Author name code: metcalf ADS astronomy entries on 2022-09-14 author:"Metcalf, Thomas R." OR author:"Metcalf, Tomas" OR author:"Metcalf, Tom" ------------------------------------------------------------------------ Title: On Friederich's New Fine-Tuning Argument Authors: Metcalf, Thomas Bibcode: 2021FoPh...51...31M Altcode: The most common objection to the Fine-Tuning Argument for the Multiverse is that the argument commits the Inverse Gambler's Fallacy. Simon Friederich has recently composed an interesting version of this fine-tuning argument that avoids this fallacy and better-matches important scientific instances of anthropic reasoning. My thesis in this paper is that this new argument, while it may avoid the fallacy, contains a disputable premise concerning the prior probabilities of the hypotheses at issue. I consider various ways to modify the argument to avoid this problem, but I argue that plausible replacements render other lines unjustified. I also briefly compare `indexical' fine-tuning arguments such as Friederich's, according to which our universe permits life, to `existential' fine-tuning arguments, according to which some universe or other permits life. I conclude that while Friederich is correct that the new fine-tuning argument avoids the Inverse Gambler's Fallacy, the argument nevertheless depends on an unjustified premise, and this is further reason for proponents of fine-tuning arguments for the multiverse to employ existential arguments rather than indexical arguments. Title: The Imaging Vector Magnetograph at Haleakalā IV: Stokes Polarization Spectra in the Sodium D1 589.6 nm Spectral Line Authors: Leka, K. D.; Mickey, Donald L.; Uitenbroek, Han; Wagner, Eric L.; Metcalf, Thomas R. Bibcode: 2012SoPh..278..471L Altcode: The Imaging Vector Magnetograph (IVM) at the Mees Solar Observatory, Haleakalā, Maui, Hawai'i, obtained many years of vector magnetic-field data in the photospheric Fe I 630.25 nm line. In the latter period of its operation, the IVM was modified to allow routine observations in the chromospheric Na I D1 line, as well as the Fe I line. We describe the sodium observational data in detail, including the data-reduction steps that differ from those employed for the Fe I 630.25 nm line, to obtain calibrated Stokes polarization spectra. We have performed a systematic comparison between the observational data and synthetic NLTE Na I D1 Stokes spectra derived for a variety of solar-appropriate atmospheric and magnetic configurations. While the Na I D1 Stokes polarization signals from the solar atmosphere are expected to be weak, they should generally be within the IVM capability. A comparison between synthetic spectra and observational data indicates that this is indeed the case. Title: Observing and Interpreting Na D1 589.6nm Stokes Spectra with the Imaging Vector Magnetograph II: The Magnetic Maps Authors: Derouich, M.; Leka, K. D.; Mickey, D. L.; Uitenbroek, H.; Metcalf, T. R. Bibcode: 2010shin.confE...5D Altcode: Following Poster I (Leka et al), we focus here on recent progress regarding the inversion algorithms and interpretation of Zeeman Na D1 589.6nm Stokes Spectra observed using the Imaging Vector Magnetograph. We present systematic tests of the inversion procedures and our interpretation of the results, relying on synthetic Na D1 589.6nm Stokes Spectra generated using known magnetic and atmospheric models described in Poster I. In this second poster, we present the results of "hare and hound" exercises which focus on (1) the optimal fitting and inversion algorithm for the Na D1 589.6nm Stokes spectra, and (2), the interpretation of the resulting active magnetic field "maps", especially as a function of inferred height of the solar atmosphere sampled by these Zeeman-polarization spectra.

This work has been supported by AFOSR contract F49620-03-C-0019, NSF space weather program grant ATM-0519107 and NSF SHINE grant ATM-0454610. Title: Observing and Interpreting Na D1 589.6nm Stokes Spectra with the Imaging Vector Magnetograph I: Polarization Spectra Authors: Leka, K. D.; Mickey, D. L.; Uitenbroek, H.; Derouich, M.; Metcalf, T. R. Bibcode: 2010shin.confE...4L Altcode: We report on progress made recently on validating and interpreting Stokes spectra from the Na D1 589.6nm line observed using the Imaging Vector Magnetograph at Mees Solar Observatory, Haleakala, Maui. While preliminary results from the dataset (which comprises daily observations of active regions spanning 2000 - 2005, plus a few additional special observation campaigns) have been reported previously (e.g., Leka & Metcalf 2003; Metcalf Leka & Mickey 2005), we focus here on systematic tests of the observed data and our interpretation of them. In this first poster, we present Non-LTE synthetic Na D1 589.6nm spectra, computed using known underlying magnetic and atmospheric models, which form the basis for various "hare and hound" exercises to test (1) the observed degree of polarization in (2) the context of the instrument response and photon noise. While we find generally excellent agreement, there are some systematic differences between the synthetic and observational data. We summarize our understandings of these differences and attempts to mitigate their effects in the context of the inversion to a magnetic field map (see Poster II, Derouich et al).

This work has been supported by AFOSR contract F49620-03-C-0019, NSF space weather program grant ATM-0519107 and NSF SHINE grant ATM-0454610. Title: Resolving the 180° Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions Authors: Leka, K. D.; Barnes, Graham; Crouch, A. D.; Metcalf, Thomas R.; Gary, G. Allen; Jing, Ju; Liu, Y. Bibcode: 2009SoPh..260...83L Altcode: The objective testing of algorithms for performing ambiguity resolution in vector magnetic field data is continued, with an examination of the effects of noise in the data. Through the use of analytic magnetic field models, two types of noise are "added" prior to resolving: noise to simulate Poisson photon noise in the observed polarization spectra, and a spatial binning to simulate the effects of unresolved structure. The results are compared through the use of quantitative metrics and performance maps. We find that while no algorithm severely propagates the effects of Poisson noise beyond very local influences, some algorithms are more robust against high photon-noise levels than others. In the case of limited spatial resolution, loss of information regarding fine-scale structure can easily result in erroneous solutions. Our tests imply that photon noise and limited spatial resolution can act so as to make assumptions used in some ambiguity resolution algorithms no longer consistent with the observed magnetogram. We confirm a finding of the earlier comparison study that results can be very sensitive to the details of the treatment of the observed boundary and the assumptions governing that treatment. We discuss the implications of these findings, given the relative sensitivities of the algorithms to the two sources of noise tested here. We also touch on further implications for interpreting observational vector magnetic field data for general solar physics research. Title: Forward Modeling of Active Region Coronal Emissions. II. Implications for Coronal Heating Authors: Lundquist, L. L.; Fisher, G. H.; Metcalf, T. R.; Leka, K. D.; McTiernan, J. M. Bibcode: 2008ApJ...689.1388L Altcode: In Paper I, we introduced and tested a method for predicting solar active region coronal emissions using magnetic field measurements and a chosen heating relationship. Here, we apply this forward-modeling technique to 10 active regions observed with the Mees Solar Observatory Imaging Vector Magnetograph and the Yohkoh Soft X-ray Telescope. We produce synthetic images of each region using four parameterized heating relationships depending on magnetic field strength and geometry. We find a volumetric coronal heating rate (dEH/dV, not to be confused with dEH/dA quoted by some authors) proportional to magnetic field and inversely proportional to field-line loop length (BL-1) best matches observed coronal emission morphologies. This parameterization is most similar to the steady-state scaling of two proposed heating mechanisms: van Ballegooijen's "current layers" theory, taken in the AC limit, and Parker's "critical angle" mechanism, in the case where the angle of misalignment is a twist angle. Although this parameterization best matches the observations, it does not match well enough to make a definitive statement as to the nature of coronal heating. Instead, we conclude that (1) the technique requires better magnetic field measurement and extrapolation techniques than currently available, and (2) forward-modeling methods that incorporate properties of transiently heated loops are necessary to make a more conclusive statement about coronal heating mechanisms. Title: Preprocessing of Hinode/SOT Vector Magnetograms for Nonlinear Force-Free Coronal Magnetic Field Modeling Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa, M. L.; Metcalf, T. R. Bibcode: 2008ASPC..397..198W Altcode: 2008arXiv0801.2884W The solar magnetic field is key to understanding the physical processes in the solar atmosphere. Nonlinear force-free codes have been shown to be useful in extrapolating the coronal field from underlying vector boundary data (for an overview see Schrijver et al. (2006)). However, we can only measure the magnetic field vector routinely with high accuracy in the photosphere with, e.g., Hinode/SOT, and unfortunately these data do not fulfill the force-free consistency condition as defined by Aly (1989). We must therefore apply some transformations to these data before nonlinear force-free extrapolation codes can be legitimately applied. To this end, we have developed a minimization procedure that uses the measured photospheric field vectors as input to approximate a more chromospheric like field (The method was dubbed preprocessing. See Wiegelmann et al. (2006) for details). The procedure includes force-free consistency integrals and spatial smoothing. The method has been intensively tested with model active regions (see Metcalf et al. 2008) and been applied to several ground based vector magnetogram data before. Here we apply the preprocessing program to photospheric magnetic field measurements with the Hinode/SOT instrument. Title: Non-Linear Force-Free Field Modeling of a Solar Active Region Around the Time of a Major Flare and Coronal Mass Ejection Authors: De Rosa, M. L.; Schrijver, C. J.; Metcalf, T. R.; Barnes, G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.; Wheatland, M.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann, M.; Kusano, K.; Régnier, S.; Thalmann, J. Bibcode: 2008AGUSMSP31A..06D Altcode: Solar flares and coronal mass ejections are associated with rapid changes in coronal magnetic field connectivity and are powered by the partial dissipation of electrical currents that run through the solar corona. A critical unanswered question is whether the currents involved are induced by the advection along the photosphere of pre-existing atmospheric magnetic flux, or whether these currents are associated with newly emergent flux. We address this problem by applying nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA Active Region 10930 around the time of a powerful X3.4 flare in December 2006. We compute 14 NLFFF models using 4 different codes having a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We do find agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, (3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the ~1032~erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection. We discuss the relative merits of these models in a general critique of our present abilities to model the coronal magnetic field based on surface vector field measurements. Title: Nonlinear Force-free Field Modeling of a Solar Active Region around the Time of a Major Flare and Coronal Mass Ejection Authors: Schrijver, C. J.; DeRosa, M. L.; Metcalf, T.; Barnes, G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.; Wheatland, M. S.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann, M.; Kusano, K.; Régnier, S.; Thalmann, J. K. Bibcode: 2008ApJ...675.1637S Altcode: 2007arXiv0712.0023S Solar flares and coronal mass ejections are associated with rapid changes in field connectivity and are powered by the partial dissipation of electrical currents in the solar atmosphere. A critical unanswered question is whether the currents involved are induced by the motion of preexisting atmospheric magnetic flux subject to surface plasma flows or whether these currents are associated with the emergence of flux from within the solar convective zone. We address this problem by applying state-of-the-art nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA AR 10930 around the time of a powerful X3.4 flare. We compute 14 NLFFF models with four different codes and a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We discuss the relative merits of these models in a general critique of present abilities to model the coronal magnetic field based on surface vector field measurements. For our application in particular, we find a fair agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, (3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the ~1032 erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection. Title: Can We Improve the Preprocessing of Photospheric Vector Magnetograms by the Inclusion of Chromospheric Observations? Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa, M. L.; Metcalf, T. R. Bibcode: 2008SoPh..247..249W Altcode: 2008arXiv0801.2707W; 2008SoPh..tmp...27W The solar magnetic field is key to understanding the physical processes in the solar atmosphere. Nonlinear force-free codes have been shown to be useful in extrapolating the coronal field upward from underlying vector boundary data. However, we can only measure the magnetic field vector routinely with high accuracy in the photosphere, and unfortunately these data do not fulfill the force-free condition. We must therefore apply some transformations to these data before nonlinear force-free extrapolation codes can be self-consistently applied. To this end, we have developed a minimization procedure that yields a more chromosphere-like field, using the measured photospheric field vectors as input. The procedure includes force-free consistency integrals, spatial smoothing, and - newly included in the version presented here - an improved match to the field direction as inferred from fibrils as can be observed in, for example, chromospheric Hα images. We test the procedure using a model active-region field that included buoyancy forces at the photospheric level. The proposed preprocessing method allows us to approximate the chromospheric vector field to within a few degrees and the free energy in the coronal field to within one percent. Title: Nonlinear Force-Free Modeling of Coronal Magnetic Fields. II. Modeling a Filament Arcade and Simulated Chromospheric and Photospheric Vector Fields Authors: Metcalf, Thomas R.; De Rosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham; van Ballegooijen, Adriaan A.; Wiegelmann, Thomas; Wheatland, Michael S.; Valori, Gherardo; McTtiernan, James M. Bibcode: 2008SoPh..247..269M Altcode: 2008SoPh..tmp...17M We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional, and Grad - Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied to both forced "photospheric" and more force-free "chromospheric" vector magnetic field boundary data derived from the model. When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field's free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though, like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various codes will not necessarily converge to the correct, or even the same, solution. Title: Can we Improve the Preprocessing of Photospheric Vectormagnetograms by the Inclusion of Chromospheric Observations? Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa, M. L.; Metcalf, T. R. Bibcode: 2007AGUFMSH51C..02W Altcode: The solar magnetic field is key to understanding the physical processes in the solar atmosphere. Unfortunately, we can measure the magnetic field vector routinely with high accuracy only in the photosphere with, e.g., Hinode/SOT and in future with SDO/HMI. These measurements are extrapolated into the corona under the assumption that the field is force-free. That condition is not fulfilled in the photosphere, but is in the chromosphere and corona. In order to make the observed boundary data consistent with the force-free assumption, we therefore have to apply some transformations before nonlinear force-free extrapolation codes can be legitimately applied. We develop a minimization procedure that uses the measured photospheric field vectors as input to approximate a more chromospheric-like field. The procedure includes force-free consistency integrals, spatial smoothing, and - newly included in the version presented here - an improved match to the field direction as inferred from fibrils as can be observed in, e.g., chromospheric H-alpha images. We test the procedure using a model active-region field that included buoyancy forces at the photospheric level. We apply the combined preprocessing and nonlinear force-free extrapolation method to compute the coronal magnetic field in an active region measured with the Hinode/SOT instrument. Title: Energy Deposition in White Light Flares with TRACE and RHESSI Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2007ASPC..368..423F Altcode: In Fletcher et al. (2007) we investigated the white light (WL) continuum during solar flares and its relationship to energy deposition by electron beams. In 9 flare events, spanning GOES classifications from C4.8 to M9.1, we have high cadence TRACE WL and RHESSI hard X-ray observations, and compare the WL radiative power output with that provided by flare electrons. Under the thick--target model assumptions, we find that the electron beam must extend down to 15--20 keV, and the energy input to the chromosphere should occur within the collisional stopping depth of these electrons - approximately 2× 10-4 g cm-2. In this short paper, we discuss some ideas on flare WL emission, summarise the results of the Fletcher et al. (2007) study and discuss their implications for chromospheric heating and white light flare emission. Title: Evidence For Reconnection In A Flare Observed With The X-ray Telescope On Hinode Authors: Reeves, Kathy; Metcalf, T.; Ishibashi, B.; Weber, M. Bibcode: 2007AAS...210.3703R Altcode: 2007BAAS...39..151R A long duration flare observed at the limb on December 17th with the X-Ray Telescope (XRT) on Hinode exhibits many features characteristic of reconnection. Cusp-shaped loops are clearly resolved by XRT, and these loops show convincing evidence of field line shrinkage. In addition, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) detects a hard X-ray loop top source that extends into the cusp structure visible in the XRT images. The implications of these observations for reconnection theories of solar flares will be discussed.

