High-resolution observations of Hα flare regions

This paper gives a review of the recent high-resolution H observations of solar flares and flare-productive active regions. From studies of the morphological and evolutional features of H flare emitting regions, two types of two-ribbon flares, which are termed separating two-ribbon flare and confined two-ribbon flare, are discussed. The former is characterized by conspicuous separating motions or expanding motions of the H two ribbons, whereas the latter shows only a short range of or no separating motions of the two ribbons. The explosive compact flares, which occur in some compact newly-emerging flux regions, are also discussed.Attention is paid to the successive and impulsive brightenings of H flare points which form the H flare kernels and the front lines of H two ribbons at the impulsive phases of flares. Temporal relationships between H line intensities or profiles and hard X-ray or microwave emissions are discussed to discriminate the energy transport mechanisms in the flare loops.H monochromatic image of high spatial resolution, at the present time, is the most sensitive detector for finding the first appearance of newly-emerging magnetic flux region and the developing features of sheared configuration of magnetic field, both of which are the key factors in flare energy build-up processes. It is suggested that the successive emergence of a twisted magnetic flux rope might be essential for the production of a major flare.Contributions from the Kwasan and Hida Observatories, Kyoto University, No. 292.     

Optical observations of solar flares

This paper gives a review of the results of optical observations of solar flares. Observations carried out in the H_α line, flare spectra, and the methods of analysing the flare spectra are briefly discussed. Great attention is paid to the continuous emission of flares in the optical spectral region. In the last section, optical aspects of proton flares are summarized.     

Varieties of Coronal Mass Ejections and Their Relation to Flares

Most coronal mass ejections (CMEs) start as coronal storms which are caused by an opening of channels of closed field lines along the zero line of the longitudinal magnetic field. This can happen along any zero line on the Sun where the configuration is destabilized. If the opening includes a zero line inside an active region, one observes a chromospheric flare. If this does not happen, no flare is associated with the CME in the chromosphere, but the process, as well as the response in the corona (a Long Decay Event in X-rays) remains the same. The only difference between flare-associated and non-flare-associated CMEs is the strength of the magnetic field in the region of the field line opening. This can explain essentially all differences which have been observed between these two kinds of CMEs. However, there are obviously also other sources of CMEs, different from coronal storms: sprays (giving rise to narrow, pointed ejections), erupting interconnecting loops (often destabilized by flares), and growing coronal holes. This paper tries to summarize and interpret observations which support this general picture, and demonstrates that both CMEs and flares must be properly discussed in any study of solar-terrestrial relations.     

The streaming of solar flare particles and plasma in interplanetary space

A review is given of the features of solar particle emissions which cause various terrestrial disturbances. Three types of corpuscular emissions, namely, solar cosmic rays, energetic storm protons and plasma clouds, are associated with intense solar flares. Outward streaming of the solar wind and of beams of enhanced activity originate from the quiescent solar corona. It is shown that these solar particles propagate through interplanetary space, being modulated in a systematic way by existing magnetic fields. Time variations of solar flare particle flux, and their energy spectrum, are discussed.     

Observational support of reconnection in solar flares

We present a detailed analysis of the magnetic topology of flaring active region. TheH kernels are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibrils or/and transverse magnetic field direction, is taken into account. We show that the kernels are magnetically connected by field lines passing close to the separator. We confirm, for other flares, previous studies which show that photospheric current concentrations are located at the borders of flare ribbons. Moreover we found two photospheric current concentrations of opposite sign, linked in the corona by field lines which follow separatrices. These give evidence that magnetic energy is released by reconnection processes in solar flares.     

Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares

High-energy X-rays and ??-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.     

