric changes occur between the times of flaring activity. AR 8210 is a case study for the Solar MURI project. Title: Detection of a Taylor-like Plasma Relaxation Process in the Sun Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.; Longcope, Dana W. Bibcode: 2003ApJ...597L..73N Altcode: The relaxation dynamics of a magnetized plasma system is a subject of fundamental importance in magnetohydrodynamics-with applications ranging from laboratory plasma devices such as the toroidal-field pinch and spheromaks to astrophysical plasmas, stellar flaring activity, and coronal heating. Taylor in 1974 proposed that the magnetic field in a plasma, subject to certain constraints, relaxes to a minimum energy state such that the final magnetic field configuration is a constant α (linear) force-free field-where α is a quantity describing the twist in magnetic field lines. While Taylor's theory was remarkably successful in explaining some intriguing results from laboratory plasma experiments, a clear signature of this mechanism in astrophysical plasmas remained undetected. Here we report observational detection of a relaxation process, similar to what Taylor envisaged, in the magnetic fields of flare-productive solar active regions. The implications of this result for magnetic reconnection and the coronal heating problem are discussed. Title: Preflare Phenomena in Eruptive Flares Authors: Des Jardins, Angela C.; Canfield, Richard C. Bibcode: 2003ApJ...598..678D Altcode: We report the results of a statistical study of the relationship between eruptive solar flares and an observed Hα preflare phenomenon we call moving blueshift events (MBSEs). The Hα data were gathered using the Mees Solar Observatory CCD imaging spectrograph (MCCD). The 16 events in our data set were observed by both the MCCD and the Yohkoh Soft X-Ray Telescope, typically for at least 3 hr prior to the flare and in some cases repeatedly for several days prior to the flare. The data set contains both eruptive and noneruptive flares, without bias. Focusing on 3 hr periods before and after the flares, we found that the average rate of MBSEs prior to the flares was ~5 times greater prior to the 11 eruptive flares than prior to the five noneruptive ones. Also, the average rate of MBSEs dropped by a factor of ~6 after the eruptive flares. Earlier studies inferred that MBSEs reflect motions that originate in the readjustment of magnetic fields after magnetic reconnection. From the high correlation between eruptive flares and preflare MBSEs in the several hours prior to such events, we conclude that reconnection in the chromosphere or low corona plays an important role in establishing the conditions that lead to solar flare eruptions. Title: What Is the Role of the Kink Instability in Solar Coronal Eruptions? Authors: Leamon, Robert J.; Canfield, Richard C.; Blehm, Zachary; Pevtsov, Alexei A. Bibcode: 2003ApJ...596L.255L Altcode: We report the results of two simple studies that seek observational evidence that solar coronal loops are unstable to the MHD kink instability above a certain critical value of the total twist. First, we have used Yohkoh soft X-ray telescope image sequences to measure the shapes of 191 X-ray sigmoids and to determine the histories of eruption (evidenced by cusp and arcade signatures) of their associated active regions. We find that the distribution of sigmoid shapes is quite narrow and the frequency of eruption does not depend significantly on shape. Second, we have used Mees Solar Observatory vector magnetograms to estimate the large-scale total twist of active regions in which flare-related signatures of eruption are observed. We find no evidence of eruption for values of large-scale total twist remotely approaching the threshold for the kink instability. Title: On the origin of activity in solar-type stars Authors: Canfield, Richard C. Bibcode: 2003AdSpR..32..985C Altcode: The magnetic flux of solar coronal active regions is thought to originate in strong toroidal magnetic fields generated by a dynamo at the base of the convection zone. Once generated, this magnetic flux rises through the convection zone as discrete buoyant flux tubes, which may be formed into S2-shaped loops by their interaction with convective: cells and strong downdrafts. The loops are prevented from fragmentation by twist and curvature of their axes, which are writhed by the Coriolis effect and helical convective turbulence. These Σ-shaped loops emerge through the photosphere to form dipolar sunspot pairs and coronal active regions. These regions' free energy, relative magnetic helicity, and tendency to flare and erupt reflect the convection zone phenomena that dominate their journey to the surface, in which helical convective turbulence appears to play a primary role. Recent research leads me to suggest a new paradigm for activity in solar-type stars with deep-seated (tachocline) dynamos. In the present paradigm, dynamo models are expected to explain the distribution of activity in the H-R diagram, as reflected in mean chromospheric emission in lower main-sequence stars. In the new paradigm, dynamo action simply generates the flux that is necessary, but not sufficient, for such activity, and the amplitude of activity depends; most importantly on the kinetic helicity and turbulence of convection zone flows. Title: Force-free Fields in Active Regions: Magnetic configurations and Energetics Authors: Regnier, S.; Canfield, R. C.; Amari, T. Bibcode: 2003SPD....34.0103R Altcode: 2003BAAS...35..805R To determine the 3D configuration of solar active regions and to define boundary conditions for MHD codes, we reconstruct the coronal magnetic field from photospheric measurements. For that, we consider the active region in an equilibrium state. Three hypotheses are then often used: the potential field (no electric current), the linear force-free field (the current density is α times the magnetic field, α is a constant), the nonlinear force-free field (α is a function of space). We here compare those three models in terms of connectivity changes and energetics for two different active regions: AR 8151 and AR 8210. AR 8151 shows dramatical changes between these models whereas AR 8210 configuration stays close to the potential field. The structure of the filament and of the sigmoid observed in AR 8151 can only be determined in the nonlinear force-free field: both structures are identified with twisted flux tubes with ∼ 0.5-0.6 turns. Interestingly, the filament is a right-handed structure whereas the sigmoid is left-handed! This work is a part of the Solar MURI project. Title: Magnetic Cloud and Active Region Topology Compared Authors: Canfield, R. C.; Leamon, R. J.; Jones, S. L.; Lambkin, K.; Lundberg, B. Bibcode: 2003SPD....34.0518C Altcode: 2003BAAS...35..817C Magnetic clouds are closely associated with Coronal Mass Ejections (CMEs). Most CMEs are associated with active regions. What is the relationship between the topology of these clouds and the associated active region? For our purposes magnetic clouds can be modeled adequately by a cylindrical force-free magnetic configuration (Lepping, 1990). We have modeled the magnetic field topology of 14 magnetic clouds and their associated active regions to determine values of the force-free field parameter for both, as well as total currents and fluxes. We find that the number of turns of the twisted magnetic field in the full length of the cloud is typically an order of magnitude greater than the same quantity across the associated active region. This finding compels us to reject models of flux rope formation that do not invoke magnetic reconnection and helicity conservation. Title: On a Cyclic Variation of the Hemispheric Helicity Rule. Authors: Pevtsov, Alexei A.; Hagyard, Mona J.; Blehm, Zachary; Smith, James E.; Canfield, Richard C.; Sakurai, Takashi; Hagino, Masaoki Bibcode: 2003IAUJD...3E..35P Altcode: We report the result of a study of magnetic helicity in solar active regions during 1980-2000 (cycles 21-23). Using the vector magnetograms from four different instruments (Haleakala Stokes Polarimeter Marshall Space Flight Center Mitaka Solar Flare Telescope and Okayama Observatory Solar Telescope) we calculated the force-free parameter alpha as in Pevtsov et al. (1995). We use alpha as the proxy for current helicity. For each instrument we computed a gradient dalpha/dL as the linear fit of alpha versus latitude L using annual subsets of data. The hemispheric helicity rule (negative/positive helicity in northern/southern hemisphere) can be expressed in terms of this gradient as dalpha/dL < 0. We find that each instrument exhibits change in sign of this gradient for some years which implies that the hemispheric helicity rule may not hold in some phases of a solar cycle (see Hagino and Sakurai 2002). However we do not see consistency between different instruments in regards to years disobeying the rule. The disagreement may be due to difference in observations and/or insufficient number of magnetograms in some years. We conclude that the present data sets do not allow to make statistically significant inference about possible cyclic variation of the hemispheric helicity rule. Title: Of Twist and Tilt Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca A.; Howard, Robert F.; Pevtsov, Alexei A. Bibcode: 2003IAUJD...3E..27H Altcode: Using Haleakala Stokes Polarimeter active-region vector magnetograms and Mt. Wilson Observatory full-disk longitudinal magnetograms we measure both the overall twist (using the force-free-field parameter alpha) and tilt of 368 active regions. This dataset clearly shows two well-known phenomena Joy's law and the hemispheric helicity rule as well as a lesser-known twist-tilt relationship which is the point of this work. Those regions that closely follow Joy's law show no twist-tilt relationship as expected if the twist originates from convective buffeting of initially untwisted and unwrithed flux tubes within the convection zone through the Sigma effect. Those regions that strongly depart from Joy's law show significantly larger than average twist and a very strong twist-tilt relationship. These properties suggest that the twist-tilt relationship in these regions is due to kinking of flux tubes that are initially highly twisted but not strongly writhed perhaps as a result of dynamo action Title: Magnetic Cloud and Active Region Topology Compared Authors: Leamon, Robert J.; Canfield, Richard C.; Jones, Sarah L.; Lundberg, Brian Bibcode: 2003IAUJD...3E..24L Altcode: Magnetic clouds are closely associated with Coronal Mass Ejections (CMEs). Most CMEs are associated with active regions. What is the relationship between the topology of these clouds and the associated active region? For our purposes magnetic clouds can be modeled adequately by a cylindrical force-free magnetic configuration (Lepping 1990). We have modeled the magnetic field topology of 14 magnetic clouds and their associated active regions to determine values of the force-free field parameter for both as well as total currents and fluxes. We find that the number of turns of the magnetic field in the full length of the cloud is typically an order of magnitude greater than the same quantity in the associated active region. This finding compels us to reject models of flux rope formation that do not invoke magnetic reconnection and helicity conservation. Title: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) Authors: Lin, R. P.; Dennis, B. R.; Hurford, G. J.; Smith, D. M.; Zehnder, A.; Harvey, P. R.; Curtis, D. W.; Pankow, D.; Turin, P.; Bester, M.; Csillaghy, A.; Lewis, M.; Madden, N.; van Beek, H. F.; Appleby, M.; Raudorf, T.; McTiernan, J.; Ramaty, R.; Schmahl, E.; Schwartz, R.; Krucker, S.; Abiad, R.; Quinn, T.; Berg, P.; Hashii, M.; Sterling, R.; Jackson, R.; Pratt, R.; Campbell, R. D.; Malone, D.; Landis, D.; Barrington-Leigh, C. P.; Slassi-Sennou, S.; Cork, C.; Clark, D.; Amato, D.; Orwig, L.; Boyle, R.; Banks, I. S.; Shirey, K.; Tolbert, A. K.; Zarro, D.; Snow, F.; Thomsen, K.; Henneck, R.; Mchedlishvili, A.; Ming, P.; Fivian, M.; Jordan, John; Wanner, Richard; Crubb, Jerry; Preble, J.; Matranga, M.; Benz, A.; Hudson, H.; Canfield, R. C.; Holman, G. D.; Crannell, C.; Kosugi, T.; Emslie, A. G.; Vilmer, N.; Brown, J. C.; Johns-Krull, C.; Aschwanden, M.; Metcalf, T.; Conway, A. Bibcode: 2002SoPh..210....3L Altcode: RHESSI is the sixth in the NASA line of Small Explorer (SMEX) missions and the first managed in the Principal Investigator mode, where the PI is responsible for all aspects of the mission except the launch vehicle. RHESSI is designed to investigate particle acceleration and energy release in solar flares, through imaging and spectroscopy of hard X-ray/gamma-ray continua emitted by energetic electrons, and of gamma-ray lines produced by energetic ions. The single instrument consists of an imager, made up of nine bi-grid rotating modulation collimators (RMCs), in front of a spectrometer with nine cryogenically-cooled germanium detectors (GeDs), one behind each RMC. It provides the first high-resolution hard X-ray imaging spectroscopy, the first high-resolution gamma-ray line spectroscopy, and the first imaging above 100 keV including the first imaging of gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec with a full-Sun (≳ 1°) field of view, and the spectral resolution is ∼ 1-10 keV FWHM over the energy range from soft X-rays (3 keV) to gamma-rays (17 MeV). An automated shutter system allows a wide dynamic range (>107) of flare intensities to be handled without instrument saturation. Data for every photon is stored in a solid-state memory and telemetered to the ground, thus allowing for versatile data analysis keyed to specific science objectives. The spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼ 0.2° and operates autonomously. RHESSI was launched on 5 February 2002, into a nearly circular, 38° inclination, 600-km altitude orbit and began observations a week later. The mission is operated from Berkeley using a dedicated 11-m antenna for telemetry reception and command uplinks. All data and analysis software are made freely and immediately available to the scientific community. Title: Non-constant-α force-free field of active region NOAA 8210 Authors: Régnier, S.; Amari, T.; Canfield, R. C. Bibcode: 2002ESASP.505...65R Altcode: 2002IAUCo.188...65R; 2002solm.conf...65R We investigate the 3D coronal magnetic configuration of the active region NOAA 8210 (AR8210). This active region observed on May 1, 1998 is the site of numerous flares. Using the non-constant-α force-free hypothesis, we determine the coronal magnetic field of AR8210. The EIT/SOHO observations and the reconstructed magnetic configuration suggest that the initiation of the eruptive events is related to the existence of a complex topology (e.g. separatrix surfaces). From some characteristic parameters before and after the eruptions, we note that the magnetic energy (the free magnetic budget) decreases by 28% (55%, respectively). Title: Properties of magnetic clouds and geomagnetic storms associated with eruption of coronal sigmoids Authors: Leamon, Robert J.; Canfield, Richard C.; Pevtsov, Alexei A. Bibcode: 2002JGRA..107.1234L Altcode: We study 46 solar coronal eruptions associated with sigmoids seen in images from the Yohkoh Soft X-ray Telescope (SXT). We relate the properties of the sigmoids to in situ measurements at 1 AU and geomagnetic storms. Our primary result is that erupting sigmoids tend to produce geoeffective magnetic clouds (MCs): 85% of the erupting sigmoidal structures studied spawned at least a "moderate" (|Dst| ≥ 50 nT) geomagnetic storm. A collateral result is that MCs associated with sigmoids do not show the same solar-terrestrial correlations as those associated with filaments and, as such, form a distinct class of events. First, rather than reversing with the global solar dipole (at solar maximum), the leading field in MCs weakly (2:1) shows a solar cycle (Hale polarity) based correlation (reversing at solar minimum). Second, whereas the handedness of MCs associated with filament eruptions is strongly (95%) related to their launch hemisphere, that of MCs associated with sigmoid eruptions is only weakly (∼70%) so related. Finally, we are unaware of any model of the magnetic fields of sigmoids and their eruption that gives a useful prediction of the leading field orientation of their associated MC. Title: The Twist and Tilt of Active Region Magnetic Fields Authors: Holder, Z. A.; McMullen, R. A.; Canfield, R. C.; Howard, R. F.; Pevtsov, A. A. Bibcode: 2002AAS...200.0305H Altcode: 2002BAAS...34..642H We combine two large datasets to study the formation of the overall twist that is present in solar active-region magnetic fields. For purposes of discussion, we adopt a simplified model in which the magnetic flux bundles that form active regions start as axisymmetric toroids, without intrinsic twist, and rise in Ω -shaped loops through the convection zone, acquiring writhe through the effect of the Coriolis force on the large-scale flows within them, as well as buffeting by turbulent convection. The tilt of active regions with respect to the equator is an observable manifestation of such writhe, at photospheric levels. Owing to magnetic helicity conservation, we expect this tilt to be related to twist of the fields of these same regions. Using Mees Solar Observatory active-region vector magnetograms and Mount Wilson Observatory full-disk longitudinal magnetograms, we have measured both the tilt (Mount Wilson) and twist (Mees) of their magnetic fields, on active-region scales. This dataset clearly shows two well-known phenomena, Joy's law and the hemispheric handedness rule. In this paper we present the relationship between twist and tilt and estimate the extent to which that relationship is due to a mutual dependence of tilt and twist on latitude. We then compare our observational results to the simplified model. Title: What is the role of the kink instability in eruption of X-ray sigmoids? Authors: Leamon, R.; Canfield, R.; Blehm, Z.; Pevtsov, A. Bibcode: 2002AGUSMSH32D..03L Altcode: Observers see ample evidence of helical structures in erupting solar filaments, X-ray sigmoids and CMEs. It has been argued that the total amount of twist in a given loop is a factor in its MHD stability [Priest, 1984]. A simple model illustrates this point. Consider a cylindrical force-free magnetic field with constant α = T /L, where L is the length of the tube and T is the total twist contained within it. The tube is stable to the MHD kink instability for total twist below a critical value Tc ~ 2 π . Rust and Kumar [1996] compared the shape of 49 transient, bright sigmoid structures to the signature of a helically kinked flux rope. From a study of the aspect ratios of these transient sigmoid brightenings, they inferred that the cause of CMEs is the eruption of an unstable, kinked magnetic field. We have analyzed 155 X-ray sigmoids in the the Yohkoh SXT data, measuring the angle γ at which the sigmoid crosses its central axis and the length of the sigmoid along that axis (which is not identical to L, but is closely related to it). In a simple 2D force-free analysis, Pevtsov et al. [1997] showed that α = ( π / L ) sin γ , implying that sin γ is a measure of the total twist T. By simple visual inspection of the Yohkoh SXT movies, we have identified well-known signatures of eruption, i.e., X-ray cusps and arcades. We find no relationship between the frequency of occurrence of such signatures of eruption and sin γ . {Pevtsov, A. A.}, {Canfield, R. C.}, and {McClymont, A. N.}, Astrophys J., 481, 973, 1997. {Priest, E. R.}, {Solar Magneto-Hydrodynamics}, {Reidel: Dordrecht}, {1984}. {Rust, D. M.}, and {Kumar, A.}, Astrophys J., 464, L199, 1996. Title: What is the role of the kink instability in eruption of X-ray sigmoids? Authors: Canfield, R. C.; Leamon, R. J.; Blehm, Z.; Pevtsov, A. A. Bibcode: 2002AAS...200.2001C Altcode: 2002BAAS...34..672C Observers see ample evidence of helical structures in erupting solar filaments, X-ray sigmoids and CMEs. It has been argued that the total amount of twist in a given loop is a factor in its MHD stability [Priest, 1984]. A simple model illustrates this point. Consider a cylindrical force-free magnetic field with constant α = T /L, where L is the length of the tube and T is the total twist contained within it. The tube is stable to the MHD kink instability for total twist below a critical value Tc ~ 2 π . Rust and Kumar [1996] compared the shape of 49 transient, bright sigmoid structures to the signature of a helically kinked flux rope. From a study of the aspect ratios of these transient sigmoid brightenings, they inferred that the cause of CMEs is the eruption of an unstable, kinked magnetic field. We have analyzed 191 X-ray sigmoids in the the Yohkoh SXT data, measuring the angle γ at which the sigmoid crosses its central axis and the length of the sigmoid along that axis (which is not identical to L, but is closely related to it). In a simple 2D force-free analysis, Pevtsov et al. [1997] showed that α = ( π / L ) sin γ , implying that sin γ is a measure of the total twist T. By simple visual inspection of the Yohkoh SXT movies, we have identified well-known signatures of eruption, i.e., X-ray cusps and arcades. We find no relationship between the frequency of occurrence of such signatures of eruption and sin γ . Title: Importance of flux emergence as triggers Authors: Canfield, Richard Bibcode: 2002ocnd.confE...4C Altcode: No abstract at ADS Title: Active region magnetic flux systems Authors: Canfield, R. Bibcode: 2002cosp...34E.688C Altcode: 2002cosp.meetE.688C Solar active regions form where and when the tops of bundles of magnetic flux, shaped like the Greek letter Omega, emerge into the solar atmosphere. The last decade has witnessed considerable progress in the understanding of the transport of these flux systems through the convection zone and their emergence into the visible atmosphere. In particular, observations with magnetographs and coronal imagers have drawn attention to the relevance of twist in these magnetic fields to both the convection zone transport of Omega loops and their eruption, in the form of coronal mass ejections. In this talk I will review these magnetic field observations and relevant theoretical models. Title: Solar magnetic fields and geomagnetic events Authors: Pevtsov, Alexei A.; Canfield, Richard C. Bibcode: 2001JGR...10625191P Altcode: Some interplanetary studies lead one to expect that the toroidal fields of individual active regions are directly related to their heliospheric structure. Other studies conclude that the large-scale solar dipolar field is more important. We have carried out two studies that bear on these apparently conflicting views. We first studied geomagnetic events temporally associated with the eruption of 18 individual coronal X-ray sigmoids, which occurred while the large-scale solar dipolar magnetic field pointed southward. We found that if a coronal flux rope model is used to interpret magnetic structure, eruptions with a southward leading magnetic field are associated with stronger geomagnetic storms, but those with a northward leading field are associated with more storms. We next studied three full magnetic cycles, solar cycles 17-22. We examined the temporal variation of the ratio of the geomagnetic Ap index to the sunspot number. We found no statistically compelling fluctuations of this quantity on solar cycle time scales that are in phase with the reversal of active region polarities. On the other hand, we found a weak tendency for fluctuations that are in phase with the reversal of the large-scale solar dipole field. From these two studies we infer that the magnetic structure of individual active regions plays a role in geomagnetic events, but their geoeffectiveness is complicated by asymmetries in the leading and following magnetic field and density. We conclude that simple cycle-dependent generalizations have only statistical significance, and cannot dependably be used to predict the geomagnetic effects of a given solar eruption. Title: Preflare Phenomena in Eruptive Flares Authors: Colman, A. M.; Canfield, R. C. Bibcode: 2001AGUSM..SH22A03C Altcode: We report the results of a statistical study of the relationship between eruptive solar flares and an observed pre-flare phenomenon we call blue shift events (BSEs). BSEs are believed to be the result of magnetic reconnections in the chromosphere. Data for the study, Hα CCD spectrograph images, were recorded at Mees Solar Observatory in Hawaii. In order for a flare to be chosen for our study, it had to be well observed by both the Mees Solar Observatory and Yohkoh. Therefore, the dataset studied contains both eruptive and non-eruptive flares, without bias. Once we finished analyzing each flare event for BSEs, we determined whether or not the flare was eruptive. Out of 16 events observed, the average rate of BSEs was five times greater prior to eruptive flares than prior to non-eruptive ones. Also, the rate of BSEs prior to eruptive flares was six times greater than after the flares. From this high correlation, we conclude that reconnections in the lower corona play a role in solar flare eruptions. Title: Properties of Magnetic Clouds Resulting from Eruption of Coronal Sigmoids Authors: Leamon, R. J.; Canfield, R. C.; Pevtsov, A. A. Bibcode: 2001AGUSM..SH31C08L Altcode: We study over 40 eruptions which originated with coronal sigmoids seen in Yohkoh SXT images, with subsequently observed in situ magnetic clouds (MCs) and geomagnetic storms at 1~AU. We correlate solar and interplanetary features so as to infer terrestrial event properties from their solar sources. A collateral result from studying this database is that CMEs and MCs resulting from erupting sigmoids seem not to adhere to rules such as leading Bz versus solar dipole orientation and, as such, form a distinct class of events. % Instead of a large-scale dipole rule, we find there is a weak (3:2) solar cycle (Hale polarity)-based rule for leading interplanetary field in MCs. We find that the helicity of magnetic clouds is much more strongly correlated (>90%) with launch hemisphere than the 60--70%\ rule of photospheric active region helicity. This rule appears to hold for all CMEs, taking the 28 years of events of Bothmer &\ Rust [``Coronal Mass Ejections,'' AGU Monograph Series 99, 139, 1997]. % At least half of Bothmer &\ Rust's events are associated with disparitions brusques, outside active regions. We therefore suggest that active region sigmoids and disappearing filaments are the origins of two different classes of CMEs. Title: Twist Propagation in Hα Surges Authors: Jibben, P.; Canfield, R. C. Bibcode: 2001AGUSM..SP51B08J Altcode: Hα surges are straight or slightly curved ejections that reach heights of 200 Mm and typically last 10-30 minutes. The surge material either fades or returns into the chromosphere along the trajectory of accent. It has been proposed that reconnection of magnetic fields accounts for Hα surges [2,3]. Previous observations demonstrated Hα surges are seen in the early stages of flux emergence [1]. We have studied 11 active regions, selected for surge activity, with observations from the Mees Solar Observatory's CCD Imaging Spectrograph and Haleakala Stokes Polarimeter (HSP). The spectroheliograms were made into movies showing Hα line center and the corresponding Doppler velocities from the shift in the \ha\ absorption line center. The direction of stored twist in the magnetic field is determined from the force-free field parameter, α , where ∇ x B =\ α B, computed from HSP vector magnetograms. The magnetogram and velocity data were coregistered and the relationship between the surge spin and the stored twist in the magnetic field was recorded. Statistical analysis of the data found with a 99% confidence level that the direction of observed spin of the surge is consistent with the relaxation of the stored twist in the magnetic field at the point where the surge originated. [1] Kurowaka, H. & Kawai, G 1993, The Magnetic and Velocity Fields of Solar Active Regions, ed. H. Zirin, G. Ai, & H. Wang, ASP Conf. Ser., 46, 507. [2] Roy, J.-R. 1973, Solar Physics, 28, 95. [3] Rust, D. M. 1968, Structure and Development of Solar Active Regions, ed. K. O. Kiepenheuer, IAU Symposium 35, p 77. Title: Hemispheric Helicity Trend for Solar Cycle 23 Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Latushko, Sergei M. Bibcode: 2001ApJ...549L.261P Altcode: Applying the same methods we used in solar cycle 22, we study active region vector magnetograms, full-disk X-ray images, and full-disk line-of-sight magnetograms to derive the helicity of solar magnetic fields in the first 4 years of solar cycle 23. We find that these three data sets all exhibit the same two key tendencies-significant scatter and weak hemispheric asymmetry-as were observed in solar cycle 22. This supports the interpretation of these tendencies as signatures of the writhing of magnetic flux by turbulence in the convection zone. Title: Solar Active Regions Authors: Canfield, R. Bibcode: 2000eaa..bookE2023C Altcode: Solar active regions form where the tops of loops of magnetic flux, shaped like the Greek letter omega (Ω), emerge into the solar atmosphere where they can be seen. These Ω loops are thought to be generated by the solar dynamo, located deep within the Sun (see DYNAMOS: SOLAR AND STELLAR). The appearance of active regions depends greatly on the radiation used to image them, as well as the angle at... Title: Vector Magnetic Fields, Sub-surface Stresses and Evolution of Magnetic Helicity Authors: Canfield, R. C.; Pevtsov, A. A. Bibcode: 2000JApA...21..213C Altcode: No abstract at ADS Title: Coronal Structures as Tracers of Sub-surface Processes Authors: Pevtsov, A. A.; Canfield, R. C. Bibcode: 2000JApA...21..185P Altcode: No abstract at ADS Title: Max Millennium/Whole Sun Month Observations of a Sigmoid Region (AR 8668) Authors: Zarro, D. M.; Canfield, R. C.; Nitta, N.; Myers, D. C.; Gregory, S. E.; Qiu, J.; Alexander, D.; Hudson, H. S.; Thompson, B. J.; LaBonte, B. J. Bibcode: 2000SPD....31.0236Z Altcode: 2000BAAS...32..817Z We report on observations of a sigmoidal region AR 8668 obtained during the Whole Sun Month #3 campaign and Max Millennium Coordinated Observing Program #2. The observations pertain to the period 1999 August 16-17 during which several GOES B and C class flares occurred in AR 8668. Near simultaneous observations were obtained by SOHO (EIT 195 Angstroms/ and MDI full-disk magnetograms), TRACE 171 Angstroms/, Yohkoh SXT, Big Bear (Hα ), and Mees (IVM vector magnetograms). The multi-wavelength nature of these data, combined with their overlapping spatial and temporal coverages, provide a unique opportunity to study the magnetic topology and flaring evolution of twisted flux structures associated with sigmoids. An objective of this study is to co-align images and magnetograms obtained before and during the observed flares, and compare the results with inferences from the topological model of Titov and Demoulin, A&A 351, 707 (1999). We will present examples of these coalignments and identify sites of magnetic energy release that are associated with topological features (e.g. separatrices) predicted by this model. Title: Simultaneous Hα and X-ray Observations of Prominence Eruption and Disappearance Authors: Tonooka, H.; Matsumoto, R.; Miyaji, S.; Martin, S. F.; Canfield, R. C.; Reardon, K.; McAllister, A.; Shibata, K. Bibcode: 2000AdSpR..26..473T Altcode: Prominence eruptions or disappearances observed with an Hα limb filtergraph can be classified into 3 categories, the eruptive prominence, the quasi-eruptive prominence, and the disappearing prominence. We investigated their mechanism by comparing the results of simultaneous observations by Yohkoh SXT and Hα. We found that soft X-ray features change in both eruptive prominences and quasieruptive prominences, whereas no significant change takes place in disappearing prominences.In one prominence eruption event soft X-ray cusp structure suggests that the reconnection point is just below the Hα prominence Title: The HESSI Coordinated Data Analysis Flare Archive Authors: Zarro, D. M.; Canfield, R. C.; Csillaghy, A. Bibcode: 1999AAS...194.7703Z Altcode: 1999BAAS...31..960Z In preparation for the NASA HESSI mission and in conjunction with the Max Millenium Program, the current capabilities of the Synoptic Data Archive at the NASA/GSFC Solar Data Analysis Center (SDAC) will be enhanced. This Flare Archive will provide two distinct functions with the same database: (1) a WWW interface to synoptic datasets (e.g. daily images) for planning coordinated observations with HESSI; (2) a central, searchable archive of ground-based observatory (GBO) and space-based datasets for convenient joint analysis. Examples of datasets that the ideal HESSI Flare Archive will benefit from including are: Datasets & Source Full Disk Coalignment Img. & Sac Peak, BBSO, NSO (SOLIS) Hα /Optic. Spect. (inc. Polariz.) & Mees, Meudon Soft/Hard X-ray, EUV Img. & SXI, SXT, HXT, EIT, TRACE Soft/Hard X-ray Lightcurve & GOES, BCS, BATSE Microwave Img/Spect. & OVRO, Nobeyama, Zurich Radio (mm, meter, decimeter) & BIMA, Zurich, Nancay, Izmiran Magnetograms (inc. Vector) & BBSO, MDI, Mees, Mitaka, MSFC, KPNO Current capabilities of the Synoptic Data Archive will be demonstrated, and methods for integrating some of the above ancillary datasets with HESSI observations will be described. Title: Max Millennium Program in 1999/2000 Authors: Canfield, R. C. Bibcode: 1999AAS...194.8006C Altcode: 1999BAAS...31..965C The primary goal of the Max Millennium program is to understand particle acceleration and explosive energy release in solar flares. Elements of the program are ground-based solar observation, data analysis, and theory. We will work closely with the NASA HESSI mission and other NASA and NOAA spacecraft during the period of high solar activity from 1999 -- 2004. A comprehensive outline of the Max Millennium program is on the WWW at http://solar.physics.montana.edu/max_millennium/ Although the HESSI instrument views the whole Sun, many ground-based instruments have a limited field of view. In order to put together a compelling dataset, it is important to have comprehensive observations of a given flare. Experience shows that the effectiveness of instruments at Ground-Based Observatories (GBOs) with a limited field of view depends on: Definition of GBO campaigns and targets of opportunity Web access to GBO and spacecraft data Documentation and evaluation of current solar conditions Definition and Web posting of GBO plans and targets Web archive of operational GBO data During the coming year the emphasis of the Max Millennium program will be on the development and testing of capabilities for coordination of ground and space-based solar flare observations and data analysis. Title: Sigmoidal morphology and eruptive solar activity Authors: Canfield, Richard C.; Hudson, Hugh S.; McKenzie, David E. Bibcode: 1999GeoRL..26..627C Altcode: Soft X-ray images of solar active regions frequently show S- or inverse-S (sigmoidal) morphology. We have studied the Yohkoh Soft X-Ray Telescope video movie for 1993 and 1997. We have classified active regions according to morphology (sigmoidal or non-sigmoidal) and nature of activity (eruptive or non-eruptive). As well, we have used NOAA sunspot areas for each region as a measure of size. We find that regions are significantly more likely to be eruptive if they are either sigmoidal or large. Title: Magnetic helicity attracts interdisciplinary participation at chapman conference Authors: Canfield, R. C.; Pevtsov, A. A. Bibcode: 1999EOSTr..80...13C Altcode: During the last 5 years interest in magnetic helicity has grown dramatically in solar physics as a result of improved capabilities to measure and image magnetic fields. Magnetic helicity was introduced by K. Moffatt in the late 1950s as a topological invariant that describes the complexity of a magnetic field. The topological aspect of helicity is readily visualized in a Moebius strip, in which the system of interest is closed and helicity takes two forms, the writhing of the central axis of the strip and the twisting of the edges of the strip about that axis.In many plasmas (but not in atmospheres like that of Earth, for example), helicity is conserved, just as the sum of twist and writhe is conserved in a Moebius strip. Mathematically, it is related to linking integrals, which K. F. Gauss employed to study asteroid paths on the sky. In the late 1970s the concept of magnetic helicity was introduced in laboratory plasma physics, turbulence theory, space physics, and statistical theory. Title: Magnetic Helicity in Space and Laboratory Plasmas Authors: Brown, Michael R.; Canfield, Richard C.; Pevtsov, Alexei A. Bibcode: 1999GMS...111.....B Altcode: 1999mhsl.conf.....B Using the concept of magnetic helicity, physicists and mathematicians describe the topology of magnetic fields: twisting, writhing, and linkage. Mathematically, helicity is related to linking integrals, which Gauss introduced in the 19th century to describe the paths of asteroids in the sky. In the late 1970s the concept proved to be critical to understand laboratory plasma experiments on magnetic reconnection, dynamos, and magnetic field relaxation. In the late 1980s it proved equally important in understanding turbulence in the solar wind and the interplanetary magnetic field. During the last five years interest in magnetic helicity has grown dramatically in solar physics, and it will continue to grow as observations of vector magnetic fields become increasingly sophisticated. Title: High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum Authors: Lin, Robert P.; Hurford, Gordon J.; Madden, Norman W.; Dennis, Brian R.; Crannell, Carol J.; Holman, Gordon D.; Ramaty, Reuven; von Rosenvinge, Tycho T.; Zehnder, Alex; van Beek, H. Frank; Bornmann, Patricia L.; Canfield, Richard C.; Emslie, A. Gordon; Hudson, Hugh S.; Benz, Arnold; Brown, John C.; Enome, Shinzo; Kosugi, Takeo; Vilmer, Nicole; Smith, David M.; McTiernan, Jim; Hawkins, Isabel; Slassi-Sennou, Said; Csillaghy, Andre; Fisher, George; Johns-Krull, Chris; Schwartz, Richard; Orwig, Larry E.; Zarro, Dominic; Schmahl, Ed; Aschwanden, Markus; Harvey, Peter; Curtis, Dave; Pankow, Dave; Clark, Dave; Boyle, Robert F.; Henneck, Reinhold; Michedlishvili, Akilo; Thomsen, K.; Preble, Jeff; Snow, Frank Bibcode: 1998SPIE.3442....2L Altcode: The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum. Title: The Eruptive Flare of 15 November 1991: Preflare Phenomena Authors: Canfield, Richard C.; Reardon, Kevin P. Bibcode: 1998SoPh..182..145C Altcode: We present and interpret observations of the preflare phase of the eruptive flare of 15 November, 1991 in NOAA AR 6919. New flux emerged in this region, indicated by arch filaments in Hα and increasing vertical flux in vector magnetograms. With increasing frequency before the eruption, transient dark Hα fibrils were observed that crossed Hα bright plage and the magnetic inversion line to extend from the region of flux emergence to the filament, whose eruption was associated with the flare. These crossing fibrils were dynamic, and were often associated with sites of propagating torsional motion. These sites propagated from the region of flux emergence into the filament flux system. We interpret these morphological and dynamic features in terms of relaxation after magnetic reconnection episodes which create longer field lines within the filament flux system, as envisioned in the tether cutting model, and transfer twist to it, as well. Title: Helicity of Solar Active-Region Magnetic Fields Authors: Canfield, Richard C.; Pevtsov, Alexei A. Bibcode: 1998ASPC..140..131C Altcode: 1998ssp..conf..131C No abstract at ADS Title: On the Origin of Helicity in Active Region Magnetic Fields Authors: Pevtsov, A. A.; Canfield, R. C. Bibcode: 1998ASSL..229...85P Altcode: 1998opaf.conf...85P No abstract at ADS Title: Soft X-Ray Features of Prominence Eruption and Disappearance Authors: Tonooka, H.; Matsumoto, R.; Miyaji, S.; Martin, S. F.; Canfield, R. C.; Reardon, K.; McAllister, A.; Shibata, K. Bibcode: 1998ASSL..229..371T Altcode: 1998opaf.conf..371T No abstract at ADS Title: The High Energy Solar Spectroscopic Imager (HESSI): A Small Explorer for the Start of the New Millennium Authors: Holman, G. D.; Lin, R. P.; Dennis, B. R.; Crannell, C. J.; Ramaty, R. R.; Rosenvinge, T. T.; Canfield, R. C.; Emslie, A. G.; Hudson, H. S.; Hurford, G. J.; Madden, N. W.; van Beek, H. F.; Benz, A.; Bornmann, P. L.; Brown, J. C.; Enome, S.; Kosugi, T.; Vilmer, N.; Zehnder, A. Bibcode: 1997AAS...191.7416H Altcode: 1997BAAS...29R1326H The High Energy Solar Spectroscopic Imager (HESSI) has been selected for launch in mid 2000, at the peak of the solar activity cycle. The primary scientific objective of HESSI is to understand particle acceleration and explosive energy release in the magnetized plasma at the Sun. HESSI will provide the first high-spectral-resolution x-ray and gamma -ray images of the Sun. It will obtain the first imaging above 100 keV, the first imaging of solar gamma -ray lines, and the first high-resolution spectroscopy of solar gamma -ray lines, including the first determination of line shapes. In two years HESSI is expected to obtain observations of tens of thousands of microflares, thousands of hard x-ray flares, and of order a hundred gamma -ray line flares. HESSI will also monitor and provide high-spectral-resolution observations of cosmic and terrestrial hard x-ray and gamma -ray transients, as well as imaging of the Crab Nebula. HESSI's high spectral, spatial, and temporal resolution and dynamic range will allow the first detailed studies of the evolution of both accelerated particles and hot, thermal plasma in solar flares. Title: The Solar-B Mission Authors: Antiochos, Spiro; Acton, Loren; Canfield, Richard; Davila, Joseph; Davis, John; Dere, Kenneth; Doschek, George; Golub, Leon; Harvey, John; Hathaway, David; Hudson, Hugh; Moore, Ronald; Lites, Bruce; Rust, David; Strong, Keith; Title, Alan Bibcode: 1997STIN...9721329A Altcode: Solar-B, the next ISAS mission (with major NASA participation), is designed to address the fundamental question of how magnetic fields interact with plasma to produce solar variability. The mission has a number of unique capabilities that will enable it to answer the outstanding questions of solar magnetism. First, by escaping atmospheric seeing, it will deliver continuous observations of the solar surface with unprecedented spatial resolution. Second, Solar-B will deliver the first accurate measurements of all three components of the photospheric magnetic field. Solar-B will measure both the magnetic energy driving the photosphere and simultaneously its effects in the corona. Solar-B offers unique programmatic opportunities to NASA. It will continue an effective collaboration with our most reliable international partner. It will deliver images and data that will have strong public outreach potential. Finally, the science of Solar-B is clearly related to the themes of origins and plasma astrophysics, and contributes directly to the national space weather and global change programs. Title: What Is the Spatial Relationship between Hard X-Ray Footpoints and Vertical Electric Currents in Solar Flares? Authors: Li, Jing; Metcalf, Thomas R.; Canfield, Richard C.; Wülser, Jean-Pierre; Kosugi, Takeo Bibcode: 1997ApJ...482..490L Altcode: We examine the spatial relationship between solar hard X-ray sources observed with the Hard X-Ray Telescope aboard Yohkoh and photospheric electric currents observed at Mees Solar Observatory. In 1993, Canfield et al. concluded that energetic electron precipitation tends to occur at the edge of sites of high vertical current. They did not, however, have a direct diagnostic of particle precipitation; they used Hα Stark-wing emission as a proxy. In this paper, we analyze hard X-ray images and vector magnetograms of six flares of M/X X-ray class to reach two basic conclusions. First, we confirm that electron precipitation avoids sites of high vertical current density at photospheric levels, preferentially occurring adjacent to these current channels. Hence, we conclude that our observations rule out flare models in which nonthermal electrons are accelerated within the large-scale active-region current systems that are observed by present vector magnetographs. Second, at conjugate magnetic footpoints the stronger hard X-ray emission is associated with smaller vertical current density and weaker magnetic field. This result is consistent with a ``cornucopia''-shaped magnetic morphology in which precipitating electrons are preferentially deflected away from the narrower footpoint by magnetic mirroring. Title: The Solar Flare Dynamics Imager: A Low-Cost Mission for This Solar Maximum Authors: Wulser, J. -P.; Bruner, M. E.; Strong, K. T.; Canfield, R. C.; Culhane, J. L.; Mariska, J. T.; Polidan, R. Bibcode: 1997SPD....28.0232W Altcode: 1997BAAS...29R.898W The Solar Flare Dynamics Imager (SolFDI) is a very low-cost flare mission proposed for this upcoming solar maximum. It is based on a small solar EUV telescope/spectrograph for Spartan Lite, NASA's new ejectable hitchhiker spacecraft. The experiment's main scientific objective is to investigate the spatial, temporal, and velocity structure of the upflowing hot plasma frequently observed in soft X-ray lines during solar flares. Past observations were spatially unresolved, and the nature of these plasma flows, and their relationship with the impulsive phase flare energy release are not well understood. The second objective is to image the spatial and velocity structure of eruptive flares. Such observations can ultimately reveal the true three-dimensional geometry of the eruptive component of the flare. The Solar Flare Dynamics Imager will observe flares in a few selected extreme ultraviolet lines, including a high temperature Fe XXIV line. The instrument uses normal incidence multilayer coated optics to provide simultaneous imaging and spectroscopy in these lines with good spatial, spectral, and temporal resolution. The instrument is designed to fit within the envelope of the Spartan Lite ejectable hitchhiker spacecraft. Spartan Lite is smaller, and costs a fraction of a SMEX spacecraft. But if launched into the upcoming solar maximum, its 6-12 month mission duration would be sufficient to achieve SolFDI's scientific objectives. Title: NOAA 7926: A Kinked, Submerging Omega -loop ? Authors: Pevtsov, Alexei A.; Canfield, Richard C. Bibcode: 1997SPD....28.0238P Altcode: 1997BAAS...29..900P Using vector magnetograms and X-ray images, we study the evolution of decaying active region NOAA AR 7926. This active region had bipolar structure with non-Hale polarity. Observations suggest that the following polarity of this active region was in fact the leading polarity of active region NOAA AR 7918 on the previous solar rotation. Analyzing the rotation rate of both active regions and the topology of their magnetic field, we conclude that they belong to a single magnetic system resembling a kinked Omega loop. During the first rotation the upper part of the loop was exposed, forming the bipolar active region of normal (Hale) polarity. The rest of the Omega loop had emerged on the time by the second rotation, giving the appearance of non-Hale polarity. The major soft X-ray coronal loops of NOAA AR 7926 closely relate to the weak magnetic field in the middle part of the region. In that area, the photospheric field stayed concave-down in the course of dissipation of the active region, and the coronal loops decreased their length. We also find that coronal loops decrease their height derived using E and W limb observations of the two active regions NOAA AR 7926 and NOAA AR 7918. Hence, we concluded that the magnetic field of the active region submerged under the photosphere. Title: On the Subphotospheric Origin of Coronal Electric Currents Authors: Pevtsov, Alexei A.; Canfield, Richard C.; McClymont, Alexander N. Bibcode: 1997ApJ...481..973P Altcode: Using photospheric vector magnetograms from the Haleakala Stokes Polarimeter and coronal X-ray images from the Yohkoh Soft X-Ray Telescope (SXT), we infer values of the force-free field parameter α at both photospheric and coronal levels within 140 active regions. We determine the value of α for a linear force-free field that best fits each magnetogram in a least-squares sense. We average values from all available magnetograms to obtain a single mean photospheric α-value <αp> for each active region. From the SXT images we estimate α in the corona by determining (π/L) sin γ for individual loops, where γ is the observed shear angle of X-ray loops of length L. We then average these values of α to obtain a single coronal α value, <αc>, for each active region.