The US XRT team is supported by a contract from NASA to SAO. Hinode is an international project supported by JAXA, NASA, PPARC and ESA. We are grateful to the Hinode team for all their efforts in the design, development and operation of the mission. Title: Non-linear Force-free Modeling Of Coronal Magnetic Fields Authors: Metcalf, Thomas R.; De Rosa, M. L.; Schrijver, C. J.; Barnes, G.; van Ballegooijen, A.; Wiegelmann, T.; Wheatland, M. S.; Valori, G.; McTiernan, J. M. Bibcode: 2007AAS...210.9102M Altcode: 2007BAAS...39..204M We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional, and Grad-Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied to both forced "photospheric'' and more force-free "chromospheric'' vector magnetic field boundary data derived from the model. When applied to the

chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field's free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric boundary data, the reference model field is not well recovered, indicating that the Lorentz forces on the photosphere severely impact the extrapolation of the field. Preprocessing of the photospheric boundary does improve the extrapolations considerably, although the results depend sensitively on the details of the numerical codes. When applying the NLFFF codes to solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various codes will not necessarily converge to the correct, or even the same, solution. Title: Is Coronal X-ray Emission Energized By Electric Currents? Authors: Ishibashi, Kazunori; Metcalf, T.; Lites, B. Bibcode: 2007AAS...210.9421I Altcode: 2007BAAS...39..220I We examine the spatial correlation between coronal X-ray emission observed with the Hinode X-Ray Telescope and electric currents observed with the Hinode Solar Optical Telescope Spectro-polarimeter. We determine to what extent the X-ray brightness is correlated with electric current density and hence to what extent the hot corona is energized by electric currents which flow through the photosphere. We will also consider whether the currents reach the corona to heat the coronal plasma or whether they predominantly close below the corona.

