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The physical processes that trigger solar flares are not well understood and significant debate remains around processes governing particle acceleration, energy partition, and particle and energy transport. Observations at high resolution in energy, time, and space are required in multiple energy ranges over the whole course of many flares in order to build an understanding of these processes. Obtaining high-quality, co-temporal data from ground- and space- based instruments is crucial to achieving this goal and was the primary motivation for starting the Max Millennium program and Major Flare Watch (MFW) alerts, aimed at coordinating observations of all flares >X1 GOES X-ray classification (including those partially occulted by the limb). We present a review of the performance of MFWs from 1 February 2001 to 31 May 2010, inclusive, that finds: (1) 220 MFWs were issued in 3,407 days considered (6.5% duty cycle), with these occurring in 32 uninterrupted periods that typically last 2-8 days; (2) 56% of flares >X1 were caught, occurring in 19% of MFW days; (3) MFW periods ended at suitable times, but substantial gain could have been achieved in percentage of flares caught if periods had started 24 h earlier; (4) MFWs successfully forecast X-class flares with a true skill statistic (TSS) verification metric score of 0.500, that is comparable to a categorical flare/no-flare interpretation of the NOAA Space Weather Prediction Centre probabilistic forecasts (TSS = 0.488)., 19 pages, 2 figures, accepted for publication in Solar Physics

We study the X17 eruptive flare on 2003 October 28 using Michelson Doppler Imager observations of photospheric magnetic and velocity fields and TRACE 1600 A images of the flare in a three-dimensional model of energy buildup and release in NOAA 10486. The most dramatic feature of this active region is the 123° rotation of a large positive sunspot over 46 hr prior to the event. We apply a method for including such rotation in the framework of the minimum current corona model of the buildup of energy and helicity due to the observed motions. We distinguish between helicity and energy stored in the whole active region and that released in the flare itself. We find that while the rotation of a sunspot contributes significantly to the energy and helicity budgets of the whole active region, it makes only a minor contribution to that part of the region that flares. We conclude that in spite of the fast rotation, shearing motions alone store sufficient energy and helicity to account for the flare energetics and interplanetary coronal mass ejection helicity content within their observational uncertainties. Our analysis demonstrates that the relative importance of shearing and rotation in this flare depends critically on their location within the parent active region topology.

We use the Michelson Doppler Imager and TRACE observations of photospheric magnetic and velocity fields in NOAA 10759 to build a three-dimensional coronal magnetic field model. The most dramatic feature of this active region is the 34 ◦ rotation of its leading polarity sunspot over 40 hr. We describe a method for including such rotation in the framework of the Minimum Current Corona model. We apply this method to the buildup of energy and helicity associated with the eruptive flare of 2005 May 13. We find that including the sunspot rotation almost triples the modeled flare energy (1.0 × 10 31 erg) and flux rope self-helicity (−7.1 × 10 42 Mx 2 ). This makes the results consistent with observations: the energy derived from GOES is 1.0 × 10 31 erg, the magnetic cloud helicity from WIND is −5 × 10 42 Mx 2 . Our combined analysis yields the first quantitative picture of the helicity and energy content processed through a flare in an active region with an obviously rotating sunspot and shows that rotation dominates the energy and helicity budget of this event.

In order to better understand magnetic reconnection and particle acceleration in solar flares, we compare the RHESSI hard X-ray (HXR) footpoint motions of three flares with a detailed study of the corresponding topology given by a Magnetic Charge Topology model. We analyze the relationship between the footpoint motions and topological spine lines and find that the examined footpoint sources move along spine lines. We present a three-dimensional topological model in which this movement can be understood. As reconnection proceeds, flux is transferred between the reconnecting domains, causing the separator to move. The movement of the separator's chromospheric ends, identified with the HXR footpoints, is along those spine lines on which the separator ends.

We examine the hard X-ray (HXR) footpoint sources of three flares, as observed by RHESSI, in combination with the topology given by the extrapolation of line-of-sight magnetograms into the corona. Assuming the HXR footpoint sources are chromospheric consequences of magnetic reconnection that takes place on separators, we further assume a relationship between the buildup of energy in stressed coronal magnetic fields and the measurement of the change in separator flux per unit length. We find that the value of this quantity is larger for the separators that connect the HXR footpoint sources than the quantity for the separators that do not. Therefore, we conclude that we are able to understand the location of HXR sources observed in flares in terms of a physical and mathematical model of the topology of the active region.

We study the statistical significance of observed temporal variations of the solar active-region hemispheric helicity rule, as measured by the latitudinal gradient of the best-fit linear force-free-field parameter, -->dα/d . Using data from four different vector magnetographs, we compute and compare average annual -->dα/d values for these instruments for 19 years from solar cycles 21, 22, and 23. We find that although every instrument shows the wrong sign for the hemispheric rule in some years, there is no agreement among the instruments on which years are abnormal. None of the four data sets shows annual values of -->dα/d departing from the hemispheric helicity rule by more than 3 σ. We conclude that because the hemispheric helicity rule is a weak tendency with significant scatter, an annual subset of active regions is likely to produce statistically unreliable results.