In active regions for which the photospheric α-map is predominantly of one sign, we find that the values of <αp> and <αc> are well correlated. Only for active regions in which both signs of α are well represented, and in which our method of analysis therefore breaks down, are the values of <αp> and <αc> poorly correlated. The former correlation implies that coronal electric currents typically extend down to at least the photosphere. However, other studies imply subphotospheric origin of the currents, and even current systems, that are observed in the photosphere. We therefore conclude that the currents responsible for sinuous coronal structures are of subphotospheric origin. Title: Coronal Structure as a Diagnostic of the Solar Dynamo Authors: Sandborgh, S.; Canfield, R. C.; Pevtsov, A. A. Bibcode: 1997SPD....28.0151S Altcode: 1997BAAS...29..888S This paper is motivated by the discovery of large-scale structures in the distribution of twist of magnetic fields with solar longitude and latitude. These structures are observed to persist for many solar rotations (Pevtsov and Latushko, BAAS 27, 978, 1995; Pevtsov, Canfield, and Metcalf, 4th SOHO Workshop: Helioseismology, Asilomar, 2-6 April 1995; Pevstov, Canfield, and Glatzmaier, Geophysical and Astrophysical Convection, NCAR, 10-13 October 1995). The large scale of these patterns, in space and time, implies that they are a deep-seated phenomenon, presumably that of the solar dynamo itself. Recent observations of twist and tilt of active regions (Pevstov and Canfield, Yohkoh Fifth Anniversary Symposium, Yoyogi, 6-8 November 1996, and Canfield and Pevtsov, this meeting) and the relationship between photospheric and coronal manifestations of twist (Pevstov, Canfield, and McClymont, ApJ 481, in press) confirm this conclusion. The Yohkoh Soft X-Ray Telescope images from the first five years of operations often clearly show the presence of topologically distinct regions. We have developed and used IDL software that allows us to identify and transfer to Carrington coordinates the boundaries of these systems, which we identify with magnetic flux systems. In this paper we present the results of our studies of these systems over many solar rotations, and relate them to the previously discovered persistent large-scale structures in the distribution of twist of active region magnetic fields with longitude and latitude. Title: On the Origin of Helicity in Active Region Magnetic Fields Authors: Canfield, Richard C.; Pevtsov, Alexei A. Bibcode: 1997SPD....28.1705C Altcode: 1997BAAS...29..921C The magnetic helicity of flux tubes consists of twist and writhe. If flux bundles at the base of the convection zone are simple axisymmetric toroids, and rise in omega-shaped loops through the convection zone, they acquire writhe through the effect of the Coriolis force on flows within them. The tilt of active regions with respect to the equator is an observable manifestation of this writhe, at photospheric levels. As a consequence of helicity conservation, we expect active regions to acquire twist to compensate for this writhe. The non-zero curl of the large-scale magnetic fields in active-region vector magnetograms is the observable manifestation of twist. Recent observations have revealed many interesting properties of the helicity of solar magnetic fields. Hemispheric preferences exist: active region fields predominantly have left-handed topology in the Northern hemisphere. Prominence structures also predominantly have left-handed writhe in the Northern hemisphere. Large-scale structures have been observed to exist in the distribution of both twist and writhe with longitude and latitude, and to persist for many solar rotations. To study the physical origin of these properties, we have used a dataset of about 100 active regions for which vector magnetograms were obtained at Mees Solar Observatory. We have measured both the overall tilt and the overall twist of these active regions. The dataset clearly shows Joy's law, the well-known dependence of tilt on latitude, as well as the hemispheric dependence of twist. However, our analysis shows that twist and tilt are not related as they should be if both twist and tilt are a consequence of the Coriolis force. Hence, we conclude that the twist seen in active regions is the consequence of a deep-seated phenomenon, presumably that of the solar dynamo itself. Title: Erratum: "The imaging vector magnetograph at Haleakala" [Sol. Phys., Vol. 168, No. 2, p. 229 - 250 (Oct 1996)]. Authors: Mickey, D. L.; Canfield, R. C.; Labonte, B. J.; Leka, K. D.; Waterson, M. F.; Weber, H. M. Bibcode: 1997SoPh..170..455M Altcode: No abstract at ADS Title: Reconnection and Helicity in a Solar Flare Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Zirin, Harold Bibcode: 1996ApJ...473..533P Altcode: Using X-ray images, Hα images, and vector magnetograms, we have studied the evolution of the coronal structure and magnetic field of NOAA Active Region 7154 during 1992 May 5-12. A two-ribbon 4B/M7.4 flare associated with an Hα filament eruption was observed on May 8, 15:13-19:16 UT. An interesting feature of the region was a long, twisted X-ray structure, which formed shortly before the flare and disappeared after it, being replaced by a system of unsheared postflare loops. Neither the X-ray nor Hα morphology nor the photo spheric magnetic field shows any indication of gradual buildup of nonpotential energy prior to the flare. Rather, the long structure appears to result from the reconnection of two shorter ones just tens of minutes before the filament eruption and flare.

Using vector magnetograms and X-ray morphology, we determine the helicity density of the magnetic field using the force-free field parameter α. The observations show that the long structure retained the same helicity density as the two shorter structures, but its greater length implies a higher coronal twist. The measured length and α value combine to imply a twist that exceeds the threshold for the MHD kink instability in a force-free cylindrical flux tube. We conclude that theoretical studies of such simple models, which have found that the MHD kink instability does not lead to global dissipation, do not adequately address the physical processes that govern coronal magnetic fields. Title: High-Energy Solar Spectroscopic Imager (HESSI) Authors: Dennis, Brian R.; Lin, Robert P.; Canfield, Richard C.; Crannell, Carol J.; Emslie, A. Gordon; Holman, Gordon D.; Hudson, Hugh H.; Hurford, Gordon J.; Ling, James C.; Madden, Norman W.; Ramaty, Reuven Bibcode: 1996SPIE.2804..228D Altcode: The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) is to understand particle acceleration and explosive energy release in the magnetized plasmas at the Sun. HESSI will provide the first hard X-ray imaging spectroscopy, the first high-resolution spectroscopy of solar gamma-ray lines from a spacecraft, the first imaging above 100 keV, and the first imaging of solar gamma- ray lines. The gamma-ray imaging spectroscopy will provide the first information on the spatial distribution of energetic (>1 MeV) protons, heavy ions, and relativistic electrons, and the first information on the angular distribution of the energetic ions. It will also provide detailed information on elemental abundances for both the accelerated ions and the ambient ions in the interaction region. HESSI uses Fourier-transform imaging spectroscopy to cover the broad energy range from soft X-rays (2 keV) to gamma-rays (20 MeV) with spatial resolutions down to 2 arcseconds and spectral resolutions down to 1 keV. This capability is achieved with 12 bi-grid rotating modulation collimators located in front of a corresponding set of 12 pairs of cooled germanium and silicon (Si(Li)) detectors to provide the wide spectral coverage. HESSI has been selected by NASA as an alternate Medium-class Explorer (MIDEX) mission, for launch in the year 2000. If it does not get funded as a flight mission, it will be descoped and proposed at a Small Explorer mission for launch in 2000 at half the MIDEX cost. Title: The Imaging Vector Magnetograph at Haleakala Authors: Mickey, D. L.; Canfield, R. C.; LaBonte, B. J.; Leka, K. D.; Waterson, M. F.; Weber, H. M. Bibcode: 1996SoPh..168..229M Altcode: We describe an instrument we have built and installed at Mees Solar Observatory on Haleakala, Maui, to measure polarization in narrow-band solar images. Observations in Zeemansensitive photospheric lines have been made for nearly all solar active regions since the instrument began operations in 1992. The magnetograph includes a 28-cm aperture telescope, a polarization modulator, a tunable Fabry-Pérot filter, CCD cameras and control electronics. Stokes spectra of a photospheric line are obtained with 7 pm spectral resolution, 1 arc sec spatial resolution over a field 4.7 arc min square, and polarimetric precision of 0.1%. A complete vector magnetogram observation can be made every eight minutes. The flexibility of the instrument encourages diverse observations: besides active region magnetograms we have made, for example, composite vector magnetograms of the full solar disk, and Hα polarization movies of flaring regions. Title: What is the spatial relationship between hard x-ray footpoints and vertical electric currents? Authors: Li, Jing; Metcalf, Thomas R.; Canfield, Richard C.; Wülser, Jean-Pierre; Kosugi, Takeo Bibcode: 1996AIPC..374..336L Altcode: 1996hesp.conf..336L We examine the spatial relationship between solar flare hard x-ray emission sites observed with the HXT aboard Yohkoh and photospheric electric currents observed at Mees Solar Observatory. Canfield et al. (3) concluded that nonthermal electron precipitation tends to occur at the edges of sites of high vertical current, not at their maxima. They did not, however, have very direct observations of the electron precipitation, since they used Hα Stark wing emission. In this work we compare hard x-ray images and vertical current maps in six large M/X-class flares. Our results confirm that electron precipitation sites avoid regions of strong photospheric vertical currents in large flares, and that magnetic mirroring strongly influences the relative HXR brightness of conjugate footpoints. Title: H alpha Surges and X-Ray Jets in AR 7260 Authors: Canfield, Richard C.; Reardon, Kevin P.; Leka, K. D.; Shibata, K.; Yokoyama, T.; Shimojo, M. Bibcode: 1996ApJ...464.1016C Altcode: We discuss nine events, observed simultaneously as jets in X-rays and surges in Hα, which are associated with moving magnetic bipoles. The X-ray jets share many features with those discovered by Yohkoh in active regions, emerging flux regions, and X-ray bright points (see paper by Shibata et al.); in particular, they originate near one end of a pair of small flaring loops. The Hα surges are adjacent to the X-ray jets. At the bases of these surges we observe both blueshifts (initially) and redshifts (1-2 minutes later). All the observed surges spin in a sense consistent with the relaxation of the twist stored in the magnetic fields of the moving magnetic bipoles. Newly discovered phenomena include footpoint convergence and moving-blueshift features.

We develop a model of the role of magnetic reconnection in these events. This model explains the temporal and spatial relationship between the jets and surges, the role of the moving bipoles, the flaring X-ray loops and their converging Hα footpoints, the Hα moving-blueshift features, the direction and amount of spin of the surges, and the relative temporal development of the Hα redshifts and blueshifts. Title: Evidence for Current-carrying Emerging Flux Authors: Leka, K. D.; Canfield, R. C.; McClymont, A. N.; van Driel-Gesztelyi, L. Bibcode: 1996ApJ...462..547L Altcode: To determine the relationship between electric currents and magnetic flux in emerging sunspots, we use observations of the morphology, proper motion, magnetic flux, and currents associated with several well-observed growing bipoles. Our target was NOAA Active Region 7260, which included a preexisting large spot and a fast-growing area of new magnetic flux. Magnetic bipoles in this region are well documented by X-ray images from the Yohkoh spacecraft and optical images and vector magnetograms from several ground-based observatories.