Hinode is an international project supported by JAXA, NASA, PPARC and ESA. We are grateful to the Hinode team for all their efforts in the design, development and operation of the mission. Title: A TRACE White Light and RHESSI Hard X-Ray Study of Flare Energetics Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2007ApJ...656.1187F Altcode: In this paper we investigate the formation of the white-light (WL) continuum during solar flares and its relationship to energy deposition by electron beams inferred from hard X-ray emission. We analyze nine flares spanning GOES classifications from C4.8 to M9.1, seven of which show clear cospatial RHESSI hard X-ray and TRACE WL footpoints. We characterize the TRACE WL/UV continuum energy under two simplifying assumptions: (1) a blackbody function, or (2) a Paschen-Balmer continuum model. These set limits on the energy in the continuum, which we compare with that provided by flare electrons under the usual collisional thick-target assumptions. We find that the power required by the white-light luminosity enhancement is comparable to the electron beam power required to produce the HXR emission only if the low-energy cutoff to the spectrum is less than 25 keV. The bulk of the energy required to power the white-light flare (WLF) therefore resides at these low energies. Since such low-energy electrons cannot penetrate deep into a collisional thick target, this implies that the continuum enhancement is due to processes occurring at moderate depths in the chromosphere. Title: An Overview of Existing Algorithms for Resolving the 180° Ambiguity in Vector Magnetic Fields: Quantitative Tests with Synthetic Data Authors: Metcalf, Thomas R.; Leka, K. D.; Barnes, Graham; Lites, Bruce W.; Georgoulis, Manolis K.; Pevtsov, A. A.; Balasubramaniam, K. S.; Gary, G. Allen; Jing, Ju; Li, Jing; Liu, Y.; Wang, H. N.; Abramenko, Valentyna; Yurchyshyn, Vasyl; Moon, Y. -J. Bibcode: 2006SoPh..237..267M Altcode: 2006SoPh..tmp...14M We report here on the present state-of-the-art in algorithms used for resolving the 180° ambiguity in solar vector magnetic field measurements. With present observations and techniques, some assumption must be made about the solar magnetic field in order to resolve this ambiguity. Our focus is the application of numerous existing algorithms to test data for which the correct answer is known. In this context, we compare the algorithms quantitatively and seek to understand where each succeeds, where it fails, and why. We have considered five basic approaches: comparing the observed field to a reference field or direction, minimizing the vertical gradient of the magnetic pressure, minimizing the vertical current density, minimizing some approximation to the total current density, and minimizing some approximation to the field's divergence. Of the automated methods requiring no human intervention, those which minimize the square of the vertical current density in conjunction with an approximation for the vanishing divergence of the magnetic field show the most promise. Title: Observations of The Chromospheric Magnetic Field In Solar Active Regions Authors: Leka, K. D.; Metcalf, T. R.; Mickey, D. L.; Barnes, G. Bibcode: 2006IAUJD...3E..53L Altcode: Measuring the magnetic field in solar active regions in all spatial and temporal dimensions is a long-standing and ambitious goal in solar physics. As the locations of complex and rapidly evolving magnetic fields and the source of geo-effective energetic events, understanding active region magnetic field generation and evolution is extremely an important goal; however, basic physics presents great challenges to achieving it. Measuring the chromospheric magnetic field in active regions is an important first step beyond routine photospheric measurements; important both for basic understanding of active region structure but also for the many ramifications coming from chromosphere being closer to a force-free state than the photosphere. However, it is also a very difficult measurement. In this talk I will describe highlights of our group's on-going efforts to understand solar active region magnetic field structure via direct observation of the vector chromospheric magnetic field. Since late 2003, the U. Hawai`i/Mees Solar Observatory's Imaging Vector Magnetograph has routinely acquired spectropolarimetry measurements of active regions across the Na-I 589.6nm line; from the polarization at the line's near-wings approximately 0.007nm from line center we deduce the vector magnetic field. The data are specific to active regions, specifically the structure, free energy storage and evolution at that low-chromospheric layer. I will present recent results from these chromospheric data with a focus on the differences between the photosphere and chromosphere, and the free energy storage in solar active regions. Title: Non-linear Force-free Modeling: Applications To Solar Data Authors: De Rosa, Marc L.; Schrijver, C. J.; Metcalf, T. R.; NLFFF Team Bibcode: 2006SPD....37.1805D Altcode: 2006BAAS...38..247D Understanding the conditions under which solar magnetic fields can destabilizeto cause flares and other eruptive events requires a quantitativeunderstanding of the coronal magnetic field and of the currents that itcarries. Because no direct measurements of magnetic fields and current withincoronal volumes exist, the coronal field is typically modeled usinginformation contained in photospheric vector magnetograms, to be compared toH-alpha images of the chromosphere and EUV and X-ray imagery of the corona.We report on recent results of a team effort to further understand theintricacies of non-linear force-free extrapolations of the coronal magneticfield, presenting results from several solar and solar-like test cases. Wealso consider the use of such coronal field modeling in the upcoming Solar-Band SDO missions. Title: Measuring the Magnetic Free Energy Available for Solar Flares Authors: Metcalf, Thomas R.; Leka, K. D.; Mickey, D. L.; Barnes, G. Bibcode: 2006SPD....37.0903M Altcode: 2006BAAS...38..236M In this poster we report on recent progress in the effort to measurethe magnetic energy available to power solar flares. To directlymeasure the free magnetic energy using the virial theorem, themagnetic field must be known at an atmospheric height where it isforce-free, i.e. J x B = 0. In Metcalf, Leka & Mickey (2005) the freeenergy of AR 10486 was determined just prior to the X10 flare at20:39UT on 29 October 2003, using vector magnetic field measurementsobtained in the solar chromosphere where the field is force-free. Theresults from this study are expanded here to a wider investigation ofthe magnetic energy storage in flare- and CME-producing activeregions. With appropriate effort and instrumentation, directlymeasuring the free energy and its evolution may provide a powerfulflare-prediction capability. This research was funded by NASAcontract NAG5-12466 and AFOSR contract F49620-03-C-0019. Title: Nonlinear Force-Free Modeling of Coronal Magnetic Fields Part I: A Quantitative Comparison of Methods Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Metcalf, Thomas R.; Liu, Yang; McTiernan, Jim; Régnier, Stéphane; Valori, Gherardo; Wheatland, Michael S.; Wiegelmann, Thomas Bibcode: 2006SoPh..235..161S Altcode: We compare six algorithms for the computation of nonlinear force-free (NLFF) magnetic fields (including optimization, magnetofrictional, Grad-Rubin based, and Green's function-based methods) by evaluating their performance in blind tests on analytical force-free-field models for which boundary conditions are specified either for the entire surface area of a cubic volume or for an extended lower boundary only. Figures of merit are used to compare the input vector field to the resulting model fields. Based on these merit functions, we argue that all algorithms yield NLFF fields that agree best with the input field in the lower central region of the volume, where the field and electrical currents are strongest and the effects of boundary conditions weakest. The NLFF vector fields in the outer domains of the volume depend sensitively on the details of the specified boundary conditions; best agreement is found if the field outside of the model volume is incorporated as part of the model boundary, either as potential field boundaries on the side and top surfaces, or as a potential field in a skirt around the main volume of interest. For input field (B) and modeled field (b), the best method included in our study yields an average relative vector error En = « |B−b|»/« |B|» of only 0.02 when all sides are specified and 0.14 for the case where only the lower boundary is specified, while the total energy in the magnetic field is approximated to within 2%. The models converge towards the central, strong input field at speeds that differ by a factor of one million per iteration step. The fastest-converging, best-performing model for these analytical test cases is the Wheatland, Sturrock, and Roumeliotis (2000) optimization algorithm as implemented by Wiegelmann (2004). Title: White-Light Flares: A TRACE/RHESSI Overview Authors: Hudson, H. S.; Wolfson, C. J.; Metcalf, T. R. Bibcode: 2006SoPh..234...79H Altcode: The Transition Region and Coronal Explorer (TRACE) instrument includes a "white light" imaging capability with novel characteristics. Many flares with such white-light emission have been detected, and this paper provides an introductory overview of these data. These observations have 0.5″ pixel size and use the full broad-band response of the CCD sensor; the images are not compromised by ground-based seeing and have excellent pointing stability as well as high time resolution. The spectral response of the TRACE white-light passband extends into the UV, so these data capture, for the first time in images, the main radiative energy of a flare. This initial survey is based on a sample of flares observed at high time resolution for which the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) had complete data coverage, a total of 11 events up to the end of 2004. We characterize these events in terms of source morphology and contrast against the photosphere. We confirm the strong association of the TRACE white-light emissions - which include UV as well as visual wavelengths - with hard X-ray sources observed by RHESSI. The images show fine structure at the TRACE resolution limit, and often show this fine structure to be extended over large areas rather than just in simple footpoint sources. The white-light emission shows strong intermittency both in space and in time and commonly contains features unresolved at the TRACE resolution. We detect white-light continuum emission in flares as weak as GOES C1.6. limited by photon statistics and background solar fluctuations, and support the conclusion of Neidig (1989) that white-light continuum occurs in essentially all flares. Title: An Improved Virial Estimate of Solar Active Region Energy Authors: Wheatland, M. S.; Metcalf, Thomas R. Bibcode: 2006ApJ...636.1151W Altcode: 2005astro.ph..9652W The MHD virial theorem may be used to estimate the magnetic energy of active regions on the basis of vector magnetic fields measured at the photosphere or chromosphere. However, the virial estimate depends on the measured vector magnetic field being force-free. Departure from the force-free condition leads to an unknown systematic error in the virial energy estimate and an origin dependence of the result. We present a method for estimating the systematic error by assuming that magnetic forces are confined to a thin layer near the photosphere. If vector magnetic field measurements are available at two levels in the low atmosphere (e.g., the photosphere and the chromosphere), the systematic error may be directly calculated using the observed horizontal and vertical field gradients, resulting in an energy estimate that is independent of the choice of origin. If (as is generally the case) measurements are available at only one level, the systematic error may be approximated using the observed horizontal field gradients together with a simple linear force-free model for the vertical field gradients. The resulting ``improved'' virial energy estimate is independent of the choice of origin but depends on the choice of the model for the vertical field gradients, i.e., the value of the linear force-free parameter α. This procedure is demonstrated for five vector magnetograms, including a chromospheric magnetogram. Title: High Spectral Resolution Cryogenic Imaging Detectors for Solar Physics Authors: Stern, R.; Martinez-Galarce, D.; Rausch, A.; Shing, L.; Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S.; Brink, P.; Irwin, K.; Ullom, J.; Alexander, D. Bibcode: 2005AGUFMSH41B1130S Altcode: X-ray microcalorimeters have the potential to substantially improve our understanding of magnetic reconnection in the early phases of flares or during microflare events. Operating in the sub-Kelvin temperature range, they provide high non-dispersive energy resolution at ~ keV energies (e.g, E/Δ E ~ 1500 at 6 keV), coupled with high (msec or better) time resolution. Coupled with grazing-incidence X-ray optics, microcalorimeter arrays or position sensitive readout schemes will form the basis for a new generation of solar imaging spectrometers. An Explorer-class solar mission within the next 5-10 years, based upon these detectors, is rapidly becoming technically feasible. LMSAL currently has both internally and NASA funded laboratory research programs to investigate TES (Transition Edge Sensor) microcalorimeters. In this presentation, we will discuss the current status of these programs and their applicability to future Explorer missions and Roadmap missions such as RAM. Title: TRACE white light and RHESSI hard X-rays Authors: Fletcher, L.; Allred, J.; Hannah, I.; Hudson, H.; Metcalf, T. Bibcode: 2005AGUFMSH13A0286F Altcode: We study the energetics of RHESSI and TRACE observations for a sample of 11 solar flares well observed in the TRACE ``white light" channel. In general, the data show excellent correlations between hard X-rays and white light. We discuss the energetics of this relationship based upon simple models for the visible/UV continuum, namely (i) a non-thermal approximation (Balmer and Paschen continuum), (ii) a thermal approximation (blackbody), and (iii) model spectra derived from radiation-hydrodynamic modeling. We relate the white-light energy and the low-energy cutoff energy of the primary electron spectrum required for energetic equivalence. This comparison will be made in the context of the flare WL morphology and in comparison with RHESSI hard X-ray images. Title: The Nonpotentiality of Active-Region Coronae and the Dynamics of the Photospheric Magnetic Field Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Title, Alan M.; Metcalf, Thomas R. Bibcode: 2005ApJ...628..501S Altcode: The magnetic field in the solar photosphere frequently carries strong electric currents, even though the global coronal configuration often resembles a potential field ringed by the heliospheric current sheet. To understand this, we compare TRACE EUV images of active-region coronae and potential-field source-surface extrapolations based on SOHO MDI magnetograms for 95 active regions. We conclude that significant nonpotentiality of the overall active-region coronal field occurs (1) when new flux has emerged within or very near a region within the last ~30 hr, resulting in complex polarity separation lines, or (2) when rapidly evolving, opposite-polarity concentrations are in contact at 4" resolution. If these criteria are met by more than 15% of the region's flux, they correctly identify the (non) potentiality of active-region coronae in 88% of the cases. Flares are found to occur 2.4 times more frequently in active regions with nonpotential coronae than in near-potential regions, while their average X-ray peak flare brightness is 3.3 times higher. We suggest that the currents associated with coronal nonpotentiality have a characteristic growth and decay timescale of ~10-30 hr. We find that shear flows drive enhanced flaring or coronal nonpotentiality only if associated with complex and dynamic flux emergence within the above timescale. We discuss the implications of this finding for the modeling of the coronal-heliospheric coupling. Title: Cryogenic 3-D Detectors for Solar Physics Authors: Stern, R. A.; Martinez-Galarce, D.; Rausch, A.; Shing, L.; Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S. W.; Brink, P.; Irwin, K.; Alexander, D. Bibcode: 2005AGUSMSP12A..02S Altcode: Cryogenic microcalorimeters operating in the sub-Kelvin temperature range provide non-dispersive energy resolution at optical through gamma ray energies (e.g, E/Δ E ~ 1500 at 6 keV). Microcalorimeters also have high time resolution (msec or better), and can be made into imaging arrays through SQUID multiplexing of individual pixels or employing position sensitive detector structures. The application of such "3-D" detector technology to solar physics will lead to significant advances in our understanding of magnetic reconnection in the Sun, including X-ray jet phenomena, and active region heating and dynamics. An Explorer-class solar mission within the next 5-10 years, based upon these detectors, is rapidly becoming technically feasible. LMSAL currently has an internally funded laboratory research program to investigate TES (Transition Edge Sensor) microcalorimeters; we recently saw our first X-ray photons using TES detectors supplied by NIST. In addition, we have recently been funded by NASA to begin work with NIST on position-sensitive X-ray strip detectors for solar physics applications. Finally, we are collaborating with with Stanford and NIST on a solar sounding rocket. In this presentation, we will discuss the current status of these programs and their applicability to future Explorer missions and Roadmap missions such as RAM. Title: Non-linear force-free field modeling: model techniques, boundary conditions, hares, and hounds Authors: Schrijver, C. J.; De Rosa, M. L.; Metcalf, T. Bibcode: 2005AGUSMSH31A..05S Altcode: Understanding the conditions under which solar magnetic fields can destabilize to erupt in flares and coronal mass ejections requires a quantitative understanding of the coronal magnetic field and of the currents that it carries. The increased availability of vector magnetograms, together with EUV and X-ray coronal images, should provide adequate constraints to model the coronal field, and thus to visualize its 3D geometry and to measure the available free energy and helicity. Non-linear force-free fields (NLFFF) are likely a useful model to use when extrapolating the solar surface field upward into the coronal volume. It may even be possible to use the observed trajectories of coronal loops, evident in EUV images of the corona, as a further constraint. We present initial results of a team effort to understand the intricacies of NLFFF modeling: we discuss and evaluate comparisons of NLFFF models computed with different models and applications of boundary conditions, and look ahead to full coronal field modeling for the upcoming Solar-B and SDO missions. Title: Microcalorimeter X-ray Detectors for Solar Physics Authors: Deiker, S.; Boerner, P.; Martinez-Galarce, D.; Metcalf, T.; Rausch, A.; Shing, L.; Stern, R.; Irwin, K.; William, D.; Reintsema, C.; Ullom, J.; Cabrera, B.; Lehman, S.; Brink, P. Bibcode: 2005AGUSMSP12A..03D Altcode: Cryogenic X-ray microcalorimeters provide high spectral resolution over a large bandwidth. They have achieved < 3 eV resolution at 5.9 keV, and can produce this performance simultaneously from 0.25 to 10 keV. Although they operate at low (< 0.1 K) temperatures, such temperature are now easily produced. Microcalorimeters cooled by adiabatic demagnetization refrigerators have already flown on sounding rocket flights to study the soft X-ray background of the interstellar medium, and will soon be launched on the ASTRO-E II satellite. Microcalorimeters based on superconducting transition edge sensors are multiplexable and may be fabricated using standard photolithographic techniques. This makes large arrays of microcalorimeters feasible. Each pixel of such an array detects the arrival time of each photon to within < 0.01 ms. Such an instrument would offer simultaneous spatial, temporal and energy resolution, bringing a wealth of new information about solar processes. Current design and performance of microcalorimeters will be presented. Future improvements required to optimize microcalorimeters for solar physics applications will also be discussed. Title: Tracing Coronal Waves Back to the Photosphere Authors: De Pontieu, B.; Erdelyi, R.; De Moortel, I.; Metcalf, T. Bibcode: 2005AGUSMSH11C..03D Altcode: There are now many observations of waves with periods around 5 minutes in the outer atmosphere of the Sun. We provide an observational overview of 5 minute periodicity in chromospheric spicules in active region plage, upper transition region moss and the low legs of coronal loops. Using a numerical model, we show that all of these phenomena are connected: normally evanescent photospheric oscillations can propagate into the low atmosphere as long as they are guided along magnetic field lines that are inclined away from the vertical. The leaked photospheric oscillations develop into shocks and lead to periodic upward chromospheric flows, which we have identified as active region spicules. These shocks continue upwards and enter into the corona. We suggest that TRACE observations of propagating acoustic waves in the corona are shocked and tunneled photospheric oscillations. Using SOHO/MDI, TRACE and Imaging Vector Magnetograph (Hawaii) data we explore how these coronal waves can be exploited to determine the connectivity between photosphere and corona,and thus allow seismology of the lower solar atmosphere. Title: TRACE and RHESSI observations of white-light flares Authors: Hudson, H. H.; Metcalf, T.; Wolfson, J.; Fletcher, L.; Khan, J. Bibcode: 2005AGUSMSP51C..03H Altcode: TRACE gives a photometrically stable, high-resolution view of the visible and UV emissions of solar flares, with enough diagnostic power to distinguish photospheric from chromospheric contributions. These emissions dominate the radiant energy of a flare and correspond well with hard X-ray emission in the impulsive phase. We survey these data with reference to well-observed events of 4~October~2002 and 4~November~2003, on the disk (M4; S19W09) and on the limb (X17; S19W83) respectively. We analyze the spatial and temporal properties of the footpoint sources in these two events, characterizing their physical parameters (contrast, image gradients, apparent motions). The brightest kernels of the 4~October~2002 event have intensities twice that of the quiet photosphere, and have sizes close to the Nyquist limit of the TRACE pixels (0.5~arc~sec). The white-light footpoint sources can be considered as a proxy for the target region of fast electrons studied via hard X-ray bremsstrahlung. We compare RHESSI hard X-ray footpoint sources with their white-light counterparts and estimate filling factors for the hard X-ray emission itself. Title: Predicting Coronal Emissions with Multiple Heating Rates Authors: Lundquist, L. L.; Fisher, G. H.; Leka, K. D.; Metcalf, T. R.; McTiernan, J. M. Bibcode: 2005AGUSMSP14A..02L Altcode: A variety of proposed coronal heating mechanisms remain prominent in the literature, with insufficient observational constraints to distinguish between them. In an attempt to add further constraints, we create predicted coronal emission maps of several active regions using simple parametrizations of differing theoretical heating mechanisms and compare the results to the observed coronal emissions. The results are interpolated to a 3-d grid, convolved with instrument response function, and integrated over line of sight to simulate satellite observation of the modeled loops. We also discuss those factors which dominate the differences in observed and predicted coronal emission. Title: Another Piece of the Elephant: Chromospheric Vector Field Observations Authors: Leka, K. D.; Metcalf, T. R.; Mickey, D. L. Bibcode: 2005AGUSMSH13C..10L Altcode: As with most solar observational questions, investigating the structure and role of the chromosphere is one of remote sensing: many investigations describing their "piece of the elephant". The goal is, of course, to form a coherent picture of the state of the magnetized plasma which resides there (or passes through). In this presentation, recent efforts to understand the chromospheric magnetic field structure via direct observation, i.e. chromospheric vector magnetograms, will be presented. Since late 2003, the U. Hawai`i/Mees Solar Observatory's Imaging Vector Magnetograph has routinely acquired spectropolarimetry measurements of active regions across the Na-I 589.6nm line; from the polarization at the line's near-wings approximately 0.007nm from line center we deduce the vector magnetic field. The data are specific to active regions, with the focus being the structure, free energy storage and evolution at that low-chromospheric layer. I will present salient aspects of the observed chromospheric magnetic field structure, to compare and contrast with the picture formed by the other methods in this session. Title: TRACE and SOHO/MDI Observations of Five X-Flares from AR10720, the "Fireworks Active Region", on January 15-20, 2005 Authors: Nightingale, R. W.; Frank, Z. A.; Metcalf, T. R.; Kang, C. A. Bibcode: 2005AGUSMSP21A..10N Altcode: Both TRACE and SOHO/MDI observed the five X-flares that were produced by AR10720 during January 15-20, 2005. This active region appears to be "special" because it is occurring so late in the solar cycle, about two-thirds of the way toward solar minimum. Also its structure is initially one large penumbral region with several umbral spots that are being stretched out east to west with rotating sunspots emerging in leading and trailing positions during the transit across the solar disk, similar to fireworks with pin-wheels attached. With a neutral line running almost east-west in between two of the stretched umbral regions and with opposite magnetic polarities on both sides of the neutral line adjacent to each other, the beta-delta region starts the "fireworks" with the first X-flare on January 15, continuing every one to one and one-half days over the next five days. Images and movies will be shown of these eruptive events in white light and 1600A from TRACE, and as magnetograms from MDI. Two of the three rotating sunspots also change their direction of rotation during this period. These X-flares add to those observed by TRACE since its launch in April 1998, all of which have been associated with rotating sunspots in the associated active regions. These X-flares consist of over half of the X-flares observed by GOES since April 1998 and many are associated with geo-effective CMEs in the form of solar energetic particle events. Rotating sunspots may be a good predictor of solar eruptions. This work has been supported by NASA, in part under the TRACE project at LMSAL (contract NAS5-38099) and in part under the MDI/SOHO project (NAG5-13261). Title: Magnetic Free Energy in NOAA Active Region 10486 on 2003 October 29 Authors: Metcalf, Thomas R.; Leka, K. D.; Mickey, D. L. Bibcode: 2005ApJ...623L..53M Altcode: We calculate the total and the free magnetic energy for solar NOAA active region 10486 on 2003 October 29 using chromospheric vector magnetograms observed with the Imaging Vector Magnetograph at Mees Solar Observatory in the Na I λ5896 spectral line. The magnetic energy is derived from the magnetic virial theorem using observations spanning the X10 flare that occurred at 20:39 UT. Although poor atmospheric seeing prevented us from discerning changes in the free energy associated with the flare, there was an unusually large amount of free magnetic energy in NOAA AR 10486: (5.7+/-1.9)×1033 ergs, which is consistent with the very high level of activity observed in this active region. It is thus plausible that the extreme activity was powered by the magnetic free energy. Title: TRACE Observations of Many Active Regions with X-flares and Rotating Sunspots in the Current Solar Cycle Authors: Nightingale, R. W.; Metcalf, T. R. Bibcode: 2004AGUFMSH13A1137N Altcode: The TRACE instrument in its 7 UV and EUV wavelength channels has observed approximately 55 X-flares throughout its mission so far in solar cycle 23 from launch in April 1998 through August 2004. About 40 of those X-flares (73%) can be associated with 17 solar active regions that contain rotating sunspots, which have been observed by TRACE in its white light channel. These sunspots, rotating about the center of their umbras, appear to be manifestations of the twist and/or writhe, i.e., the helicity, instilled in the rising active region omega loops as they travel through the convection region below the photosphere. Of the 40 X-flaring active regions observed, 26 (65%) can be further associated with large, fast Coronal Mass Ejections (CMEs) leaving the sun, as recorded by the LASCO instrument on SoHO. Furthermore, 24 of these 26 events appear to be geo-effective because the energetic proton fluxes, observed by instruments on the GOES geosynchronous spacecraft, increased by several orders of magnitude shortly after the solar events occurred. The data and the importance of such a high percentage of rotating sunspots being associated with X-flaring active regions and CMEs will be discussed. This work was supported by NASA under the TRACE contract NAS5-38099. Title: Photospheric Oscillations in the Solar Atmosphere: Driving Chromospheric Spicules and Coronal Waves Authors: De Pontieu, B.; Erdelyi, R.; De Moortel, I.; Metcalf, T. Bibcode: 2004AGUFMSH13A1142D Altcode: There are now many observations of oscillations and waves with periods around 5 minutes in the solar transition region and corona. We provide an observational overview of 5 minute periodicity in upper transition region moss, the low legs of coronal loops, and chromospheric spicules in active region plage. The source of the 5 minute periodicity is unclear, since photospheric p-modes are evanescent in the upper photosphere which should prevent them from propagating into the chromosphere, transition region and corona. Using a numerical model we show that photospheric oscillations can propagate into the low atmosphere as long as they are guided along a magnetic flux tube that is inclined away from the vertical. The leaked photospheric oscillations develop non-linearly into shocks at low chromospheric heights because of the density decrease with height. The upward traveling shocks and resulting rebound shocks of the chromosphere lead to periodic upward chromospheric flows, which in a recent paper we have identified as the periodic spicules that we observe in active region plage. After passage through the spicule, these shocked photospheric oscillations propagate into the corona. We suggest that TRACE observations of propagating acoustic waves in the corona are shocked and tunneled photospheric oscillations. We also explore whether these coronal waves can be exploited to determine the connectivity between photosphere and corona, and thus perform seismology of the lower solar atmosphere. Title: STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images Authors: Aschwanden, M. J.; Lemen, J.; Nitta, N.; Metcalf, T.; Wuelser, J.; Alexander, D. Bibcode: 2004AGUFMSH22A..02A Altcode: We simulate 3D models of EUV images of flare and CME events, using TRACE EUV movies. TRACE movies show 2D images in projection along a particular line-of-sight. We simulate 3D models of erupting filaments, flare loops, and postflare loops using: (1) a ``finger printing'' technique to trace linear structures in 2D images; (2) geometric 3D models based on force-free fields and curvature radius maximization of flare loop and flux rope structures; (3) conservation of velocity and acceleration parameters; (4) multi-temperature plasma filling according to hydrodyamic scaling laws; and (5) 2D projections from secondary line-of-sights that correspond to viewpoints of the secondary STEREO spacecraft. From such simulations we envision to illustrat 3D time-dependent models, what would be observed at the two STEREO spacecraft positions as well as from a near-Earth spacecraft such as SoHO. These simulations are used to test STEREO analysis software and to investigate what physical parameters and geometric 3D reconstructions can be retrieved from STEREO/SECCHI data. Title: ``Transition-Edge Sensors for Solar X-ray Spectral Observations - An Update'' Authors: Martinez-Galarce, D. S.; Stern, R.; Cabrera, B.; Deiker, S.; Alexander, D.; Metcalf, T.; Irwin, K.; Brink, P.; Boerner, P. F.; Morse, K.; Leman, S.; Shing, L.; Rausch, A.; Nichols, T. D.; Chakraborty, S. Bibcode: 2004AGUFMSH13A1147M Altcode: ABSTRACT: The advent of cryogenic microcalorimeters (operating at temperatures of ∼ 0.1 K) in ground-based and space-based astronomy promises a revolution of new discoveries. Particularly, Transition-Edge Sensors (TES) have demonstrated high-energy resolution measurements of soft X-rays of up to E/Δ E ∼ 1500 (at 6keV) and with high temporal resolution of a msec or less in photon pulse detections. Fabricated into multiplexed arrays of single detectors, or position sensing macropixels, and placed at the focus of a Wolter optic would further yield high spatial resolution capability of 2 arcsec or less, thus producing unprecedented "3-D" solar observations. We report herein, on progress to date in the development of these detectors and particularly, with on-going work with the sounding rocket payload, the Advanced Technology Solar Spectroscopic Imager, which will debut a TES instrument operating in the 300eV - 1.5keV range to study active region magnetic reconnection. Furthermore, as part of our larger effort, we discuss also current technical developments and plans at the Lockheed Martin Solar & Astrophysics Laboratory to design a TES instrument (3 - 8keV range) for realization into a dedicated Explorer-class solar observatory in the next 5 - 10 years. Title: Energy partition in two solar flare/CME events Authors: Emslie, A. G.; Kucharek, H.; Dennis, B. R.; Gopalswamy, N.; Holman, G. D.; Share, G. H.; Vourlidas, A.; Forbes, T. G.; Gallagher, P. T.; Mason, G. M.; Metcalf, T. R.; Mewaldt, R. A.; Murphy, R. J.; Schwartz, R. A.; Zurbuchen, T. H. Bibcode: 2004JGRA..10910104E Altcode: Using coordinated observations from instruments on the Advanced Composition Explorer (ACE), the Solar and Heliospheric Observatory (SOHO), and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we have evaluated the energetics of two well-observed flare/CME events on 21 April 2002 and 23 July 2002. For each event, we have estimated the energy contents (and the likely uncertainties) of (1) the coronal mass ejection, (2) the thermal plasma at the Sun, (3) the hard X-ray producing accelerated electrons, (4) the gamma-ray producing ions, and (5) the solar energetic particles. The results are assimilated and discussed relative to the probable amount of nonpotential magnetic energy available in a large active region. Title: Hard X-ray Footpoint Asymmetries in Solar Flares Authors: Chollet, E.; Alexander, D.; Metcalf, T. R.; Pollack, L. Bibcode: 2004AAS...204.5406C Altcode: 2004BAAS...36R.758C RHESSI observations of a M4.0 solar flare on 17 March, 2002 are presented. This flare exhibits footpoint asymmetries in hard X-ray emission, allowing us to study the energy and time dependence of particle transport in this flare. The data, originally presented in Alexander and Metcalf (2002), is reanalyzed with new calibrations, shorter integration times and better spatial resolution, allowing for more accurate determination of the time and energy dependence of the asymmetries. While the previous work suggested that this was a simple flare with two footpoints, the new analysis suggests a more complex structure with three or more footpoints, confirmed by force free magnetic field extrapolations. The time development of the evolving flare indicates a transition from one flaring structue to another which significantly complicates the interpretation of the measured asymmetry. The implications of this new interpretation for the previously reported energy dependence will be discussed. We gratefully acknowledge support from NASA (NAS5-02048). Title: Non-Relativistic Electron Beam Stability in Solar Flares Authors: Daou, A. G.; Alexander, D.; Metcalf, T. R. Bibcode: 2004AAS...204.0203D Altcode: 2004BAAS...36..667D The thick-target electron beam model has been used for decades as a viable description for the production of solar flare hard X-ray emission. The required very rapid transport of energy to the footpoints during solar flares is achieved in this model by fast electrons traversing the loop to deposit their energy in the dense chromosphere. For some of the largest flares the currents (up to 1017 Amps or 1036 electrons/sec) inferred can significantly exceed the Alfven-Lawson limit suggesting that the assumed electron beam is inherently unstable. In this paper, we use the spectral and spatial resolution of RHESSI to explore whether the conditions for a stable non-thermal electron beam exist in large solar flares. The incident electron spectra at flaring footpoints are derived from the RHESSI photon spectra while an upper limit to the footpoint area is detremined form the hard X-ray images. We determine the electron beam density needed to produce the hard X-ray emission for two large flares, July 17, 2002 (M8.5) and October 28, 2003 (X17.2) and determine whether such beams are viable in these cases.