The properties and dynamics of magnetic fields on the Sun's photosphere and outer layers—notably those within solar active regions—govern the eruptive activity of the Sun. These photospheric magnetic fields also act as the evolving lower boundary of the Sun–Earth coupled system. Quantifying the physical attributes of these magnetic fields and exploring the mechanisms underlying their influence on the near-Earth space environment are of vital importance for forecasting and mitigating adverse space weather effects. In this context, we discuss here a novel technique for measuring twist in the magnetic field lines of solar active regions that does not invoke the force-free field assumption. Twist in solar active regions can play an important role in flaring activity and the initiation of CMEs via the kink instability mechanism; we outline a procedure for determining this solar active region eruptive potential. We also discuss how twist in active region magnetic fields can be used as inputs in simulations of the coronal and heliospheric fields; specifically, we explore through simulations, the formation, evolution and ejection of magnetic flux ropes that originate in twisted magnetic structures. The results and ideas presented here are relevant for exploring the role of twisted solar active region magnetic fields and flux ropes as drivers of space weather in the Sun–Earth system.

Aims. Coronal sigmoids are important sources of eruptions into interplanetary space, and a handful of models have been proposed to explain their characteristic S shape. However, the coronal X-ray images available to date have generally not had sufficient resolution to distinguish between these models. The goal of the present investigation is to determine whether the new observations from Hinode can help us to make such a distinction. Methods. We present the first observations of a persistent coronal sigmoid obtained with the Hinode X-Ray Telescope (XRT). The excellent angular resolution of XRT (1 arcsec per pixel) and the sigmoid’s location near disk center combined to provide an unprecedented view of the formation and eruption of this phenomenon. We compared the observed morphology with expectations inferred from two popular models of sigmoid formation, the bald-patch separatrix surface model and the kinking flux rope model. Results. The images during the pre-eruptive phase show that the overall S shape of the sigmoid comprises two separate J-shaped bundles of many loops. The straight sections of the two J patterns lie anti-parallel to one another in the middle of the S, on opposite sides of the magnetic polarity inversion line. The images during the eruptive phase reveal that, before any soft X-ray flaring begins, a diffuse linear structure almost as long as the sigmoid lifts off from the middle of the S. It shows slight clockwise rotation. The X-ray flare begins with the appearance of a sheared arcade of short loops, in the area centered between the two J-shaped patterns of the sigmoid. Conclusions. Taken together, the observational findings provide strong support for the bald-patch separatrix surface model for this sigmoid.

We study the structure of 107 bright sigmoids using full-resolution (2.5'' pixels) images from the Yohkoh Soft X-Ray Telescope (SXT) obtained between 1991 December and 2001 December. We find that none of these sigmoids are made of single loops of S or inverse-S shape; all comprise a pattern of multiple loops. We also find that all S-shaped sigmoids are made of right-bearing loops and all inverse-S-shaped sigmoids of left-bearing loops, without exception. We co-align the SXT images with Kitt Peak magnetograms to determine the magnetic field directions in each sigmoid. We use a potential-field source surface model to determine the direction of the overlying magnetic field. We find that sigmoids for which the relative orientation of these two fields has a parallel component outnumber antiparallel ones by more than an order of magnitude. We find that the number of sigmoids per active region varies with the solar cycle in a manner that is consistent with this finding. Finally, those few sigmoids that are antiparallel erupt roughly twice as often as those that are parallel. We briefly discuss the implications of these results in terms of formation and eruption mechanisms of flux tubes and sigmoids.

We explore the properties of the fragmentation of magnetic flux in solar active regions. We apply gradient-based tessellation to magnetograms of 59 active regions to identify flux fragments. First, we find that the distribution function of flux fragments in these regions is highly consistent with lognormal form, which is the most direct evidence yet obtained that repeated random bifurcation dominates fragmentation and coalescence in all active regions. Second, we apply nonparametric statistical methods to the variance of the lognormal distribution of fragment flux, the flare X-ray energy output of the active regions, and kinetic helicity measurements in the upper convection zone to show that there is no significant statistical relationship between the amount of fragmentation of an active region's flux at photospheric levels and the amplitude of either its average kinetic helicity density in the upper convection zone or its X-ray flare energy output.