In this paper we show that (1) the Hα and X-ray structures associated with these bipoles do not agree with potential-field extrapolations of magnetograms; (2) proper motions imply that the flux bundles that make up these new bipoles are twisted before they emerge; (3) these new bipoles are cospatial with significant vertical electric currents; (4) the morphology, proper motion, and measured currents of these bipoles all imply the same sense of twist; (5) this sense of twist is the same as the large-scale twist of the preexisting large spot; and (6) the increase of these currents, as new flux emerges, is not consistent with their generation by photospheric motions.

We conclude that the new magnetic flux that emerged in this active region carried currents generated below the photosphere. Title: The High Energy Solar Spectroscopic Imager - HESSI Authors: Dennis, B. R.; Crannell, C. J.; Holman, G. D.; Ramaty, R.; von Rosenvinge, T. T.; Benz, A.; Bornmann, P. L.; Brown, J. C.; Canfield, R. C.; Emslie, A. G.; Enome, S.; Kosugi, T.; Hudson, H. S.; Hurford, G. J.; Lin, R. P.; Ling, J. C.; Madden, N. W.; van Beek, H. F.; Vilmer, N. Bibcode: 1996AAS...188.7016D Altcode: 1996BAAS...28..941D HESSI will investigate the physics of particle acceleration and energy release in solar flares through an unprecedented combination of high resolution imaging and spectroscopy of X-rays and gamma rays from 2 keV to 20 MeV during the next solar maximum. It uses Fourier-transform imaging with 12 bi-grid modulation collimators and cooled germanium and silicon detectors mounted on a Sun-pointed spin-stabilized spacecraft in a low-altitude equatorial orbit. HESSI will carry out the first imaging spectroscopy in hard X-rays with 2 arcseconds angular resolution, time resolution to tens of ms, and ~ 1 keV energy resolution; the first gamma-ray line spectroscopy from a spacecraft with ~ 1 keV energy resolution; and the first gamma-ray line and continuum imaging with 20 arcseconds angular resolution. Title: Magnetic Chirality and Coronal Reconnection Authors: Pevtsov, A. A.; Canfield, R. C.; McClymont, A. N. Bibcode: 1996AAS...188.3503P Altcode: 1996BAAS...28..871P We have used Mees Solar observatory vector magnetograms and Yohkoh Soft X-Ray Telescope images to study the role of magnetic chirality (handedness) in the trans-equatorial reconnection of active regions. Transequatorial reconnections are identified using SXT images. The chirality of the active regions is inferred from vector magnetograms and SXT images. Our results indicate that active regions reconnect preferentially with others of the same chirality. We have identified the 9 closest pairs of active regions separated by up to +/- 20° in latitude and longitude. All six pairs of active regions having the same sign of chirality showed transequatorial connection. All three pairs of active regions of opposite chirality indicated no inter-connection. Less close pairs of active regions, with separation up to +/- 40° in latitude and +/- 20° in longitude, showed the same tendency: 15 of 17 regions with the same chirality were inter-connected in the corona, and 8 of 10 regions having the opposite chirality did not show inter-connection. We explain this result with a simple model of current-system closure. Title: H alpha Surges and X-ray Jets in AR7260 Authors: Canfield, R. C.; Reardon, K. P.; Leka, K. D.; Shibata, K.; Yokoyama, T.; Shimojo, M. Bibcode: 1996mpsa.conf...49C Altcode: 1996IAUCo.153...49C No abstract at ADS Title: A Search for Asymmetric Flows in Young Active Regions Authors: Cauzzi, G.; Canfield, R. C.; Fisher, G. H. Bibcode: 1996ApJ...456..850C Altcode: We have studied the temporal evolution of photospheric velocities in young active regions that show evidence of ongoing magnetic flux emergence. We searched for asymmetries in the vertical plasma flows between the leading and following legs of the magnetic flux tubes. Such asymmetries are predicted in models of flux tubes rising in the convection zone (see the recent work of Fan, Fisher, & DeLuca). These models, which successfully describe several aspects of active region formation, predict plasma flows from the leading to the following leg of a magnetic flux loop, driven by the Coriolis force acting on the rising loop. These flows contribute to an excess of gas pressure in the following leg with respect to the leading one.

Our results show a predominance of downflow in the leading part of three young regions with respect to the following part, contrary to the model predictions. The observed asymmetries, obtained by averaging over the totality of the magnetic structures, range from 60 to 150 m s-1. Their real value, however, could be higher if the age and effective magnetic filling factor were taken into account. The flow asymmetry seems to disappear when the active regions enter a phase of magnetic stability.

We suggest two possible interpretations of these results in terms of the dynamics of emerging magnetic flux tubes as the most plausible ones. One possibility is that the rising flux tube experiences severe fragmentation during the last stages of emergence through the convection zone. After fragmentation, the greater effect of aerodynamic drag strongly reduces the rise speed of the smaller flux tubes and hence the Coriolis force that drives the flows from the leading to the following leg of the magnetic loop. Since the higher gas pressure present in the following leg is no longer balanced, it will then drive a flow in the opposite direction, i.e., from the following to the leading side. Estimates of these pressure-driven flow velocities are consistent with the observed values. A second possibility is that the asymmetric flows originate from a preexisting superrotational velocity within high-field strength toroidal flux rings near the base of the solar convection zone. As pointed out in the recent work of Moreno-Insertis, Schussler, & Ferriz-Mas, such superrotational velocities are required to maintain toroidal flux rings in dynamical equilibrium. Title: Magnetic Chirality and Coronal Reconnection Authors: Canfield, Richard C.; Pevtsov, Alexei A.; McClymont, Alexander N. Bibcode: 1996ASPC..111..341C Altcode: 1997ASPC..111..341C The authors have used Mees Solar Observatory vector magnetograms and Yohkoh Soft X-ray Telescope images to study the role of magnetic chirality in the trans-equatorial reconnection of active regions. They conclude that active regions reconnect preferentially with others of the same chirality. They explain this result with a simple model of the closure of their current systems. Title: YOHKOH SXT Observations of Prominence Eruption and Disappearance Authors: Tonooka, H.; Matsumoto, R.; Miyaji, S.; Martin, S. F.; Canfield, R. C.; Reardon, K.; McAllister, A.; Shibata, K. Bibcode: 1996mpsa.conf..493T Altcode: 1996IAUCo.153..493T No abstract at ADS Title: Coordinated X-ray and Hα Observations of Eruptive Flares Authors: Wülser, J. -P.; Canfield, R. C.; Sakao, T.; Acton, L. W. Bibcode: 1995SPD....26.1315W Altcode: 1995BAAS...27..990W No abstract at ADS Title: Latitudinal Variation of Helicity of Photospheric Magnetic Fields Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Metcalf, Thomas R. Bibcode: 1995ApJ...440L.109P Altcode: Using a 1988-1994 data set of original photospheric vector magnetograms as well as published data, we have studied the average magnetic helicity of 69 diverse active regions, adopting the linear force-free field parameter alpha as a measure. This average value was determined by minimizing the differences between the computed constant-alpha force-free and observed horizontal magnetic fields. The average magnetic helicity shows a sign difference at the 2 sigma level in opposite hemispheres. In our data set, 76% of the active regions in the northern hemisphere have negative helicity, and 69% in the southern hemisphere, positive. Although the data show considerable variation from one active region to the next, the data set as a whole suggest that the magnitude of the average helicity increases with solar latitude, starting at zero near the equator, reaches a maximum near 15 deg - 25 deg in both hemispheres, and drops back toward smaller values avove 35 deg - 40 deg. Qualitative comparison with published models shows that such latitudinal variation of the average magnetic helicity may result from either turbulent convective motions or differential rotation, although our studies of rotating sunspots lead us to favor the former. Title: The 1991 October 24 Flare: A Challenge for Standard Models Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Hudson, H. S.; Wulser, J. -P.; Acton, L.; Kosugi, T.; Masuda, S. Bibcode: 1995ApJ...440..386D Altcode: The M9.8 solar flare of 1991 October 24 22:30 UT presents several interesting characteristics: (1) energy release starts high in the corona; (2) the primary chromospheric ribbons are initially well separated and do not move apart at an observable rate; (3) no evidence is found for an erupting filament or other driver. To explain this flare, we consider several canonical flare models, including a filament eruption, a confined filament eruption, current interruption, and interacting loops. We conclude that none of these scenarios unequivocally explains this flare. Two possibilities which cannot be ruled out are (1) the eruption of a filament unobservable in H-alpha which starts high in the corona and produces no ribbon motions smaller than our detection threshold and no perceptible expansion of the coronal X-ray source, and (2) energy release due to spontaneous, propagating reconnection which allows the system to essentially brighten in place. Title: Is the Solar Chromospheric Magnetic Field Force-free? Authors: Metcalf, Thomas R.; Jiao, Litao; McClymont, Alexander N.; Canfield, Richard C.; Uitenbroek, Han Bibcode: 1995ApJ...439..474M Altcode: We use observations of the Na I lambda 5896 spectral line, made with the Stokes Polarimeter at Mees Solar Observatory, to measure the chromospheric vector magnetic field in NOAA active region 7216. We compute the magnetic field from observations of the Stokes parameters at six wavelengths within this spectral line using a derivative method and calculate the height dependence of the net Lorentz force in the photosphere and low chromosphere. We conclude that the magnetic field is not force-free in the photosphere, but becomes force-free roughly 400 km above the photosphere. Title: The Magnetic Evolution of the Activity Complex AR:7260 - a Roadmap Authors: Leka, K. D.; Canfield, R. C.; Mickey, D. L.; van Driel-Gesztelyi, L.; Nitta, N.; Sakurai, T.; Ichimoto, K. Bibcode: 1994SoPh..155..301L Altcode: The active region NOAA 7260 rotated onto the north solar hemisphere as a mature bipole: a dominant negative-polarity sunspot with trailing plage and scattered small spots in attendance. The dominantp spot itself had strong magnetic fields and covered almost 400 × 10−6 of a solar hemisphere. For a period of seven days beginning 14 August, 1992 this active region displayed rapid and drastic evolution: no fewer than 50 magnetic bipoles emerged in the area trailing the large sunspot, increasing the region's magnetic flux by more than 1022 Mx. This new group of sunspots formed a complexβγδ configuration with twoδ spots and a high degree of magnetic shear. Title: Transient Microwave Brightenings in Solar Active Regions: Comparison between VLA and YOHKOH Observations Authors: Gopalswamy, N.; Payne, T. E. W.; Schmahl, E. J.; Kundu, M. R.; Lemen, J. R.; Strong, K. T.; Canfield, R. C.; de La Beaujardiere, J. Bibcode: 1994ApJ...437..522G Altcode: We report observations of transient microwave (2 cm) brightenings and their relationship with brightenings in soft X-rays. The peak flux of the microwave brightenings observed by the Very Large Array (VLA) is smaller than the previously reported fluxes by two orders of magnitude. The microwave sources were highly polarized (up to 100%) and were situated on the periphery of a sunspot umbra. Among the many transients observed in X-rays by Yohkoh, two were observed simultaneously in microwaves. The microwave sources were found to be closer to the umbra of the sunspot than were the X-ray loops. It seems that the microwave sources are located at the footpoints of the looplike X-ray transients. Using the combined VLA, Yohkoh, and Mees data set, we determine the physical parameters of the loop in which the brightenings occur. We find that an increase in emission measure accompanied by small-scale heating can account for the X-ray brightening. The microwave emission can be interpreted as thermal gyroresonance or nonthermal gyrosynchrotron processes during the X-ray brightening. The magnetic field in the microwave-source region is found to be 1200-1800 G. The observations also provide evidence for temperature gradient in the coronal loops. Title: The NASA High-Energy Solar Physics Mission (HESP) Authors: Dennis, B. R.; Emslie, A. G.; Canfield, R.; Doschek, G.; Lin, R. P.; Ramaty, R. Bibcode: 1994AIPC..294..230D Altcode: 1994hesp.conf..230D The NASA High Energy Solar Physics (HESP) mission offers the opportunity for major breakthroughs in our understanding of the fundamental energy release and particle acceleration processes at the core of the solar flare problem. HESP's primary strawman instrument, the High Energy Imaging Spectrometer (HEISPEC), will provide X-ray and gamma-ray imaging spectroscopy, i.e., high-resolution spectroscopy at each spatial point in the image. It has the following unique capabilities: (1) high-resolution (~keV) spectroscopy from 2 keV-20 MeV to resolve flare gamma-ray lines and sharp features in the continuum; (2) hard X-ray imaging with 2` angular resolution and tens of millisecond temporal resolution, commensurate with the travel times and stopping distances for the accelerated electrons; (3) gamma-ray imaging with 4-8` resolution with the capability of imaging in specific lines or continuum regions; (4) moderate resolution measurements of energetic (20 MeV to ~1 GeV) gamma-rays and neutrons.