We gratefully acknowledge support from NASA (NAS5-02048). Title: TRACE Observations of a Torsionally Rotating Sunspot Reversing Direction Authors: Nightingale, R. W.; Metcalf, T. R.; Ullman, J. R. Bibcode: 2004AAS...204.3716N Altcode: 2004BAAS...36..711N For almost a week, starting on July 30, 2003, TRACE observed AR10421 in white light (WL), ultra-violet (UV), and extreme ultra-violet (EUV) passbands. Analysis of the photospheric WL images shows that the leading sunspot was torsionally rotating about the center of its umbra, in a clockwise direction on and before July 31 and counter-clockwise after a reversal of direction on August 1st. The sunspot appears to be fragmenting with clumps of small pores moving away toward the east during the first 2 days, followed by another clump that appears to be drawn into the sunspot after the rotational direction change on August 1st. Twisted coronal fans can be seen above the rotating sunspot in the EUV images on and after August 1st. Movies of this rather unique oscillatory behavior will be shown in 2 or more passbands, along with corresponding magnetogram data, as observed by SOHO/MDI.

This work has been supported by NASA, in part under the TRACE project at LMSAL (contract NAS5-38099) and in part under the MDI/SOHO project (NAG5-13261). Title: Nonlinear Force Free Field Models of AR 0486 Authors: McTiernan, J. M.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2004AAS...204.0204M Altcode: 2004BAAS...36Q.668M In this work we present nonlinear force free field (NLFFF) extrapolations from vector magnetograms of AR0486 taken before, during and after the X11 flare of 29 October 2003. The extrapolations will be used to interpret the different source patterns and motions seen in RHESSI and TRACE images of the flare. In particular we will examine the footpoint motions observed in the flare by RHESSI, and also the expansion and other changes in the high temperature thermal sources visible in low energy (less than 20 keV) RHESSI images and TRACE 195 A images.

This work is funded by NASA contract NAS 5-98033. Title: The Magnetic Free Energy in AR0486 Authors: Metcalf, T. R.; Leka, K. D.; Mickey, D. L. Bibcode: 2004AAS...204.0208M Altcode: 2004BAAS...36S.668M During October/November 2003, the dramatic active region 0486 traversed the solar disk and produced many large solar flares. During this time, we obtained chromspheric vector magnetic field data for AR0486 using the Imaging Vector Magnetograph (IVM) at Mees Solar Observatory, Haleakala, Hawaii. We will describe these vector field data and will use them to compute the magnetic free energy, and its time variation, for AR0486. This large, complex active region contained an unusually large amount of free magnetic energy, not surprising considering the level of activity it produced.

This work was supported by NASA grant NAG5-12466. Title: On the Resolution of the Azimuthal Ambiguity in Vector Magnetograms of Solar Active Regions Authors: Georgoulis, Manolis K.; LaBonte, Barry J.; Metcalf, Thomas R. Bibcode: 2004ApJ...602..446G Altcode: We introduce a ``structure minimization'' technique to resolve the azimuthal ambiguity of 180°, intrinsic in solar vector magnetic field measurements. We resolve the 180° ambiguity by minimizing the inhomogeneities of the magnetic field strength perpendicular to the magnetic field vector. This relates to a minimization of the sheath currents that envelope the solar magnetic flux tubes, thus allowing for more space-filling and less complex magnetic fields. Structure minimization proceeds in two steps: First, it derives a local solution analytically, by means of a structure minimization function. Second, it reaches a global solution numerically, assuming smoothness of the magnetic field vector. Structure minimization (i) is disentangled from any use of potential or linear force-free extrapolations and (ii) eliminates pixel-to-pixel dependencies, thus reducing exponentially the required computations. We apply structure minimization to four active regions, located at various distances from disk center. The minimum structure solution for each case is compared with the ``minimum energy'' solution obtained by the slower simulated annealing algorithm. We find correlation coefficients ranging from significant to excellent. Moreover, structure minimization provides an ambiguity-free vertical gradient of the magnetic field strength that reveals the variation of the magnetic field with height. The simplicity and speed of the method allow a near real-time processing of solar vector magnetograms. This task was not possible in the past and may be of interest to both existing and future solar missions and ground-based magnetographs. Title: On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy Authors: Aschwanden, Markus J.; Metcalf, Thomas R.; Krucker, Säm; Sato, Jun; Conway, Andrew J.; Hurford, G. J.; Schmahl, Edward J. Bibcode: 2004SoPh..219..149A Altcode: 2003astro.ph..9499A We compare the photometric accuracy of spectra and images in flares observed with the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft. We test the accuracy of the photometry by comparing the photon fluxes obtained in different energy ranges from the spectral-fitting software SPEX with those fluxes contained in the images reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the fidelity of source geometries, and spatial spectra reconstructed with the five image reconstruction algorithms. We investigate the effects of grid selection, pixel size, field of view, and time intervals on the quality of image reconstruction. The detailed parameters and statistics are provided in an accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean have a robust convergence behavior and a photometric accuracy in the order of a few percent, while MEM does not converge optimally for large degrees of freedom (for large field of view and/or small pixel sizes), and MEM-Vis suffers in the case of time-variable sources. This comparative study documents the current status of the RHESSI spectral and imaging software, one year after launch. Title: Radiative Hydrodynamic Models of Solar White Light Flares Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.; Fisher, G. H.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2003AGUFMSH22A0175A Altcode: We report on theoretical radiative hydrodynamic simulations of solar white light flares. The solar atmosphere is modeled in detail from the transition region to the photosphere. The coronal pressure and X-ray backheating are included self-consistently. Flare heating is assumed to be from an electron beam which is modeled for several white light flares using data from RHESSI, TRACE and Yohkoh. We also investigate the possibility that the 511 keV line width is produced from a significant column depth of atmosphere at transition region temperatures. We compare our new solar flare models to previous results, and to models of M dwarf stellar flares. Title: The Relationship Between X-Ray Radiance and Magnetic Flux Authors: Pevtsov, Alexei A.; Fisher, George H.; Acton, Loren W.; Longcope, Dana W.; Johns-Krull, Christopher M.; Kankelborg, Charles C.; Metcalf, Thomas R. Bibcode: 2003ApJ...598.1387P Altcode: We use soft X-ray and magnetic field observations of the Sun (quiet Sun, X-ray bright points, active regions, and integrated solar disk) and active stars (dwarf and pre-main-sequence) to study the relationship between total unsigned magnetic flux, Φ, and X-ray spectral radiance, LX. We find that Φ and LX exhibit a very nearly linear relationship over 12 orders of magnitude, albeit with significant levels of scatter. This suggests a universal relationship between magnetic flux and the power dissipated through coronal heating. If the relationship can be assumed linear, it is consistent with an average volumetric heating rate Q~B/L, where B is the average field strength along a closed field line and L is its length between footpoints. The Φ-LX relationship also indicates that X-rays provide a useful proxy for the magnetic flux on stars when magnetic measurements are unavailable. Title: Observational consequences of a magnetic flux rope topology Authors: Gibson, S.; Barnes, G.; Demoulin, P.; Fan, Y.; Fisher, G.; Leka, K.; Longcope, D.; Mandrini, C.; Metcalf, T. Bibcode: 2003AGUFMSH42B0516G Altcode: We consider the implications of a magnetic flux rope topology for the interpretation of observations of sigmoidal active regions. A region of tangential magnetic discontinuities can be identified using techniques that determine a bald patch (BP) and corresponding separatrices or a quasi-separatrix layer (QSL) -- for a flux rope this region can be S-shaped, or sigmoidal. If such a region is physically driven, current sheets can form yielding conditions appropriate for reconnective heating. Using a numerical simulation of an emerging flux rope driven by the kink instability, Fan and Gibson (ApJL, 2003) showed that current sheets indeed formed a sigmoidal surface. In this poster we will demonstrate that the current sheets formed on the BP and BP separatrices. Moreover, we will use the results of the numerical simulation as proxies for observations: specifically the simulated field at the photosphere as proxy for the magnetic boundary condition, the sigmoidal current sheets as proxy for the X-ray active region emission, and the location of dipped magnetic field lines as proxy for a filament. We will then consider to what extent such observations might be used to understand and constrain the basic properties of the coronal field. Title: The Relation Between Coronal Mass Ejections and Outward Motions in the Low Corona Authors: Metcalf, T. R. Bibcode: 2003AGUFMSH22B..02M Altcode: Outward motions are frequently observed in the low corona, especially in the early phase of solar flares. Some of these motions, observed in X-ray, EUV, and radio wavelengths, appear to be intimately linked to coronal mass ejections (CMEs) as observed by white-light coronagraphs, typically above 2 Rsun. In principle, a combination of X-ray/EUV/radio and coronagraph data should give many constraints on models of CME initiation and early propagation. However, such attempts are not straightforward because of the diversity of low coronal motions, which often have unclear relationships, if any, to the CME. In this presentation, we compare many examples of low coronal motions observed by TRACE and Yohkoh with CMEs observed by LASCO, and discuss what we can objectively conclude from the data. Errors resulting from the inclusion of different and possibly incompatible data in a single so-called height-time plot are discussed. We also study the reality of the flare association of CMEs, which, according to the height-time plots, precede the flare and hence the low coronal motions by more than an hour. Title: Cryogenic 3-D Detectors for Solar Physics Using Position Sensitive Transition-Edge Sensors Authors: Stern, R. A.; Martinez-Galarce, D.; Metcalf, T.; Lemen, J.; Cabrera, B.; Brink, P.; Leman, S.; Deiker, S.; Irwin, K.; Alexander, D. Bibcode: 2003AAS...203.1805S Altcode: 2003BAAS...35Q1237S Space and ground-based astronomy is currently undergoing a revolution in detector technology with the advent of cryogenic sensors operating in the sub-Kelvin temperature range. These detectors provide non-dispersive energy resolution at optical through gamma ray energies (e.g, E/Δ E ∼ 1500 at 6 keV), high time resolution (msec or better), and can be made into imaging arrays through SQUID multiplexing of individual pixels or employing macropixels with position sensing capability. The application of such ``3-D'' detector technology to solar physics will lead to significant advances in our understanding of magnetic reconnection phenomena in the Sun, including flares and microflares, X-ray jets, and active region dynamics. An Explorer-class solar mission based upon these detectors is a distinct possibility within the next 5-10 years. In this poster, we will describe some of the recent advances in cryogenic detector technology with particular applicability to solar physics, and future technical developments required to make such a mission a reality.

This work was supported in part by the Lockheed Martin Independent Research Program. Title: Data inversion for the Multi-Order Solar Extreme-Ultraviolet Spectrograph Authors: Fox, J. Lewis; Kankelborg, Charles C.; Metcalf, Tomas R. Bibcode: 2003SPIE.5157..124F Altcode: The Multi-Order Solar Extreme Ultraviolet Spectrograph (MOSES) is a high resolution, slitless imaging spectrometer that will observe the Sun in extreme ultraviolet near 304A. MOSES will fly on a NASA sounding rocket launch in spring 2004. The instrument records spatial and spectral information into images at three spectral orders. To recover the source spectrum, an ill-posed inversion must be performed on these data. We will explore two of the techniques by which this may be accomplished: Fourier backprojection and Pixons, constrained by the spatially integrated spectrum of the Sun. Both methods produce good results, including doppler shifts measured to 1/3-pixel accuracy. The Pixon code better reproduces the line widths. Title: TRACE and Yohkoh Observations of a White-Light Flare Authors: Metcalf, Thomas R.; Alexander, David; Hudson, Hugh S.; Longcope, Dana W. Bibcode: 2003ApJ...595..483M Altcode: We present observations of a large solar white-light flare observed on 2001 August 25, using data from the Transition Region and Coronal Explorer (TRACE) white-light channel and Yohkoh/HXT. These emissions are consistent with the classic type I white-light flare mechanism, and we find that the enhanced white-light emission observed by TRACE originates in the chromosphere and temperature minimum regions via nonequilibrium hydrogen ionization induced by direct collisions with the electron beam and by back-warming of the lower atmosphere. The three flare kernels observed in hard X-rays and white light are spatially associated with magnetic separatrices, and one of the kernels is observed to move along a magnetic separatrix at 400 km s-1. This is evidence in favor of particle acceleration models, which energize the electrons via magnetic reconnection at magnetic separators. Title: Observations of Rotating Sunspots from TRACE Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver, C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J. Bibcode: 2003SoPh..216...79B Altcode: Recent observations from TRACE in the photospheric white-light channel have shown sunspots that rotate up to 200° about their umbral centre over a period of 3-5 days. The corresponding loops in the coronal fan are often seen to twist and can erupt as flares. In an ongoing study, seven cases of rotating sunspots have been identified, two of which can be associated with sigmoid structures appearing in Yohkoh/SXT and six with events seen by GOES. This paper analyzes the rotation rates of the sunspots using TRACE white-light data. Observations from AR 9114 are presented in detail in the main text and a summary of the results for the remaining six sunspots is presented in Appendixes A-F. Discussion of the key results, particularly common features, are presented, as well as possible mechanisms for sunspot rotation. Title: A Statistical Study of Limb Flares Observed by RHESSI: Imaging Authors: Liu, W.; Jiang, Y. W.; Petrosian, V.; Metcalf, T. R. Bibcode: 2003SPD....34.1802L Altcode: 2003BAAS...35..839L Hard X-ray observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provide unprecedented opportunities to understand the underlying physics driving solar flares. We have embarked on an investigation of a sample of \it limb flares observed by RHESSI. RHESSI flare images, combined with TRACE EUV observations and SoHO/MDI magnetograms, are used to infer flaring loop structures and to distinguish between footpoint, loop top and other possible types of sources. We will present an analysis of the lightcurves and images for these features and will show statistics on the occurrence of loop top and footpoint sources. These will be compared with previous studies utilizing YOHKOH/HXT observations, and the implications for particle acceleration processes and other theoretical aspects of flares will be discussed.