To better understand eruptive events in the solar corona, we combine sequences of multi-wavelength observations and modelling of the coronal magnetic field of NOAA AR 8210, a highly flare-productive active region. From the photosphere to the corona, the observations give us information about the motion of magnetic elements (photospheric magnetograms), the location of flares (e.g., H$\alpha$, EUV or soft X-ray brightenings), and the type of events (H$\alpha$ blueshift events). Assuming that the evolution of the coronal magnetic field above an active region can be described by successive equilibria, we follow in time the magnetic changes of the 3D nonlinear force-free (nlff) fields reconstructed from a time series of photospheric vector magnetograms. We apply this method to AR 8210 observed on May 1, 1998 between 17:00 UT and 21:40 UT. We identify two types of horizontal photospheric motions that can drive an eruption: a clockwise rotation of the sunspot, and a fast motion of an emerging polarity. The reconstructed nlff coronal fields give us a scenario of the confined flares observed in AR 8210: the slow sunspot rotation enables the occurence of flare by a reconnection process close to a separatrix surface whereas the fast motion is associated with small-scale reconnections but no detectable flaring activity. We also study the injection rates of magnetic energy, Poynting flux and relative magnetic helicity through the photosphere and into the corona. The injection of magnetic energy by transverse photospheric motions is found to be correlated with the storage of energy in the corona and then the release by flaring activity. The magnetic helicity derived from the magnetic field and the vector potential of the nlff configuration is computed in the coronal volume. The magnetic helicity evolution shows that AR 8210 is dominated by the mutual helicity between the closed and potential fields and not by the self helicity of the closed field which characterizes the twist of confined flux bundles. We conclude that for AR 8210 the complex topology is a more important factor than the twist in the eruption process.

Together with the magnetic energy, the magnetic helicity is an important quantity used to describe the nature of a magnetic field configuration. In the following, we propose a new technique to evaluate various components of the total magnetic helicity in the corona for an equilibrium reconstructed magnetic field. The most meaningful value of helicity is the total relative magnetic helicity which describes the linkage of the field lines even if the volume of interest is not bounded by a magnetic surface. In addition if the magnetic field can be decomposed into the sum of a closed field and a reference field (following , Berger 1999 in Magnetic Helicity in Space and Laboratory Plasmas, ed. M. R. Brown, R. C. Canfield, & A. A. Pevtsov, 1), we can introduce three other helicity components: the self helicity of the closed field, the mutual helicity between the closed field and the reference field, and the vacuum helicity (self helicity of the reference field). To understand the meaning of those quantities, we derive them from the potential field (reference) and the force-free field computed with the same boundary conditions for three different cases: a single twisted flux tube derived from the extended Gold-Hoyle solutions, a simple magnetic configuration with three balanced sources and a constant distribution of the force-free parameter, and the AR 8210 magnetic field observed from 17:13 UT to 21:16 UT on May 1, 1998. We analyse the meaning of the self and mutual helicities: the self and mutual helicities correspond to the twist and writhe of confined flux bundles, and the crossing of field lines in the magnetic configuration respectively. The main result is that the magnetic configuration of AR 8210 is dominated by the mutual helicity and not by the self helicity (twist and writhe). Our results also show that although not gauge invariant the vacuum helicity is sensitive to the topological complexity of the reference field.

Twisted magnetic field lines in solar active regions constitute stressed flux systems, the reconnection of which can release the stored (excess) magnetic energy in the form of solar flares. Using co-registered photospheric vector magnetograms and chromospheric Hα images for 29 flares, we explore the spatial relationship between these flares and the magnetic topology of the active regions in which they occur. We find two dominant trends. First, flares are preferentially initiated in subregions that have a high gradient in twist. Second, flare initiation occurs close to chirality inversion lines (which separate regions with twist of opposite handedness). Our results demonstrate that magnetic helicity, as manifested in the twist parameter, plays an important role in magnetic reconnection and solar flaring activity.

Tilt and twist are two measurable characteristics of solar active regions that can give us information about subsurface physical processes associated with the creation and subsequent evolution of magnetic flux tubes inside the Sun. Using Mees Solar Observatory active region vector magnetograms and Mount Wilson Observatory full-disk longitudinal magnetograms, we measure the magnetic twist and tilt angles of 368 bipolar active regions. In addition to two well-known phenomena, Joy's law and the hemispheric helicity rule, this data set also shows a lesser known twist-tilt relationship, which is the focus of this study. We find that those regions that closely follow Joy's law do not show any twist-tilt dependence. The dispersion in tilt angles and the dispersion in twist are also found to be uncorrelated with each other. Both of these results are predicted consequences of convective buffeting of initially untwisted and unwrithed flux tubes through the Σ-effect. However, we find that regions that strongly depart from Joy's law show significantly larger than average twist and very strong twist-tilt dependence—suggesting that the twist-tilt relationship in these regions is due to the kinking of flux tubes that are initially highly twisted, but not strongly writhed. This implies that some mechanism other than the Σ-effect (e.g., the solar dynamo itself or the process of buoyancy instability and flux tube formation) is responsible for imparting the initial twist (at the base of the solar convection zone) to the flux tubes that subsequently become kink-unstable.