Addtional strawman instruments include a Bragg crystal spectrometer for diagnostic information and a soft X-ray/XUV/UV imager to map the flare coronal magnetic field and plasma structure. The HESP mission also includes extensive ground-based observational and supporting theory programs. Recently, the HESP mission has been adapted to ``lightsats''-lighter, smaller, cheaper spacecraft that can be built faster-and the baseline plan now includes two Taurus-class and one Pegasus-class spacecraft. A launch by the end of the year 2000 is desirable to be in time for the next solar activity maximum. Title: A search for low-energy protons in a solar flare from October 1992: Preliminary results Authors: Metcalf, T.; Mickey, D.; Canfield, R.; Wülser, J. -P. Bibcode: 1994AIPC..294...59M Altcode: 1994hesp.conf...59M We give preliminary results from the first use of the University of Hawaii's new Imaging Vector Magnetograph (IVM) to search for linear polarization in the H-alpha spectral line during solar flares. Such polarization has previously been interpreted as impact polarization from 100 keV protons impacting the chromosphere. The new data set has several advantages over previous data. First, the field of view is substantially larger than that used by Metcalf et al., and, second, the temporal resolution (16 s) is a factor of two better than that previously obtained. We show a preliminary comparison between the flare Hα polarization and hard X-rays observed with the Compton Observatory. Title: H_alpha and X-ray Signatures of Chromospheric Heating Observed in Solar Flares Authors: Wuelser, J. -P.; Canfield, R. C.; Sakao, T.; Masuda, S.; Kosugi, T.; Tsuneta, S. Bibcode: 1994kofu.symp..195W Altcode: We have studied the spatial and temporal relationship between thermal and non-thermal energy transport, and the chromospheric response in solar flares. H_alpha imaging spectra from Mees Solar Observatory provided the information on the heating and dynamics in the chromosphere, soft X-ray images from Yohkoh provided the conditions of the thermal plasma in the corona, and hard X-ray data from Yohkoh provided the diagnostics of the non-thermal particles. We present some preliminary results for several large flares, and discuss their implications for the chromospheric flare heating mechanism. Title: Study of Active Region Magnetic Field Structures Using VLA Radio, YOHKOH X-ray and MEES Optical Observations Authors: Gopalswamy, N.; Schmahl, E. J.; Kundu, M. R.; Lemen, J. R.; Strong, K. T.; Canfield, R. C.; de La Beaujardiere, J. Bibcode: 1994kofu.symp..347G Altcode: We report on the observation of compact magnetic flux tubes from the boundary between the umbra and penumbra of a large sunspot in AR 7135 on April 24, 1992. The structure and geometry of one such flux tube was determined using the coordinated observations obtained by the Very Large Array, the Yohkoh Soft X-ray Telescope and the Mees Solar Observatory. From radio observations we infer that the magnetic field of the flux tube at the spot-side footpoint is ~ 1300-1800 G. Title: Electric Currents and Coronal Heating in NOAA Active Region 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Wulser, J. -P.; Martens, P. C. H.; Tsuneta, S. Bibcode: 1994ApJ...428..860M Altcode: We examine the spatial and temporal relationship between coronal structures observed with the soft X-ray telescope (SXT) on board the Yohkoh spacecraft and the vertical electric current density derived from photospheric vector magnetograms obtained using the Stokes Polarimeter at the Mees Solar Observatory. We focus on a single active region: AR 6952 which we observed on 7 days during 1991 December. For 11 independent maps of the vertical electric current density co-aligned with non-flaring X-ray images, we search for a morphological relationship between sites of high vertical current density in the photosphere and enhanced X-ray emission in the overlying corona. We find no compelling spatial or temporal correlation between the sites of vertical current and the bright X-ray structures in this active region. Title: Electron precipitation and mass motion in the 1991 June 9 white-light flare Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Metcalf, T. R.; Hiei, E.; Sakurai, T.; Ichimoto, K. Bibcode: 1994SoPh..151..389D Altcode: We use Hα line profiles as a diagnostic of mass motion and nonthermal electron precipitation in the white-light flare (WLF) of 1991 June 9 01:34 UT. We find only weak downflow velocities (≈10 km s−1) at the site of white-light emission, and comparable velocities elsewhere.We also find that electron precipitation is strongest at the WLF site. We conclude that continuum emission in this flare was probably caused by nonthermal electrons and not by dynamical energy transport via a chromospheric condensation. Title: Patterns of Helicity in Solar Active Regions Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Metcalf, Thomas R. Bibcode: 1994ApJ...425L.117P Altcode: Using 46 vector magnetograms from the Stokes Polarimeter of Mees Solar Observatory (MSO), we studied patterns of local helicity in three diverse solar active regions. From these magnetograms we computed maps of the local helicity parameter alpha = Jz/Bz. Although such maps are noisy, we found patterns at the level approximately 2 to 3 sigmaJ(sub z), which repeat in successive magnetograms for up to several days. Typically, the alpha maps of any given active region contain identifiable patches with both positive and negative values of alpha. Even within a single sunspot complex, several such alpha patches can often be seen. We followed 68 alpha patches that could be identified on at least two successive alpha maps. We found that the persistence fraction of such patches decrease exponentially, with a characteristic time approximately 27 hr. Title: A Purely Polarized S-Component at 17 GHz Authors: Shibasaki, Kiyoto; Enome, Shinzo; Nakajima, Hiroshi; Nishio, Masanori; Takano, Toshiaki; Hanaoka, Yoichiro; Torii, Chikayoshi; Sekiguchi, Hideaki; Kawashima, Susumu; Bushimata, Takeshi; Shinohara, Noriyuki; Koshiishi, Hideki; Shiomi, Yasuhiko; Irimajiri, Yoshihisa; Leka, K. D.; Canfield, Richard C. Bibcode: 1994PASJ...46L..17S Altcode: A purely polarized bright radio source was found at 17 GHz by the Nobeyama Radioheliograph. This source was associated with a large sunspot. The source structure of this S-component was resolved due to high spatial resolution of the radioheliograph. A soft X-ray image of this active region taken by Yohkoh Satellite shows no counterpart for the radio source. Emission mechanism of the radio source is identified as gyroresonance. Magnetic field of the sunspot was measured by the Haleakala Vector Magnetograph at Mees Solar Observatory. The field strength at the half power level of the radio source was 2000 gauss at the photospheric level. This corresponds to the third harmonic layer. A bright soft X-ray loop, whose footpoint is at the penumbra of the large sunspot, can also be seen in the radio map. This loop is strongly curved, to form a part of spiral, which reflects strong electric current. Vector magnetogram shows strong rotation of the transverse field in the sunspot, which also corresponds to strong electric current. Due to this current and also to the density and the temperature enhancement near the X-ray loop, the radio peak is shifted toward the loop and has no dip. Title: Radio Imaging Observations of the Evolution of Thermal and Nonthermal Sources during a Gradual Solar Burst Authors: Nishio, Masanori; Nakajima, Hiroshi; Enome, Shinzo; Shibasaki, Kiyoto; Takano, Toshiaki; Hanaoka, Yoichiro; Torii, Chikayoshi; Sekiguchi, Hideaki; Bushimata, Takeshi; Kawashima, Susumu; Shinohara, Noriyuki; Irimajiri, Yoshihisa; Choi, Yong-Seok; Koshiishi, Hideki; Shiomi, Yasuhiko; Metcalf, Thomas R.; Canfield, Richard C. Bibcode: 1994PASJ...46L..11N Altcode: With the Nobeyama Radioheliograph, evolution of thermal and nonthermal radio sources was observed during a gradual solar radio burst. Two thermal sources were located near the top of an arcade-like soft X-ray structure observed by the Yohkoh satellite, while a nonthermal source was observed near the foot points of the arcade-like structure. During the early phase of the burst, an elongated structure was observed connecting one of the thermal sources and the nonthermal source. The elongation started from the location of the nonthermal source and gradually progressed to the thermal source placed just radially above the nonthermal source with the velocity of about 170 km s(-1) . These observations imply that the thermal source and the nonthermal source are connected by a single magnetic loop, and that the elongation of the source structure is an indication of chromospheric evaporation by radio imaging observations. The energetics of the thermal and nonthermal sources was analyzed using the radio and soft X-ray data. We suggest that a continuous supply of accelerated electrons for about 1000 s can maintain the hot plasma observed at the foot points of the loop. Title: Multispectral Observations of Chromospheric Evaporation in the 1991 November 15 X-Class Solar Flare Authors: Wuelser, Jean-Pierre; Canfield, Richard C.; Acton, Loren W.; Culhane, J. Leonard; Phillips, Andrew; Fludra, Andrzej; Sakao, Taro; Masuda, Satoshi; Kosugi, Takeo; Tsuneta, Saku Bibcode: 1994ApJ...424..459W Altcode: We analyze simultaneous H(alpha) images and spectra (from Mees Solar Observatory), and soft and hard X-ray images and spectra (from YOHKOH) during the early phase of an X1.5/3B flare. We investigate the morphological relationship between chromospheric downflows, coronal upflows, and particle precipitation sites, and the energetic relationship between conductive heating, nonthermal particle heating, and the chromospheric response. We find that the observations consistently fit the chromospheric evaporation model. In particular, we demonstrate that the observed upflowing coronal and downflowing chromospheric plasma components originate in the same locations, and we show that our unique set of optical and X-ray observations can clearly distinguish between conductively driven and electron beam driven evaporation. Title: Rapid Sunspot Motion Associated with Large Solar Flares Authors: Reardon, K. W.; Canfield, R. C.; McClymont, A. N.; Hudson, H. S. Bibcode: 1994ASPC...68..336R Altcode: 1994sare.conf..336R No abstract at ADS Title: Filament Tether Cutting Before a Major Eruptive Flare Authors: Canfield, R. C.; Blais, K. A.; Reardon, K. P.; Acton, Loren; Kurokawa, H. Bibcode: 1994ASPC...68..411C Altcode: 1994sare.conf..411C No abstract at ADS Title: Evidence for Twisted Emerging Flux in NOAA AR 7260 Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Canfield, R. C. Bibcode: 1994ASPC...68..145L Altcode: 1994sare.conf..145L No abstract at ADS Title: Photospheric Magnetic Field Measurement Errors and the Inferred Properties of Coronal Magnetic Fields Authors: Klimchuk, James A.; Canfield, Richard C. Bibcode: 1994ASPC...68..233K Altcode: 1994sare.conf..233K No abstract at ADS Title: The NASA High Energy Solar Physics (HESP) mission for the next solar maximum. Authors: Lin, R. P.; Dennis, B. R.; Ramaty, R.; Emslie, A. G.; Canfield, R.; Doschek, G. Bibcode: 1994GMS....84..283L Altcode: The NASA High Energy Solar Physics (HESP) mission offers the opportunity for major breakthroughs in the understanding of the fundamental energy release and particle acceleration processes at the core of the solar flare problem. Recently, the HESP mission has been adapted to Lightsats, lighter, smaller, cheaper spacecraft: the baseline HESP mission now includes two Pegasus-class spacecraft. A launch by the end of the year 2000 is desirable to be in time for the next solar activity maximum. Title: The X Flare of 15 November, 1991: Preflare Flux Emergence, Heating and Filament Eruption Authors: Canfield, R. C.; Blais, K. A.; McClymont, A. N.; Metcalf, T. R.; Reardon, K. P.; Wülser, J. -P.; Acton, L. W.; Kurokawa, H.; Hirayama, T. Bibcode: 1994xspy.conf..153C Altcode: No abstract at ADS Title: Diagnostics of Twisted Flux Emergence (noaa AR7260) Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Anwar, B.; Canfield, R. C.; Hudson, H. S.; Metcalf, T. R.; Mickey, D. L.; Nitta, N.; Kurokawa, H. Bibcode: 1994xspy.conf...25L Altcode: No abstract at ADS Title: Flares in Active Region NOAA 7260 Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Sakurai, T.; Shibata, K.; Ichimoto, K.; Canfield, R. C.; Wülser, J. -P.; Metcalf, T. R.; Mickey, D. L. Bibcode: 1994xspy.conf..111N Altcode: No abstract at ADS Title: Hα and X-Ray Signatures of Chromospheric Evaporation Observed during the Early Phase of the 15 November 1991 Flare Authors: Wülser, J. -P.; Canfield, R. C.; Acton, L. W.; Culhane, J. L.; Phillips, A.; Fludra, A.; Sakao, T.; Masuda, S.; Kosugi, T. Bibcode: 1994xspy.conf...75W Altcode: No abstract at ADS Title: Electric Currents and Coronal Structures in NOAA Active Region 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Wülser, J. -P.; Tsuneta, S. Bibcode: 1994xspy.conf...51M Altcode: No abstract at ADS Title: Solar imaging vector magnetograph Authors: Canfield, Richard C. Bibcode: 1993huha.rept.....C Altcode: This report describes an instrument which has been constructed at the University of Hawaii to make observations of the magnetic field in solar active regions. Detailed knowledge of active region magnetic structures is crucial to understanding many solar phenomena, because the magnetic field both defines the morphology of structures seen in the solar atmosphere and is the apparent energy source for solar flares. The new vector magnetograph was conceived in response to a perceived discrepancy between the capabilities of X ray imaging telescopes to be operating during the current solar maximum and those of existing magnetographs. There were no space-based magnetographs planned for this period; the existing ground-based instruments variously suffered from lack of sensitivity, poor time resolution, inadequate spatial resolution or unreliable sites. Yet the studies of flares and their relationship to the solar corona planned for the 1991-1994 maximum absolutely required high quality vector magnetic field measurements. By 'vector' measurements we mean that the observation attempts to deduce the complete strength and direction of the field at the measurement site, rather than just the line of sight component as obtained by a traditional longitudinal magnetograph. Knowledge of the vector field permits one to calculate photospheric electric currents, which might play a part in heating the corona, and to calculate energy stored in coronal magnetic fields as the result of such currents. Information about the strength and direction of magnetic fields in the solar atmosphere can be obtained in a number of ways, but quantitative data is best obtained by observing Zeeman-effect polarization in solar spectral lines. The technique requires measuring the complete state of polarization at one or more wavelengths within a magnetically sensitive line of the solar spectrum. This measurement must be done for each independent spatial point for which one wants magnetic field data. All the measurements need to be done in a time short compared to the time scale for changes of the solar features being observed. Were it possible, one would want to record all the needed data simultaneously, since temporal variation of atmospheric seeing degrades both the image and the polarization sensitivity. Since the measurements must span four dimensions, two spatial plus polarization and wavelength, we had some freedom to design the instrument to favor some dimensions over others in terms of simultaneity. Our earlier instrument, the Haleakala Stokes Polarimeter, records a range of wavelengths spanning two spectral lines in each reading, but requires two seconds to determine the polarization state and obtains spatial information only by assembling a long sequence of measurements at single locations on the sun. The new instrument sacrifices spectral detail and accuracy in favor of greatly improved imaging characteristics. The scientific goals for this instrument were to measure surface magnetic fields with enough accuracy to permit calculations of photospheric currents, but with a field of view covering an entire typical active region, high spatial resolution, and a fast enough temporal cadence for detecting flare-associated changes in magnetic structures. Title: The NASA high energy solar physics (HESP) mission for the next solar maximum Authors: Lin, R. P.; Dennis, B. R.; Emslie, A. G.; Ramaty, R.; Canfield, R.; Doschek, G. Bibcode: 1993AdSpR..13i.401L Altcode: 1993AdSpR..13..401L The NASA High Energy Solar Physics (HESP) mission offers the opportunity for major breakthroughs in our understanding of the fundamental energy release and particle acceleration processes at the core of the solar flare problem. HESP's primary strawman instrument, the High Energy Imaging Spectrometer (HEISPEC), will provide X-ray and γ-ray imaging spectroscopy, i.e., high-resolution spectroscopy at each spatial point in the image. It has the following unique capabilities; (1) high-resolution (~keV) spectroscopy from 2 keV - 20 MeV to resolve flare gamma-ray lines and sharp features in the continuum; (2) hard X-ray imaging with 2'' angular resolution and tens of millisecond temporal resolution, commensurate with the travel and stopping distances and times for the accelerated electrons; (3) gamma-ray imaging with 4''-8'' resolution with the capability of imaging in specific lines or continuum regions; (4) moderate resolution imaging of energetic (20 MeV to ~1 GeV) gamma-rays and neutrons. Additional strawman instruments include a Bragg crystal spectrometer for diagnostic information and a soft X-ray/XUV/UV imager to map the flare coronal magnetic field and plasma structure. The HESP mission also includes extensive ground-based observational and supporting theory programs. Presently HESP is planned for a FY 1995 new start and late 1999 launch, in time for the next solar activity maximum. Title: The Morphology of Flare Phenomena, Magnetic Fields, and Electric Currents in Active Regions. I. Introduction and Methods Authors: Canfield, Richard C.; de La Beaujardiere, J. -F.; Fan, Yuhong; Leka, K. D.; McClymont, A. N.; Metcalf, Thomas R.; Mickey, Donald L.; Wuelser, Jean-Pierre; Lites, Bruce W. Bibcode: 1993ApJ...411..362C Altcode: Electric current systems in solar active regions and their spatial relationship to sites of electron precipitation and high-pressure in flares were studied with the purpose of providing observational evidence for or against the flare models commonly discussed in the literature. The paper describes the instrumentation, the data used, and the data analysis methods, as well as improvements made upon earlier studies. Several flare models are overviewed, and the predictions yielded by each model for the relationships of flares to the vertical current systems are discussed. Title: The Morphology of Flare Phenomena, Magnetic Fields, and Electric Currents in Active Regions. II. NOAA Active Region 5747 (1989 October) Authors: Leka, K. D.; Canfield, Richard C.; McClymont, A. N.; de La Beaujardiere, J. -F.; Fan, Yuhong; Tang, F. Bibcode: 1993ApJ...411..370L Altcode: The paper describes October 1989 observations in NOAA Active Region 5747 of the morphology of energetic electron precipitation and high-pressure coronal flare plasmas of three flares and their relation to the vector magnetic field and vertical electric currents. The H-alpha spectroheliograms were coaligned with the vector magnetograms using continuum images of sunspots, enabling positional accuracy of a few arcsec. It was found that, during the gradual phase, the regions of the H-alpha flare that show the effects of enhanced pressure in the overlying corona often encompass extrema of the vertical current density, consistent with earlier work showing a close relationship between H-alpha emission and line-of-sight currents. The data are also consistent with the overall morphology and evolution described by erupting-filament models such as those of Kopp and Pneuman (1976) and Sturrock (1989). Title: The Morphology of Flare Phenomena, Magnetic Fields, and Electric Currents in Active Regions. III. NOAA Active Region 6233 (1990 August) Authors: de La Beaujardiere, J. -F.; Canfield, Richard C.; Leka, K. D. Bibcode: 1993ApJ...411..378D Altcode: We investigate the spatial relationship between vertical electric currents and flare phenomena in NOAA Active Region 6233, which was observed 1990, August 28-31 at Mees Solar Observatory. The two flares studied are the 1N/M1.8 flare on August 28, 22:30 UT and the 1N/M1.6 flare on August 29, 20:35 UT. Using Stokes polarimetry we make magnetograms of the region and compute the vertical current density. Using H-alpha imaging spectroscopy we identify sites of intense nonthermal electron precipitation or of high coronal pressure. The precipitation in these flares is barely strong enough to be detectable. We find that both precipitation and high pressure tend to occur near vertical currents, but that neither phenomenon is cospatial with current maxima. In contrast with the conclusion of other authors, we argue that these observations do not support a current-interruption model for flares, unless the relevant currents are primarily horizontal. The magnetic morphology and temporal evolution of these flares suggest that an erupting filament model may be relevant, but this model does not explicitly predict the relationship between precipitation, high pressure, and vertical currents. Title: The Confined Two-Ribbon Flare of 1991 October 24 Authors: de La Beaujardiere, J. F.; Canfield, R. C.; Hudson, H. S.; Wuelser, J. -P.; Kosugi, T.; Masuda, S.; Acton, L. W. Bibcode: 1993BAAS...25.1178D Altcode: No abstract at ADS Title: Electric Currents and Coronal Structures in NOAA Active Region 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Martens, P. C. H.; Tsuneta, S. Bibcode: 1993BAAS...25.1179M Altcode: No abstract at ADS Title: Hα Impact Polarization Observations in Solar Flares Authors: Mickey, D. L.; Metcalf, T.; Canfield, R. C.; Wuelser, J. -P. Bibcode: 1993BAAS...25.1223M Altcode: No abstract at ADS Title: Filament Tether Cutting Before a Major Eruptive Flare Authors: Canfield, R. C.; Blais, K. A.; McClymont, A. N.; Metcalf, T. R.; Reardon, K. P.; Wuelser, J. -P.; Acton, L. W.; Kurokawa, H. Bibcode: 1993BAAS...25.1188C Altcode: No abstract at ADS Title: A Magnetograph Comparison Workshop Authors: Jones, H.; Bogart, R.; Canfield, R.; Chapman, G.; Henney, C.; Kopp, G.; Lites, B.; Mickey, D.; Montgomery, R.; Pillet, V.; Rabin, D.; Ulrich, R.; Walton, S. Bibcode: 1993BAAS...25.1216J Altcode: No abstract at ADS Title: Evidence for Twisted Emerging Flux: NOAA AR 7260 Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Canfield, R. C.; Anwar, B.; Metcalf, T. R.; Mickey, D. L.; Nitta, N. Bibcode: 1993BAAS...25R1187L Altcode: No abstract at ADS Title: Invited Talk: (The NASA High Energy Solar Physics (HESP) Mission for the Next Solar Maximum) Authors: Lin, R. P.; Dennis, B. R.; Ramaty, R.; Emslie, A. G.; Canfield, R.; Doschek, G. Bibcode: 1993BAAS...25.1180L Altcode: No abstract at ADS Title: Coordinated Spectral and Temporal H alpha Observations of a Solar Flare Authors: de La Beaujardiere, J. -F.; Kiplinger, Alan L.; Canfield, Richard C. Bibcode: 1992ApJ...401..761D Altcode: We report simultaneous observations of a flare (1991 January 11 18:25 UT) with a combination of spectra and high time resolution images in H-alpha. The Mees Solar Observatory CCD Imaging Spectrograph obtains spectra and spectroheliograms with a repetition rate of 14 s. These data permit the identification of sites of nonthermal electron precipitation or of high coronal pressure. We observe extremely strong electron precipitation in this flare; we find reasonable agreement between the observed profiles and theoretical precipitation spectra. We discuss the possibility that the spectra classed as precipitation signatures are in fact the result of a projection effect, and we indicate limitations of the one-dimensional theoretical models. Title: Joint vector magnetograph observations at BBSO, Huairou Station and Mees Solar Observatory Authors: Wang, Haimin; Varsik, John; Zirin, Harold; Canfield, Richard C.; Leka, K. D.; Wang, Jingxiu Bibcode: 1992SoPh..142...11W Altcode: Joint vector magnetograph observations were carried out at Big Bear Solar Observatory (BBSO), Huairou Solar Observing Station (Huairou), and Mees Solar Observatory (MSO) in late September 1989. Comparisons of vector magnetograms obtained at the three stations show a high degree of consistency in the morphology of both longitudinal and transverse fields. Quantitative comparisons show the presence of noise, cross-talk between longitudinal field and transverse field, Faraday rotation and signal saturation effects in the magnetograms. We have tried to establish how the scatter in measurements from different instruments is apportioned between these sources of error. Title: The X Flare of 1991 November 15: Coordinated Mees/Yohkoh Observations Authors: Canfield, Richard C.; Hudson, Hugh S.; Leka, K. D.; Mickey, Donald L.; Metcalf, Thomas R.; Wuelser, Jean-Pierre; Acton, Loren W.; Strong, Keith T.; Kosugi, Takeo; Sakao, Taro; Tsuneta, Saku; Culhane, J. Leonard; Phillips, Andrew; Fludra, Andrzej Bibcode: 1992PASJ...44L.111C Altcode: This is a preliminary report on two unique new results from coordinated observations at Mees Solar Observatory and Yohkoh of the X1.5 flare of 1991 November 15, using vector magnetograms, Hα imaging spectra, X-ray images, and X-ray spectra. First, we find a close spatial relationship between Hα redshifts and X-rays from a flare loop and its footpoints at a time of large X-ray blueshifts. Second, we find that impulsive-phase hard X-rays originate in regions that are near, but not coincident with, the peaks of the vertical electrical current density distribution in AR 6919. Title: The YOHKOH mission for high-energy solar physics Authors: Acton, L.; Tsuneta, S.; Ogawara, Y.; Bentley, R.; Bruner, M.; Canfield, R.; Culhane, L.; Doschek, G.; Hiei, E.; Hirayama, T. Hudson, H.; Kosugi, T.; Lang, J.; Lemen, J.; Nishimura, J.; Makishima, K.; Uchida, Y.; Watanabe, T. Bibcode: 1992Sci...258..618A Altcode: 1992Sci...258..591A Data on solar flare mechanisms and the sun's corona will be generated by Japan's Yohkoh satellite's X-ray imaging sensors and X-ray and gamma-ray spectrometers. It is noted that the X-ray corona above active regions expands, in some cases almost continually, in contradiction of the widely accepted model of magnetohydrostatic equilibrium in such regions. Flaring X-ray bright points have been discovered to often involve ejecta into an adjacent, much larger and fainter magnetic loop, which brightens along its length at speeds up to 1000 km/sec. Title: Hard X-Ray Imaging Observations by YOHKOH of the 1991 November 15 Solar Flare Authors: Sakao, Taro; Kosugi, Takeo; Masuda, Satoshi; Inda, Mika; Makishima, Kazuo; Canfield, Richard C.; Hudson, Hugh S.; Metcalf, Thomas R.; Wuelser, Jean-P.; Acton, Loren W.; Ogawara, Yoshiaki Bibcode: 1992PASJ...44L..83S Altcode: We report on hard X-ray imaging observations of the 1991 November 15 flare with the HXT instrument aboard {Yohkoh}. Distributions of the hard X-ray sources at various stages of the flare, together with an overlay of the white-light flare, are presented. Attention is concentrated on the behavior of hard X-ray sources during the impulsive phase. The hard X-ray source appeared initially as a single source near the magnetic neutral line, then evolved into a double-source shape with the separation increasing with time. We believe that this is evidence for a multiple loop system flaring successively with a rising energy-release site. At the minima between the individual spikes of the time profile, the hard X-rays at 20--30 keV were concentrated near the apex of the flaring loop, whereas the hard X-rays above 30 keV originated from the footpoints. These observations are compared with the existing models. Title: CELIAS: charge, element and isotope analysis system for the SOHO mission. Authors: Hovestadt, D.; Geiss, J.; Gloeckler, G.; Axford, W. I.; Balsiger, H.; Bochsler, P.; Bürgi, A.; Canfield, R.; Coplan, M.; Dinse, H.; Galvin, A. B.; Gliem, F.; Gringauz, K. I.; Grünwaldt, H.; Hilchenbach, M.; Hsieh, K. C.; Ipavich, F. M.; Judge, D.; Klecker, B.; Kühne, M.; Lee, M. A.; Livi, S.; Managadze, G. G.; Marsch, E.; Neugebauer, M.; Möbius, E.; Reiche, K. V.; Scholer, M.; Wilken, B. Bibcode: 1992ESASP.346..343H Altcode: 1992ssts.rept..343H The status and further development as well as first calibration of the CELIAS experiment on SOHO is presented. CELIAS is designed to measure the mass, ionic charge, and energy of low and high speed solar wind of suprathermal ions and of low energy flare particles. Through analysis of the elemental and isotopic abundances (the ionic charge state and velocity distributions of ions originating on the solar atmosphere), the investigation focuses on studying the plasma processes in various temporal and spatial scales in the solar chromosphere, transition zone, and corona. Additionally, the CELIAS experiment includes the Solar EUV Monitor (SEM). This unit, which consists of an EUV transmission grating spectrometer and three isolated silicon photodiodes as detectors, is described. The three mass and charge discriminating sensors based on the time of flight technique are also described. Title: The X10 Flare of 1991 June 9: White Light, H-alpha, Magnetic Fields, and Electric Currents Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Metcalf, T. R.; Hiei, E.; Sakurai, T.; Ichimoto, K. Bibcode: 1992AAS...180.4108D Altcode: 1992BAAS...24..795D We present observations by several instruments of the white-light flare (WLF) of 1991 June 9 01:34 UT. A white-light image from the National Astronomical Observatory of Japan indicates the location of the WLF within the active region (NOAA 6659). Stokes polarimetry from Mees Solar Observatory (University of Hawai`i) yields a vector magnetogram and a map of the vertical electric currents. Also from Mees comes Hα imaging spectroscopy, which is an indicator of such specific chromospheric processes as nonthermal electron precipitation, high coronal pressure, and mass motion. Both Mees instruments provide continuum images, allowing coregistration of the various datasets. The white-light emission arises from an extended area including both sunspot umbra and penumbra. The active region magnetic field is strongly nonpotential and has numerous vertical electric current channels. The WLF is situated in an area of relatively low current density at the edge of the strongest current in the active region, and the nearby magnetic neutral line is highly sheared. The WLF site shows electron precipitation, but only in its penumbral portion; stronger magnetic mirroring in the umbra may inhibit precipitation there. Also, the precipitation is not especially strong (relative to that observed elsewhere in this flare), suggesting that the electron-beam model for WLFs is not appropriate in this case. Also, the lack of strong redshifts argues against a dynamical energy transport model. Title: November 15, 1991 X Flare -- The Movie: Hα , Soft X-rays, and Hard X-rays and Magnetic Fields Authors: Wulser, J. -P.; Acton, L.; Sakao, T.; Canfield, R.; Kosugi, T.; Slater, G.; Strong, K.; Tsuneta, S. Bibcode: 1992AAS...180.3003W Altcode: 1992BAAS...24..775W The X1.5/3B flare on 1991 November 15, 22:33 UT was well observed by the Hα Imaging Spectrograph and the Vector Magnetograph (Stokes Polarimeter) at Mees Solar Observatory, and by the Soft- and Hard X-ray Telescopes (SXT and HXT) aboard YOHKOH. We have combined this multispectral dataset into a series of temporally and spatially co-aligned video movies and analyzed the morphological and temporal relationships of the various flare emissions. The earliest manifestations of this flare include unresolved preflare SXR brightenings very close to the magnetic neutral line and preflare motions of filaments seen in Hα . In the flare core, SXR and Hα emission show moving and rotating coronal structures which we interpret as a successive brightening of adjacent loops during the main phase of the flare. The HXR source shows much more dramatic variability than the SXR source, and they are clearly not cospatial. On the other hand, there is a close spatial relationship between the HXR and Hα blue wing emission sites. The Hα , HXR, and SXR images all point to acceleration and heating in a region that starts close to the neutral line and moves outward during each HXR burst and during the gradual phase. Spectacular mass ejections are seen in both SXR and Hα , with clear unwinding of tightly coiled structures, acceleration of X-ray and Hα material to velocities of order 1000 km/s, and a striking thermal bifurcation between hot and cold plasma. Title: Electric Currents and Coronal Structures in Two Flare- Productive Active Regions, AR 6850 and AR 6952 Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey, D. L.; Strong, K. T.; Tsuneta, S. Bibcode: 1992AAS...180.3004M Altcode: 1992BAAS...24R.775M In this study, we examine the spatial and temporal relationship between coronal structures observed with the Soft X-Ray Telescope (SXT) on board the YOHKOH spacecraft and vertical electric currents derived from vector magnetograms obtained at the Mees Solar Observatory, Haleakala, Hawaii. We have focused on two active regions, AR 6850 (October 1991) and AR 6952 (December 1991). In both active regions, we observed significant current structures which persisted over time scales of days. The SXR emitting coronal structures, however, changed on much shorter time scales, indicating that there is no compelling, direct spatial and temporal relationship between the non-flaring SXR structures and the long-lived electric current systems. We have seen at least one case (in AR 6952) where a SXR brightening was associated spatially with a change in the vertical electric current. In this case, the the vertical current dissipated between December 8, 00:35 UT and the next observation at 00:48 UT on December 9, leaving a bright SXR structure which was observed at 24:27 UT on December 8. Hence, although more data must be analyzed to make a compelling case, it is possible that the SXR emission is related more closely to changes in the electric current systems rather than simply to the presence of these currents. Title: The Haleakala Imaging Vector Magnetograph Authors: Mickey, D. L.; Canfield, R. C.; Labonte, B. J.; Metcalf, T. R. Bibcode: 1992AAS...180.4005M Altcode: 1992BAAS...24..793M The Imaging Vector Magnetograph, now in daily use at Mees Solar Observatory, Haleakala, Hawaii, extends our capabilities in the measurement of solar vector magnetic fields by providing high spatial and temporal resolution, together with extended area coverage. The instrument is based on a fast-readout CCD camera as its primary detector, a tunable air-spaced Fabry-Perot filter for spectral selection, and variable nematic liquid-crystal retarders for polarization selection. A 28 cm aperture reflecting telescope provides an image the size of a large active region; it is coupled to the magnetograph in a configuration compact enough to mount on the Observatory's 3.6 m equatorial spar. The assembly can be pointed independently of the spar to select a region of interest, so the telescope is always used on-axis. A tip-tilt image stabilizer corrects for image displacement due to spar shake or large-scale atmospheric turbulence. A workstation-based computer control system, incorporating separate processors for user interface, process management and device control, permits accurate process timing along with a flexible user interface. The latter is implemented in an X-windows framework, so that in fact we have found it quite reasonable to operate the instrument from an X terminal at a remote location. The raw data images are stored on 8mm tape for off-line processing, or can be reduced in a few minutes using the instrument's built-in array processor to provide near-real-time magnetograms. Instrument operational parameters can be adjusted in several ways to favor spatial resolution, spatial field, temporal resolution or magnetic sensitivity, but typically we have a 4.5 x 4.5 arcminute field, 1 arcsecond spatial sampling, and an observation interval of five minutes. Initial solar observations showing current capabilities will be presented. Title: Co-Registered High-Time-Resolution Images and Spectra of the 1991 January 11 18:25 UT flare. Authors: de La Beaujardiere, J. -F.; Kiplinger, A. L.; Canfield, R. C. Bibcode: 1992AAS...180.4506D Altcode: 1992BAAS...24..803D The 1B/M1.0 flare of 1991 January 11 18:25 UT was simultaneously observed in Hα by two complementary instruments. Spectra and spectroheliograms were obtained from the Mees Solar Observatory CCD Imaging Spectrograph (MCCD) at the University of Hawai`i, while line-center and blue-wing (-1.3 Angstroms) images were made at 0.1 s time resolution by the NASA/GSFC High-Speed Hα Camera (HSHC) at the University of Colorado at Boulder. >From the MCCD data we identify sites showing the spectral signatures of nonthermal electron precipitation, high coronal pressure, or mass motions. We detect strong precipitation, and find reasonable agreement between the observed line profiles and theoretical precipitation spectra. We consider the possibility that the observed spectra actually arise from a projection effect and we indicate limitations of the one-dimensional theoretical models. We also observe redshifted emission which may be the signature of a chromospheric condensation, and we compare the downflow velocity with that predicted by the numerical model of Fisher. The slowing-down time of the observed downflow is about four times longer than in the model, perhaps because of the superposition of several unresolved condensations. Title: SPAM: A Canned Internet-Accessible Database of Interest to Solar Flare Researchers Authors: Canfield, R. C.; Hudson, H. S.; Kiernan, E.; Metcalf, T. R.; Wulser, J. -P. Bibcode: 1992AAS...180.5103C Altcode: 1992BAAS...24..813C We have established a searchable database, called SPAM (Spectroscopy and Polarimetry at Mees), which contains logs of observations made at Mees Solar Observatory (Haleakala, Maui). Of more general interest, the database also includes the Events List and Region Report from the Space Environment Laboratory (Boulder). Logs from YOHKOH are currently being added. Hence, SPAM can be used to determine, for example, whether Mees has vector magnetograms of a certain NOAA AR or whether YOHKOH has certain types of observations in specified time ranges. As well, it can be used to search the SEL database for flares with selected attributes. Included logs (and searchable attributes, in addition to date, day of year, and time) are: Mees Solar Observatory Log (instrument, NOAA AR, data type, observing setup), SEL Event List (NOAA AR, X-ray Class), SEL Region Report (NOAA AR), YOHKOH Orbit Summary, YOHKOH SXT Quiet Mode PFI Observations (latitude, longitude, X-ray and optical image size), YOHKOH Flare Observations (latitude, longitude, specific channel counts or ratios). SPAM runs