The work at Stanford is supported by NASA grants NAG5-12111, NAG5 11918-1, and SUB2001-402-01 through the University of Alabama in Huntsville (PI: J. Miller). T. Metcalf would like to acknowledge support from grant NAS-98033-05/03. Title: RHESSI Observations of Electron Beam Stability Authors: Metcalf, T. R.; Alexander, D. Bibcode: 2003SPD....34.1805M Altcode: 2003BAAS...35..840M The standard thick-target electron beam model for solar flare hard X-ray production typically requires electron beams with a current of 1017 Amperes in order to explain the observed hard X-ray emission. Such high beam currents can exceed the Alfven-Lawson limit causing the beam to essentially 'self-destruct'. One way of circumventing this problem is to impose a converging field geometry which reduces the area in the hard X-ray production region relative to that in the electron acceleration region. In this paper, we use the highest spatial resolution RHESSI observations of flare footpoints, with inferred electron spectral and flux properties, to test whether the conditions for stability of the electron beam are met. Title: The role of magnetic separators in solar flare particle acceleration Authors: Alexander, D.; Metcalf, T. R. Bibcode: 2003SPD....34.0418A Altcode: 2003BAAS...35..813A In recent years, the development of theoretical models of separatrices, separators, and quasi-separatrix layers, coupled to observations of flare brightenings, have led to the notion that these topological structures are the natural locations for current sheets to form and magnetic reconnection to occur. We report on correlated hard X-ray and vector magnetic field data from RHESSI and Mees IVM which show the detailed spatial and temporal relationships between the magnetic topology and the expected sites of reconnection. Title: Transition-Edge Sensors for Solar Atmospheric Observations A Technology Overview &Update Authors: Martinez-Galarce, D.; Stern, R.; Alexander, D.; Metcalf, T.; Cabrera, B.; Brink, P.; Deiker, S.; Irwin, K. Bibcode: 2003EAEJA....14311M Altcode: Transition-Edge Sensors (TES) are a novel class of quantum microcalorimeters that promise to revolutionize astrophysics. Presently they are being developed for use as the next generation detectors for many classes of astrophysical observations. Singe pixel X-ray measurements at the Al-Kα line (1.5keV) have been measured with 2.1 eV precision, rivaling Bragg Crystal Spectrometers resolution. As these detectors are inherent spectrometers (with near 100 quantum efficiency in X-ray energies), configured in an array, they promise to provide near-perfect 4-dimensional (space, time and energy) observations of astrophysical phenomena. We discuss the current level of advancement of this technology and as an example we discuss some of the technical challenges faced to design and fabricate an Explorer-class satellite to observe the solar atmosphere. Title: Active-Region Magnetic Structure Observed in the Photosphere and Chromosphere Authors: Leka, K. D.; Metcalf, Thomas R. Bibcode: 2003SoPh..212..361L Altcode: The full magnetic vector has been measured in both the photosphere and chromosphere across sunspots and plage in NOAA Active Region 8299. We investigate the vertical magnetic structure above the umbral, penumbral and plage regions using quantitative statistical comparisons of the photospheric and chromospheric magnetic data. The results include: (1) a general decrease in average magnetic flux density with height; (2) the direct detection of the superpenumbral canopy in the chromosphere; (3) values for dB/dz which are consistent with earlier investigations when derived from a straight difference between the two measurements, but which are somewhat small when derived from the ∇⋅B=0 condition, (4) a monolithic structure in the umbrae which extends well into the upper chromosphere, with a very complex and varied structure in penumbrae and plage, as evidenced by (5) a uniform magnetic scale height in the umbrae with an abrupt jump to widely varying scale heights in penumbral and plage regions. Further, we find (6) evidence that field extrapolations using the photospheric flux as the boundary may not agree with expectations or with observed coronal structures as well as those which use the chromospheric magnetic flux as the extrapolation starting point. Title: Mapping RHESSI footpoints with potential-field models Authors: Fletcher, L.; Hudson, H. S.; Metcalf, T. R. Bibcode: 2002AGUFMSH52A0452F Altcode: RHESSI hard X-ray observations help us to identify the locations of magnetically conjugate footpoints, and to study their apparent motions during the evolution of the impulsive phase of a flare. We put this information into the context of an M-class flare that occurred 2002 March 14 01:50 UT (GOES peak time) at S12, E23 (NOAA region 9866) by making potential-field mappings of the coronal magnetic structure. In principle the hard X-ray sources (plus the mapping) constrain the site of magnetic energy release, and the maps reveal the location of the stored energy. The RHESSI source centroids can be determined to better than 1'' (rms) for an M-class flare. This analysis is an exploration of the feasibility of such an approach, since full success would require understanding the magnetic restructuring in detail. If suitable X-class RHESSI flares occur we will be able to present data with better precision. Title: The RHESSI Imaging Concept Authors: Hurford, G. J.; Schmahl, E. J.; Schwartz, R. A.; Conway, A. J.; Aschwanden, M. J.; Csillaghy, A.; Dennis, B. R.; Johns-Krull, C.; Krucker, S.; Lin, R. P.; McTiernan, J.; Metcalf, T. R.; Sato, J.; Smith, D. M. Bibcode: 2002SoPh..210...61H Altcode: The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV with spatial resolution as high as 2.3 arc sec. Instead of focusing optics, imaging is based on nine rotating modulation collimators that time-modulate the incident flux as the spacecraft rotates. Starting from the arrival time of individual photons, ground-based software then uses the modulated signals to reconstruct images of the source. The purpose of this paper is to convey both an intuitive feel and the mathematical basis for this imaging process. Following a review of the relevant hardware, the imaging principles and the basic back-projection method are described, along with their relation to Fourier transforms. Several specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable to RHESSI imaging are briefly described. The characteristic strengths and weaknesses of this type of imaging are summarized. Title: Energy dependence of electron trapping in a solar flare Authors: Alexander, David; Metcalf, Thomas R. Bibcode: 2002SoPh..210..323A Altcode: Observations of an energy-dependent asymmetry in footpoint hard X-ray emission by RHESSI for the M4.0 solar flare of 17 March 2002 allows us to probe the dynamics of particle transport with energy and time. The presence of such an asymmetry is most readily explained by the effects of a converging magnetic field with different rates of convergence at the different footpoints, as would be expected from realistic surface field distributions. Such a geometry has been discussed in the context of a trap-plus-precipitation model where the transport of energetic particles in the flare is governed by the precipitation out of the coronal trap via collisions, wave-particle interactions or some other scattering process, into the high-density chromosphere. Comparison of RHESSI observations with a trap-plus-precipitation model allows us to use the energy dependence of the asymmetry and the observed ratio of footpoint to coronal emission at the different energies to assess the role of the trapping in the transport of energetic electrons and to probe the nature of the particle precipitation process inside the loss cone. 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: Observations of rotating sunspots and their effect in the corona Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver, C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J. Bibcode: 2002ESASP.505..261B Altcode: 2002IAUCo.188..261B; 2002solm.conf..261B Recent observations from TRACE have seen sunspots, in the photospheric white light filter, rotate up to 180 degrees about their umbral centre. The corresponding loops in the coronal fan are seen to twist and can erupt. In an ongoing study, five cases of rotating sunspots have been identified, three of which can be identified with sigmoid structures appearing in Yohkoh/SXT. This paper will present images from one of these events, showing the coupling between the photosphere and the corona, and observational analysis deducing the rotation speeds and how they change through time and with radius of the sunspot. In particular, the paper will focus on the best example of a rotating sunspot observed so far, that of AR 9114 which occurred over 8-10 August 2000 and was observed by TRACE, SoHO/MDI and Yohkoh/SXT. The sunspot rotated 150 degrees within this time and is associated with a sigmoid. Title: Helicity driven sigmoid evolution Authors: Alexander, D.; Nightingale, R.; Metcalf, T. R.; Brown, D. Bibcode: 2002AAS...200.3608A Altcode: 2002BAAS...34..694A Recent observations of rotating sunspots in TRACE white light images and their apparent association with soft X-ray sigmoids have led to the intriguing possibility that the sunspot rotation serves as the driver for both sigmoid formation and their potential eruption. We discuss the energization of the corona resulting from currents generated by the vortex motions of the rotating sunspot. We will present data from events for which we have good white light coverage of the sunspot, an evolving sigmoid and an associated CME (in those cases where the sigmoid erupts). We investigate the relationship between the sunspot rotation and the evolution of the sigmoid structure and attempt to determine the key physical conditions which result in a sigmoid destabilizing and ultimately producing a CME. Title: The Electron Number Problem Revisited with RHESSI Flare Observations Authors: Aschwanden, M. J.; Alexander, D.; Metcalf, T.; Nitta, N. Bibcode: 2002AAS...200.7608A Altcode: 2002BAAS...34..776A The Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observed a number of solar flares since its launch on February 5, 2002. We analyze first images and spectra from this new mission, using a variety of image reconstruction methods, such as Clean, Pixon, Maximum Entropy (MEM), and Forward-Fitting. We obtain context images of the flare regions in EUV from SoHO/EIT and TRACE, and soft X-ray light curves from GOES. From RHESSI we reconstruct hard X-ray images with full uv-coverage in time intervals of the spin period ( 4 s). RHESSI provides a substantially higher spatial resolution ( 2") and spectral resolution than previous data from Yohkoh/HXT. A longstanding problem in solar flare physics is the so-called electron number problem, which challenges a reconciliation between (1) the rate of injected electrons inferred from the observed hard X-ray photon spectrum in the framework of the thick-target model and the (2) the replenishment rate of electrons in the coronal acceleration volume. Thanks to the high spectral resolution of RHESSI we can for the first time properly separate the thermal and the non-thermal electron spectral components, and this way determine the injection rate of nonthermal electrons more accurately. In addition, imaging in many energy bands allows us to trace propagation and energy loss between the coronal acceleration site and the chromospheric thick-target site with higher accuracy, taking into account the partial energy loss of trapped and precipitating electrons. The goal of this investigation is to quantify the geometry, density, and inflows in the coronal acceleration region, which provides crucial constraints for the underlying magnetic reconnection process. Title: Hard X-ray and White Light Observations of the August 25, 2001 X Flare Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. S.; Longcope, D.; Myers, D. Bibcode: 2002AAS...200.6803M Altcode: 2002BAAS...34..755M An X5.3 flare occurred about 16:31 UT on 2001 August 25 and was well observed by the Yohkoh and TRACE spacecraft. The flare showed gamma-ray emission, nuclear lines, and was a dramatic white light flare seen in TRACE data. A preliminary analysis of the hard X-ray images from the Yohkoh/HXT instrument shows two clear footpoints and a moving HXR source in this very energetic flare. The moving hard X-ray source appears to move along a magnetic separatrix at 400 km/sec. We will discuss the hard X-ray and white light structure of this flare and discuss the energetics and possible mechanisms for the formation of the TRACE white light emission. Title: Fast acceleration of a CME-related X-ray structure in the low solar corona Authors: Alexander, David; Metcalf, Thomas R.; Nitta, Nariaki V. Bibcode: 2002GeoRL..29.1403A Altcode: 2002GeoRL..29j..41A We report on a unique observation of the early and rapid acceleration of a fast CME in the low solar corona. The coronal disturbance associated with a LASCO CME and concurrent X1.2 flare on 1998 April 23 was well-observed by the Yohkoh Soft X-ray Telescope. The X-ray observations clearly show an accelerating structure, reaching 800-1100 km/s in ~500s at a de-projected height of only 280 Mm, indicating an acceleration well in excess of 1700 m/s2. The soft X-ray observations are coincident with the onset of the CME, which ultimately attained a velocity of 1390 km/s and generated a number of interplanetary radio signatures, frequently associated with fast events. Assuming a constant acceleration, a0, throughout the duration of the X-ray observations, we determine a best-fit value of a0 = 1756 m/s2, while application of a power-law acceleration model yields a better fit to the data with a peak acceleration of ~4865 m/s2 within the first 280 Mm of the solar corona. Title: The Magnetic Free Energy in Active Regions Authors: Metcalf, Tom Bibcode: 2002smra.progE..16M Altcode: No abstract at ADS Title: Concurrent Rotating Sunspots, Twisted Coronal Fans, Simgoid Structures and Coronal Mass Ejections Authors: Nightingale, R. W.; Brown, D. S.; Metcalf, T. R.; Schrijver, C. J.; Shine, R. A.; Title, A. M.; Wolfson, C. J. Bibcode: 2002mwoc.conf..149N Altcode: In an on-going study, several sunspots, in apparent rotation, have been identified in TRACE photospheric white light (WL) images with accompanying twisting of coronal fans in the corresponding EUV (171, 195 AA) images. These observations can also be temporally and spatially associated with S or inverse-S shaped regions (sigmoid structures) appearing in Yohkoh SXT images and with concurrent coronal mass ejections (CMEs) and/or flares. We have determined the rotational speed of the apparently rotating sunspot in AR 9114 over 8-10 August 2000, established the inverse S shape observed in the SXT data, and viewed a rapid, bright flash of possible reconnection in a TRACE EUV movie. A CME was observed during the 15-18 August 1999 event, which also included an inverse S shaped region in the SXT data, and a rotating sunspot and twisting coronal fans in the TRACE data. The large Bastille Day CME event of 14 July 2000 was accompanied by one or more apparently rotating sunspots as observed in TRACE WL and by an inverse S shaped region as seen in a difference SXT image. Movies and plots of some of these data will be shown along with flow maps and a list of the pertinent parameters for several rotating sunspots. We will report on our attempt to determine the vertical electric current flowing through the 8 August 2000 sunspot utilizing the Mees vector magnetograph data in order to better understand the apparent rotation "driver". These observations display the coupling of the solar magnetic field from the photosphere into the corona. Title: How the corona depends on magnetic variables Authors: Metcalf, Thomas Bibcode: 2002ocnd.confE..21M Altcode: No abstract at ADS Title: The Yohkoh Public Outreach Project Authors: Larson, M. B.; Slater, T.; McKenzie, D.; Acton, L.; Alexander, D.; Lemen, J.; Freeland, S.; Metcalf, T. Bibcode: 2002mwoc.conf..117L Altcode: The NASA funded Yohkoh Public Outreach Project (YPOP) began in 1995 with the goal of providing public access to high quality Yohkoh SXT data via the World Wide Web. The project utilizes the intrinsic excitement of the SXT data, and in particular the SXT movies, to develop science learning tools and classroom activities. The WWW site at URL: http://www.lmsal.com/YPOP/ uses a movie theater theme to highlight available Yohkoh movies in a format that is entertaining and inviting to non-scientists and well received by scientists. We will discuss the wide range of people YPOP has reached over the past six years, as well as lessons learned during the development of the project. Title: Yohkoh: A Decade of Discovery [Invited] Authors: Alexander, D.; Metcalf, T. R. Bibcode: 2002mwoc.conf..103A Altcode: Data from the Yohkoh satellite has led the way in the dissemination of scientific knowledge to the general public over the last decade. This talk will discuss the role that solar physics in general, and Yohkoh in particular, have played in stimulating a public interest in science and the effect they have had in the teaching of science in the classroom. To highlight Yohkoh we will discuss the extremely successful Yohkoh Public Outreach Project and its daughter Solar Week with particular emphasis on the role played by scientists in the effective dissemination of the solar science. Title: The Magnetic Free Energy and a CME in Active Region 8299 Authors: Metcalf, T. R.; Mickey, D. L.; Labonte, B. J.; Ryder, L. A. Bibcode: 2002mwoc.conf..249M Altcode: We calculate the magnetic free energy as a function of time for NOAA active region 8299 on 1998 August 11 using vector magnetic field measurements in the ion Na 1 5896Å sepctral line observed with the Imaging Vector Magnetograph at Mees Solar Observatory. The free energy in this active region is significant with a magnitude of about 1033 ergs, though the active region was not flare productive. This amount of free energy is more than enough to explain the enhanced heating of the active region corona. The free energy dipped to a value consistent with zero for one hour during the observation. Yohkoh/SXT images reveal that during this dip in the free energy, the coronal structure of AR 8299 and the nearby AR8297 changed significantly. SXT observed the brightening of a coronal loop connecting AR 8299 and AR 8297 and observed coronal dimming and the formation of a cusp structure in AR 8297, suggesting that a gradual CME was launched as the magnetic energy dipped. Unfortunately, LASCO data were not available to confirm the existence of a halo CME. However, the circumstantial evidence points to the magnetic free energy as the energy source for the postulated CME. Title: The Yohkoh Public Outreach Project: A Space Science Resource for Formal and Informal Education Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.; Acton, L. W.; Larson, M.; McKenzie, D.; Slater, T. Bibcode: 2001AGUFMED12A0160L Altcode: The Yohkoh Public Outreach Project (YPOP) is a NASA-funded web site maintained by scientists and educators at Lockheed Martin Solar and Astrophysics Lab. and Montana State