We have studied Mees Solar Observatory Hα spectroheliogram and Doppler velocity movies of surges in 11 active regions. We used these movies to observe the surges' rotating motion, direction of propagation, and the implied handedness of that motion. We were able to coregister movies containing 47 surges with Haleakala Stokes Polarimeter vector magnetograms. We could hence determine the direction of twist stored in the magnetic field at the point of origin of each surge. We found (with a 99% confidence) that the direction of observed spin of these surges is consistent with the relaxation of the stored twist in the magnetic field. Magnetic reconnection of twisted flux tubes with their less twisted surroundings can account for the production and rotating motion of these surges.

Using photospheric line-of-sight magnetograms from the National Solar Observatory (NSO) Kitt Peak and coronal X-ray images from the Yohkoh Soft X-Ray Telescope (SXT), we have determined the value of the constant α of the linear force-free field model (∇ × = α) that gives the best visual fit to the overall coronal X-ray structure (αc) of 34 flare-productive active regions of relatively simple bipolar morphology. Vector magnetograms for 24 of these active regions are available from the Haleakala Stokes Polarimeter at Mees Solar Observatory. For each of them, we determine the single best-fit value of α in the photosphere (αp) by three quite different methods and show that these methods give statistically consistent values. By combining this data set with that of the NSO and SXT, we are able to compare for the first time quantitatively and statistically the observed values of α in the photosphere and corona of these regions. We find that the distribution of αp and αc values is fully consistent with the hypothesis that the overall twist density of the magnetic fields of active regions, as measured by the linear force-free field parameter α, is the same in the photosphere and the corona. We therefore conclude that the electric currents that create the nonpotential structure of such solar coronal active regions are of subphotospheric origin and pass without significant modification through the photosphere.

We report the results of a statistical study of the relationship between eruptive solar flares and an observed Hα preflare phenomenon we call moving blueshift events (MBSEs). The Hα data were gathered using the Mees Solar Observatory CCD imaging spectrograph (MCCD). The 16 events in our data set were observed by both the MCCD and the Yohkoh Soft X-Ray Telescope, typically for at least 3 hr prior to the flare and in some cases repeatedly for several days prior to the flare. The data set contains both eruptive and noneruptive flares, without bias. Focusing on 3 hr periods before and after the flares, we found that the average rate of MBSEs prior to the flares was ~5 times greater prior to the 11 eruptive flares than prior to the five noneruptive ones. Also, the average rate of MBSEs dropped by a factor of ~6 after the eruptive flares. Earlier studies inferred that MBSEs reflect motions that originate in the readjustment of magnetic fields after magnetic reconnection. From the high correlation between eruptive flares and preflare MBSEs in the several hours prior to such events, we conclude that reconnection in the chromosphere or low corona plays an important role in establishing the conditions that lead to solar flare eruptions.

The relaxation dynamics of a magnetized plasma system is a subject of fundamental importance in magnetohydrodynamics—with applications ranging from laboratory plasma devices such as the toroidal-field pinch and spheromaks to astrophysical plasmas, stellar flaring activity, and coronal heating. Taylor in 1974 proposed that the magnetic field in a plasma, subject to certain constraints, relaxes to a minimum energy state such that the final magnetic field configuration is a constant α (linear) force-free field—where α is a quantity describing the twist in magnetic field lines. While Taylor's theory was remarkably successful in explaining some intriguing results from laboratory plasma experiments, a clear signature of this mechanism in astrophysical plasmas remained undetected. Here we report observational detection of a relaxation process, similar to what Taylor envisaged, in the magnetic fields of flare-productive solar active regions. The implications of this result for magnetic reconnection and the coronal heating problem are discussed.

We report the results of two simple studies that seek observational evidence that solar coronal loops are unstable to the MHD kink instability above a certain critical value of the total twist. First, we have used Yohkoh soft X-ray telescope image sequences to measure the shapes of 191 X-ray sigmoids and to determine the histories of eruption (evidenced by cusp and arcade signatures) of their associated active regions. We find that the distribution of sigmoid shapes is quite narrow and the frequency of eruption does not depend significantly on shape. Second, we have used Mees Solar Observatory vector magnetograms to estimate the large-scale total twist of active regions in which flare-related signatures of eruption are observed. We find no evidence of eruption for values of large-scale total twist remotely approaching the threshold for the kink instability.