on a Sun workstation at Mees Solar Observatory, and is available over Internet. Simply access (e.g., telnet) koa.ifa.hawaii.edu (128.171.167.1) from any vt100, Sun, or xterm emulator. Log on as spam (lower case); there is no password. New users are asked to read release notes and hints. Title: Soft X-ray Blueshifts and Hα Redshifts in the November 15, 1991 X-Class Flare Authors: Wulser, J. -P.; Acton, L.; Canfield, R.; Culhane, L.; Fludra, A.; Masuda, S.; Phillips, A.; Sakao, T. Bibcode: 1992AAS...180.1805W Altcode: 1992BAAS...24..755W We analyzed simultaneous Hα line profile observations (from Mees Solar Observatory), CaXIX line profile observations (from the Bragg Crystal Spectrometer aboard YOHKOH), and soft- and hard X-ray images (from SXT and HXT on YOHKOH) of the November 15, 1991 X-class flare. The observed CaXIX emission shows strong blueshifts very early in the flare. The soft X-ray images suggest that this blueshifted emission originates from one or more of three localized soft X-ray bright points. At the same time, the Hα line profile shows redshifted and blueshifted emission in several locations. Two Hα redshift kernels are associated with the two brightest soft X-ray sources. On the basis of their relationship to the magnetic neutral line and their subsequent development, we conclude that these Hα redshift kernels are the footpoints of a single loop, which initially exhibits two soft X-ray bright points in the loop legs. The results suggest that chromospheric evaporation is the responsible mechanism for the observed Hα redshifts and CaXIX blueshifts in the early stage of the flare. The independent hard X-ray (HXT) and Hα observations both indicate that this chromospheric evaporation is not associated with strong non-thermal electron precipitation. The third soft X-ray bright point, the faintest of the three, is not associated with redshifted Hα emission. This bright spot develops into a connection between the second Hα redshift kernel, and another Hα kernel with strong blueshifts. The Hα blueshift is associated with a mass ejection phenomenon. Title: Ground-Based Context Observations for the High Energy Solar Physics (HESP) Mission Authors: Canfield, R. C.; Hurford, G. Bibcode: 1992AAS...180.3305C Altcode: 1992BAAS...24..780C The core instrument in the HESP strawman payload is an X-ray and gamma-ray imaging spectrometer. Physical interpretation of this instrument's data will require a knowledge of the magnetic and thermal context in which the radiating ions and electrons are accelerated and thermalized. Many of the required observations can effectively and economically be made with ground-based optical and radio imagers, spectrometers, and magnetographs. For these reasons, the HESP Science Study Group has included ground-based instruments and observations as an integral part of the baseline HESP mission. The nature of HESP's needs for ground-based instrumentation is well understood as the result of continuing experience with collaborative use of ground-based instruments in coordination with long-duration balloons, SMM, and YOHKOH. Ground-based instruments relevant to HESP include magnetographs (both longitudinal/full disk and vector/active-region), optical and microwave high-resolution imagers and imaging spectrographs, and coronagraphs. The key to a successful HESP mission is adequate ground-based support in three respects: (1) a capability for observations with the appropriate spatial, spectral, and temporal resolution, and polarimetric accuracy, from the point of view of the HESP scientific requirements; (2) sufficiently extensive coordinated ground-based coverage so that complementary data for HESP-selected events is likely to be available throughout the mission; (3) adequate support and the mechanisms so that reduced ground-based data is conveniently available as an integral part of the HESP data base. Title: The Center-to-Limb Dependence of Flare Hα Redshift Velocities Authors: Blais, K. A.; Canfield, R. C. Bibcode: 1992AAS...180.4103B Altcode: 1992BAAS...24Q.794B It is well known that impulsive phase flare spectra show redshifts in Hα and blueshifts in soft X-ray lines. Various authors have argued that this is a consequence of chromospheric evaporation and momentum balance. During the Solar Maximum Mission it was found that a center-to-limb dependence exists in X-ray blueshifts. This is understood to imply that the motions of the X-ray emitting material have a significant radial preference. If this is true, and the motions are due to chromospheric evaporation (so momentum balance should obtain in the observed Hα and X-ray plasmas), we should expect to see a center-to-limb dependence in Hα redshifts, as well. In this paper we present the results of a statistical study of the Hα spectra of ~ 35 flares observed with the CCD Imaging Spectrograph at Mees Solar Observatory. For purposes of comparison of Hα to the X-ray data, we have spatially averaged the spectrum of each flare, subtracted off the preflare spectrum, and determined the line-of-sight velocity of the usually redshifted component as a function of time using a two-Gaussian fit. We force one component to appear in the preflare line center, and determine the maximum observed velocity of the second (shifted) component for each flare. No center-to-limb dependence is obvious when we plot these velocities as a function of the usual direction cosine mu = cos theta . But when we compare this velocity to a modeled one based on the chromospheric evaporation model, a relationship emerges between the observed and modeled velocity. Finally, the relationship is improved still further when we take disk position into account, using a linear mu relation. We conclude that the center-to-limb dependence of Hα Doppler shifts in these flares supports the chromospheric evaporation model. As well, we infer that there is considerable statistical departure from strictly radial motion due to non-radial magnetic field geometry in the chromosphere at the sites of these flares. Title: Electric Currents and Hard X-ray Images in the X Class Flare of November 15, 1991 Authors: Metcalf, T. R.; Sakao, T.; Acton, L. W.; Canfield, R. C.; Hudson, H. S.; Inda, M.; Kosugi, T.; Wulser, J. P. Bibcode: 1992AAS...180.3005M Altcode: 1992BAAS...24..776M We present co-aligned observations of hard x-rays observed with the Hard X-ray Telescope (HXT) on board the YOHKOH spacecraft and vertical electric currents derived from a vector magnetogram obtained at the Mees Solar Observatory, Haleakala, Hawaii. Previous work comparing the wings of the Hα line to vertical electric currents has suggested that electron precipitation in flares occurs at the edges of these currents. The Stark wings of Hα were interpreted as a signature of non-thermal electrons penetrating the relatively dense chromosphere and used as a proxy for direct observation of the non-thermal electrons. The hard X-rays used in this study provide a direct determination of the locations of the electron energy losses. In the X class flare of November 15, 1991, we find the same relation between hard X-ray emission and vertical electric currents as was found between Hα Stark wing emission and vertical currents: the hard x-ray emission occurs predominantly at the edges of the vertical current sites, and not spatially on top of these currents. Canfield, R. C., de La Beaujardiere, J., and Leka, K. D., in ``The Physics of Solar Flares", ed. Culhane and Jordan, The Royal Society, London, 1991 Canfield, R. C., Leka, K. D., and Wulser,J. P., in ``Flare Physics in Solar Activity Maximum 22", ed. Uchida, Canfield, Watanabe, and Hiei, Springer-Verlag, Berlin, 1991 Title: Search for evidence of low energy protons in solar flares. Authors: Metcalf, Thomas R.; Wuelser, Jean-Pierre; Canfield, Richard C.; Hudson, Hugh S. Bibcode: 1992NASCP3137..536M Altcode: 1992como.work..536M We searched for linear polarization in the H alpha line using the Stokes Polarimeter at Mees Solar Observatory and present observations of a flare from NOAA active region 6659 which began at 01:30 UT on 14 Jun. 1991. Our dataset also includes H alpha spectra from the Mees charge coupled device (MCCD) imaging spectrograph as well as hard x ray observations from the Burst and Transient Source Experiment (BATSE) instrument on board the Gamma Ray Observatory (GRO). The polarimeter scanned a 40 x 40 inch field of view using 16 raster points in a 4 x 4 grid. Each scan took about 30 seconds with 2 seconds at each raster point. The polarimeter stopped 8.5 inches between raster points and each point covered a 6 inch region. This sparse sampling increased the total field of view without reducing the temporal cadence. At each raster point, an H alpha spectrum with 20 mA spectral sampling is obtained covering 2.6 A centered on H alpha line center. The preliminary conclusions from the research are presented. Title: Energetics and Dynamics in a Large Solar Flare of 1989 March Authors: Wulser, Jean-Pierre; Canfield, Richard C.; Zarro, Dominic M. Bibcode: 1992ApJ...384..341W Altcode: Solar Maximum Mission X-ray observations and National Solar Observatory/Sacramento Peak H alpha spectra are combined in a large (X1.2) solar flare to test predictions of chromospheric heating and evaporation by nonthermal thick-target electrons. It is demonstrated that the ratio of H alpha flare energy flux to the energy flux deposited by thick-target electrons obeys a power-law dependence on electron heating flux, with a slope that is consistent with that predicted by a thick-target electron transport and heating model in a 1D hydrostatic atmosphere. It is concluded that the thick-target model satisfactorily accounts for the observed magnitude of chromospheric H alpha emission, and the amplitudes and timing of oppositely directed plasma motions during the impulsive phase of this X flare. Title: The Practical Application of the Magnetic Virial Theorem Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E. Bibcode: 1992ApJ...385..327K Altcode: The magnetic energy of solar active regions is computed via the application of the magnetic virial theorem together with vector magnetograph data. In order to investigate how errors in the vector magnetograph measurements produce errors in the virial theorem energy, the effects of realistic errors on known magnetic fields are simulated numerically. These include errors due to random polarization noise, crosstalk between different polarization signals, systematic polarization bias, and seeing-induced crosstalk. Analytical expressions for the energy errors which apply under certain idealized conditions are derived. The results are useful for evaluating the ability of vector magnetographs to provide suitable data for the accurate determination of magnetic energies using the virial theorem. Title: Inferring Chromospheric Flare Heating from Hydrogen-Line Wings Authors: Gayley, K. G.; Canfield, R. C. Bibcode: 1991ApJ...380..660G Altcode: Strong flare heating that penetrates deeply into the solar chromosphere is examined, and a simple model that is supported by a numerical radiative transfer simulation is applied to study how elevated free electron densities produce markedly increased Stark emission in the wings of hydrogen Lyman and Balmer lines. The proportionality that exists between the observed Stark wing intensity and the total energy deposition in the partially ionized layer is calculated for various Lyman and Balmer lines. It is concluded that when the effects of background opacity sources on the wing enhancements are not important, the magnitude of the nonthermal heating that penetrates the chromosphere may be inferred from the observed line wings. It is predicted that Ly-alpha extended wings will exhibit enhancement of Stark shape, even when inherent damping wing opacity exceeds Stark wing opacity over the entire absorption profile. Title: The Mees CCD imaging spectrograph Authors: Penn, Matthew J.; Mickey, Donald L.; Canfield, Richard C.; Labonte, Barry J. Bibcode: 1991SoPh..135..163P Altcode: The Mees CCD (MCCD) instrument is an imaging spectroscopy device which uses the 25 cm coronagraph telescope and the 3.0 m Coudé spectrograph at Mees Solar Observatory (MSO) on Haleakala, Maui. The instrument works with resolving power up to R ≈ 200 000 with significant throughput from λ3934 Å (CaII K) to λ ≈ 10 000 Å. A fast guiding active mirror stabilizes the image during observations. A rapidly writing magnetic tape storage system allows observations to be recorded at 256 kbytes s−1. Currently, the MCCD is used for imaging spectroscopy of solar flares at λ6563 Å (Hα), and velocity measurements of umbral oscillations; future plans include emission line studies of active region coronae, and photospheric studies of solar oscillations. Title: Flare Energy Release: Observational Consequences and Signatures Authors: Canfield, Richard C.; de La Beaujardiere, J. -F.; Leka, K. D. Bibcode: 1991RSPTA.336..381C Altcode: It is generally accepted, but not yet compellingly demonstrated, that the energy released in solar flares is stored in stressed magnetic fields. Little is known, at present, about how the most obvious manifestations of flare energy release - heating, mass motion, magnetic field reconfiguration and particle acceleration - are related to the spatial distribution of free energy within those fields. To address this issue we have underway at Mees Solar Observatory a programme of simultaneous polarimetric and spectroscopic observations that allow us to explore the spatial relation between active region currents, flare particle acceleration and flare heating. In this paper we discuss several days observations of two flare-productive active regions. By using the Haleakala Stokes polarimeter, we observed the spatial distribution of the Stokes profiles of two photospheric FeI lines, from which we inferred the spatial distribution of the vector magnetic field and the vertical current density. In flares that were observed on the same days, we then compared the locations of vertical currents to the sites of non-thermal electron precipitation and high coronal pressure inferred from Hα line profiles and spectroheliograms obtained with the Mees charge coupled device imaging spectrograph. Without exception we found that the sites of significant energetic electron precipitation into the chromosphere were at the edges of regions of vertical current, not within them. In contrast, we found that the footpoints of high-pressure flare plasmas during the main phase of the observed flares all coincided very well with such currents. Title: The Magnetic Morphology of High-Pressure Plasmas in Three October 1989 (AR5747) Flares Authors: Leka, K. D.; Canfield, R. C. Bibcode: 1991BAAS...23.1066L Altcode: No abstract at ADS Title: The Derivation of Vector Magnetic Fields from Stokes Profiles: Derivative versus Least Squares Fitting Techniques Authors: Metcalf, T. R.; Canfield, R. C.; Mickey, D. L. Bibcode: 1991BAAS...23.1054M Altcode: No abstract at ADS Title: Energetics and Dynamics in an Explosive Flare of March 1989 Authors: Zarro, D. M.; Wüsler, J. -P.; Canfield, R. C. Bibcode: 1991BAAS...23..924Z Altcode: No abstract at ADS Title: The Practical Application of the Magnetic Virial Therom: Analytical Results Authors: Rhoads, J. E.; Klimchuk, J. A.; Canfield, R. C. Bibcode: 1991BAAS...23.1055R Altcode: No abstract at ADS Title: Electric Currents and Flares in Solar Active Regions Authors: Canfield, R. C. Bibcode: 1991BAAS...23.1025C Altcode: No abstract at ADS Title: Current Systems and Particle Precipitation in AR 6233 Authors: de La Beaujardière, J. -F.; Canfield, R. C. Bibcode: 1991BAAS...23.1031D Altcode: No abstract at ADS Title: The Practical Application of the Magnetic Virial Theorm: Simulated Magnetograph Observations Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E. Bibcode: 1991BAAS...23.1031K Altcode: No abstract at ADS Title: Hα Imaging Spectroscopy during the Max '91 Campaign Authors: de La Beaujardière, J. -F.; Blais, K. A.; Canfield, R. C. Bibcode: 1991BAAS...23.1071D Altcode: No abstract at ADS Title: Observations of Linear Hα Polarization in Flares Authors: Metcalf, T. R.; Wulser, J. P.; Canfield, R. C. Bibcode: 1991BAAS...23.1073M Altcode: No abstract at ADS Title: A Study of Solar Flare Energy Transport Based on Coordinated H alpha and X-Ray Observations Authors: Canfield, Richard C.; Wulser, Jean-Pierre; Zarro, Dominic M.; Dennis, Brian R. Bibcode: 1991ApJ...367..671C Altcode: The temporal evolution of the ratio between H-alpha to nonthermal hard X-ray emission was investigated using coordinated H-alpha and hard- and soft-X-ray observations of five solar flares (on May 7, June 23, June 24, and June 25, 1980 and on April 30, 1985). These observations were used to estimate the emitted flare energy flux F(H-alpha) in H-alpha, the flux of F(2O) energy deposited by nonthermal electrons with energies above 20 keV, and the pressure p(c) of soft X-ray-emitting plasma as functions of time during the impulsive phase of each flare. It was found that the F(H-alpha)/F(2O) ratio shows a power-law dependence on F(2O), with a slope that differs slightly from that predicted by the static thick-target model of solar transport. Results also indicate that the power-law dependence is modified by hydrostatic pressure effects. Title: The derivation of vector magnetic fields from Stokes profiles: derivative versus least squares fitting techniques. Authors: Metcalf, T. R.; Canfield, R. C.; Mickey, D. L.; Lites, B. W. Bibcode: 1991sopo.work..376M Altcode: The authors present a comparison of solar magnetic fields calculated using the weak field equations of Jefferies, Lites, and Skumanich and the least squares fitting method of Skumanich and Lites. The two calculations used Fe I 6302 data from June, 1985, and are found to agree quite well up to at least 1200 G. Title: Chromospheric Plasma Motions in the X 1.2 Flare of 1536 UT 1989 March 11 Authors: Zarro, D. M.; Wülser, J. -P.; Canfield, R. C. Bibcode: 1991max..conf...77Z Altcode: No abstract at ADS Title: Solar astronomy Authors: Rosner, Robert; Noyes, Robert; Antiochos, Spiro K.; Canfield, Richard C.; Chupp, Edward L.; Deming, Drake; Doschek, George A.; Dulk, George A.; Foukal, Peter V.; Gilliland, Ronald L. Bibcode: 1991aap..reptR....R Altcode: An overview is given of modern solar physics. Topics covered include the solar interior, the solar surface, the solar atmosphere, the Large Earth-based Solar Telescope (LEST), the Orbiting Solar Laboratory, the High Energy Solar Physics mission, the Space Exploration Initiative, solar-terrestrial physics, and adaptive optics. Policy and related programmatic recommendations are given for university research and education, facilitating solar research, and integrated support for solar research. Title: Flare Physics in Solar Activity Maximum 22 Authors: Uchida, Yutaka; Canfield, Richard C.; Watanabe, Tetsuya; Hiei, Eijiro Bibcode: 1991LNP...387.....U Altcode: 1991fpsa.conf.....U No abstract at ADS Title: Chromospheric Response During the Gamma Ray Flare on March 10, 1989 Authors: Wülser, J. -P.; Canfield, R. C.; Rieger, E. Bibcode: 1991max..conf..149W Altcode: No abstract at ADS Title: The Practical Application of the Magnetic Virial Theorem Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E. Bibcode: 1991LNP...387..219K Altcode: 1991fpsa.conf..219K We have performed simulated vector magnetograph observations to study the effects of random and systematic magnetic field measurement errors on the magnetic energies that may be estimated using the virial theorem. Title: Flare energy release: observational consequences and signatures. Authors: Canfield, R. C.; de La Beaujardiere, J. -F.; Leka, K. D. Bibcode: 1991psf..conf..381C Altcode: At Mees Solar Observatory a programme is underway of simultaneous polarimetric and spectroscopic observations that allow to explore the spatial relation between active region currents, flare particle acceleration and flare heating. The authors discuss several days observations of two flare-productive active regions. They compared the locations of vertical currents to the sites of non-thermal electron precipitation and high coronal pressure inferred from Hα line profiles and spectroheliograms. Without exception they found that the sites of significant energetic electron precipitation into the chromosphere were at the edges of regions of vertical current, not within them. In contrast, they found that the footpoints of high-pressure flare plasmas during the main phase of the observed flares all coincided very well with such currents. Title: Magnetic Morphology of Nonthermal Electron Precipitation During Three Flares in a Highly Nonpotential Active Region Authors: Canfield, Richard C.; Leka, K. D.; Wülser, Jean-Pierre Bibcode: 1991LNP...387...96C Altcode: 1991fpsa.conf...96C NOAA Active region 5747, during its October 1989 transit across the solar disk, showed highly nonpotential photospheric vector magnetic field structure and produced many solar flares, three of which we observed at Mees Solar Observatory. After resolution of the 180° ambiguity, we determined the photospheric distribution of the vertical current density. We then compared the locations of the major current systems to sites of nonthermal electron precipitation inferred from H profiles of three flares observed using the Mees CCD Imaging Spectrograph. We found that the sites of energetic electron precipitation are at the edges of these currents, not at their peaks. Title: Flare Heating and Ionization of the Low Solar Chromosphere. II. Observations of Five Solar Flares Authors: Metcalf, Thomas R.; Canfield, Richard C.; Saba, Julia L. R. Bibcode: 1990ApJ...365..391M Altcode: Two neutral Mg spectral lines formed in the temperature-minimum region and the low chromosphere, at 4571 and 5173 A, are used to quantify the changes in the atmospheric structure as a function of time during five solar flares. Eight proposed flare heating and ionization mechanisms and predictions of the effects of each on the temperature minimum region are discussed. Two Mg spectral observations made at the National Solar Observatory (Sacramento Peak), along with observations of hard and soft X-rays from the SMM and GOES satellites, are compared to the predictions of the eight proposed mechanisms. The initial effects in all five flares are consistent with backwarming by enhanced Balmer- and Paschen-continuum radiation originating in the upper chromosphere. Extended heating observed in two of the flares is most likely due to UV irradiation. In all cases heating by the dissipation of nonreversed electric currents, collisions with an electron or proton beam, irradiation by soft X-rays, and dissipation of Alfven waves are eliminated. Title: H alpha Spectra of Dynamic Chromospheric Processes in Five Well-observed X-Ray Flares Authors: Canfield, Richard C.; Penn, Matthew J.; Wulser, Jean-Pierre; Kiplinger, Alan L. Bibcode: 1990ApJ...363..318C Altcode: Simultaneous H-alpha and hard X-ray (HXR) spectra were obtained for five solar flares to determine the relationship of H-alpha profiles and the nonthermal part of the flare represented by the hard X-ray burst. All five flares exhibited impulsive-phase redshifted H-alpha in emission, which was temporarily and spatially associated with intense HXR emission and broad impulsive-phase H-alpha wings. A few small regions within two flares showed a blueshifted H-alpha emission which appeared only early in the impulsive phase and was temporally correlated with the HXR emission but not with broad H-alpha wings. Finally, there were both redshifted and blueshifted absorption spectra with properties fully consistent with those known for erupting and untwisting filaments. Title: Flare Heating and Ionization of the Low Solar Chromosphere Authors: Metcalf, T. R.; Canfield, R. C. Bibcode: 1990BAAS...22Q.891M Altcode: No abstract at ADS Title: Chromospheric Mass Motions in the X1 Flare of 1533 UT 11 March 1989 Authors: Canfield, R. C.; Zarro, D. M.; Wülser, J. -P. Bibcode: 1990BAAS...22..824C Altcode: No abstract at ADS Title: Vertical Currents in a Flare-Productive Active Region Authors: Fan, Y.; Canfield, R. C.; McClymont, A. N. Bibcode: 1990BAAS...22..827F Altcode: No abstract at ADS Title: Capabilities of the MCCD System at Mees Solar Observatory Authors: Penn, M. J.; Mickey, D. L.; Canfield, R. C. Bibcode: 1990BAAS...22..880P Altcode: No abstract at ADS Title: Chromospheric response during the gamma ray flare on March 10, 1989. Authors: Wülser, J. -P.; Canfield, R. C.; Rieger, E. Bibcode: 1990BAAS...22..890W Altcode: No abstract at ADS Title: Hα Flare Observations with the Mees CCD Imaging Spectrograph Authors: Canfield, R. C.; Mickey, D. L.; Penn, M. J.; Wülser, J. -P. Bibcode: 1990BAAS...22..809C Altcode: No abstract at ADS Title: Chromospheric Response During the Gamma Ray Flare on March 10, 1989 Authors: Wülser, J. -P.; Canfield, R. C.; Rieger, E. Bibcode: 1990BAAS...22R.890W Altcode: No abstract at ADS Title: The Magnetic Morphology of Chromospheric Particle Precipitation in Three October 1989 (AR 5747) Flares Authors: Leka, K. D.; Canfield, R. C.; Wülser, J. -P.; Fan, Y. Bibcode: 1990BAAS...22..824L Altcode: No abstract at ADS Title: Coordinated Optical and Hard X-ray Flare Observations: A GRO Guest Investigator Proposal Authors: Metcalf, T. R.; Wülser, J. P.; Canfield, R. C. Bibcode: 1990BAAS...22..791M Altcode: No abstract at ADS Title: Flare Heating and Ionization of the Low Solar Chromosphere. I. Inversion Methods for MG i lambda lambda 4571 and 5173 Authors: Metcalf, Thomas R.; Canfield, Richard C.; Avrett, Eugene H.; Metcalf, Frederic T. Bibcode: 1990ApJ...350..463M Altcode: Various methods of inverting solar Mg I 4571 and 5173 spectral line observations are examined to find the best method of using these lines to calculate the vertical temperature and electron density structure around the temperature minimum region. Following a perturbation analysis by Mein (1971), a Fredholm integral equation of the first kind is obtained which can be inverted to yield these temperature and density structures as a function of time. Several inversion methods are tested and compared. The methods are used to test data as well as to a subset of observations of these absorption lines taken on February 3, 1986 before and during a solar flare. A small but significant increase is found in the temperature and a relatively large increase in the electron density during this flare. The observations are inconsistent with heating and ionization by an intense beam of electrons and with ionization by UV photoionization of Si I. Title: Momentum Balance in Four Solar Flares Authors: Canfield, Richard C.; Zarro, Dominic M.; Metcalf, Thomas R.; Lemen, James R. Bibcode: 1990ApJ...348..333C Altcode: Solar Maximum Mission soft X-ray spectra and National Solar Observatory (Sacramento Peak) H-alpha spectra were combined in a study of high-speed flows during the impulsive phase of four solar flares. In all events, a blue asymmetry (indicative of upflows) was observed in the coronal Ca XIX line during the soft X-ray rise phase. In all events a red asymmetry (indicative of downflows) was observed simultaneously in chromospheric H-alpha. These oppositely directed flows were concurrent with impulsive hard X-ray emission. Combining the velocity data with estimates of the density based on emission measurements and volume estimates, it is shown that for the impulsive phase as a whole the total momentum of upflowing soft X-ray plasma equaled that of the downflowing H-alpha plasma, to within an order of magnitude, in all four events. Only the chromospheric evaporation model predicts equal total momentum in the upflowing soft X-ray-emitting and downflowing H-alphba-emitting materials. Title: On the Stability of Proton Beams against Resonant Scattering by Alfven Waves in Solar Flare Loops Authors: Tamres, David H.; Melrose, D. B.; Canfield, Richard C. Bibcode: 1989ApJ...342..576T Altcode: The growth of Alfven waves in magnetized hydrogen plasma at flare-loop densities and magnetic field strengths driven by a dilute population of streaming protons in the energy range 10-1000 keV is investigated. The streaming distribution induces wave growth principally through resonant interactions between forward-moving protons and forward-propagating waves. All other factors constant, the maximum growth rate increases with mean beam energy and with increased narrowness of the beam distribution. The frequency of the most rapidly growing waves is inversely related to the mean beam energy. Under typical flare conditions, thermal damping when mean energy equals the Alfven speed effectively stabilizes proton beams of moderate collimation. At beam energies only one order of magnitude larger, thermal damping under flare loop conditions is insufficient to prevent highly restrictive upper limits on the beam-to-background proton density ratio for even mildly collimated proton beams. Title: On the stability of super-Alfvénic proton beams in solar flare loops. Authors: Tamres, D. H.; Melrose, D. B.; Canfield, R. C. Bibcode: 1989BAPS...34.1284T Altcode: No abstract at ADS Title: H alpha Redshifts as a Diagnostic of Solar Flare Heating Authors: Zarro, Dominic M.; Canfield, Richard C. Bibcode: 1989ApJ...338L..33Z Altcode: The dynamics of chomospheric condensations formed during chromospheric evaporation are studied using coordinated X-ray and H-alpha observations of five solar flares. It is shown that the peak downflow velocity of condensations predicted by simple hydrodynamic compression of the chromosphere is in good agreement with empirical downflow velocities implied by impulsive phase H-alpha red wing Doppler shifts. It is found that the H-alpha wing redshift provides a diagnostic of the pressure excess in the evaporating region and the coronal energy flux driving chromospheric evaporation. Because preflare coronal pressures derived for each of the flares are high, it is suggested that the chromosphere succeeds in radiating excess flare heat energy without undergoing explosive chromospheric evaporation. Title: Hα Imaging Spectroscopy of Three of the March 1989 X Flares Authors: Wuelser, J. -P.; Canfield, R. C. Bibcode: 1989BAAS...21R.835W Altcode: No abstract at ADS Title: Energetics of Hα and Hard X-Ray Emission in Solar Flares Authors: Canfield, R. C.; Zarro, D. M.; Wuelser, J. -P.; Dennis, B. R. Bibcode: 1989BAAS...21..846C Altcode: No abstract at ADS Title: An Imaging vector Magnetograph for the Next Solar Maximum Authors: Mickey, D. L.; Canfield, R. C. Bibcode: 1989BAAS...21..863M Altcode: No abstract at ADS Title: Lyα Fermi Acceleration in Chromospheric Shocks Authors: Gayley, K. G.; Canfield, R. C. Bibcode: 1989BAAS...21..851G Altcode: No abstract at ADS Title: An imaging vector magnetograph for the next solar maximum. Authors: Canfield, R. C.; Mickey, D. L. Bibcode: 1989GMS....54...37C Altcode: 1989sspp.conf...37C The authors describe the conceptual design of a new imaging vector magnetograph that combines a modest solar telescope with a rotating quarter-wave plate, an acousto-optic tunable prefilter as a blocker for a servo-controlled Fabry-Perot etalon, charge-coupled device cameras, and a rapid digital tape recorder. Its high spatial resolution (0.5 arc sec pixel size) over a large field-of-view (4 by 5 arc min) is expected to be sufficient to significantly measure, for the first time, the magnetic energy dissipated in major solar flares. Its millisecond tunability and wide spectral range (5000 - 7000 Å) enable nearly simultaneous vector magnetic field measurements in the gas-pressure-dominated photosphere and magnetically-dominated, chromosphere, as well as effective co-alignment with Solar-A's X-ray images. Title: Chromospheric downflows as a diagnostic of solar flare heating. Authors: Zarro, D. M.; Canfield, R. C. Bibcode: 1989sasf.confP.203Z Altcode: 1989IAUCo.104P.203Z; 1988sasf.conf..203Z Using coordinated X-ray and Hα observations of five solar flares, the authors investigate the dynamics of chromospheric condensations formed during chromospheric evaporation. They show that the peak downflow velocity of condensations predicted by simple hydrodynamic compression of the chromosphere is in good agreement with empirical downflow velocities implied by impulsive phase Hα redwing Doppler shifts. This agreement indicates that the Hα wing redshift provides a useful diagnostic of the pressure excess in the evaporating region and, hence, the energy flux driving chromospheric evaporation. Title: Impulsive phase transport. Authors: Canfield, R. C.; Bely-Dubau, E.; Brown, J. C.; Dulk, G. A.; Emslie, A. G.; Enome, S.; Gabriel, A. H.; Kundu, M. R.; Melrose, D.; Neidig, D. F.; Ohki, K.; Petrosian, V.; Poland, A.; Rieger, E.; Tanaka, K.; Zirin, H. Bibcode: 1989epos.conf..225C Altcode: The work of this group was concerned with how the energy released in a solar flare is transported through the solar atmosphere before escaping in the form of radiant and mechanical energy. Title: An imaging vector magnetograph for the next solar maximum Authors: Mickey, D. L.; Labonte, B. J.; Canfield, R. C. Bibcode: 1989dots.work..324M Altcode: Researchers describe the conceptual design of a new imaging vector magnetograph currently being constructed at the University of Hawaii. The instrument combines a modest solar telescope with a rotating quarter-wave plate, an acousto-optical tunable prefilter as a blocker for a servo-controlled Fabry-Perot etalon, CCD cameras, and on-line digital image processing. Its high spatial resolution (1/2 arcsec pixel size) over a large field of view (5 by 5 arcmin) will be sufficient to significantly measure, for the first time, the magnetic energy dissipated in major solar flares. Its millisecond tunability and wide spectral range (5000 to 7000 A) enable nearly simultaneous vector magnetic field measurements in the gas-pressure-dominated photosphere and magnetically-dominated chromosphere, as well as effective co-alignment with Solar-A's X ray images. Researchers expect to have the instrument in operation at Mees Solar Observatory (Haleakala) in early 1991. They have chosen to use tunable filters as wavelength-selection elements in order to emphasize the spatial relationships between magnetic field elements, and to permit construction of a compact, efficient instrument. This means that spectral information must be obtained from sequences of images, which can cause line profile distortions due to effects of atmospheric seeing. Title: Max 1991: Flare Research at the Next Solar Maximum. Workshop 1: Scientific Objectives Authors: Canfield, Richard C.; Dennis, Brian R. Bibcode: 1988fnsm.work.....C Altcode: No abstract at ADS Title: An imaging vector magnetograph for the next solar maximum Authors: Canfield, Richard C.; Mickey, Donald L. Bibcode: 1988fnsm.work...81C Altcode: Measurements of the vector magnetic field in the solar atmosphere with high spatial and temporal resolution over a large field of view are critical to understanding the nature and evolution of currents in active regions. Such measurements, when combined with the thermal and nonthermal X-ray images from the upcoming Solar-A mission, will reveal the large-scale relationship between these currents and sites of heating and particle acceleration in flaring coronal magnetic flux tubes. The conceptual design of an imaging vector magnetograph that combines a modest solar telescope with a rotating quarter-wave plate, an acousto-optical tunable prefilter as a blocker for a servo-controlled Fabry-Perot etalon, CCD cameras, and a rapid digital tape recorder are described. Its high spatial resolution (1/2 arcsec pixel size) over a large field of view (4 x 5 arcmin) will be sufficient to significantly measure, for the first time, the magnetic energy dissipated in major solar flares. Its millisecond tunability and wide spectra range (5000 to 8000 A) enable nearly simultaneous vector magnetic field measurements in the gas-pressure-dominated photosphere and magnetically dominated chromosphere, as well as effective co-alignment with Solar-A's X-ray images. Title: The Dependence of Hα Redshifts on Coronal Heating in Solar Flares Authors: Zarro, D. M.; Canfield, R. C. Bibcode: 1988BAAS...20.1028Z Altcode: No abstract at ADS Title: An Imaging Vector Magnetograph for the Next Solar Maximum Authors: Canfield, R. C.; Mickey, D. L. Bibcode: 1988BAAS...20..745C Altcode: No abstract at ADS Title: Determination of the Temperature Structure of the Temperature Minimum Region during Solar Flares Authors: Metcalf, T. R.; Metcalf, F. T.; Canfield, R. C.; Avrett, E. H. Bibcode: 1988BAAS...20..688M Altcode: No abstract at ADS Title: Momentum Balance in Four Solar Flares Authors: Canfield, R. C.; Zarro, D. M.; Metcalf, T. R.; Lemen, J. R. Bibcode: 1988BAAS...20..688C Altcode: No abstract at ADS Title: On the Stability of Superof-Alfvénic Proton Beams in Solar Flare Loops Authors: Tamres, D. H.; Melrose, D. B.; Canfield, R. C. Bibcode: 1988BAAS...20..739T Altcode: No abstract at ADS Title: Explosive Plasma Flows in a Solar Flare Authors: Zarro, Dominic M.; Canfield, Richard C.; Strong, Keith T.; Metcalf, Thomas R. Bibcode: 1988ApJ...324..582Z Altcode: Solar Maximum Mission soft X-ray data and Sacramento Peak Observatory H-alpha observations are combined in a study of the impulsive phase of a solar flare. A blue asymmetry, indicative of upflows, was observed in the coronal Ca XIX line during the soft X-ray rise phase. A red asymmetry, indicative of downflows, was observed simultaneously in chromospheric H-alpha emitted from bright flare kernels during the period of hard X-ray emission. Combining the velocity data with a measurement of coronal electron density, it is shown that the impulsive phase momentum of upflowing soft X-ray-emitting plasma equalled that of the downflowing H-alpha-emitting plasma to within one order of magnitude. In particular, the momentum of the upflowing plasma was 2 x 10 to the 21st g cm/s while that of the downflowing plasma was 7 x 10 to the 21st g cm/s, with a factor of 2 uncertainty on each value. This equality supports the explosive chromospheric evaporation model of solar flares, in which a sudden pressure increase at the footprint of a coronal loop produces oppositely directed flows in the heated plasma. Title: Coordinated soft X-ray and Hα observations of solar flares Authors: Zarro, D. M.; Canfield, R. C.; Metcalf, T. R.; Lemen, J. R. Bibcode: 1988AdSpR...8k.149Z Altcode: 1988AdSpR...8..149Z We have analyzed a unique set of four solar flares for which coordinated soft X-ray Ca XIX and Hα spectral observations were obtained during the impulsive phase. In all events, a blue asymmetry (indicative of upflows) was observed in the coronal Ca XIX line during the soft X-ray rise phase. In all events, a red asymmetry (indicative of downflows) was observed simultaneously in chromospheric Hα at spatial locations associated with enhanced flare heating. Combining the velocity data with estimates of the evaporated mass based on soft X-ray emission measure, we demonstrate that the impulsive phase momentum of upflowing soft X-ray plasma equalled that of the downflowing Hα plasma, to within an order of magnitude. This equality supports the explosive chromospheric evaporation model of solar flares - the only model that predicts equal momentum content in upflowing coronal and downflowing chromospheric plasmas.