University. YPOP includes a range of activities for youngsters, parents, teachers and anyone interested in learning more about the Sun. YPOP utilizes a number of approaches to the dissemination of solar data which incorporates elements of both formaleducation, via a number of lesson plans and classroom activities, and informal education, via access to the latest solar images, a solar tour, and updated movies. This combination has proved extremely effective in providing quality access to scientific data for a broad audience with a wide range of interests. The Yohkoh Public Outreach Project can be found at http://www.LMSAL.com/YPOP. Title: Energization of Rotating Sunspots, Twisted Coronal Fans, Sigmoid Structures, and Coronal Mass Ejections Authors: Nightingale, R. W.; Alexander, D.; Brown, D. S.; Metcalf, T. R. Bibcode: 2001AGUFMSH11C0724N Altcode: In an on-going study, several sunspots, rotating about their umbral centers, have been identified in TRACE photospheric white light (WL) images with accompanying twisting of coronal fans connected to penumbral magnetic field lines in the corresponding EUV (171, 195 Å) images. These observations can also be temporally and spatially associated with S or inverse-S shaped regions (sigmoid structures) appearing in Yohkoh SXT images and with concurrent coronal mass ejections (CMEs) and/or flares. We have determined the rotational speed of the sunspot in AR9114 over 8-10 August 2000, established the inverse-S shape observed in the SXT data, and viewed a rapid flash of possible reconnection in a TRACE EUV movie. A CME was observed in AR8667 during the 15-18 August 1999 event, which also included an inverse-S shaped region in the SXT data, and a rotating sunspot and twisting coronal fans in the TRACE data. The large Bastille Day CME event of 14 July 2000 in AR9077 was accompanied by one or more rotating sunspots as observed in TRACE WL and by an inverse-S shaped region as seen in a difference SXT image. We will utilize these data to estimate the energization of the twisted coronal fans resulting from the sunspot rotation and compare this with the temporal evolution of the sigmoid structures. We will investigate the possibility of a direct role of the observed sunspot rotation and the potential for a sigmoid to erupt as a CME. This work was supported by NASA under contract NAS5-38099. Title: Hard X-ray Observations of the August 25, 2001 X Flare Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. H. Bibcode: 2001AGUFMSH42A0776M Altcode: The X5.3 flare which occurred about 16:31 UT on 2001 August 25 was well observed by the Yohkoh spacecraft. The flare showed gamma-ray emission, nuclear lines and was a dramatic white light flare. A preliminary analysis of the hard X-ray images from the Yohkoh/HXT instrument show two clear footpoints to this very energetic flare, one nearly stationary and the other apparently moving as the flare progresses. We will discuss the hard X-ray and soft X-ray structure of this flare and compare the hard X-ray images to the TRACE and MDI white light data both spatially and temporally. Title: Evidence for the Flare Trigger Site and Three-Dimensional Reconnection in Multiwavelength Observations of a Solar Flare Authors: Fletcher, L.; Metcalf, T. R.; Alexander, D.; Brown, D. S.; Ryder, L. A. Bibcode: 2001ApJ...554..451F Altcode: Based on a multiwavelength data set and a topological model for the magnetic field, we argue that a M1.9 flare which occurred on 1993 May shows evidence of three-dimensional coronal reconnection in a spine-fan configuration. Images from the Transition Region and Coronal Explorer allow the detailed examination of the structures involved in the flare and preflare in the 171 Å (1 MK) EUV passband and the Lyα (10,000-20,000 K) passband. Yohkoh Hard X-ray Telescope maps the position of nonthermal electron precipitation and the Soft X-ray Telescope reveals preflare and flare heating on large and small scales. While the flare appears to be driven by changes in small-scale field close to the photosphere, near the interface between strong opposite magnetic polarities, the result is the disruption of large-scale field. We demonstrate how this observed activity on large and small scales, along with many other aspects of the flare, suggests a qualitative explanation in the three-dimensional reconfiguration of coronal magnetic field, following a small-scale flux cancellation at the photosphere. Title: A Comparison of the Active Region Magnetic Field in the Photosphere and Chromosphere Authors: Metcalf, T. R.; Leka, K. D. Bibcode: 2001AGUSM..SP41B07M Altcode: During the Whole Sun Fortnight, the Imaging Vector Magnetograph at Mees Solar Observatory obtained vector magnetic field maps of AR 8299 in the Na-D line (the core of which is formed in the lower chromosphere). At almost the same time, the HAO/NSO Advanced Stokes Polarimeter obtained data in the Fe-I line, formed in the photosphere. We present a comparison of the structures observed in AR 8299's main sunspots on 18 August 1998 at the different atmospheric heights. The data suggest that the Na-D magnetic field maps are formed 2.5 Mm above the Fe-I magnetic field maps. At this height in the atmosphere, the magnetic field is force-free and we will explore the implications of this for the extrapolation of the magnetic field in the corona. Title: Analysis of X-ray counterparts for Fast and Slow Halo CMEs Authors: Alexander, D.; Metcalf, T. R.; Nitta, N. Bibcode: 2001AGUSM..SH22B05A Altcode: In many cases, coronal mass ejections exhibit a strong counterpart in the X-ray corona, particularly in flare-related events. Observations by the Soft X-ray Telescope on Yohkoh have exhibited a number of manifestations in association with CME eruptions. These include the well-known dimming events, post-event arcades and expanding loop systems. We examine the X-ray signatures of a number of fast and slow halo CME events for evidence of two-classes of CME eruption in accord with the observed velocity profiles determined from white-light data. Flare-associated CMEs, which have a tendency to exhibit constant velocity, necessarily undergo a rapid acceleration in the low corona and should exhibit enhanced heating of the ambient corona to X-ray emitting temperatures. Slow CMEs, on the other hand, are expected to display a more subtle, if any, effect in the hot corona. We examine the Yohkoh database for evidence of a dichotomy in the X-ray signatures of halo CMEs. Title: Instrumental Effects and their Removal from TRACE Images Authors: Tarbell, T. D.; Nightingale, R. N.; Metcalf, T. R.; Frank, Z. A.; TRACE Team Bibcode: 2000SPD....31.0291T Altcode: 2000BAAS...32..827T TRACE raw images often show fine structures at such high contrast that compensation for instrumental effects is not necessary to study their morphology and evolution. Nevertheless, TRACE team members have gradually been developing the techniques and calibrations necessary to understand and (sometimes) remove the principal instrumental degradations. These degradations include CCD pedestal variation with temperature and time, flat-field response, CCD sensitivity loss varying with position and wavelength, electrical interference during CCD readout, permanent dim pixels, intermittent hot pixels, overall system point spread function, diffraction of EUV radiation by the front entrance filters, scattered light at off-limb pointings, tracks of electrons and protons from the radiation belts, and artifacts from JPEG compression of solar strucures and the above defects. Characterization and removal of some of these will be presented in the poster, such as the following. A deconvolution routine can partially compensate for the EUV diffraction, which is described in the adjacent poster by Frank et al. White light flat fields are derived using the Kuhn-Lin algorithm. CCD sensitivity degradation (presumably lumogen damage) is measured both from crude UV and EUV flat fields and from mission-long analysis of synoptic disk center images; combining these results with the WL flats yields flat fields at all wavelengths. Some information on scattered light and point spread functions are obtained from the August, 1999, eclipse observations and the Mercury transit. SSW routines for dealing with some of these degradations will be identified. This work is supported by the TRACE project at LMSAL (contract NAS5-38099). Title: Coronal Heating and Sheared Neutral Line Length in Solar Active Regions Authors: Griffiths, N. W.; Fisher, G. H.; Metcalf, T. R.; Falconer, D. A. Bibcode: 2000SPD....31.0209G Altcode: 2000BAAS...32..812G We present a comparison of X-ray and magnetic data from a sample of several hundred solar active regions, with the aim of finding which global magnetic properties are most strongly related to observed levels of coronal heating. The study includes SFD images from several different bandpasses of the Soft X-ray Telescope on the Yohkoh spacecraft, and vector magnetogram data from the Haleakala Stokes Polarimeter at the University of Hawaii's Mees Solar Observatory. Following upon earlier work by Falconer, we will examine the role that sheared neutral lines (as well as other magnetic quantities, such as magnetic flux), might play in determining the level of coronal heating. In particular, we will test whether sheared neutral line length within active regions provides a better correlation with X-ray luminosity than the established relationship between unsigned magnetic flux and X-rays. Title: Evidence for the Flare Trigger Site and 3-D Reconnection in Multi-Wavelength Observations of a Solar Flare Authors: Metcalf, T. R.; Fletcher, L.; Alexander, D.; Brown, D. S.; Ryder, L. A. Bibcode: 2000SPD....31.0261M Altcode: 2000BAAS...32..822M Based on a multi-wavelength data set and a topological model for the magnetic field, we argue that a M1.9 flare which occurred on 3-May-99 shows evidence of 3-d reconnection through a coronal null. Images from the Transition Region and Coronal Explorer allow the detailed examination of the structures involved in the flare and pre-flare in the 171 Angstroms (1MK) EUV passband, and the Lyman α (10,000-20,000K) passband. Yohkoh Hard X-ray Telescope maps the position of non-thermal electron precipitation and the Soft X-ray Telescope reveals pre-flare and flare heating on large and small-scales. While the flare appears to trigger in small scale field close to the photosphere, near the interface between strong opposite magnetic polarities, the large-scale field is directly involved in or affected by all phases of the flare. We demonstrate how this observed activity on large and small scales, along with many other aspects of the flare find a qualitative explanation in the three-dimensional reconfiguration of coronal magnetic field, following a small-scale flux cancelation at the photosphere. This work is supported by the TRACE and SXT projects at LMSAL (contract NAS5-38099 and NAS8-40801). Title: Observations of H-alpha Polarization in Flares Authors: Mickey, D. L.; Metcalf, T. R.; Fletcher, L. Bibcode: 2000SPD....31.0255M Altcode: 2000BAAS...32R.820M Max Millennium Coordinated Observing Plan #006 has as its goal the measurement of H-alpha linear polarization, a possible signature of low-energy proton beams in solar flares. Such observations have been attempted in the past, but the results were inconclusive. A campaign involving Mees Solar Observatory and Big Bear Solar Observatory, together with Yohkoh, SOHO and TRACE spacecraft, was carried out during the latter half of March 2000. We present preliminary results from the observations with the Imaging Vector Magnetograph at Mees Solar Observatory. The IVM was operated in a standard mode, except that the spectral scan was limited to one point in the core of H-alpha and one in the blue wing. This limited spectral sampling, together with a recently upgraded data acquisition system, permitted a complete measurement of Stokes vectors every 3.5 seconds. The field of view was 280 arc sec square, with one arc sec pixels. A second camera, exposed simultaneously but with a broad-band filter, provides images which allow compensation for relative image motion and stretch between exposures. Approximately two dozen flares were observed by the IVM during the campaign, including one X-class and one M-class flare. We present samples of the observations, including relevant spacecraft observations, and discuss the sensitivity of the IVM to linear polarization under these conditions. This work was supported in part by the SXT project at LMSAL (contract NAS5-38099) and by NASA grant NAG5-4941. Title: Anticipating HESSI's Spatially Resolved View of Spectral Evolution Authors: Newton, E.; Giblin, T.; Metcalf, T. Bibcode: 2000ASPC..206..272N Altcode: 2000hesp.conf..272N No abstract at ADS Title: Coronal Trapping of Energetic Flare Particles: Yohkoh/HXT Observations Authors: Metcalf, Thomas R.; Alexander, David Bibcode: 2000ASPC..206..233M Altcode: 2000hesp.conf..233M No abstract at ADS Title: Coronal Trapping of Energetic Flare Particles:Yohkoh/HXT Observations Authors: Metcalf, Thomas R.; Alexander, David Bibcode: 1999ApJ...522.1108M Altcode: We examine spectroscopic data from the Yohkoh Hard X-Ray Telescope in a search for spectral evidence of the coronal trapping of energetic particles during solar flares. Two distinct particle populations with significantly different spectral properties are found to be present in three of the six flares studied; the first population is trapped in the corona, where it encounters a ``thick-thin'' target, while the second population precipitates directly to the footpoints. In the remaining three events, a single population of energetic particles appear to be responsible for the observed hard X-ray emission, either via thermal bremsstrahlung (one case) or nonthermal thin-target emission (two cases). For the three events in which a trapped population is observed, the spectroscopic observations imply first that there is likely to be a single acceleration mechanism for both the trapped and the precipitating populations and second that the magnetic field geometry in these flares is conducive to trapping in a confined region high in the corona, above the soft X-ray loops. Both conditions are consistent with magnetic reconnection models of flares in which energetic particles are trapped between MHD slow-mode shocks attached to the reconnection region and a fast-mode shock formed by the reconnection outflow jet. Title: Making YOHKOH SXT Images Available to the Public: The YOHKOH Public Outreach Project Authors: Larson, M. B.; McKenzie, D.; Slater, T.; Acton, L.; Alexander, D.; Freeland, S.; Lemen, J.; Metcalf, T. Bibcode: 1999AAS...194.7024L Altcode: 1999BAAS...31..941L The NASA funded Yohkoh Public Outreach Project (YPOP) provides public access to high quality Yohkoh SXT data via the World Wide Web. The products of this effort are available to the scientific research community, K-12 schools, and informal education centers including planetaria, museums, and libraries. The project utilizes the intrinsic excitement of the SXT data, and in particular the SXT movies, to develop science learning tools and classroom activities. The WWW site at URL: http://solar.physics.montana.edu/YPOP/ uses a movie theater theme to highlight available Yohkoh movies in a format that is entertaining and inviting to non-scientists. The site features informational tours of the Sun as a star, the solar magnetic field, the internal structure and the Sun's general features. The on-line Solar Classroom has proven very popular, showcasing hand-on activities about image filtering, the solar cycle, satellite orbits, image processing, construction of a model Yohkoh satellite, solar rotation, measuring sunspots and building a portable sundial. The YPOP Guestbook has been helpful in evaluating the usefulness of the site with over 300 detailed comments to date. Title: The Magnetic Free Energy in Active Regions Authors: Metcalf, T. R.; Mickey, D. L. Bibcode: 1999AAS...194.9407M Altcode: 1999BAAS...31..992M The energy source for the heating of the solar corona and for solar flares is widely believed to be the solar magnetic field. While most observations of the solar magnetic field are of the photospheric magnetic field, there are a number of advantages to observing the magnetic field in the chromosphere. Most of these derive from the fact that the magnetic field in the chromosphere is force-free more than a few hundred km above the photosphere. This was first demonstrated by Metcalf et al. (ApJ, 439, 474, 1995) using NaI D-line observations from the Haleakala Stokes Polarimeter. When analyzing the magnetic field in an active region, the field is usually assumed to be force-free. The fact that it is not restricts the usefulness of such analyses. For example, the magnetic field is universally assumed to be force-free when extrapolations of the field into the corona are computed. Clearly, if the measured field really is force-free, such extrapolations are more robust. Also, when the field can be shown to be force-free, new analysis tools are available. For example, the free energy in the magnetic field can be measured using the magnetic virial theorem. This is impossible with photospheric measurements of the magnetic field (Metcalf et al., 1995). The objective of this project is to measure the solar chromospheric magnetic field using the NaI D line observed with the University of Hawaii's Imaging Vector Magnetograph (IVM). Since the magnetic field observed in the chromosphere is known to be force-free, we plan to use the data to measure the magnetic free energy in active regions. This data set will enable us to look for relationships between the free energy and coronal heating rates and flare rates. This work is supported by NASA contract NAG5-7438. Title: High Cadence EUV Imaging Spectrograph for the 2010 Solar Maximum Authors: Wuelser, J. -P.; Metcalf, T. R. Bibcode: 1999AAS...194.7616W Altcode: 1999BAAS...31..959W Recent observations by TRACE clearly show that the solar corona is both highly structured and highly time variable. To understand the physics behind the dynamical coronal processes, future observations should determine not only the morphology, but also the physical condition of the plasma. This task is best accomplished by imaging spectroscopy of the corona. However, currently planned and operating instruments fail to provide the necessary measurements at the relevant spatial and temporal scales. This paper presents the concept of an EUV imaging spectrograph that overcomes many of the limitations of current designs. The concept focusses on high sensitivity over a limited spectral range, and achieves spectroscopy with full spatial coverage at TRACE-like resolution, cadence, and signal-to-noise ratio. Title: Coronal Heating in Active Regions as a Function of Global Magnetic Variables Authors: Fisher, George H.; Longcope, Dana W.; Metcalf, Thomas R.; Pevtsov, Alexei A. Bibcode: 1998ApJ...508..885F Altcode: A comparison of X-ray images of the Sun and full disk magnetograms shows a correlation between the locations of the brightest X-ray emission and the locations of bipolar magnetic active regions. This correspondence has led to the generally accepted idea that magnetic fields play an essential role in heating the solar corona.