The magnetic flux of solar coronal active regions is thought to originate in strong toroidal magnetic fields generated by a dynamo at the base of the convection zone. Once generated, this magnetic flux rises through the convection zone as discrete buoyant flux tubes, which may be formed into S2-shaped loops by their interaction with convective: cells and strong downdrafts. The loops are prevented from fragmentation by twist and curvature of their axes, which are writhed by the Coriolis effect and helical convective turbulence. These Σ-shaped loops emerge through the photosphere to form dipolar sunspot pairs and coronal active regions. These regions' free energy, relative magnetic helicity, and tendency to flare and erupt reflect the convection zone phenomena that dominate their journey to the surface, in which helical convective turbulence appears to play a primary role. Recent research leads me to suggest a new paradigm for activity in solar-type stars with deep-seated (tachocline) dynamos. In the present paradigm, dynamo models are expected to explain the distribution of activity in the H-R diagram, as reflected in mean chromospheric emission in lower main-sequence stars. In the new paradigm, dynamo action simply generates the flux that is necessary, but not sufficient, for such activity, and the amplitude of activity depends; most importantly on the kinetic helicity and turbulence of convection zone flows.

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.

Some interplanetary studies lead one to expect that the toroidal fields of individual active regions are directly related to their heliospheric structure. Other studies conclude that the large-scale solar dipolar field is more important. We have carried out two studies that bear on these apparently conflicting views. We first studied geomagnetic events temporally associated with the eruption of 18 individual coronal X-ray sigmoids, which occurred while the large-scale solar dipolar magnetic field pointed southward. We found that if a coronal flux rope model is used to interpret magnetic structure, eruptions with a southward leading magnetic field are associated with stronger geomagnetic storms, but those with a northward leading field are associated with more storms. We next studied three full magnetic cycles, solar cycles 17–22. We examined the temporal variation of the ratio of the geomagnetic Ap index to the sunspot number. We found no statistically compelling fluctuations of this quantity on solar cycle timescales that are in phase with the reversal of active region polarities. On the other hand, we found a weak tendency for fluctuations that are in phase with the reversal of the large-scale solar dipole field. From these two studies we infer that the magnetic structure of individual active regions plays a role in geomagnetic events, but their geoeffectiveness is complicated by asymmetries in the leading and following magnetic field and density. We conclude that simple cycle-dependent generalizations have only statistical significance, and cannot dependably be used to predict the geomagnetic effects of a given solar eruption.

Observations of the strength and spatial distribution of vector magnetic fields in active regions have revealed several fundamental properties of the twist of their magnetic fields. First, the handedness of this twist obeys a hemispheric rule: left-handed in the northern hemisphere, right-handed in the southern. Second, the rule is weak; active regions often disobey it. It is statistically valid only in a large ensemble. Third, the rule itself, and the amplitude of the scatter about the rule, are quantitatively consistent with twisting of fields by turbulence as flux tubes buoy up through the convection zone. Fourth, there is considerable spatial variation of twist within active regions. However, relaxation to a linear force-free state, which has been documented amply in laboratory plasmas, is not observed.

The solar corona – one of the most spectacular celestial shows and yet one of the most challenging puzzles – exhibits a spectrum of structures related to both the quiet Sun and active regions. In spite of dramatic differences in appearance and physical processes, all these structures share a common origin: they are all related to the solar magnetic field. The origin of the field is beneath the turbulent convection zone, where the magnetic field is not a master but a slave, and one can wonder how much the coronal magnetic field “remembers” its dynamo origin. Surprisingly, it does. We will describe several observational phenomena that indicate a close relationship between coronal and sub-photospheric processes.

Prominence eruptions or disappearances observed with an Hα limb filtergraph can be classified into 3 categories, the eruptive prominence, the quasi-eruptive prominence , and the disappearing prominence . We investigated their mechanism by comparing the results of simultaneous observations by Yohkoh SXT and Hα. We found that soft X-ray features change in both eruptive prominences and quasieruptive prominences , whereas no significant change takes place in disappearing prominences . In one prominence eruption event soft X-ray cusp structure suggests that the reconnection point is just below the Hα prominence.

Observations of the strength and spatial distribution of vector magnetic fields in active regions have revealed several fundamental properties of the twist of their magnetic fields. First, the handedness of this twist obeys a hemispheric rule: left-handed in the northern hemisphere, right-handed in the southern. Second, the rule is weak; active regions often disobey it. It is statistically valid only in a large ensemble. Third, the rule itself, and the amplitude of the scatter about the rule, are quantitatively consistent with twisting of fields by turbulence as flux tubes buoy up through the convection zone. Fourth, there is considerable spatial variation of twist within active regions. However, relaxation to a linear force-free state, which has been documented amply in laboratory plasmas, is not observed.