under contract with Lockheed Palo Alto Research Laboratory Title: Max '91: Flare research at the next solar maximum Authors: Dennis, Brian; Canfield, Richard; Bruner, Marilyn; Emslie, Gordon; Hildner, Ernest; Hudson, Hugh; Hurford, Gordon; Lin, Robert; Novick, Robert; Tarbell, Ted Bibcode: 1988STIN...8814919D Altcode: To address the central scientific questions surrounding solar flares, coordinated observations of electromagnetic radiation and energetic particles must be made from spacecraft, balloons, rockets, and ground-based observatories. A program to enhance capabilities in these areas in preparation for the next solar maximum in 1991 is recommended. The major scientific issues are described, and required observations and coordination of observations and analyses are detailed. A program plan and conceptual budgets are provided. Title: Impulsive H alpha Diagnostics of Electron-Beam--heated Solar Flare Model Chromospheres Authors: Canfield, Richard C.; Gayley, Kenneth G. Bibcode: 1987ApJ...322..999C Altcode: Time-dependent H-alpha profiles were computed for the dynamic model atmospheres of Fisher, Canfield, and McClymont (1985) simulating the effects of an intense impulsively initiated power-law beam of electrons incident on the chromosphere. The temporal response of H-alpha arises from three separate physical mechanisms, whose relative importance varies over the line profile. The fastest variations (typically less than 0.1 s for the explosive heating discussed here) arise from energy imbalance; these are apparent on chromospheric heating and cooling time scales and have their greatest amplitude at line center. Slower variations arise from ionization imbalance and are most apparent in the blue wing. The slowest variations arise from hydrodynamic effects and are related to the formation of a chromospheric condensation; these are most apparent in the red wing. These results provide a basis for the design and analysis of observations of H-alpha, in coordination with hard X-rays or microwaves, to test mechanisms of energy transport in solar flares. Title: The H alpha Spectral Counterparts of Solar Hard X-Rays Microflares Authors: Canfield, Richard C.; Metcalf, Thomas R. Bibcode: 1987ApJ...321..586C Altcode: X-ray observations have revealed energetically significant numbers of very small hard X-ray bursts, termed microflares by Lin et al. The authors have simultaneously observed the Hα counterparts of serveral of these microflares. It is found that microflares occur in regions that are also productive of larger flares, suggesting that they may be components of the larger flare event. All but the weakest miroflares show pronounced impulsive-phase red asymmetry in Hα. Their energetics, interpreted using the thick-target nonthermal model of electron transport, implies that these events are substantially underresolved at the authors' spatial resolution and have a true area of 1015- 1016cm-2. Title: Alfvén Wave Growth Driven by Streaming Protons in Coronal Loops Authors: Tamres, D. H.; Melrose, D. B.; Canfield, R. C. Bibcode: 1987BAAS...19R.943T Altcode: No abstract at ADS Title: Explosive Mass Motions in the Flare of 1456 UT 7 May 1980 Authors: Canfield, Richard C.; Zarro, Dominic M. Bibcode: 1987BAAS...19..921C Altcode: No abstract at ADS Title: A novel observational test of momentum balance in a solar flare Authors: Canfield, Richard C.; Metcalf, Thomas R.; Strong, Keith T.; Zarro, Dominic M. Bibcode: 1987Natur.326..165C Altcode: A unique combination of Solar Maximum Mission X-ray spectra and Sacramento Peak Observatory Hα imaging spectra has enabled us, for the first time, to measure and compare momentum values of upflowing and downflowing plasmas during the impulsive phase of a solar flare. We observed the well-known blue asymmetry of X-ray spectral lines1, indicative of upflow, in the coronal Ca XIX line. We simultaneously observed the red asymmetry of Hα line profiles, indicative of downflow, in bright Ha kernels. We find that, to within observational uncertainty, the momentum transported by the upflowing X-ray plasma was the same as that of the downflowing Hα material. Of the several physical mechanisms advanced2 to explain the observed blue asymmetry of X-ray lines, only explosive chromospheric evaporation predicts oppositely directed momenta of equal amplitude. Title: Alfven wave growth driven by streaming protons in coronal loops Authors: Tamres, David H.; Melrose, D. B.; Canfield, Richard C. Bibcode: 1987SoPh..113..313T Altcode: 1982SoPh..113..313T No abstract at ADS Title: Optical imaging spectroscopy Authors: Canfield, R. C. Bibcode: 1987SoPh..113...95C Altcode: 1982SoPh..113...95C No abstract at ADS Title: Impulsive Phase Observations and Their Interpretation Authors: Canfield, R. C.; Bely-Dumau, E.; Brown, J. C.; Dulk, G. A.; Emslie, A. G.; Enome, S.; Gabriel, A. H.; Kundu, M. R.; Melrose, D.; Neidig, D. F.; Ohki, K.; Petrosian, V.; Poland, A.; Rieger, E.; Tanaka, K.; Zirin, H. Bibcode: 1986epos.conf..3.4C Altcode: 1986epos.confC...4C No abstract at ADS Title: Theoretical Studies of Transport Processes Authors: Canfield, R. C.; Bely-Dumau, E.; Brown, J. C.; Dulk, G. A.; Emslie, A. G.; Enome, S.; Gabriel, A. H.; Kundu, M. R.; Melrose, D.; Neidig, D. F.; Ohki, K.; Petrosian, V.; Poland, A.; Rieger, E.; Tanaka, K.; Zirin, H. Bibcode: 1986epos.conf.3.34C Altcode: 1986epos.confBC.34C No abstract at ADS Title: Impulsive phase transport Authors: Canfield, Richard C.; Bely-Dubau, Francoise; Brown, John C.; Dulk, George A.; Emslie, A. Gordon; Enome, Shinzo; Gabriel, Alan H.; Kundu, Mukul R.; Melrose, Donald; Neidig, Donald F. Bibcode: 1986epos.conf..3.1C Altcode: 1986epos.confC...1C The transport of nonthermal electrons is explored. The thick-target electron beam model, in which electrons are presumed to be accelerated in the corona and typically thermalized primarily in the chromosphere and photosphere, is supported by observations throughout the electromagnetic spectrum. At the highest energies, the anisotropy of gamma-ray emission above 10 MeV clearly indicates that these photons are emitted by anisotropically-directed particles. The timing of this high-energy gamma-radiation with respect to lower-energy hard X-radiation implies that the energetic particles have short life-times. For collisional energy loss, this means that they are stopped in the chromosphere or below. Stereoscopic (two-spacecraft) observations at hard X-ray energies (up to 350 keV) imply that these lower-energy (but certainly nonthermal) electrons are also stopped deep in the chromosphere. Hard X-ray images show that, in spatially resolved flares whose radiation consists of impulsive bursts, the impulsive phase starts with X-radiation that comes mostly from the foot-points of coronal loops whose coronal component is outlined by microwaves. Title: Beam-induced Pressure Gradients in the Early Phase of Proton-heated Solar Flares Authors: Tamres, David H.; Canfield, Richard C.; McClymont, A. N. Bibcode: 1986ApJ...309..409T Altcode: The pressure gradient induced in a coronal loop by proton beam momentum deposition is calculated and compared with the thermal pressure gradient arising from nonuniform deposition of beam energy; it is assumed that the transfer of momentum and energy from beam to target occurs via the Coulomb interaciton. Results are presented for both a low mean energy and a high mean energy proton beam injected at the loop apex and characterized by a power-law energy spectrum. The present treatment takes account of the breakdown of the cold target approximation for the low-energy proton beam in the corona, where the thermal speed of target electrons exceeds the beam speed. It is found that proton beam momentum deposition plays a potentially significant role in flare dynamics only in the low mean energy case and only in the corona, where it may dominate the acceleration of target material for as long as several tens of seconds. This conclusion suggest that the presence of low-energy nonthermal protons may be inferred from velocity-sensitive coronal observations in the early impulsive phase. Title: Action and Reaction Observed in a Solar Flare Authors: Zarro, D. M.; Canfield, R. C.; Strong, K. T.; Metcalf, T. R. Bibcode: 1986BAAS...18..966Z Altcode: No abstract at ADS Title: The Solar Flare Extreme Ultraviolet to Hard X-Ray Ratio Authors: McClymont, A. N.; Canfield, R. C. Bibcode: 1986ApJ...305..936M Altcode: Simultaneous measurements of the peak 10-1030 A extreme ultraviolet (EUV) flux enhancement and more than 10 keV hard X-ray (HXR) peak flux of many solar flare bursts, ranging over about four orders of magnitude in HXR intensity, are studied. A real departure from linearity is found in the relationship between the peak EUV and HXR fluxes in impulsive flare bursts. This relationship is well described by a given power law. Comparison of the predictions of the impulsive nonthermal thick-target electron beam model with observations shows that the model satisfactorily predicts the observed time differences between the HXR and EUV peaks and explains the data very well under given specific assumptions. It is concluded that the high-energy fluxes implied by the invariant area thick-target model cannot be completely ruled out, while the invariant area model with smaller low cutoff requires impossibly large beam densities. A later alternative thick-target model is suggested. Title: Optical imaging spectroscopy Authors: Canfield, R. C. Bibcode: 1986sfcp.nasa..120C Altcode: During the recent solar maximum the combination of imaging and spectroscopy in the visible part of the spectrum became a powerful tool for observational study of flares primarily because of the development of two-dimensional charge-coupled-device (CCD) arrays. In combination with appropriate new operational methods, this has led to the ability to observe, for the first time, the preflare and impulsive-phase physical processes associated with spatially resolved features of flare loops. As a result of concurrent theoretical developments, modeling progressed from an empirical to a physical level. This made it possible to interpret imaging spectra in terms of coronal pressure and heat flux, particle beam heating, chromospheric evaporation, and explosive chromospheric dynamics at the footpoints of flare loops. There is clear potential for further advances in the near future, taking advantage of improvements in digital recording speed (approx. 10-fold), number of photosensitive elements per array (approx. 10-fold), real-time data pre-reduction (potentially 10- to 100-fold), and using multiple CCD arrays. By the time of the next solar maximum imaging spectroscopy is expected to achieve spatial resolution or approx. arc 1 arc s, temporal resolution or approx. 5 s, and simultaneous critically-sampled spectroscopy of several lines and continua. As a result, continued increase in our understanding of the physical processes and configurations of solar flares in the chromosphere, temperature minimum region, and photosphere can be anticipated. Even greater progress toward a more global understanding of flares will obviously come about when simultaneous optical, X-ray, and gamma-ray imaging spectroscopy are possible. Title: H-α Response to Rapid Flare Heating Fluctuations Authors: Gayley, K. G.; Canfield, R. C. Bibcode: 1986BAAS...18..698G Altcode: No abstract at ADS Title: The Role of Nonclassical Electron Transport in the Lower Solar Transition Region Authors: Owocki, S. P.; Canfield, R. C. Bibcode: 1986ApJ...300..420O Altcode: One problem in solar physics is concerned with an understanding of the observed brightness of the quiet solar atmosphere in spectral lines which are formed in the lower solar transition region. The present paper has the objective to examine the possibility that the observed line emission results from nonclassical electron transport effects which are associated with the inherently steep temperature gradients in the solar transition region. The height variation of the electron temperature is parameterized to enable correspondence with a variety of one-dimensional constant pressure transition region models. The models include empirical models, theoretical models, and the constant classical heat fluxx model used by Shoub (1983). The electron velocity distribution function is considered along with the effect on collisional excitation and ionization rates, and effects on heat transport. Title: Impulsive phase chromospheric flare dynamics. Authors: Canfield, Richard C. Bibcode: 1986lasf.conf...10C Altcode: 1986lasf.symp...10C The study of the dynamics of chromospheric material in solar flares has a long history, which is primarily observational. The author briefly summarizes what was known on this topic prior to the recent solar maximum, as a standard against which to measure recent work. Then he discusses the most relevant theoretical work on chromospheric aspects of impulsive phase dynamics and singles out chromospheric spectroscopy that is different from older work in an important respect: the Hα spectroscopy and imaging is accompanied by hard X-ray and/or microwave spectroscopy (and in some cases, also imaging), which uniquely pins down the temporal (and in some cases, spatial) relationship of chromospheric dynamics to high-energy (nonthermal) impulsive phase phenomena. Discussion of what a comparison of the new theory and observation implies, and what it does not imply, is found, leading to requirements for future progress in theory and observation. Title: Observations of chromospheric flare dynamics at the next solar maximum specific recommendations of the Chromospheric Flare Dynamics Group. Authors: Canfield, R. C.; Gaizauskas, V.; Kurokawa, H.; Martin, S. F.; Svestka, Z. Bibcode: 1986lasf.conf..489C Altcode: No abstract at ADS Title: Evidence for explosive chromospheric evaporation in a solar flare observed with SMM Authors: Zarro, D. M.; Strong, K. T.; Canfield, R. C.; Metcalf, T.; Saba, J. L. R. Bibcode: 1986AdSpR...6f.155Z Altcode: 1986AdSpR...6..155Z Solar Maximum Mission soft X-ray data and Sacramento Peak Observatory Hα observations are combined in a study of the impulsive phase of a solar flare. A blue asymmetry, indicative of upflow motions, was observed in the coronal Ca XIX line during the soft X-ray rise phase. Hα redshifts, indicative of downward motions, were observed simultaneously in bright flare kernels during the period of hard X-ray emission. We show that, to within observational errors, the impulsive phase momentum transported by the upflowing soft X-ray plasma is equivalent to that of the downward moving chromospheric material. Title: Impulsive phase transport. Authors: Canfield, R. C.; Bely-Dubau, E.; Brown, J. C.; Dulk, G. A.; Emslie, A. G.; Enome, S.; Gabriel, A. H.; Kundu, M. R.; Melrose, D.; Neidig, D. F.; Ohki, K.; Petrosian, V.; Poland, A.; Rieger, E.; Tanaka, K.; Zirin, H. Bibcode: 1986NASCP2439....3C Altcode: Contents: 1. Introduction: motivation for transport studies, historical perspective, overview of the chapter. 2. Impulsive phase observations and their interpretation: gamma-ray emission above 10 MeV, hard X-ray and microwave morphology, combined soft and hard X-ray spectra, iron Kα emission, ultraviolet and hard X-ray emission, white light emission, Hα emission. 3. Theoretical studies of transport processes: electron beams and reverse currents, proton transport, radiative energy transport by amplified decimetric waves. 4. Summary. Title: Impulsive phase explosive dynamics Authors: Canfield, Richard C. Bibcode: 1986AdSpR...6f.167C Altcode: 1986AdSpR...6..167C As the result of observational and theoretical research carried out during the last solar cycle, we have seen a dramatic increase in our physical understanding of explosive mass motions during the impulsive phase. One of the major discoveries was the dramatic blueshift of 107 K X-ray lines during the impulsive phase of many solar flares. The temporal and spectral character of these blueshifts is well observed, but their physical interpretation has been quite controversial. Another interesting discovery made during the last solar maximum was strong redshift of 104 K Hα emission during the impulsive phase, which is closely temporally correlated with X-ray and microwave emission. The evidence that redshifts are a consequence of chromospheric explosions is quite strong-stronger, in fact, than that for X-ray blueshifts because of better spatial resolution. Theoretical hydrodynamic simulations of impulsive-phase nonthermal electron transport, spanning the temperature range from 104 to 107 K, show upward-moving 107 K material and downward-moving 104 K material caused by chromospheric evaporation driven by either thermal conduction from a hot flare corona and Coulomb heating by energetic flare electrons of coronal origin. Of the various physical mechanisms that figure in the controversies, only explosive chromospheric evaporation has been shown to explain impulsive-phase Hα redshifts and X-ray blueshifts in a simple and compelling manner. Title: Optical imaging spectroscopy. Authors: Canfield, R. C. Bibcode: 1986NASCP2421..120C Altcode: The author reviews the technological and operational advances, some of the physical understanding that has come about as a result, and the possibilities for the future. Title: The sensitivity of Hα profiles to rapid electron beam fluctuations. Authors: Canfield, Richard C.; Gayley, Kenneth G. Bibcode: 1986NASCP2449..249C Altcode: 1986rfsf.nasa..249C The authors find an Hα response to an instantaneously initiated intense beam of nonthermal electrons that is rapid compared to the timescale associated with the propagation of these electrons over characteristic flare loop dimensions. The amplitude and timescale of this response vary over the Hα profile, and show effects which arise from three different physical mechanisms. It is concluded that observational efforts to detect impulsive Hα brightenings associated with implusive hard X-ray or microwave bursts should initially focus their attention on line center. Title: Ly-alpha and H-alpha emission by superthermal proton beams Authors: Canfield, R. C.; Chang, C. -R. Bibcode: 1985ApJ...295..275C Altcode: Simnett and Harrison (1984) have presented a model in which 100-1000 keV protons are an energy transfer agent linking coronal mass ejections and solar flares. Orrall and Zirker (1976) suggested that such protons, incident upon the chromosphere, would produce nonthermal Ly-alpha emission after charge exchange with ambient chromospheric hydrogen atoms. The present investigation is concerned with a study of the charge-exchange mechanism proposed by Orral and Zirker. The physical theory of the formation of nonthermal Ly-alpha (and H-alpha) emission is considered, taking into account photon emission, atomic transitions, atomic equilibrium, the dominant atomic processes, and the stopping of superthermal protons. Computational results presented by Orrall and Zirker are extended. Title: On the Detectability of Kev-Mev Solar Protons Through Their Nonthermal Lyman-Alpha Emission Authors: Canfield, R. C.; Chang, C. R. Bibcode: 1985ICRC....4...86C Altcode: 1985ICRC...19d..86C The intensity and timescale of nonthermal Doppler-shifted hydrogen L alpha photon emission as diagnostics of 10 keV to 10 MeV protons bombarding the solar chromosphere during flares are investigated. The steady-state excitation and ionization balance of the proton beam are determined, taking into account all important atomic interactions with the ambient chromosphere. For a proton energy flux comparable to the electron energy flux commonly inferred for large flares, L alpha wing intensities orders of magnitude larger than observed nonflaring values were found. Investigation of timescales for ionization and charge exchange leads researchers to conclude that over a wide range of values of mean proton energy and beam parameters, Doppler-shifted nonthermal L alpha emission is a useful observational diagnostic of the presence of 10 keV to 10 MeV superthermal proton beams in the solar flare chromosphere. Title: The Importance of Proton Beam Pressure in Solar Flares Authors: Tamres, D. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1985BAAS...17..634T Altcode: No abstract at ADS Title: Coulomb and Ohmic Flare Heating by Nonthermal Electrons Authors: McClymont, A. N.; Canfield, R. C.; Brown, J. C. Bibcode: 1985BAAS...17..635M Altcode: No abstract at ADS Title: Observations of Impulsive-Phase Chromospheric Dynamics in Energetic-Electron-Heated Flare Kernels Authors: Canfield, R. C.; Gunkler, T. A.; Fiplinger, A. T. Bibcode: 1985BAAS...17..628C Altcode: No abstract at ADS Title: Hα Microflares at the Limit of Hard X-ray Detectability Authors: Metcalf, T. R.; Canfield, R. C. Bibcode: 1985BAAS...17..644M Altcode: No abstract at ADS Title: Flare Loop Radiative Hydrodynamics - Part Seven - Dynamics of the Thick Target Heated Chromosphere Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1985ApJ...289..434F Altcode: It is shown that hydrodynamic phenomena in the chromospheric portion of the flaring solar atmosphere depend dramatically on whether chromospheric evaporation by thick-target fast-electron heating is "gentle" or "explosive." In the case of gentle evaporation, velocities in the upper chromosphere are upward. In the case of explosive evaporation, the overpressure of the evaporated material drives downward motion in the residual flare chromosphere. The plasma driven downward by explosive evaporation is cool and dense in comparison with the chromospheric material ahead of it. We review previous discussions of these "chromospheric condensations" and conclude that physical understanding has been incomplete. We suggest that these condensations are an inevitable consequence of compression of a thermally stable heated plasma. We then investigate the nature of hydrodynamic waves in a heated, strongly radiating, optically thin plasma. It is first shown that acoustic waves in the flare chromosphere travel more slowly than adiabatic or even isothermal sound waves. Next, a simple model for the formation and propagation of chromospheric condensations is developed. This model is based on the propagation of a compression wave into the chromosphere, with quasi-steady equilibrium between flare heating and radiative losses on each side of the compression front. We derive jump conditions and accretion rates for the compression wave. We find that this simple model agrees well with our numerical simulations. Title: Flare loop radiative hydrodynamics. V - Response to thick-target heating. VI - Chromospheric evaporation due to heating by nonthermal electrons. VII - Dynamics of the thick-target heated chromosphere Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1985ApJ...289..414F Altcode: The results of thick-target model simulations of the hydrodynamic and radiative response of the solar-loop atmosphere to short bursts of energetic nonthermal electrons are presented in extensive graphs and diagrams and characterized in detail. The physical basis and numerical techniques of the simulations, which continue the program initiated by McClymont and Canfield (1983), are explored; the radiative-loss approximations employed are compared with those of Ricchiazzi (1982); the dynamics of coronal mass motion driven by chromospheric evaporation are investigated; and gentle and explosive evaporation regimes associated with upward and downward velocities in the upper chromosphere, respectively, are differentiated. Consideration is given to the propagation of hydrodynamic waves in heated strongly radiating optically thin plasmas, the formation and propagation of chromospheric condensations, and jump conditions and accretion rates for the compression wave. Title: Flare Loop Radiative Hydrodynamics - Part Six - Chromospheric Evaporation due to Heating by Nonthermal Electrons Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1985ApJ...289..425F Altcode: The response of the solar chromosphere to flare heating by nonthermal electrons is examined. A number of interesting phenomena appear in our numerical solutions of the equations of hydrodynamics and radiative transfer. Here we discuss one aspect of these results: the phenomenon of chromospheric evaporation. We present results for a range of heating fluxes and show how these may be understood in simple terms. Our major conclusions are as follows: (1) There is an energy flux threshold for "explosive" evaporation. Explosive evaporation occurs when the upper chromosphere is unable to radiate the flare energy deposited there, and is therefore heated rapidly to coronal temperatures. Energy fluxes less than this threshold produce "gentle" evaporation, in which the chromosphere is eaten away by conduction at a much slower rate. (2) The expansion velocity of explosively evaporated plasma cannot exceed ∼ 2.35cs , where cs is the sound speed in the evaporated material. (3) We derive a simple analytic model for the temporal variation of velocity in explosively evaporated plasma. This "gasbag" model, based on isothermal expansion of an impulsively heated mass of plasma, is used successfully to reproduce our own numerical results, as well as those of MacNeice et al. (1984). (4) The lower transition region, in both gentle and explosive evaporation, quickly reaches a quasisteady balance between conduction and radiation, so that the conductive flux at 105 K is given by 3.42 x 105 P ergs cm -2 s-1, where P(dyn cm-2) is the pressure in the flare transition region. In the case of explosive evaporation, a short powerful pulse of EUV radiation is emitted from plasma with temperatures near 105 K during the adjustment to this equilibrium. Title: Energetic electron heating and chromospheric evaporation during a well-observed compact flare Authors: Canfield, R. C.; Gunkler, T. A. Bibcode: 1985ApJ...288..353C Altcode: A previous analysis of chromospheric evaporation in the solar flare of May 7, 1980, in which it was shown that the chromospheric evaporation can account for the thermal X-ray plasma of the flare, is extended. The way the H-alpha profile should respond to thick-target nonthermal electron heating is discussed using previous theoretical modelling. It is shown that broad Stark wings are the theoretically expected response above moderately high values of the input nonthermal electron energy flux. The observed spatial and temporal behavior of H-alpha profiles and hard X-rays during the impulsive phase is exmined, and it is concluded that broad H-alpha wings are closely related observationally to hard X-rays. Finally, it is shown that the observed width of impulsive-phase H-alpha wings supports the thick-target nonthermal electron heating model. Title: A consistent picture of coronal and chromospheric processes in a well-observed solar flare Authors: Gunkler, T. A.; Canfield, R. C.; Acton, L. W.; Kiplinger, A. L. Bibcode: 1984ApJ...285..835G Altcode: The solar flare of 15:22 UT on June 24, 1980 is analyzed using simultaneous observations in hard X-rays, soft X-rays, and H-alpha line profiles obtained from instruments aboard the Solar Maximum Mission and ground-based instruments. The theoretical H-alpha profiles of Canfield, Gunkler, and Ricchiazzi (1984) are used to analyze the H-alpha data, and the work of Hummer and Rybicki (1968) is used to provide qualitative velocity information. The soft X-ray data are employed to obtain coronal measurements of parameters of interest, while the flux and spectrum of the hard X-rays are used to calculate the peak power of nonthermal electrons. Various flare phenomena are studied, including heating of the chromosphere by nonthermal electrons, enhanced coronal pressure, enhanced thermal conduction, chromospheric evaporation and mass motion. It is shown that the observations strongly suggest a scenario in which two large magnetic loop systems interact to provide the flare energy. Title: The Role of Non-Classical Transport in the Formation of the Ly-α Temperature Plateau Authors: Owocki, S. P.; Canfield, R. C.; McClymont, A. N. Bibcode: 1984BAAS...16..992O Altcode: No abstract at ADS Title: The Hα Spectral Counterpart of Hard X-ray Microflares Authors: Canfield, R. C.; Metcalf, T. R. Bibcode: 1984BAAS...16..891C Altcode: No abstract at ADS Title: The H-alpha spectral signatures of solar flare nonthermal electrons, conductive flux, and coronal pressure Authors: Canfield, R. C.; Gunkler, T. A.; Ricchiazzi, P. J. Bibcode: 1984ApJ...282..296C Altcode: During a solar flare, a number of mechanisms heat the chromosphere above its preflare temperature. Ricchiazzi and Canfield (1983) have modeled the response of the chromosphere to processes related to energetic electrons, thermal conductivity, and enhanced coronal pressure. The present investigation is concerned with the computation of H-alpha profiles for various models, thus providing their H-alpha spectral signatures. It is shown that H-alpha responds sensitively to these processes. The investigation is based on static models of flare chromospheres which have been obtained for two limiting assumptions, taking into account hydrostatic equilibrium and the assumption that the flare heating has just been turned on. After computing the model atmospheres, theoretical H-alpha line profiles are generated for each model. Title: The unimportance of beam momentum in electron-heated models of solar flares Authors: McClymont, A. N.; Canfield, R. C. Bibcode: 1984A&A...136L...1M Altcode: In a previous letter to this journal, Brown and Craig (1984) called attention to the possible importance of the hitherto neglected direct collisional acceleration of flare atmospheres heated by particle beams. For the case commonly believed to be of most physical interest - electron-beam heating in closed flare loops - these arguments are found to be incomplete. Both the early (impulsive) and late (equilibrium) phases of the atmospheric response are treated self-consistently, and in addition the intermediate phase (in which pressure equilibrium, but not energetic equilibrium, has been established) is considered. It is concluded that acceleration due to beam pressure is important for only a few seconds after the electron beam is switched on and is unlikely to have a significant influence on the global flare evolution. Brown and Craig's conclusions should, however, be taken into account in models of proton beam heating, because of the much higher momentum-to-energy ratio of proton beams. Title: Studies of solar flares and coronal loops Authors: Canfield, R. C. Bibcode: 1984ucsd.reptQ....C Altcode: The objectives of this research were to improve our understanding of solar flares and solar coronal loops. The specific approach to the flare objective was to analyze and interpret solar flare data, using theoretical methods developed as part of the research. The specific approach to the coronal loop objective was to investigate their thermal and magnetohydrodynamic stability for various physical models. The principal result of the flare research was to demonstrate that, in two well observed flares, the mechanism of chromospheric evaporation accounts for the observed amount of flare X-ray plasma. The dominant energy transport mechanism is thermal conduction. Heating by energetic electrons is of secondary importance. The principal results of the magnetohydrodynamic stability analyses were demonstrations by the role of radiative energy loss, compressibility, magnetic field line twist, foot point magnetic field line tying, and radial plasma pressure gradient. Title: Chromospheric evaporation velocities in solar flares Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1984ApJ...281L..79F Altcode: It is shown that the upper limit to chromospheric evaporation velocities in solar flare loops is 2.35 Cs, where Cs is the sound speed in the evaporated material. Upward velocities greater than this limit would imply the existence of additional nonhydrodynamic forces. Several hydrodynamic calculations are made of collisional heat produced by nonthermal energetic electrons, and upward velocities are found which are at the upper limit of large (less than 3 x 10 to the 10th ergs/sec/sq cm) energy flux values. At more modest fluxes (10 to the 10th or 10 to the 9th ergs/sec/sq cm) both thermal and thick target models yield upward velocities which are within 10 to 20 percent of the upper limit. When both temperature and velocity of the evaporated plasma are considered, the thermal model is found to be the most consistent with recent observational data. Title: On the Formation of Temperature Plateaus in the Solar Transition Region Authors: Owocki, S. P.; McClymont, A. N.; Canfield, R. C. Bibcode: 1984BAAS...16..729O Altcode: No abstract at ADS Title: Erratum - a Static Model of Chromospheric Heating in Solar Flares Authors: Ricchiazzi, P. J.; Canfield, R. C. Bibcode: 1984ApJ...279..463R Altcode: No abstract at ADS Title: Electron Beam Heating During a Well-Observed Compact Flare Authors: Canfield, R. C.; Gunkler, T. A. Bibcode: 1984BAAS...16..544C Altcode: No abstract at ADS Title: Nonthermal Lyman - α Emission by 3 KeV-300 MeV Protons Authors: Chang, C. -R.; Canfield, R. C. Bibcode: 1984BAAS...16..535C Altcode: No abstract at ADS Title: Chromospheric Evaporation in Flares Due to Heating by Nonthermal Electrons Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1984BAAS...16..543F Altcode: No abstract at ADS Title: Observational evidence for chromospheric footpoint penetration of nonthermal electrons during two well-observed flares Authors: Canfield, R. C.; Gunkler, T. A.; Kiplinger, A. L. Bibcode: 1984AdSpR...4g.255C Altcode: 1984AdSpR...4..255C Recent advances have enabled simultaneous Hα and X-ray observations with substantially improved spatial, spectral, and temporal resolution. In this paper we study two events observed as part of a coordinated observing program between the Solar Maximum Mission and Sacramento Peak Observatory: the flares of 1456 UT, 7 May 1980 and 1522 UT, 24 June 1980. Using recently-developed physical models of static flare chromospheres, and corresponding theoretical Hα line profiles, we can distinguish effects of intense nonthermal electron heating from those of high conduction and pressure from the overlying flare corona. Both flares show the signature of intense chromospheric heating by fast electrons, temporally correlated with X-ray light curves at E > 27keV, and spatially associated with X-ray emission sites at E >62; 16 keV. Interpreting the Hα line profile observations using the theoretical Hα line profiles, we infer values of the thick-target input power contained in nonthermal electrons that are observationally indistinguishable (within a factor of 2-3) from those inferred from the X-ray data. Although these events are small, the energy flux values are large: of order 1011 ergs cm-2 s-1 above 20 keV. Title: Probabilistic radiative transfer. Authors: Canfield, R. C.; McClymont, A. N.; Puetter, R. C. Bibcode: 1984mrt..book..101C Altcode: 1984mrt..conf..101C The authors have developed a computationally efficient method for highly nonlinear problems in which radiative transfer is an important aspect of the heating and cooling of the medium. This paper summarizes all essential aspects of the method. The authors derive an approximate probabilistic radiative transfer equation for one-dimensional plane-parallel atmospheres of finite or semi-infinite extent, for both spectral lines and bound-free continua. They also discuss boundary conditions, accuracy, escape probabilities, and practical aspects of complete linearization, which is a key element of the method. Title: Hα Signatures of Impulsive Flare Heating by Energetic Nonthermal Electrons and Thermal Conduction Authors: Canfield, R. C.; Gunkler, T. A.; Ricchiazzi, P. J. Bibcode: 1983BAAS...15Q.918C Altcode: No abstract at ADS Title: A static model of chromospheric heating in solar flares Authors: Ricchiazzi, P. J.; Canfield, R. C. Bibcode: 1983ApJ...272..739R Altcode: The response of the solar chromosphere to flare processes, namely nonthermal electrons, thermal conduction, and coronal pressure, is modeled. Finite difference methods employing linearization and iteration are used in obtaining simultaneous solutions to the equations of steady-state energy balance, hydrostatic equilibrium, radiative transfer, and atomic statistical equilibrium. The atmospheric response is assumed to be confined to one dimension by a strong vertical magnetic field. A solution is obtained to the radiative transfer equation for the most important optically thick transitions of hydrogen, magnesium, and calcium. The theoretical atmospheres discussed here are seen as elucidating the role of various physical processes in establishing the structure of flare chromospheres. At low coronal pressures, conduction is found to be more important than nonthermal electrons in establishing the position of the transition region. Only thermal conduction can adequately account for the chromospheric evaporation in compact flares. Of the mechanisms considered, only nonthermal electrons bring about significant heating below the flare transition region. Title: A Consistent Picture of Coronal and Chromospheric Processes in a Well-Observed Flare Authors: Gunkler, T. A.; Canfield, R. C.; Acton, L. W.; Kiplinger, A. L. Bibcode: 1983BAAS...15Q.919G Altcode: No abstract at ADS Title: Maximum Upward Velocities of Chromospheric Evaporation in Flares Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1983BAAS...15..918F Altcode: No abstract at ADS Title: Flare loop radiative hydrodynamics. IV - Dynamic evolution of unstable semiempirical loop models Authors: An, C. -H.; Canfield, R. C.; Fisher, G. H.; McClymont, A. N. Bibcode: 1983ApJ...267..421A Altcode: The evolution of the unstable solar atmosphere into the nonlinear phase, in response to various perturbations, is followed. The initial dynamic evolution of the atmosphere follows the predictions of linear stability analysis. In the nonlinear phase, rapid changes are confined to the transition region; these changes are manifested as a propagation of the transition region through the plasma, i.e., chromospheric evaporation or condensation. Global evolution therefore proceeds on the coronal conductive time scale. The rate of propagation of the transition region is determined by the imbalance between the energy supplied by thermal conduction from the corona and radiative cooling within the transition region itself. Flow velocities in the lower corona during evaporation or condensation are, in the cases studied, of order 3 km/s. The observed dynamic evolution is consistent with the existence of relatively long-lived coronal loops whose brightnesses vary on the evaporative time scale. Title: Chromospheric Evaporation in Flare Loops Heated Impulsively by Nonthermal Electrons Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1983BAAS...15..708F Altcode: No abstract at ADS Title: The H-alpha Spectral Signature of Several Flare Processes Authors: Gunkler, T. A.; Canfield, R. C.; Ricchiazzi, P. J. Bibcode: 1983BAAS...15Q.697G Altcode: No abstract at ADS Title: Emission from Flare Loops Heated Impulsively by Nonthermal Electrons Authors: Canfield, R. C.; Fisher, G. H.; McClymont, A. N. Bibcode: 1983BAAS...15R.708C Altcode: No abstract at ADS Title: Impulsive EUV and Hard X-ray Flare Emission Authors: McClymont, A. N.; Canfield, R. C.; Fisher, G. H. Bibcode: 1983BAAS...15R.711M Altcode: No abstract at ADS Title: Flare loop radiative hydrodynamics. I - Basic methods Authors: McClymont, A. N.; Canfield, R. C. Bibcode: 1983ApJ...265..483M Altcode: The study presented here has two goals. The first is to examine the role of energy transport mechanisms in flare dynamics, for instance, thermal cone conduction in the corona and radiative transfer in the chromosphere. The second is to provide diagnostics of flare energization processes by predicting the evolutionary behavior of concurrent emissions from all regions of the flaring atmosphere. To attain these ends, a numerical method is developed for the simultaneous solution of thy continuity, momentum, and energy equations; the time-dependent atomic rate equations describing ionization and excitation; and the radiative transfer equations. With the physical model and computational methods used here, all components of the plasma move as a single fluid and in the initial studies are assumed to have a common kinetic temperature. The plasma is constrained to move in one dimension along the axis of a loop of nonuniform cross section, the geometry of which is defined by a sufficiently strong magnetic field. Title: Flare loop radiative hydrodynamics. III - Nonlocal radiative transfer effects Authors: Canfield, R. C.; Fisher, G. H.; McClymont, A. N. Bibcode: 1983ApJ...265..507C Altcode: The study has three goals. The first is to demonstrate that processes exist whose intrinsic nonlocal nature cannot be represented by local approximations. The second is to elucidate the physical nature and origins of these nonlocal processes. The third is to suggest that the methods and results described here may prove useful in constructing semiempirical models of the chromosphere by means more efficient than trial and error. Matrices are computed that describe the effect of a temperature perturbation at an arbitrary point in the loop on density, hydrogen ionized fraction, total radiative loss rate, and radiative loss rate of selected hydrogen lines and continua at all other points. It is found that the dominant nonlocal radiative transfer effects can be separated into flux divergence coefficient effects and upper level population effects. The former are most important when the perturbation takes place in a region of significant opacity. Upper level population effects arise in both optically thick and thin regions in response to nonlocal density, ionization, and interlocking effects. Title: Flare Loop Radiative Hydrodynamics - Part Two - Thermal Stability of Empirical Models Authors: McClymont, A. N.; Canfield, R. C. Bibcode: 1983ApJ...265..497M Altcode: The importance of loop structures in the corona, both for flares and for the quiet Sun, has stimulated considerable attention to questions of their thermal stability. Previous studies have focused attention on the coronal part of the loop. In this paper we examine loop stability by treating the entire observable loop, from its photospheric footpoints to its coronal apex. This approach allows the chromosphere and corona to interact naturally, thus avoiding possibly artificial boundary conditions imposed at transition region footpoints.