To quantify the relationship between magnetic fields and coronal heating, the X-ray luminosity of many different active regions is compared with several global (integrated over entire active region) magnetic quantities. The X-ray measurements were made with the SXT Telescope on the Yohkoh spacecraft; magnetic measurements were made with the Haleakala Stokes Polarimeter at the University of Hawaii's Mees Solar Observatory.

The combined data set consists of 333 vector magnetograms of active regions taken between 1991 and 1995; X-ray luminosities are derived from time averages of SXT full-frame desaturated (SFD) images of the given active region taken within +/-4 hours of each magnetogram. Global magnetic quantities include the total unsigned magnetic flux Φtot ≡ \smallint dA|Bz|, B2z,tot≡ dAB2z, Jtot ≡ \smallint dA|Jz|, and B2⊥,tot≡ dAB2⊥, where Jz is the vertical current density and Bz and B⊥ are the vertical and horizontal magnetic field amplitudes, respectively.

The X-ray luminosity LX is highly correlated with all of the global magnetic variables, but it is best correlated with the total unsigned magnetic flux Φtot. The correlation observed between LX and the other global magnetic variables can be explained entirely by the observed relationship between those variables and Φtot. In particular, no evidence is found that coronal heating is affected by the current variable Jtot once the observed relationship between LX and Φtot is accounted for. A fit between LX and Φtot yields the relationship LX ~= 1.2 × 1026 ergs s-1(Φtot/1022 Mx)1.19.

The observed X-ray luminosities are compared with the behavior predicted by several different coronal heating theories. The Alfvén wave heating model predicts a best relationship between LX and Φtot, similar to what is found, but the observed relationship implies a heating rate greater than the model can accommodate. The ``Nanoflare Model'' of Parker predicts a best relationship between LX and B2z,tot rather than Φtot, but the level of heating predicted by the model can still be compared to the observed data. The result is that for a widely used choice of the model parameters, the nanoflare model predicts 1.5 orders of magnitude more heating than is observed. The ``Minimum Current Corona'' model of Longcope predicts a qualitative variation of LX with Φtot that agrees with what is observed, but the model makes no quantitative prediction that can be tested with the data. A comparison between LX and the magnetic energy Emag in each active region leads to a timescale that is typically 1 month, or about the lifetime of an active region, placing an important observational constraint on coronal heating models.

Comparing the behavior of solar active regions with nearby active stars suggests that the relationship observed between LX and Φtot may be a fundamental one that applies over a much wider range of conditions than is seen on the Sun. Title: Microwave and Hard X-Ray Observations of an X-Class Limb Flare Authors: Nakajima, H.; Fujiki, K.; Metcalf, T. R.; Kane, S. R.; Akioka, M. Bibcode: 1998ASSL..229..295N Altcode: 1998opaf.conf..295N No abstract at ADS Title: A Spectral Analysis of the Masuda Flare Using Yohkoh Hard X-Ray Telescope Pixon Reconstruction Authors: Alexander, David; Metcalf, Thomas R. Bibcode: 1997ApJ...489..442A Altcode: Masuda's discovery of a compact hard X-ray impulsive source at the apex of a flaring coronal loop has received a great deal of recent attention in the solar physics community. The Masuda flare, which occurred on 1992 January 13, exhibited evidence of energy deposition in a compact region some distance above the soft X-ray loop, suggesting, to some authors, a flare process similar to the classical model for two-ribbon flares proposed by Shibata et al. These conclusions were made on the basis of a maximum entropy method (MEM) reconstruction of the Yohkoh Hard X-Ray Telescope (HXT) observations. Recently, a new approach has been developed for reconstructing the spatial information from the HXT: that of pixon reconstruction, proposed by Metcalf et al.

In this paper, we apply the pixon reconstruction technique to the event of 1992 January 13 and determine the temporal and spectral characteristics of the loop-top source. While our emphasis here is on the spectral properties of the Masuda flare, we also provide a brief comparison between the pixon reconstruction and that of MEM for the hard X-ray loop top. In carrying out the comparison between the methods, we have applied recent improvements to the instrument response functions and reconstruction algorithms. We have also identified a previously unknown effect of weak source suppression that was inherent in previous analyses and that significantly compromised the ability to study weak sources of hard X-ray emission in the presence of strong sources. The improved response functions and the better flux estimation used in this paper reduce (but do not eliminate) the effects of this suppression, and consequently, it should be noted that the MEM analysis presented in this paper is quite distinct from any that have been carried out previously.

Our conclusions are that (a) a compact loop-top hard X-ray source exists with an impulsive temporal profile spanning the peak of the flare; (b) the loop-top source is nonthermal in nature at the peak of the flare; (c) there is a distinct dearth of HXT LO channel emission, relative to the higher energy channels, from the loop-top region, indicating either a very hard spectrum or the presence of a low-energy cutoff in the energetic electron spectrum; (d) the footpoint and loop-top emission during the impulsive phase of the flare are produced by two distinct particle populations; (e) following the main phase of this flare, the loop top is clearly thermal in nature with a peak temperature of ~40 MK that decreases with time as the event proceeds; and (f) the disparity between the present pixon results and previous MEM results is primarily due to the intrinsically better photometry achieved by the pixon method and the avoidance of suppression effects in the present analysis. These conclusions therefore support, in part, those made in previous works, confirming the existence of an impulsive source of hard X-rays in the corona above a flaring loop. Our analysis does, however, allow for a more comprehensive understanding of the temporal and spectral development of this event in the context of an alternative reconstruction technique. 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: Evolution of a Delta Group in the Photosphere and Corona Authors: Van Driel-Gesztelti, L.; Csepura, G.; Schmieder, B.; Malherbe, J. -M.; Metcalf, T. Bibcode: 1997SoPh..172..151V Altcode: 1997ESPM....8..151V We present a study of the evolution of NOAA AR 7205 in the photosphere and corona, including an analysis of sunspot motions, and show the evolutionary aspects of flare activity using full-disc white-light observations from Debrecen, vector magnetograms from Mees Observatory, Hawaii, and Yohkoh soft X-ray observations. NOAA AR 7205 was born on the disc on 18 June, 1992. During the first 3 days it consisted of intermittent minor spots. A vigorous evolution started on 21 June when, through the emergence and merging (v ≈ 100-150 m s-1) of several bipoles, a major bipolar sunspot group was formed. Transverse magnetic fields and currents indicated the presence of shear (clockwise twist) already on 21 June (with α ≈ 0.015 Mm-1). On 23 June, new flux emerged in the trailing part of the region with the new negative polarity spot situated very close to the big positive polarity trailing spot of the main bipole. The secondary bipole seemed to emerge with high non-potentality (currents). From that time the AR became the site of recurrent flare activity. We find that all 14 flares observed with the Yohkoh satellite occurred between the highly sheared new bipole and the double-headed principal bipole. Currents observed in the active region became stronger and more extended with time. We propose that the currents have been (i) induced by sunspot motions and (ii) increased by non-potential flux emergence leading to the occurrence of energetic flares (X1.8 and X3.9). This observation underlines the importance of flare analysis in the context of active region evolution. Title: A Comparison of the MEM and Pixon Algorithms for HXT Image Reconstruction Authors: Metcalf, Thomas R.; Alexander, David; Nitta, Nariaki; Kosugi, Takeo Bibcode: 1997SPD....28.0217M Altcode: 1997BAAS...29Q.896M Recently a workshop was held in Palo Alto, CA to discuss image reconstruction for the Hard X-ray Telescope (HXT) on the Yohkoh satellite. At the workshop, the participants concluded that a detailed comparison of the primary reconstruction algorithms should be undertaken. We will report on the results of a comparison of the Maximum Entropy and Pixon algorithms using pseudo data. The comparison will check photometric accuracy, speed, and image quality using a number of test images. The test images utilized in the comparison will examine a broad range of reconstruction problems, including the ability of the algorithms to accurately reconstruct single sources, multiple sources and loop-like features, as well as the ability to reconstruct weak sources in the presence of spatially distinct bright sources. Title: Coordinated SOHO, Yohkoh, and Magnetogram Observations Of Transient Loop Brightenings Authors: Zarro, D. M.; Metcalf, T. R.; Fisher, G. H.; Siegmund, O.; Longcope, D. W.; Kucera, T.; Griffiths, N. W. Bibcode: 1997SPD....28.0503Z Altcode: 1997BAAS...29..909Z Transient soft X-ray brightenings occur frequently in solar active regions, with typical durations of 2-10 minutes. They have been observed with the Yohkoh Soft X-ray Telescope (SXT) and appear to be associated primarily with interactions of multiple loops that brighten initially near their footpoints (Shimuzu et al. 1994, Ap.J., 422, 906). Suggested mechanisms for the production of soft X-ray emission include: conduction-driven chromospheric evaporation; Alfvenic outflows from reconnection of colliding field lines; and expulsion of untwisting loop material in emerging flux tubes (Uchida and Shibata 1988, Solar Phys., 116, 291). To further study the dynamics of transient soft X-ray brightenings and their relationship to the lower atmospheric magnetic field, we have conducted a coordinated SOHO/Yohkoh campaign to observe soft X-ray brightenings in a small active region at disk center on 1996 June 6. The region was observed simultaneously by Yohkoh SXT, the Coronal Diagnostic Spectrometer (CDS) on SOHO, and the Imaging Vector Magnetograph (IVM) at Mees Observatory, Hawaii. In particular, the CDS instrument obtained Mg X (609 Angstroms) line spectra with 2-3 arcsec spatial resolution in a 2x2 arcmin field with approximately 8 minute cadence. The Mg X line is formed in the low corona at approximately 10(6) K. Individual Mg X spectra were obtained with 5 second exposures per slit position. The SXT and CDS observations show evidence of soft X-ray brightness variations on timescales of 5-10 minutes. The CDS Mg X spectra show a mixture of red and blue Doppler shifts (< 100 km s(-1) ) that are spatially associated with loop footpoints indicated by the IVM. Based on the observed temporal and spatial variations of the implied plasma upflows and downflows, we investigate the validity of different proposed models of transient soft X-ray brightenings. Title: A Reconnection Model for Observed Transient Loop Brightenings Authors: Longcope, D. W.; Fisher, G. H.; Metcalf, T. R.; Lemen, J.; Zarro, D. M.; Kucera, T.; Griffiths, N.; Siegmund, O. H. W. Bibcode: 1997SPD....28.0128L Altcode: 1997BAAS...29..884L Several recent theoretical models explain coronal activity in terms of magnetic reconnection at ``separator'' field lines. These are field lines lying at the boundary between domains of coronal flux with distinct photospheric origin. Transient brightenings of X-ray loops (Shimizu et al. 1992) may be the manifestations of such localized events (Longcope 1996). Their relative simplicity, compared to large events such as flares, makes them ideal objects for exploring reconnection models. Toward this end, a campaign of coordinated observations of loop brightenings in a small active region was undertaken on June 6, 1996. High time cadence observations were made of the transition region and low corona in EUV (SUMER and CDS) and of the high corona in soft X-rays (Yohkoh), while high cadence, high resolution vector magnetograms were obtained with the Imaging Vector Magnetograph at the University of Hawaii. This series of magnetograms is used to calculate the magnetic topology of the coronal field, and to locate the separator field lines. The high time cadence of the magnetograms allows the estimation of reconnection rates: the rate at which flux must be exchanged between domains. The "minimum current corona" model (Longcope 1996) is then used to provide quantitative predictions of energy released on each separator due to this reconnection. The observational results of the campaign are described in a companion paper by Zarro et al at this meeting; we discuss our predictions in the context of their results. Title: Movies of flares observed by YOHKOH/HXT Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.; Nitta, N. Bibcode: 1997SPD....28.0169L Altcode: 1997BAAS...29..892L The Yohkoh Hard X-ray Telescope (HXT) has revealed new information about solar flares by providing high spatial resolution (5 arcsec) images in four energy bands covering 14 to 99 keV. The comparison of images obtained in hard X-rays with those obtained with the Yohkoh Soft X-ray Telescope (SXT) has led to important interpretations of flare heating mechanisms (e.g., Masuda et al, 1994, Nature, 374, 495). The HXT images are formed by reconstructing data obtained from 64 detectors located behind a bi-grid modulation pattern. Reconstruction algorithms using Maximum Entropy and PIXON methods have been successfully applied to the HXT data (Alexander and Metcalf, 1997, ApJ, submitted). At the recent HXT Coordinated Data Analysis Workshop held in January 1997, new calibration data for the response of the grids were incorporated and an error in the application of the reconstruction algorithms was identified and corrected (see Kosugi et al 1997 in these proceedings). As a result, it is now possible to reconstruct the HXT images semi-automatically to produce quick-look movies. We are in the process of generating movies for all flares obtained with the HXT using an MEM reconstruction. The automated procedure selects time intervals on the basis of the count rate statistics. We present a selection of reconstructed images and movies from this on-going project. We expect that the ability to view easily time sequences from many flares observed with the HXT will provide new insights for flare studies. Title: Using the WWW to Make YOHKOH SXT Images Available to the Public: The YOHKOH Public Outreach Project Authors: Larson, M.; McKenzie, D.; Slater, T.; Acton, L.; Alexander, D.; Freeland, S.; Lemen, J.; Metcalf, T. Bibcode: 1997SPD....28.0231L Altcode: 1997BAAS...29..898L The Yohkoh Public Outreach Project (YPOP) is funded by NASA as one of the Information Infrastructure Technology and Applications Cooperative Agreement Teams to create public access to high quality Yohkoh SXT data via the World Wide Web. These products are being made available to the scientific research community, K-12 schools, and informal education centers including planetaria, museums, and libraries. The project aims to utilize the intrinsic excitement of the SXT data, and in particular the SXT movies, to develop science learning tools and classroom activities. The WWW site at URL: http://www.space.lockheed.com/YPOP/ uses a movie theater theme to highlight available Yohkoh movies in a non-intimidating and entertaining format for non-scientists. The site features lesson plans, 'solar' activities, slide shows and, of course, a variety of movies about the Sun. Classroom activities are currently undergoing development with a team of scientists and K-12 teachers for distribution in late 1997. We will display the products currently online, which include a solar classroom with activities for teachers, background resources, and a virtual tour of our Sun. Title: Automated Identification of Soft X-ray Coronal Loops Authors: Alexander, David; Metcalf, Thomas R. Bibcode: 1997SPD....28.0138A Altcode: 1997BAAS...29..886A The presence of loop-like structures in the solar corona is clearly evident in soft X-ray images such as those from the Soft X-ray Telescope on board the Yohkoh satellite. These structures were first discovered in Skylab data and are thought to represent the enhanced heating of the coronal plasma confined in magnetic fluxtubes. In many cases the heating is not confined to a single well defined fluxtube nor is it always strong enough to dilineate the structure sharply against the diffuse coronal background, which is itself presumably composed of loop structures. In these cases it is often extremely difficult to identify the structures involved in coronal activity. We have developed a technique which uses a Pixon reconstruction of the soft X-ray images to search specifically for loop-like structures. This allows us to pick out faint loops against a strong background and to identify multiple loop structures in bright regions. We are, therefore, better able to address the heating of the solar corona both in the diffuse and active regions. Title: Using the World Wide Web to Make YOHKOH SXT Images Available to the Public: The YOHKOH Public Outreach Project Authors: McKenzie, D.; Larson, M. B.; Slater, T.; Acton, L.; Alexander, D.; Freeland, S.; Lemen, J.; Metcalf, T. Bibcode: 1997ESASP.404..561M Altcode: 1997cswn.conf..561M No abstract at ADS Title: Pixon-based Multiresolution Image Reconstruction for Yohkoh's Hard X-Ray Telescope Authors: Metcalf, Thomas R.; Hudson, Hugh S.; Kosugi, Takeo; Puetter, R. C.; Pina, R. K. Bibcode: 1996ApJ...466..585M Altcode: We present results from the application of pixon-based multiresolution image reconstruction to real and simulated data from Yohkoh's Hard X-ray Telescope (HXT). The goal of the pixon algorithm is to minimize the number of degrees of freedom used to describe an image within the accuracy allowed by the noise. This leads to a reconstruction that is optimally constrained. We apply the pixon code to two solar flares in the HXT database and compare the results of the pixon reconstruction to the results of a direct, linear, smoothed inversion of the HXT Fourier synthesis data and to a maximum entropy reconstruction. The maximum entropy reconstruction is vastly better than the direct inversion, but the pixon reconstruction gives superior noise suppression and photometry. Further, the pixon reconstruction does not suffer from overresolution of the images. 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: Microwave and hard X-ray sources in two X-class limb flares Authors: Nakajima, Hiroshi; Metcalf, Thomas R. Bibcode: 1996AIPC..374..393N Altcode: 1996hesp.conf..393N We have analyzed two intense, extended microwave and hard X-ray flares which occurred slightly behind the west limb and were accompanied by X-class, long-duration events (LDEs) in soft X-rays. We have found that: (1) Both events have typical soft X-ray properties of LDEs. (2) A major hard X-ray source at 23-33 keV and 33-53 keV is located in the high-temperature region, considerably higher than the corresponding soft X-ray loop, while a 17 GHz microwave source, where electrons at energy ≥ several hundreds keV mainly contribute to the 17 GHz emission, nearly coincides with the soft X-ray loop in extent. (3) The electron spectrum, which is derived from the microwave spectrum, is significantly harder than the electron spectrum at energy ≤100 keV, which is derived from the hard X-ray data. (4) The time profile of total intensity at 17 GHz is delayed by about 25 s with respect to that of the hard X-rays for one of the events. The above results show evidence that lower-energy (≤100 keV) electrons with a softer spectrum are accelerated at the top of the cusp, and that higher-energy (≫100 keV) electrons are accelerated at the top of the soft X-ray loop and trapped. Probably, the lower-energy electrons accelerated in the cusp are transferred by a reconnection outflow into the second acceleration region at the top of the soft X-ray loop, and further accelerated to higher energy. Title: A Test of a New Flare Loop Scaling Law Using YOHKOH SXT and GOES Observations Authors: Metcalf, Thomas R.; Fisher, George H. Bibcode: 1996ApJ...462..977M Altcode: We compare the lengths of coronal loops observed on the Sun using the Soft X-Ray Telescope on Yohkoh with those predicted from the theoretical model of Hawley et al. This model relates the temporal behavior of coronal emission observed during flares on active stars to the length of the loops undergoing flaring. In that model, the footpoint-to-footpoint loop length 2L (measured in kilometers) should obey the relationship 2L ≃ 0.01Y where Y = l.25πr3/7τd4/7TA½, τr is the "rise time" (in seconds) from flare onset to flare peak, τd is the "decay time" (s) from flare peak to the time when the emission measure is 25% of its peak value, and TA (K) is the temperature at the top of the loop at flare peak. The observations show a strong correlation between 2L and Y, with the theoretical relationship consistent with observed loop lengths in most cases. For a few loops, the theoretical model tends to somewhat over-predict the loop length. When the observed loop lengths are fitted to a single power-law relationship in Y, we find the data are best fitted by log 2L = log (0.44±0.06) + (0.75±0.05) log Y. We offer no quantitative theoretical justification for this relationship, but we do suggest several reasons why the Hawley et al. model over-predicts loop lengths in some cases. Title: Coronal Heating in Active Regions as a Function of Global Magnetic Variables Authors: Fisher, G. H.; Longcope, D. W.; Metcalf, T. R.; Pevtsov, A. A. Bibcode: 1996AAS...188.3304F Altcode: 1996BAAS...28..868F A comparison of X-ray images of the Sun and full disk magnetograms shows a correlation between the locations of the brightest X-ray emission and the locations of bipolar magnetic regions. This correspondence has led to the generally accepted idea that magnetic fields play an essential role in heating the Solar corona. To quantify the relationship between magnetic fields and coronal heating, we have compared the X-ray luminosity of many different Active Regions with several global (integrated over entire active region) magnetic quantities. The X-ray measurements were made with the SXT Telescope on the Yohkoh spacecraft; magnetic measurements were made with the Haleakala Stokes Polarimeter at the University of Hawaii's Mees Solar Observatory. Our combined dataset consists of 333 vector magnetograms of active regions taken between 1991 and 1995; SXT luminosities consist of time averages of SFD images of the given active region taken within +/- 4 hours of each magnetogram. Global magnetic quantities include the total unsigned magnetic flux, area integrals of B(2) , J_z(2) (J_z is the vertical component of the electric current density), and the best-fit alpha of the linear force-free field for the entire active region (nabla x B = alphaB ). Our results show clear and unmistakable relationships between the X-ray luminosity and most of these magnetic variables. The relationship between total unsigned magnetic flux and X-ray luminosity is especially compelling, holding over 2 orders magnitude in both quantities. These measurements provide important contraints on coronal heating mechanisms. This work was supported in part by NASA grant NAGW-3429, NSF grant AST-9218085, and Cal Space grant CS-17-95. Title: Pixon Reconstruction and the Masuda Event of 1992 January 13 Authors: Alexander, D.; Metcalf, T.; Hudson, H. S. Bibcode: 1996ASPC..111..253A Altcode: 1997ASPC..111..253A The data set of localised hard X-ray sources observed by the Yohkoh/HXT has been analysed using maximum entropy methods (MEM) to reconstruct HXT images. Recently, an alternative method, that of fractal pixon reconstruction, has been developed for use with the HXT. The authors have reanalysed the event of 13 January 1992 (the Masuda event), comparing the MEM and pixon methods. There are distinct differences in the two sets of results. The pixon method, favoured by the authors, indicates a less impulsive coronal source than the MEM reconstruction and also a relatively weaker coronal/footpoint emission ratio. Title: Evolution of two small solar flares. Authors: Fludra, A.; Doyle, J. G.; Metcalf, T.; Lemen, J. R.; Phillips, K. J. H.; Culhane, J. L.; Kosugi, T. Bibcode: 1995A&A...303..914F Altcode: Data from the YOHKOH satellite have been analysed for two small flares (GOES class C) of total duration of 10 and 60 minutes. Upflows in S XV, Ca XIX and Fe XXV lines were compared and the presence of a range of upflow velocities was found. Emission from flare loop footpoints corresponding to plasma moving with a typical velocity of 200-400km/s is seen in soft X-ray images. In one of these events (23 June 1992), which occurred in sheared loops being part of a sparse magnetic arcade, with initial energy release taking place near one of the footpoints, a large proportion of upflowing plasma was seen at least 1 minute before the first peak in hard X-rays. In the second event (13 July 1992), the increase of soft X-ray emission began more than 3 minutes before, and weak mass upflows one minute before the rapid increase of temperature and the onset of the detectable hard X-ray emission. This event was probably triggered by emerging magnetic flux and accompanied by heating and restructuring of two nearby magnetic loops. In both events the emission measure of upflowing plasma is present simultaneously to, and is very well correlated in time with the hard X-ray flux in the 14-23keV band. Differential emission measure in the temperature range 5-60x10^6^K was derived from S XV, Ca XIX and Fe XXV line and continuum fluxes, and from images in two broad band soft X-ray filters, and used to analyse the thermal contribution to the hard X-ray emission. A non-thermal component of the hard X-ray emission is found at the peak of the 23 June 1992 flare. The hard X-ray emission in the 13 July 1992 flare is primarily thermal, however, a possibility of an enhanced tail of the electron energy distribution above 14keV is also indicated. The chromospheric evaporation in these flares was driven both by electron beams and thermal conduction, with conduction predominating during most of the rise phase of the 13 July 1992 flare. In both events, the soft X-ray emission measure at flare maximum was a few times 10^48^cm^-3^ with an electron temperature 19 and 24x10^6^K; the estimated lower limit of the electron density is ~10^11^cm^-3^. The broadening of Ca XIX spectral lines in the decay phase of these flares indicates persisting random motions with a velocity of 60km/s, which is very similar to the non-thermal broadening observed previously by SMM in M and X class flares. Title: Fractal Pixon Image Reconstruction for Yohkoh's Hard X-Ray Telescope Authors: Metcalf, T. R.; Hudson, H. S.; Kosugi, T.; Puetter, R. C.; Piña, R. K. Bibcode: 1995SPD....26.1314M Altcode: 1995BAAS...27..990M 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: 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: Resolving the 180-degree ambiguity in vector magnetic field measurements: The `minimum' energy solution Authors: Metcalf, Thomas R. Bibcode: 1994SoPh..155..235M Altcode: I present a robust algorithm that resolves the 180-deg ambiguity in measurements of the solar vector magnetic field. The technique simultaneously minimizes both the divergence of the magnetic field and the electric current density using a simulated annealing algorithm. This results in the field orientation with approximately minimum free energy. The technique is well-founded physically and is simple to implement. 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: Temperature Analysis of the Post-Flare Loops of June 25-26, 1992 Authors: Anwer, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Lemen, J.; Metcalf, T. R. Bibcode: 1994kofu.symp..137A Altcode: We have performed an analysis of temperatures and emission measures of thermal plasma on a post--flare loop system following an X3.9 flare of June 25, 1992, at 20:14 UT in NOAA active region 7205 near the west limb (N09, W67). The filter ratio method was applied to the data sets taken using the Al 0.1 micron (thin Al) and Al 12 micron (thick Al) filters of the Yohkoh Soft X-ray Telescope (SXT). We found that the plasma temperature of the top of loops was in the range 5 - 8 x 10^6 K and log emission measure between 44.6 and 46.7 cm^(-3) for data sets taken from 22:56:57 UT of June 25 to 09:00 UT of June 26. Furthermore, the occurrence of a C1-class flare at the top of the flare loops increased the plasma temperature from 5.5 x 10^6 K to 6.6 x 10^6 K at 06:57:11 UT. The loops top was much brighter than the legs and footpoints, with delta_T was about 0.1 x 10^6 K. Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux Authors: Nitta, N.; Driel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.; Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K. Bibcode: 1994kofu.symp..385N Altcode: Active region NOAA 7260 exhibited remarkable flare activity as an emerging flux region appeared in the following part and evolved into the delta configuration. While it is difficult to associate an emerging bipole with a flare both temporally and spatially, there is an overall correlation of the total darkness integrated over of the sunspot area, as measured in the Yohkoh/SXT white-light images, with the soft X-ray flux and flare occurrence. It appears that the flares in the emerging flux region occurred preferentially at locations close to the spot of preceding polarity that emerged in the earliest evolution of the region. 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: 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: Bayesian Timing Analysis of Three Cataclysmic Binary Systems Authors: Freeman, P. E.; Metcalf, T. H.; Lamb, D. Q. Bibcode: 1994AIPC..313..294F Altcode: 1994sxrc.conf..294F The identification of periodic signals from cataclysmic binary systems is crucial for classifying them and interpreting their behavior. We use the Bayesian timing analysis technique of Gregory & Loredo to attempt to detect signals in the ROSAT PSPC data of three such systems. This technique, conceptually similar to, but more sensitive than, epoch folding, involves directly comparing a model with no count rate variability with one describing the count rate as a stepwise distribution with m bins per period of length P, and phase φ (relative to the start of observation). This model comparison allows the computation of the odds favoring the more complex model. If the odds strongly support variability, one then estimates the period of variation by integrating the likelihood function over all values of m and φ for each value of P.