Coronal mass ejections (CMEs) appear to originate preferentially in regions of the Sun's corona that are sigmoidal, i.e., have sinuous S or reverse-S shapes. Yohkoh solar X-ray images have been studied before and after a modest number of Earth-directed (halo) CMEs. These images tend to show sigmoidal shapes before the eruptions and arcades, cusps, and transient coronal holes after. Using such structures as proxies, it has been shown that there is a relationship between sigmoidal shape and tendency to erupt. Regions in the Sun's corona appear sigmoidal because their magnetic fields are twisted. Some of this twist may originate deep inside the Sun. However, it is significantly modulated by the Coriolis force and turbulent convection as this flux buoys up through the Sun's convection zone. As the result of these phenomena, and perhaps subsequent magnetic reconnection, magnetic flux ropes form. These flux ropes manifest themselves as sigmoids in the corona. Although there are fundamental reasons to expect such flux ropes to be unstable, the physics is not as simple as might first appear, and there exist various explanations for instability. Many gaps need to be filled in before the relationship between sigmoids and CMEs is well enough understood to be a useful predictive tool.

Soft X-ray images of solar active regions frequently show S- or inverse-S (sigmoidal) morphology. We have studied the Yohkoh Soft X-Ray Telescope video movie for 1993 and 1997. We have classified active regions according to morphology (sigmoidal or non-sigmoidal) and nature of activity (eruptive or non-eruptive). As well, we have used NOAA sunspot areas for each region as a measure of size. We find that regions are significantly more likely to be eruptive if they are either sigmoidal or large.

We present and interpret observations of the preflare phase of the eruptive flare of 15 November, 1991 in NOAA AR 6919. New flux emerged in this region, indicated by arch filaments in Hα and increasing vertical flux in vector magnetograms. With increasing frequency before the eruption, transient dark Hα fibrils were observed that crossed Hα bright plage and the magnetic inversion line to extend from the region of flux emergence to the filament, whose eruption was associated with the flare. These crossing fibrils were dynamic, and were often associated with sites of propagating torsional motion. These sites propagated from the region of flux emergence into the filament flux system. We interpret these morphological and dynamic features in terms of relaxation after magnetic reconnection episodes which create longer field lines within the filament flux system, as envisioned in the tether cutting model, and transfer twist to it, as well.

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.

Using photospheric vector magnetograms from the Haleakala Stokes Polarimeter and coronal X-ray images from the Yohkoh Soft X-Ray Telescope (SXT), we infer values of the force-free field parameter α at both photospheric and coronal levels within 140 active regions. We determine the value of α for a linear force-free field that best fits each magnetogram in a least-squares sense. We average values from all available magnetograms to obtain a single mean photospheric α-value αp for each active region. From the SXT images we estimate α in the corona by determining (π/L) sin γ for individual loops, where γ is the observed shear angle of X-ray loops of length L. We then average these values of α to obtain a single coronal α value, αc, for each active region. In active regions for which the photospheric α-map is predominantly of one sign, we find that the values of αp and αc are well correlated. Only for active regions in which both signs of α are well represented, and in which our method of analysis therefore breaks down, are the values of αp and αc poorly correlated. The former correlation implies that coronal electric currents typically extend down to at least the photosphere. However, other studies imply subphotospheric origin of the currents, and even current systems, that are observed in the photosphere. We therefore conclude that the currents responsible for sinuous coronal structures are of subphotospheric origin.

Measurements of the vector magnetic field in the solar atmosphere with high spatial and temporal resolution over a large field of view are critical to understanding the nature and evolution of currents in active regions. Such measurements, when combined with the thermal and nonthermal X-ray images from the upcoming Solar-A mission, will reveal the large-scale relationship between these currents and sites of heating and particle acceleration in flaring coronal magnetic flux tubes. The conceptual design of an imaging vector magnetograph that combines a modest solar telescope with a rotating quarter-wave plate, an acousto-optical tunable prefilter as a blocker for a servo-controlled Fabry-Perot etalon, CCD cameras, and a rapid digital tape recorder are described. Its high spatial resolution (1/2 arcsec pixel size) over a large field of view (4 x 5 arcmin) will be sufficient to significantly measure, for the first time, the magnetic energy dissipated in major solar flares. Its millisecond tunability and wide spectra range (5000 to 8000 A) enable nearly simultaneous vector magnetic field measurements in the gas-pressure-dominated photosphere and magnetically dominated chromosphere, as well as effective co-alignment with Solar-A's X-ray images.

Using X-ray images, Hα images, and vector magnetograms, we have studied the evolution of the coronal structure and magnetic field of NOAA Active Region 7154 during 1992 May 5-12. A two-ribbon 4B/M7.4 flare associated with an Hα filament eruption was observed on May 8, 15:13-19:16 UT. An interesting feature of the region was a long, twisted X-ray structure, which formed shortly before the flare and disappeared after it, being replaced by a system of unsheared postflare loops. Neither the X-ray nor Hα morphology nor the photospheric magnetic field shows any indication of gradual buildup of nonpotential energy prior to the flare. Rather, the long structure appears to result from the reconnection of two shorter ones just tens of minutes before the filament eruption and flare.Using vector magnetograms and X-ray morphology, we determine the helicity density of the magnetic field using the force-free field parameter α. The observations show that the long structure retained the same helicity density as the two shorter structures, but its greater length implies a higher coronal twist. The measured length and α value combine to imply a twist that exceeds the threshold for the MHD kink instability in a force-free cylindrical flux tube. We conclude that theoretical studies of such simple models, which have found that the MHD kink instability does not lead to global dissipation, do not adequately address the physical processes that govern coronal magnetic fields.