We develop a numerical eigenfunction method for the study of stability, which is based on the methods discussed in a previous paper. For exemplary purposes, we have applied these methods to several loop models based on semiempirical model chromospheres, under the assumption that the rate of ambient energy input per unit mass of plasma depends only on column depth. Our principal study is of a loop model based on the semiempirical model F of Vernazza, Avrett, and Loeser.

We find that this loop model has one unstable eigenmode, with a growth time of 2 minutes. This mode appears in the transition region, centered on the peak of the optically thin radiative loss function at T ≍ 105 K. However, we provide evidence that suggests that this instability may not be a feature of real loops. More importantly, we find that (1) this atmosphere is stable to the hydrogen-induced radiative instability of optically thin gases at temperatures around 104.3 K; (2) were it not for radiative transfer effects, this atmosphere would be dramatically unstable, with growth times in the range 1 ≤ r ≤ 18 s; and (3) the stability when radiative transfer is taken into account can be understood primarily as a result of the reduction of the peak in the radiative loss rate at 104.3 K, due to hydrogen, that would exist if the chromosphere were optically thin. This reduction is due to the significant optical depth, and consequent low escape probability, of radiation of the dominant coolant, Lyα, at upper temperatures. Title: Chromospheric evaporation in a well-observed compact flare Authors: Acton, L. W.; Leibacher, J. W.; Canfield, R. C.; Gunkler, T. A.; Hudson, H. S.; Kiplinger, A. L. Bibcode: 1982ApJ...263..409A Altcode: Hudson and Ohki (1972) pointed out that the increase of the soft X-ray emission measure during flares might be accounted for in two different ways, either by 'coronal condensation', or by what they termed 'chromospheric rarefaction', now more commonly called 'chromospheric evaporation'. They ruled out coronal condensation on the basis of cornal mass content arguments. Moore et al. (1980) found it highly probable that the bulk of the mass of the soft X-ray emitting plasma is supplied during the rise phase by chromospheric evaporation from the feet of the soft X-ray loops. On the other hand, Cheng et al. (1981) argued that chromospheric evaporation is not important as a source of soft X-ray plasma. The present investigation is concerned with an event in which direct chromospheric observations contradict the conclusions reached by Cheng et al. Up to now chromospheric evaporation has always been an inference, without compelling positive evidence. In the current investigation, observations are considered which constitute such evidence. Title: A Static Model of Chromospheric Heating in Solar Flares Authors: Canfield, R. C.; Ricchiazzi, P. J. Bibcode: 1982BAAS...14..898C Altcode: No abstract at ADS Title: Theoretical quasar emission-line ratios. VI - A probabilistic radiative transfer equation for finite slab atmospheres Authors: Puetter, R. C.; Hubbard, E. N.; Ricchiazzi, P. J.; Canfield, R. C. Bibcode: 1982ApJ...258...46P Altcode: Previous papers in this series have been based on an approximation in which the line ratios were inferred from those computed for a semi-infinite cloud model. In this paper we present a superior method, which permits the treatment of the emission-line clouds as slab atmospheres of finite thickness. In common with our previous semi-infinite approach, it is based on photon escape probabilities, yet it recognizes the important distinction between the photon escape probability and the flux divergence. This distinction is neglected in all existing models of energy balance in QSO emission-line clouds. This neglect can lead to order of magnitude errors in the cooling rates, casting doubt on the results of past models.

The present method reduces to the previous one in the semi-infinite case. It not only produces the correct source function S∞ at the surface of a semi-infinite atmosphere S∞(τ-0) = ɛ1/2B for a constant Planck function B and photon destruction probability ɛ, but it derives the empirical relationship ST(τ=0) ≍ ɛ1/2S∞(τ=T) proposed by Avrett and Hummer relating the source function at the surface of a finite slab of optical thickness T to the source function at τ = T in a semi-infinite slab. We show that the method provides a solution that departs from the exact solution by at most a few tens of percent in cases of physical interest, while retaining all the advantages of the previous method. Title: Erratum - Theoretical Quasar Emission-Line Ratios - Part Three - Flux Divergence and Photon Escape Authors: Canfield, R. C.; Puetter, R. C.; Ricchiazzi, P. J. Bibcode: 1982ApJ...256..798C Altcode: No abstract at ADS Title: Solar flare studies Authors: Canfield, R. C. Bibcode: 1982ucsd.rept.....C Altcode: The primary objective of the research described in this report was increased understanding of solar flares. In the course of the research, many tasks were carried out, which achieved not only the primary objective, but also secondary objectives in related areas. The research program started with active participation in the Skylab Solar Flare Workshop. New observations of solar flare spectra were obtained and interpreted in terms of basic solar flare mechanisms. It was shown that the basic process by which the X-ray radiation of flares is created is by heating the flare plasma to temperatures of about ten million degrees, through evaporation of the chromosphere. This process is driven both by beams of accelerated electrons and by thermal conduction. However, in the major flare for which data were interpreted, the principal energy release mechanism was found to be thermal in nature, implying that most of the flare energy is released in the form of heat, and not charged particles. Theoretical modeling methods were developed for understanding the spectra solar flares. These methods were applied to flare loop dynamics. The spectral signatures of both chromospheric evaporation and beams of accelerated electrons were established. Finally, a theorectical program of theoretical magnetohydrodynamic stability studies was begun. Title: A qualitative interpretation of 7 August 1972 impulsive phase flare Hα line profiles Authors: Canfield, R. C. Bibcode: 1982SoPh...75..263C Altcode: Tanaka's (1977) unique Hα profiles of the kernels of the 7 August 1972 flare were quantitatively interpreted by Brown et al. (1978; henceforth BCR) in terms of a thick target electron beam model. They found that this interpretation required beam inhomogeneity and/or partial precipation and large (60-100 km s−1) macroturbulence. The latter requirement is somewhat suspect, since the only independent evidence also comes from efforts to understand the profiles of optically thick chromospheric lines. Relationships between model atmosphere parameters and line profile parameters calculated by Dinh (1980) show that these requirements could be considerably reduced, if not totally eliminated, if the actual chromospheric flare heating mechanism were simultaneously capable of pushing the flare transition region to greater column density and causing less heating of the residual chromosphere than the BCR models. This then implies that the chromosphere is heated primarily by a mechanism through which the heating effects do not penetrate as far below the flare transition region as is the case for a power-law spectrum of non-thermal electrons whose parameters are chosen appropriate to the nonthermal thick target interpretation of hard X-rays. Thermal conduction and optically thick radiation are examples of such a mechanism. Title: Direct evidence for chromospheric evaporation in a well-observed compact flare Authors: Canfield, R. C.; Gunkler, T. A.; Hudson, H. S.; Acton, L. W.; Leibacher, J. W.; Kiplinger, A. L. Bibcode: 1982AdSpR...2k.145C Altcode: 1982AdSpR...2..145C Observations of the solar flare of May 7, 1980 using several Solar Maximum Mission instruments are presented as an investigation of the phenomenon of chromospheric evaporation. The total amount of plasma at temperatures greater than 2 x 10 to the 6th K were determined from the X-ray data, and the amount of plasma that was evaporated from the chromosphere was determined from the H-alpha data. The H-alpha profiles indicate that for the flare as a whole, at the time of peak soft X-ray emission measure, the number of atoms evaporated from the chromosphere was 7 x 10 to the 37th. The soft X-ray emission measure of 1 x 10 to the 49th/cu cm, coupled with the flare volume estimate of 10 to the 26th cu cm, indicates that there were 3 x 10 to the 37th electrons in the soft X-ray plasma with temperatures greater than 2 x 10 to the 6th K. These results indicate that enough material had been evaporated from the chromosphere to account for the X-ray plasma. Taken together, the H-alpha, soft X-ray, and hard X-ray images indicate that chromospheric evaporation is driven both by flare-accelerated electrons during the impulsive phase and by conduction during the thermal phase. Title: The Lyman-alpha/H-alpha ratio in solar flares and quasars Authors: Canfield, R. C.; Puetter, R. C.; Ricchiazzi, P. J. Bibcode: 1981ApJ...249..383C Altcode: Constant temperature and density solar flare models are constructed with temperature and hydrogen density values that reflect reasonable nonlinear averages of those parameters in the depth dependent solar flare chromosphere models of Lites and Cook (1979). Acceptable values of the intensity ratios L-alpha/H-alpha and H-beta/H-alpha correspond to temperatures from about 9000 to 13,000 K, and hydrogen densities from 10 to the 11th to 10 to the 15th cu cm. The H-alpha and Ly-alpha source functions are thermalized at depths consistent with those inferred from independent studies, although the observed Ly-alpha/H-alpha ratio does not necessarily imply an electron temperature appropriate to the Planck function ratio. It is also shown that the value of Ly-alpha/H-alpha depends on the temperature, hydrogen density, and the optical depth of the emitting chromospheric layer. Title: Theoretical quasar emission-line ratios. III - Flux divergence and photon escape Authors: Canfield, R. C.; Puetter, R. C.; Ricchiazzi, P. J. Bibcode: 1981ApJ...248...82C Altcode: In this paper we develop a computationally useful version of the probabilistic first-order differential radiative transfer equation of Frisch and Frisch. This approximate radiative transfer equation is especially appropriate for the evaluation of radiative transfer effects in multilevel atomic systems due to its extreme computational efficiency and reasonably accurate description of the physics of radiation transfer. In particular, it recognizes the distinction between the flux divergence coefficient, ρ, and the photon escape probability, Pe. We show that this distinction is crucial for calculations that attempt to construct self-consistent energy balance models since substitution of Pe for ρ in such models leads to large errors (of sign as well as magnitude) in the local cooling rate. Title: A Probabilistic Radiative Transfer Equation for Finite Slab Models of QSO Emission Line Regions Authors: Ricchiazzi, P. J.; Puetter, R. C.; Hubbard, E. N.; Canfield, R. C. Bibcode: 1981BAAS...13..788R Altcode: No abstract at ADS Title: Direct Evidence for Chromospheric Evaporation in a Well-Observed Compact Flare Authors: Canfield, R. C.; Acton, L. W.; Gunkler, T. A.; Hudson, H. S.; Kiplinger, A. L.; Leibacher, J. W. Bibcode: 1981BAAS...13R.819C Altcode: No abstract at ADS Title: Non-Local Effects of Radiative Transfer on Radiative Hydrodynamic Stability Authors: Fisher, G. H.; Canfield, R. C.; McClymont, A. N. Bibcode: 1981BAAS...13..846F Altcode: No abstract at ADS Title: Models of Electron-Heated Solar Flare Chromospheres Authors: Ricchiazzi, P. J.; Canfield, R. C. Bibcode: 1981BAAS...13..819R Altcode: No abstract at ADS Title: Non-Linear Development of the Radiative Hydrodynamic Instability in Empirical Solar Loop Models Authors: An, C. -H.; Canfield, R. C.; McClymont, A. N.; Fisher, G. H. Bibcode: 1981BAAS...13..837A Altcode: No abstract at ADS Title: Theoretical Quasar Emission Line Ratios - Part Two - Hydrogen Lyman-Alpha Balmer and Paschen Lines and the Balmer Continuum Authors: Canfield, R. C.; Puetter, R. C. Bibcode: 1981ApJ...243..390C Altcode: We explore the formation of the hydrogen La, Balmer, and Paschen lines and the Balmer continuum in highly idealized QSO broad emission line clouds (ELCs) of constant temperature and density irradiated by an external source of power-law spectral form. We simultaneously solve the equations governing excitation, ionization, and transfer of both external and diffuse radiation fields. Our calculations show that the typical observed broad emission line ratios of Lα/Hα, Hβ/Hα, Pα/Hα, and Balmer continuum/Hα can be understood for ELC conditions in the temperature range 7 × 103 ≲ Te ≲ 2 × 104 K, hydrogen density range 108 ≲ nH ≲ 1012 cm-3, and for external fluxes F ≲-6 ergs cm-2 s-1 Hz-1 at the Lyman continuum limit.

Important results are:

1. The Lα/Hα ratio is very sensitive to the optical thickness of the cloud at the optical thickness that we think are appropriate for QSOs.

2. It is not necessary to postulate dust either internal or external to the ELC.

3. The ELCs are very optically thick; the Balmer lines and Pa originate in a region for which the Lyman-limit optical depth τcl ≳ 102.

4. A very extended ionized zone results from ionization from excited states.

5. The radiation from various transitions arises from very extended and sometimes quite different regions of the cloud. This renders a mean escape probability approach inappropriate.