With this technique, we attempt to find the signal which Wood et al. interpret as that of the orbital period, Porb, of the He white dwarf PG 1346+082. We also attempt to find the signals at the spin period, Pspin, and beat period, Pbeat, of the DQ Her-type source 1H0542-407 and the possibly DQ Her-type source V426 Oph. Detection of a spin and beat period in V426 Oph would indicate that the binary system is not synchronized, aiding the DQ Her classification. We find that the odds do not favor a variable rate model for PG1346+082. The odds do strongly favor variable rate models for the latter two sources, but we cannot conclusively determine periods of variation, because of insufficient data and large gaps in the observations. Title: Dynamics of a Low Energetic Solar Flare Authors: Doyle, J. G.; Fludra, A.; Bentley, R. D.; Culhane, J. L.; Metcalf, T.; Lemen, J. R.; Kosugi, T. Bibcode: 1994ASPC...64..402D Altcode: 1994csss....8..402D No abstract at ADS 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: 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: The 1992 January 5 Flare at 13.3 UT: Observations from YOHKOH Authors: Doschek, G. A.; Strong, K. T.; Bentley, R. D.; Brown, C. M.; Culhane, J. L.; Fludra, A.; Hiei, E.; Lang, J.; Mariska, J. T.; Phillips, K. J. H.; Pike, C. D.; Sterling, A. C.; Watanabe, T.; Acton, L. W.; Bruner, M. E.; Hirayama, T.; Tsuneta, S.; Rolli, E.; Kosugi, T.; Yoshimori, M.; Hudson, H. S.; Metcalf, T. R.; Wuelser, J. -P.; Uchida, Y.; Ogawara, Y. Bibcode: 1993ApJ...416..845D Altcode: We discuss X-ray spectra and soft X-ray images of an M1.9 flare that occurred on 1992 January 5 near 13.3 UT. These data were obtained with instrumentation on the Japanese Yohkoh spacecraft. They cover the entire rise phase of the flare. To supplement these data we have ground-based magnetograms and Hα spectroheliograms. We calculate the electron temperature and emission measure of the flare as a function of time during the early rise phase using X-ray spectral line intensities and line ratios. Using spectral line widths, line profile asymmetries, and wavelength shifts due to the Doppler effect, we calculate the dynamical properties of the flare. The time development of the morphology of the flare, as revealed by the soft X-ray images and the Hα spectroheliograms, and the physical quantities inferred from the X-ray spectra, are compared with chromospheric evaporation models. There is an enhancement of blueshifted emission that is closely correlated with the hard X-ray bursts. Heating of one loop in the flare is consistent with a conduction-evaporation model, but heating is found in several structures that do not appear to be physically associated with each other. No standard evaporation model can adequately explain all of the observations. 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: 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: On the Measurement of the Chromospheric Magnetic Field Using the Na I γ5896Å Spectral Line Authors: Jiao, L.; Metcalf, T. R.; Uitenbroek, H. Bibcode: 1993BAAS...25.1206J 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: Full-Disk Magnetogram Cross-correlations at Long Time Lags Authors: Snodgrass, H. B.; Metcalf, T.; Vandekop, T. Bibcode: 1993BAAS...25.1194S Altcode: No abstract at ADS Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux and Reconnection Authors: Nitta, N.; Drel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.; Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K. Bibcode: 1993BAAS...25.1223N Altcode: No abstract at ADS Title: Correlation between X-ray Temporal Variability and Magnetic Environment in Solar Flares Authors: Nitta, N.; Harvey, K.; Hudson, H.; Ichimoto, K.; Metcalf, T.; Mickey, D.; Sakai, J. -I.; Sakao, T.; Sakurai, T.; Takahashi, M. Bibcode: 1992AAS...181.5503N Altcode: 1992BAAS...24.1211N The X-ray time history of a solar flare can reflect basic processes of heating and/or acceleration, which in turn may depend on the magnetic environment of the site. Some flares show a simple rise and fall temporal behavior, whereas others show more than one peak. Comparisons of images taken by the Soft X-ray Telescope (SXT) aboard the Yohkoh spacecraft with ground-based magnetic data (Hawaii, Kitt Peak and Mitaka) reveal that, at least for a flare-productive active region (NOAA 7260), flares with double-peaked and single-peaked time profiles occurred at systematically different locations within the region. We discuss this result in terms of theoretical models, especially those of coalescence of two current loops. 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: 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: 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: 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: 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: 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: Simultaneous EUV, Microwave, and Magnetic Field Observations of Solar Active Regions Authors: Brosius, J. W.; Davila, J. M.; Thompson, W. T.; Gopalswamy, N.; White, S. M.; Jones, H. P.; Metcalf, T. R. Bibcode: 1991BAAS...23.1388B Altcode: No abstract at ADS Title: 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: 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: 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: 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: 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: 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: Flare Heating and Ionization of the Low Solar Chromosphere. Authors: Metcalf, Thomas Robert Bibcode: 1990PhDT.........6M Altcode: In this dissertation, the long-standing question of what mechanisms are responsible for observations of flare heating at the temperature minimum region of the solar atmosphere is examined. Two neutral magnesium spectral lines formed in the temperature minimum region and the low chromosphere, lambdalambda4571 and 5173 A, were observed at the Sacramento Peak Observatory in February, 1986, and are used to quantify the changes in the atmospheric structure as a function of time. To accomplish this, a technique for inverting the Mg scI lines to yield the vertical temperature and electron density structure in the atmosphere is described. The inversion uses a first order perturbation analysis applied to a model atmosphere to determine the changes in the model's temperature and electron density structures required to bring the calculated and observed spectral lines into agreement. Since small perturbations in the observed lines can yield large changes in the required perturbation of the model atmosphere, the inversion must be stabilized and a smoothness constraint is imposed on the solution to ensure stability. After a discussion of eight proposed heating and ionization mechanisms and predictions of the effects of each on the temperature minimum region, this inversion technique is applied to five separate solar flares, yielding the changes in temperature and electron density as a function of time and atmospheric depth. These observations, 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 to show that 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 we eliminate heating by the dissipation of non -reversed electric currents, collisions with an electron or proton beam, irradiation by soft X-rays, and dissipation of Alfven waves. 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: The Global Energy Balance in Flares Authors: Simnett, G. M.; Linford, G. A.; Metcalf, T. R.; Henoux, J. C.; Spicer, D. S. Bibcode: 1989tnti.conf....4S 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: 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: 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: 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: 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: 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: 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: 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