We describe an instrument we have built and installed at Mees Solar Observatory on Haleakala, Maui, to measure polarization in narrow-band solar images. Observations in Zeemansensitive photospheric lines have been made for nearly all solar active regions since the instrument began operations in 1992. The magnetograph includes a 28-cm aperture telescope, a polarization modulator, a tunable Fabry-Perot filter, CCD cameras and control electronics. Stokes spectra of a photospheric line are obtained with 7 pm spectral resolution, 1 arc sec spatial resolution over a field 4.7 arc min square, and polarimetric precision of 0.1%. A complete vector magnetogram observation can be made every eight minutes. The flexibility of the instrument encourages diverse observations: besides active region magnetograms we have made, for example, composite vector magnetograms of the full solar disk, and Hα polarization movies of flaring regions.

The active region NOAA 7260 rotated onto the north solar hemisphere as a mature bipole: a dominant negative-polarity sunspot with trailing plage and scattered small spots in attendance. The dominant p spot itself had strong magnetic fields and covered almost 400 x 10(exp -6) of a solar hemisphere. For a period of seven days beginning 14 August, 1992 this active region displayed rapid and drastic evolution: no fewer than 50 magnetic bipoles emerged in the area trailing the large sunspot, increasing the region's magnetic flux by more than 10(exp 22) Mx. This new group of sunspots formed a complex Beta gamma delta configuration with two delta spots and a high degree of magnetic shear. This region was very well observed by Yohkoh and various ground-based instruments. It presented opportunities to study new emerging flux, flares, and also the decay of a large sunspot. For the benefit of later studies we present a description of the global characteristics of this active region, a detailed 'roadmap' of its evolution during disk passage including the development of the two delta regions. We compare proper motion trends and flaring activity to observations of other delta-spots reported in the literature. We also comment on the observed outflow of magnetic elements from the decaying p spot.

We use H alpha 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 (approximately equals 10km/s) 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.

n order to better understand the solar genesis of interplanetary magnetic clouds (MCs) we model the magnetic and topological properties of four large eruptive solar flares and relate them to observations. We use the three-dimensional Minimum Current Corona model \cite{Longcope1996d} and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity and orientation of the flux rope poloidal field. We compare model predictions of those quantities to flare and MC observations and within the estimated uncertainties of the methods used find the following. The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC's poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancellation, the energy and helicity we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity., 24 pages, 6 figures

The NASA High Energy Solar Physics (HESP) mission offers the opportunity for major breakthroughs in our understanding of the fundamental energy release and particle acceleration processes at the core of the solar flare problem. HESP's primary strawman instrument, the High Energy Imaging Spectrometer (HEISPEC), will provide X-ray and γ-ray imaging spectroscopy, i.e., high-resolution spectroscopy at each spatial point in the image. It has the following unique capabilities; (1) high-resolution (∼keV) spectroscopy from 2 keV - 20 MeV to resolve flare gamma-ray lines and sharp features in the continuum; (2) hard X-ray imaging with 2″ angular resolution and tens of millisecond temporal resolution, commensurate with the travel and stopping distances and times for the accelerated electrons; (3) gamma-ray imaging with 4″–8″ resolution with the capability of imaging in specific lines or continuum regions; (4) moderate resolution imaging of energetic (20 MeV to ∼1 GeV) gamma-rays and neutrons. Additional strawman instruments include a Bragg crystal spectrometer for diagnostic information and a soft X-ray/XUV/UV imager to map the flare coronal magnetic field and plasma structure. The HESP mission also includes extensive ground-based observational and supporting theory programs. Presently HESP is planned for a FY 1995 new start and late 1999 launch, in time for the next solar activity maximum.

Applying the same methods we used in solar cycle 22, we study active region vector magnetograms, full-disk X-ray images, and full-disk line-of-sight magnetograms to derive the helicity of solar magnetic fields in the first 4 years of solar cycle 23. We find that these three data sets all exhibit the same two key tendencies—significant scatter and weak hemispheric asymmetry—as were observed in solar cycle 22. This supports the interpretation of these tendencies as signatures of the writhing of magnetic flux by turbulence in the convection zone.

The Japanese Yohkoh satellite is now in orbit observing the sun with a set of x-ray imagers and x-ray and gamma-ray spectrometers. The data from this successful mission provide new information on solar flares and the sun's corona. This paper discusses the Yohkoh observations and presents a sample of the first scientific results from the mission.