6. Predicted line ratios and cooling rates depend critically on the functional form of the photon escape probability.

7. Acceptable values of the area covering factor and static energy balance require temperatures somewhat in excess of 104 K. Title: Theoretical quasar emission line ratios. I - Transfer and escape of radiation. II - Hydrogen L-alpha, Balmer, and Paschen lines, and the Balmer continuum Authors: Canfield, R. C.; Puetter, R. C. Bibcode: 1981ApJ...243..381C Altcode: We describe a formalism for the solution of the frequency-integrated radiative transfer equation and the atomic steady state equation appropriate to an externally irradiated, semi-infinite medium. The source functions are cast into equivalent two-level forms, and the equations linking the emergent flux, the line center source function, the flux divergence coefficient ρ, and the atomic steady state equations are given. Asymptotic forms for scaling law solutions to the radiative transfer equation are developed. Escape probabilities appropriate to the above scaling law solutions are discussed for subordinate lines, strongly interlocked resonance lines, weakly interlocked resonance lines, and the bound-free continua. Title: A Method for Combined Hydrodynamics and Probabilistic Radiative Transfer Authors: McClymont, A. N.; Canfield, R. C. Bibcode: 1980BAAS...12..915M Altcode: No abstract at ADS Title: CCD Observations of the Profile of H-alpha Throughout the Flare of 1456 UT 07 May 1980 Authors: Gunkler, T. A.; Canfield, R. C. Bibcode: 1980BAAS...12R.905G Altcode: No abstract at ADS Title: A Sufficient Condition for Evaluation of the Stability of Solar Coronal Loops Authors: An, C. -H.; McClymont, A. N.; Canfield, R. C. Bibcode: 1980BAAS...12..913A Altcode: No abstract at ADS Title: Observed Lα profiles for two solar flares: 14∶12 UT 15 June, 1973 and 23∶16 UT 21 January, 1974 Authors: Canfield, R. C.; van Hoosier, M. E. Bibcode: 1980SoPh...67..339C Altcode: Photographic observations of the time development of the profile of the Lα line of hydrogen during flares were obtained with the NRL spectrograph on ATM. The profiles for the 15 June, 1973 and 21 January, 1974 flares reported here cover both core and wings of the line. The time sequences begin before flare maximum, and continue well into the decay phase. Careful attention has been given to photometry and absolute calibration. In the case of the 15 June, 1973 flare, data are presented both first-order corrected and uncorrected for incomplete filling of the spectrograph slit by flaring material. Correction of the 21 January, 1974 flare was not possible. We discuss core symmetry and shift, and show that our observations imply integrated flare Lα/Hα intensity ratios within a factor of two of unity for these two flares. Title: A probabilistic approach to radiative energy loss calculations for optically thick atmospheres - Hydrogen lines and continua Authors: Canfield, R. C.; Ricchiazzi, P. J. Bibcode: 1980ApJ...239.1036C Altcode: An approximate probabilistic radiative transfer equation and the statistical equilibrium equations are simultaneously solved for a model hydrogen atom consisting of three bound levels and ionization continuum. The transfer equation for L-alpha, L-beta, H-alpha, and the Lyman continuum is explicitly solved assuming complete redistribution. The accuracy of this approach is tested by comparing source functions and radiative loss rates to values obtained with a method that solves the exact transfer equation. Two recent model solar-flare chromospheres are used for this test. It is shown that for the test atmospheres the probabilistic method gives values of the radiative loss rate that are characteristically good to a factor of 2. The advantage of this probabilistic approach is that it retains a description of the dominant physical processes of radiative transfer in the complete redistribution case, yet it achieves a major reduction in computational requirements. Title: Lα/Hα in Solar Flares and QSO's Authors: Canfield, R. C.; Puetter, R. C. Bibcode: 1980BAAS...12..517C Altcode: No abstract at ADS Title: Line and Continuum Cooling in QSO Emission Line Regions Authors: Canfield, R. C.; Puetter, R. C.; Ricchiazzi, P. J. Bibcode: 1980BAAS...12..536C Altcode: No abstract at ADS Title: The implications of hydrogen emission line ratios in quasi-stellar objects Authors: Canfield, R. C.; Puetter, R. C. Bibcode: 1980ApJ...236L...7C Altcode: The results of multilevel, depth-dependent, fully interlocked radiative transfer calculations for hydrogen emission line strengths in a single QSO emission line cloud (ELC) are summarized. The hydrogen-line forming region of the ELC is found to be quite thick (tauel between 1,000 and 100,000), which is consistent with heating of a pure hydrogen cloud by photoionization. Results indicate that the volume-averaged escape probability approach introduces large errors by assuming, in effect, that a single point in the ELC is representative of the emergent radiation; that the influence of frequency redistribution on the photon escape probability in resonance and subordinate lines must be explicitly recognized, and that full consistency between excitation and ionization processes must be maintained. Title: Radiative hydrodynamics of flares - Preliminary results and numerical treatment of the transition region Authors: McClymont, A. N.; Canfield, R. C. Bibcode: 1980IAUS...91..313M Altcode: No abstract at ADS Title: The chromosphere and transition region Authors: Canfield, R. C.; Brown, J. C.; Craig, I. J. D.; Brueckner, G. E.; Cook, J. W.; Doschek, G. A.; Emslie, A. G.; Machado, M. E.; Henoux, J. -C.; Lites, B. W. Bibcode: 1980sfsl.work..231C Altcode: 1980sofl.symp..231C The physical processes occurring as a result of the transfer of energy and momentum from the primary solar flare energy release site in the corona to the underlying chromosphere and transition region during the course of the flare are investigated through a comparison of theoretical models and observational data. Static, dynamic and hydrodynamic models of the lower-temperature chromospheric flare are reviewed. The roles of thermal conduction, radiation, fast particles and mass motion in chromosphere-corona interactions are analyzed on the basis of Skylab UV, EUV and X-ray data, and empirical and synthetic models of the chromospheric and upper photospheric responses to flares are developed. The canonical model of chromospheric heating during flares as a result of primary energy release elsewhere is found to be justified in the chromosphere as a whole, although not entirely as the temperature minimum, and a simplified model of horizontal chromospheric flare structure based on results obtained is presented. Title: Radiative energy output of the 5 September 1973 flare Authors: Canfield, R. C.; Cheng, C. -C.; Dere, K. P.; Dulk, G. A.; McLean, D. J.; Schmahl, E. J.; Robinson, R. D., Jr.; Schoolman, S. A. Bibcode: 1980sfsl.work..451C Altcode: 1980sofl.symp..451C Measurements of the radiative energy output of the solar flare of Sept. 5, 1973, over a wavelength range of more than ten decades, from below 1 A to above 1 m are presented. Observations of soft X-rays (0.5-20 A), XUV and EUV lines (171-1863 A) and EUV continua (1400-1960 A), H alpha radiation, visible lines and continua (3700-8700 A) and radio emission (centimeter to meter wavelengths) were obtained concurrently by Skylab and ground-based instruments. Estimates of power output at flare maximum are obtained for the observed wavelengths with uncertainty of at least half an order of magnitude, due to corresponding uncertainties in EUV and visible fluxes. Taking into account energy radiated at unobserved wavelengths and the characteristic time of the best-reduced data (the soft X-ray), calculations indicate a total radiated flare energy of approximately 4 x 10 to the 29th erg. Title: The Thermal vs. Non-Thermal Flare Controversy: More Fuel for the Fire Authors: Canfield, R. C. Bibcode: 1979BAAS...11..650C Altcode: No abstract at ADS Title: On the Formation of Lyman α, the Balmer and Paschen Lines and the Balmer Continuum in Quasars Authors: Canfield, R. C.; Puetter, R. C. Bibcode: 1979BAAS...11..670C Altcode: No abstract at ADS Title: The implications of hydrogen emission-line ratios in quasi-stellar objects. Authors: Canfield, R. C.; Puetter, R. C. Bibcode: 1979PASP...91R.609C Altcode: No abstract at ADS Title: The Height Variation of Velocity and Temperature Fluctuations in the Solar Photosphere Authors: Keil, S. L.; Canfield, R. C. Bibcode: 1978A&A....70..169K Altcode: Summary. The Vacuum Tower Telescope of Sacramento Peak Observatory is used to observe intensity and velocity fluctuations in several Fe I lines as functions of heliocentric angle. We derive the vertical and horizontal components of the velocity fluctuations, using the technique developed by Canfield (1976) to separate granular and oscillatory velocities. We also find a set of height dependent temperature perturbations which are capable of reproducing the observed intensity fluctuations. The horizontal component of the granular velocity is found to be between one and two km 1 greater than the vertical component (depending on height in the atmosphere). A temperature perturbation (constant with height) of 175 1 25 K in the upper layers of the atmosphere [Tsooo 0.1] is sufficient to reproduce the intensity fluctuations in the strong lines. In deeper layers the temperature perturbations must increase rapidly with depth to reproduce the observed intensity fluctuations in the continuum and weak lines. Key words: solar atmosphere - solar velocity fluctuations - solar temperature structure Title: Indirect estimation of energy disposition by non-thermal electrons in solar flares. Authors: Hudson, H. S.; Canfield, R. C.; Kane, S. R. Bibcode: 1978SoPh...60..137H Altcode: The broad-band EUV and microwave fluxes correlate strongly with hard X-ray fluxes in the impulsive phase of a solar flare. This note presents numerical aids for the estimation of the non-thermal electron fluxes from these correlations, using the SFD (sudden frequency deviation) ionospheric data to measure the EUV flux. Title: ATM evidence for a nonthermal proton/electron energy flux ratio in solar flares. Authors: Canfield, R. C.; Cook, J. W. Bibcode: 1978ApJ...225..650C Altcode: An observational search has been carried out for asymmetry in the wings of L-alpha during flares, produced by beams of nonthermal protons injected into the chromosphere from the corona as suggested by Orrall and Zirker (1976). The data base is the ATM/Skylab EUV spectrograms from the NRL S082B spectrograph. The asymmetries expected to be present in the normal thermal profile are discussed, and detailed consideration is given to the flare that occurred at 1551 UT on August 9, 1973, which was observed during the nonthermal phase. In this flare only very small L-alpha asymmetries are observed, not large enough to be statistically significant. It is shown that this result, combined with microwave radio data for information on nonthermal electrons, implies that the energy flux of nonthermal protons injected into the chromosphere at energies above 20 keV is less than approximately 0.02 times that of electrons of the same energy range in the observed events. Title: A Probabilistic Approach to Radiative Energy Loss Calculations for Optically Thick Atmospheres: Hydrogen Lines and Continua Authors: Canfield, R. C.; Ricchiazzi, P. J. Bibcode: 1978BAAS...10..683C Altcode: No abstract at ADS Title: Theoretical Intensities of Lyman α, Balmer α and Paschen α in QSO Clouds Authors: Canfield, R. C.; Puetter, R. C. Bibcode: 1978BAAS...10R.691C Altcode: No abstract at ADS Title: Spatial structure in lines in the 3398 3526 å region at the extreme limb: Observation, identification and interpretation Authors: Canfield, R. C.; Pasachoff, J. M.; Stencel, R. E.; Beckers, J. M. Bibcode: 1978SoPh...58..263C Altcode: We have obtained spectrograms of high spatial and spectral resolution of the extreme solar limb, using the vacuum tower telescope of Sacramento Peak Observatory. We have identified emission lines in the range 3398-3526 Å, and classified them according to intensity, spatial structure (intensity variation), and profile. Some lines show spatial intensity variation; others do not. We show that this effect is related to the abundance of the element responsible for the line and the mean lower-level excitation potential of interlocked lines. We explain the effect in terms of radiative interlocking with other lines, as well as the characteristic size of the volume contributing to the mean intensity. Title: Hα profiles from electron-heated solar flares Authors: Brown, John C.; Canfield, Richard C.; Robertson, Matthew N. Bibcode: 1978SoPh...57..399B Altcode: We briefly review the status of models of optical flare heating by electron bombardment. We recompute Brown's (1973a) flare model atmospheres using considerably revised radiative loss rates, based on Canfield's (1974b) method applied to α, Lα, and H−. Profiles of α are computed and compared with observation. The computed profiles agree satisfactorily with those observed during the large 1972 August 7 flare, if spatial and velocity inhomogeneities are assumed. The electron injection rate inferred from α is one order of magnitude less than that inferred from hard X-rays, for this event. This may be due to either (1) the neglect of a mechanism that reduces the thick-target electron injection rate or (2) failure to incorporate important radiative loss terms. Title: ATM Evidence for a Low Non-Thermal Proton/Electron Energy Flux Ratio in Solar Flares. Authors: Canfield, R. C.; Cook, J. W. Bibcode: 1978BAAS...10..441C Altcode: No abstract at ADS Title: Analysis of the solar magnesium I spectrum. II. Sensitivity of lambda 2852 to partial redistribution effects. Authors: Canfield, R. C.; Cram, L. E. Bibcode: 1977ApJ...216..654C Altcode: We have computed theoretical profiles of the Mg 1 A2852 resonance line by using various models for the frequency redistribution of the scattered radiation. We find that throughout the line core and inner wings these profiles are highly sensitive to the assumed extent of redistribution. In the line core the profile computed allowing partial redistribution has emission peaks at AA + 0.1 A, while the profile computed for complete redistribution has no peaks at all. In the inner wings (0.1 < i AAI < 5.0 A) the residual intensity with partial redistribution falls as much as a factor of 3 below the complete redistribution profiles. We conclude that partial redistribution effects in the formation of this line must be taken into account in subsequent calculations. Subject headings: line formation - line profiles - radiative transfer - Sun: spectra - ultraviolet: spectra Title: Magnetic field reconnection in the flare of 18:28 UT 1975 August 10. Authors: Canfield, R. C.; Fisher, R. R. Bibcode: 1976ApJ...210L.149C Altcode: The letter discusses observations of the cited flare made with the Sacramento Peak Observatory 512-diode array, which simultaneously measures photospheric magnetic fields, photospheric and chromospheric velocities, and chromospheric brightness in several lines. The observations suggest triggering of the flare by emergence of new magnetic flux, as well as the geometry of the reconnected field during the flare. Implications of the present observations regarding the site of the initial instability are discussed as well as the relationship to X-ray observations. Title: The height variation of granular and oscillatory velocities. Authors: Canfield, R. C. Bibcode: 1976SoPh...50..239C Altcode: Previous observations of spatially-resolved vertical velocity variations in ten lines of Fe I spanning the height range 0 ≲ h ≲ 1000 km are re-analyzed using velocity weighting functions. The amplitudes and scale heights of granular and oscillatory velocities are determined, as well as those of the remaining unresolved velocities. I find that the optimal representation of the amplitude of the outward-decreasing granular velocities is an exponentially decreasing function of height, with a scale height of 150 km and a velocity at zero height of 1.27 km s−1. The optimal representation of the same quantities for oscillatory velocities is an exponential increase with height, with a scale height of 1100 km and a velocity at zero height of 0.35 km s−1. The remaining unresolved velocities decrease with height, with a scale height of 380 km and a velocity at zero height of 2.3 km s−1. Title: Emission lines in the wings of Ca II H and K. I. Initial solar observations and implications. Authors: Canfield, R. C.; Stencel, R. E. Bibcode: 1976ApJ...209..618C Altcode: We apply solar observations to the problem of proper identification of the atomic species and the mechanism of formation of emission lines in the wings of the Ca ii H and K lines. Emission lines of both rare earths and metals appear to be present in the Sun. Their behavior in the solar spectrum implies that emission lines of metals will be useful in studies of chromospheres of other stars in which they are observed. Subject headings: line identifications - Sun: chromosphere - Sun: spectra Title: Fine Structure Variations in High-Spatial-Resolution Solar Spectra. Authors: Pasachoff, J. M.; Canfield, R. C.; Stencel, R. E.; Beckers, J. M. Bibcode: 1976BAAS....8..501P Altcode: No abstract at ADS Title: Stellar Emission Lines in the Wings of Calcium H and K Authors: Stencel, R. E.; Canfield, R. C. Bibcode: 1976BAAS....8..307S Altcode: No abstract at ADS Title: Evidence for Magnetic Field Reconnection in the Flare Process? Diode Array Observations of the 18:28 U.T. Flare, 10 August 1976 Authors: Fisher, R. R.; Canfield, R. C. Bibcode: 1976BAAS....8..374F Altcode: No abstract at ADS Title: Observational evidence for unresolved motions in the solar atmosphere Authors: Canfield, R. C.; Beckers, J. M. Bibcode: 1976pmas.conf..291C Altcode: No abstract at ADS Title: Spatially resolved motions in stellar atmospheres Authors: Beckers, J. M.; Canfield, R. C. Bibcode: 1976pmas.conf..207B Altcode: No abstract at ADS Title: Motions in the solar atmosphere Authors: Beckers, J. M.; Canfield, R. C. Bibcode: 1975STIN...7630135B Altcode: The report presents two papers on observational evidence for large and small scale motions in the solar atmosphere. Title: A model for the solar flare Authors: Canfield, R. C.; Priest, E. R.; Rust, D. M. Bibcode: 1975STIN...7615007C Altcode: It is suggested that many solar flares occur due to an interaction between newly emerging magnetic flux and an active region filament. A current sheet forms between the new and old flux and, when its electric current density exceeds a critical value, rapid magnetic fields reconnection takes place. Electrons are then accelerated to high energies and follow the magnetic field lines down to the chromosphere where they produce several (typically three) bright H alpha knots. Magnetic energy is continuously released as reconnected prominence fields are allowed to untwist. Two ribbons of H alpha emission are produced by one or more of three mechanisms: energetic particles, thermal conduction or shocks due to infalling material. Title: A simplified method for calculation of radiative energy loss due to spectral lines. Authors: Canfield, R. C. Bibcode: 1974ApJ...194..483C Altcode: In problems of atmospheric structure, numerical complexity often precludes exact treatment of the radiative transfer problem, making approximate methods necessary. Such a method, outlined by the author, makes it possible to estimate radiative losses in spectral lines in atmospheres of nonnegligible optical depth. The technique uses approximate solutions of the transfer equation to obtain source functions and photon escape probabilities as a function of position in the atmosphere. An example of the application of this method to hydrogen excitation, ionization, and radiative losses in a solar flare model is presented. The results are then compared with an exact calculation. The method reduces computing time by over four orders of magnitude. Title: Analysis of the solar magnesium I spectrum. Authors: Altrock, R. C.; Canfield, R. C. Bibcode: 1974ApJ...194..733A Altcode: Without assuming local thermodynamic equilibrium (LTE), we simultaneously solve the equations of statistical equilibrium and radiative transfer for a model Mg I atom that includes the lines 4571, 5172 (b2), and 2852 A, which we then compare with observations. From this comparison we determine by trial and error an optimum model of run with height of electron temperature, electron density, and total hydrogen density, and microturbulent velocity in the solar atmosphere up to approximately h = 1000 km. In addition, we show that the assumption of LTE for the 4571 A source function is valid to a high degree of precision. Title: Theoretical Hydrogen Spectra of Chromospheric Flares Authors: Canfield, Richard C. Bibcode: 1974BAAS....6R.285C Altcode: No abstract at ADS Title: A simplified method for computing radiative energy loss due to spectral lines. 2: Programs for solar flare models Authors: Canfield, R. C. Bibcode: 1974erp..reptQ....C Altcode: The paper lists FORTRAN-IV programs that implement a simplified method for computing radiative loss rates due to spectral lines. As it is planned to apply these techniques to solar flare models, the programs are set up to take into account Lyman alpha, Balmer-alpha and H-, the principal contributors to radiative energy loss in solar flares. Title: Theoretical Chromospheric Flare Spectra. II: Hydrogen Equilibrium for Brown's (1973) Models for Heating by Non-Thermal Electrons Authors: Canfield, Richard C. Bibcode: 1974SoPh...34..339C Altcode: We obtain simultaneous solutions of the equations of radiative transfer and statistical equilibrium for hydrogen excitation and ionization. The model atom includes Lα, Lβ, Balmer-α and the Lyman, Balmer and Paschen continua. The model atmospheres are these of Brown (1973) representating the effects of heating by non-thermal electrons on the Harvard-Smithsonian Reference Atmosphere. Title: A Non-L.T.E. Analysis of the Solar Mg I Spectrum. Authors: Altrock, R. C.; Canfield, R. C. Bibcode: 1974BAAS....6..220A Altcode: No abstract at ADS Title: Theoretical Chromospheric Flare Spectra. I: Hydrogen Equilibrium for the Kinematic Flare-Shock Models of Nakagawa et al. (1973) Authors: Canfield, Richard C.; Athay, R. Grant Bibcode: 1974SoPh...34..193C Altcode: We simultaneously solve the equations of radiative transfer and statistical equilibrium for a model hydrogen atom including Lyman-α, Lyman-β, Balmer-α and the Lyman, Balmer and Paschen continua. The model atmospheres we use are the results of Nakagawa et al. (1973) for a kinematic model of the chromospheric solar flare. Title: A non-L.T.E. analysis of the solar Mg I spectrum. Authors: Altrock, R. C.; Canfield, R. C. Bibcode: 1974BAAS....6..219A Altcode: No abstract at ADS Title: Fluctuations of Brightness and Vertical Velocity at Various Heights in the Photosphere Authors: Canfield, Richard C.; Mehltretter, J. P. Bibcode: 1973SoPh...33...33C Altcode: In a spectrogram of exceptionally high spatial resolution, brightness and velocity fluctuations in seven weak to medium-strong Fe I lines have been measured and analyzed. Heights of formation of these lines have been computed using the Harvard-Smithsonian Reference Atmosphere (Gingerich et al., 1972), taking into account departures from LTE. Title: A Scaling-law Approach to the Calculation of Radiative Losses in Optically Thick Atmospheres. Authors: Canfield, R. C. Bibcode: 1973BAAS....5..443C Altcode: No abstract at ADS Title: Vertical Phase Variation and Mechanical Flux in the Solar 5-MINUTE Oscillation Authors: Canfield, Richard C.; Musman, Steven Bibcode: 1973ApJ...184L.131C Altcode: We have made observations of the solar five-minute oscillation that allow measurement of the difference of phase between absorption lines formed over a range of almost 1000 km in the photosphere and chromosphere. We derive phase velocities, group velocities, and mechanical fluxes. The amount of mechanical flux lost by the five-minute oscillation matches the estimated chromospheric radiative loss in quantity but has a different height distribution. This difference implies that a change in the mode of energy propagation may take place below 500 km. Subject headings:chromosphere, solar - solar atmospheric motions Title: Observation and Interpretation of Phase Lags in the Five-Minute Oscillation Authors: Canfield, Richard C.; Musman, Steven Bibcode: 1973BAAS....5..269C Altcode: No abstract at ADS Title: Observations of the Variation of Temperature with Latitude in the Upper Solar Photosphere. 11. Magnetic-Field Comparison, Implications for Solar-Oblateness Measurements, and Harmonic Analysis Authors: Canfield, Richard C. Bibcode: 1973ApJ...179..643C Altcode: It is shown that there is a close relationship between the latitudinal variation of upper-photospheric temperature and that of photospheric magnetic field. This correlation, when used with 1966 magnetic-field data, implies that very little of Dicke and Goldenberg's solar-oblateness signal was due to pole-equator temperature differences at small optical depths. In the Appendix, Legendre-polynomial representations of the temperature-difference data are given. Subject headings: atmospheres, solar - magnetic fields, solar - rotation, solar Title: Observations on the Relationship between the Latitudinal Variations of Temperature and Magnetic Field Authors: Canfield, R. C. Bibcode: 1972BAAS....4..378C Altcode: No abstract at ADS Title: Observations of Photospheric Pole-Equator Temperature Differences Authors: Altrock, Richard C.; Canfield, Richard C. Bibcode: 1972SoPh...23..257A Altcode: Using photoelectric methods we have repeated Plaskett's (1970) measurements of poleequator temperature differences. We average many limb-darkening scans to reduce statistical errors. We then analyze the differences between the average polar and equatorial scans. Plaskett's large poleequator temperature differences are not confirmed. Our data yield a pole-equator temperature difference of 1.5K±0.6K, although we cannot rule out systematic errors of 3-4 K. Title: Observations of the variation of temperature with latitude in the upper solar photosphere. Authors: Altrock, R. C.; Canfield, R. C. Bibcode: 1972BAAS....4..268A Altcode: No abstract at ADS Title: Observations of the Variation of Temperature with Lattitude in the Upper Solar Photosphere Authors: Altrock, Richard C.; Canfield, Richard C. Bibcode: 1972ApJ...171L..71A Altcode: We made photoelectric meridional and equatorial limb-darkening scans during the period 15-22 June 1971, using a spectral region of 30 mA width centered 0.77 A to the red of Ca II K3. The radiation observed originates at r5000 10 . At the time of our observations the temperature relative to the equatorial temperature was enhanced by 8 K + 2.5 K at active-region latitudes and 5 K + 2 K at latitude 50 N. Title: A Measurement of the Non-Thermal Velocity in the Low Chromosphere Authors: Canfield, Richard C. Bibcode: 1971SoPh...20..275C Altcode: I have determined horizontally averaged non-thermal velocities from Jensen and Orrall's (1963) observations of Doppler widths of weak rare-earth emission lines in the wings of H and K. Combining these results with previous rare-earth line results, I conclude that this velocity in the low chromosphere (300-600 km) is 2.0 ± 0.2 km/s, and changes little with height. Title: Theory of formation of solar rare-earth lines. Authors: Canfield, R. C. Bibcode: 1971BAAS....3Q.260C Altcode: No abstract at ADS Title: Formation of Solar Rare-Earth Lines inside and outside H and K Authors: Canfield, R. C. Bibcode: 1971A&A....10...64C Altcode: The solar spectra of many lanthanide rare-earths consist entirely of weak lines, and the atomic structure of the ions of interest in the visible solar spectrum encourages interlocking effects. In this paper calculations are made that mimic the essential features of these ions in the solar atmosphere. It is shown that the mechanism of interlocking via photo-excitations and de-excitations can quantitatively explain the characteristic behavior of solar lines of rare-earth ions, both in normal continuum regions in the visible spectrum and in the wings of the H and K lines. Key words: rare-earth lines Title: Deviations from Local Thermodynamic Equilibrium in Weak Complex Spectra Authors: Canfield, R. C. Bibcode: 1971A&A....10...54C Altcode: The equations of statistical equilibrium are solved for an atom or ion whose energy level structure is complex and whose spectral lines are all weak. Semi-empirical expressions are used to calculate atomic cross-sections for idealized multi-level atoms. A linear Planck function B(T) is assumed, i.e. there is no chromospheric temperature rise in the atmospheric model. Solutions of the statistical equilibrium equations are obtained for a variety of model atoms and atmospheres. When interlocking via permitted radiative excitations and de-excitations can take place, the source function is photo-excitation controlled. As a result, LTE does not normally obtain. Calculations made for atoms of up to 24 levels plus continuum demonstrate the extent of representative deviations from LTE. Key words: interlocking - weak-line source functions Title: Deviations from Local Thermodynamic Equilibrium in Weak Complex Stellar Spectra Authors: Canfield, Richard C. Bibcode: 1970BAAS....2Q.301C Altcode: No abstract at ADS Title: On the Frequency Dependence of the Line Source Function Authors: Canfield, Richard C. Bibcode: 1970SoPh...12...63C Altcode: No abstract at ADS Title: A Self-Consistent Model Atmosphere Program with Applications to Solar OI Resonance Lines Authors: Athay, R. Grant; Canfield, R. C. Bibcode: 1970sfss.coll...65A Altcode: 1970IAUCo...2...65A No abstract at ADS Title: Reversals of Selected CE II Solar Lines Authors: Canfield, Richard C. Bibcode: 1969ApJ...157..425C Altcode: Observations of six Ce ii lines in the solar spectrum show that the lines reverse from absorption to emission on the disk, inside the limb. Furthermore, the position of this reversal varies with wavelength. Analysis implies that this behavior is due to the dominance of the scattering term in the line source function. The absorption-line profiles favor a non-thermal velocity field that is anisotropic. The emission-line profiles require a horizontal non-thermal velocity of 2 0 ± 0.2 km sec', averaged over heights 0 < h < 400 km. The equivalent widths and central intensities of the absorption lines require a cerium abundance log = 1.4 ± 0.3 (log NH 12 0), and favor the Bilderberg temperature distribution over that of Hoiweger Title: Computed Profiles for Solar Mg b and Na D Lines Authors: Athay, R. Grant; Canfield, Richard C. Bibcode: 1969BAAS....1..272A Altcode: No abstract at ADS Title: Computed Profiles for Solar MG b- and NA D-Lines Authors: Athay, R. Grant; Canfield, Richard C. Bibcode: 1969ApJ...156..695A Altcode: Profiles are computed for the Doppler cores of solar Mg b- and Na D-lines for multilevel model atoms and for selected ranges of chromospheric parameters. Comparison of computed and observed profiles from center to limb on the solar disk yields the depth variation of the horizontal and vertical components of microturbulence. The two components are found to be of unequal amplitude for r~> 1O~ and pass through minima near r~ = 1O~. The profiles also suggest that fle(Tc) at Tc = 104_106 obtained from eclipse data is more reasonable than that of the Bilderberg model Title: Computed Profiles of Mg b and Na D Lines Authors: Athay, R. G.; Canfield, R. C. Bibcode: 1968rla..conf..363A Altcode: No abstract at ADS Title: Observation and Interpretation of Rare Earth Spectra. Authors: Canfield, Richard C. Bibcode: 1968AJS....73Q..57C Altcode: We have observed spectral lines of the rare earth ion Ce II in the wavelength range 4000-4700 A. Photoelectric and photographic observations cover the disk from its center to beyond the limb. The height resolution of the observations near the limb is enhanced through Fourier transform techniques. The residual spread function has 0.7-1.4 sec of arc half-width at half-maximum, and low-amplitude wings. The observed chromospheric emission line profiles yield 2.0+0.1 km/ sec non-thermal broadening velocities in the height range 0 <h <400 km. The observed Ce II lines reverse from absorption to emission on the disk. The reversal height depends on wavelength. At shorter wavelengths emission lines extend farther onto the disk. The center-limb variation of absorption line equivalent widths is analyzed by Altrock's method (Ph.D. thesis, University of Colorado, 1967). The ratio of the line- to-continuum source functions S/B >1, and increases with decreasing wavelength. This suggests that scattering contributes to the line source function. Athay and Skumanich's method (Ann. Astrophys. 30, 669,1967) is used with recent solar models to calculate pure absorption and pure noncoherent scattering line profiles. Noncoherent scattering produces better agreement with the observed profiles. The atmospheric parameters that provide the best agreement with the observed absorption lines are then applied to the extreme limb spectra. Using these parameters, noncoherent scattering also correctly predicts both the existence of emission lines on the disk and the observed wavelength dependence of the reversal height. Title: Solar Extreme Limb Spectrum of the Rare-Earth Cerium. Authors: Canfield, Richard Charles Bibcode: 1968PhDT.........3C Altcode: No abstract at ADS