It is generally accepted, but not yet compellingly demonstrated, that the energy released in solar flares is stored in stressed magnetic fields. Little is known, at present, about how the most obvious manifestations of flare energy release — heating, mass motion, magnetic field reconfiguration and particle acceleration — are related to the spatial distribution of free energy within those fields. To address this issue we have underway at Mees Solar Observatory a programme of simultaneous polarimetric and spectroscopic observations that allow us to explore the spatial relation between active region currents, flare particle acceleration and flare heating. In this paper we discuss several days observations of two flare-productive active regions. By using the Haleakala Stokes polarimeter, we observed the spatial distribution of the Stokes profiles of two photospheric Fe 1 lines, from which we inferred the spatial distribution of the vector m agnetic field and the vertical current density. In flares that were observed on the same days, we then compared the locations of vertical currents to the sites of non-thermal electron precipitation and high coronal pressure inferred from Hα line profiles and spectroheliograms obtained with the Mees charge coupled device imaging spectrograph. Without exception we found that the sites of significant energetic electron precipitation into the chromosphere were at the edges of regions of vertical current, not within them. In contrast, we found that the footpoints of high-pressure flare plasmas during the main phase of the observed flares all coincided very well with such currents.

The Mees CCD (MCCD) instrument is an imaging spectroscopy device which uses the 25 cm coronagraph telescope and the 3.0 m Coude spectrograph at Mees Solar Observatory (MSO) on Haleakala, Maui. The instrument works with resolving power up to R ≈ 200 000 with significant throughput from λ3934 A (Caii K) to λ ≈ 10 000 A. A fast guiding active mirror stabilizes the image during observations. A rapidly writing magnetic tape storage system allows observations to be recorded at 256 kbytes s−1. Currently, the MCCD is used for imaging spectroscopy of solar flares at λ6563 A (Hα), and velocity measurements of umbral oscillations; future plans include emission line studies of active region coronae, and photospheric studies of solar oscillations.

NOAA Active region 5747, during its October 1989 transit across the solar disk, showed highly nonpotential photospheric vector magnetic field structure and produced many solar flares, three of which we observed at Mees Solar Observatory. After resolution of the 180° ambiguity, we determined the photospheric distribution of the vertical current density. We then compared the locations of the major current systems to sites of nonthermal electron precipitation inferred from Hα profiles of three flares observed using the Mees CCD Imaging Spectrograph. We found that the sites of energetic electron precipitation are at the edges of these currents, not at their peaks.

We have performed simulated vector magnetograph observations to study the effects of random and systematic magnetic field measurement errors on the magnetic energies that may be estimated using the virial theorem.

[1] In this work we relate the magnetic and topological parameters of twelve interplanetary magnetic clouds to associated solar active regions. We use a cylindrically symmetric constant-α force-free model to derive field line twist, total current, and total magnetic flux from in situ observations of magnetic clouds. We compare these properties with those of the associated solar active regions, which we infer from solar vector magnetograms. Our comparison of fluxes and currents reveals: (1) the total flux ratios ΦMC/ΦAR tend to be of order unity, (2) the total current ratios IMC/IAR are orders of magnitude smaller, and (3) there is a statistically significant proportionality between them. Our key findings in comparing total twists αL are that (1) the values of (αL)MC are typically an order of magnitude greater than those of (αL)AR and (2) there is no statistically significant sign or amplitude relationship between them. These findings compel us to believe that magnetic clouds associated with active region eruptions are formed by magnetic reconnection between these regions and their larger-scale surroundings, rather than simple eruption of preexisting structures in the corona or chromosphere.

[1] We study 46 solar coronal eruptions associated with sigmoids seen in images from the Yohkoh Soft X-ray Telescope (SXT). We relate the properties of the sigmoids to in situ measurements at 1 AU and geomagnetic storms. Our primary result is that erupting sigmoids tend to produce geoeffective magnetic clouds (MCs): 85% of the erupting sigmoidal structures studied spawned at least a “moderate” (|Dst| ≥ 50 nT) geomagnetic storm. A collateral result is that MCs associated with sigmoids do not show the same solar-terrestrial correlations as those associated with filaments and, as such, form a distinct class of events. First, rather than reversing with the global solar dipole (at solar maximum), the leading field in MCs weakly (2:1) shows a solar cycle (Hale polarity) based correlation (reversing at solar minimum). Second, whereas the handedness of MCs associated with filament eruptions is strongly (95%) related to their launch hemisphere, that of MCs associated with sigmoid eruptions is only weakly (∼70%) so related. Finally, we are unaware of any model of the magnetic fields of sigmoids and their eruption that gives a useful prediction of the leading field orientation of their associated MC.