GeV Particle Acceleration in Solar Flares and Ground Level Enhancement (GLE) Events

Ground Level Enhancement (GLE) events represent the most energetic class of solar energetic particle (SEP) events, requiring acceleration processes to boost ?1?GeV ions in order to produce showers of secondary particles in the Earth’s atmosphere with sufficient intensity to be detected by ground-level neutron monitors, above the background of cosmic rays. Although the association of GLE events with both solar flares and coronal mass ejections (CMEs) is undisputed, the question arises about the location of the responsible acceleration site: coronal flare reconnection sites, coronal CME shocks, or interplanetary shocks? To investigate the first possibility we explore the timing of GLE events with respect to hard X-ray production in solar flares, considering the height and magnetic topology of flares, the role of extended acceleration, and particle trapping. We find that 50% (6 out of 12) of recent (non-occulted) GLE events are accelerated during the impulsive flare phase, while the remaining half are accelerated significantly later. It appears that the prompt GLE component, which is observed in virtually all GLE events according to a recent study by Vashenyuk et al. (Astrophys. Space Sci. Trans. 7(4):459–463, 2011), is consistent with a flare origin in the lower corona, while the delayed gradual GLE component can be produced by both, either by extended acceleration and/or trapping in flare sites, or by particles accelerated in coronal and interplanetary shocks.     

Solar Gamma-Ray Spectroscopy

Interactions of ions accelerated in solar flares produce gamma-ray lines and continuum and neutrons. These emissions contain a rich set of observables that provides information about both the accelerated ions and the environment where the ions are transported and interact. Ion interactions with the various nuclei present in the ambient medium produce gamma-ray lines at unique energies. How abundance information is extracted from the measurements is discussed and results from analyses of a number of solar flares are presented. The analyses indicate that the composition of the ambient gas where the ions interact (typically at chromospheric densities) is different from that of the photosphere and more like the composition of the corona, exhibiting low-FIP elemental enhancements that may vary from flare to flare. Evidence for increased Ne/O and the photospheric ³He abundance is also discussed.     

Energy Release and Particle Acceleration in Flares: Summary and Future Prospects

RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI??s precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent electron spectrum. Taking into account the effects of albedo, these show that the low energy cut-off to the electron power-law spectrum is typically ?tens of keV, confirming that the accelerated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy density is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI??s pioneering ??-ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. Flare particle acceleration is shown to have a close relationship to impulsive Solar Energetic Particle (SEP) events observed in the interplanetary medium, and also to both fast coronal mass ejections and gradual SEP events. New instrumentation to provide the high sensitivity and wide dynamic range hard X-ray and ??-ray measurements, plus energetic neutral atom (ENA) imaging of SEPs above ??2 R_??, will enable the next great leap forward in understanding particle acceleration and energy release is large solar eruptions??solar flares and associated fast coronal mass ejections (CMEs).     

Solar flares and particle acceleration

Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 10¹⁰ cm^–3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 10³⁰ erg, and an average number of 10³⁸ electrons with E 15 keV is accelerated. In subsecond pulses these values are about 10³⁵ electrons and about 10²⁷ erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 10⁴ km above the solar photosphere, a region permeated by a large number ( 10) of fluxthreads, each carrying electric currents of 10¹⁰–10¹¹ A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of ³He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived ( 0.1 s) superhot ( 10⁸ K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs by fast reconnection in magnetic fields 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities ( 10¹⁰ cm^–3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and vestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9).     

Properties of Ground Level Enhancement Events and the Associated Solar Eruptions During Solar Cycle 23

Solar cycle 23 witnessed the most complete set of observations of coronal mass ejections (CMEs) associated with the Ground Level Enhancement (GLE) events. We present an overview of the observed properties of the GLEs and those of the two associated phenomena, viz., flares and CMEs, both being potential sources of particle acceleration. Although we do not find a striking correlation between the GLE intensity and the parameters of flares and CMEs, the solar eruptions are very intense involving X-class flares and extreme CME speeds (average ～2000?km/s). An M7.1 flare and a 1200?km/s CME are the weakest events in the list of 16 GLE events. Most (80?%) of the CMEs are full halos with the three non-halos having widths in the range 167 to 212?degrees. The active regions in which the GLE events originate are generally large: 1290?msh (median 1010?msh) compared to 934?msh (median: 790?msh) for SEP-producing active regions. For accurate estimation of the CME height at the time of metric type?II onset and GLE particle release, we estimated the initial acceleration of the CMEs using flare and CME observations. The initial acceleration of GLE-associated CMEs is much larger (by a factor of 2) than that of ordinary CMEs (2.3?km/s² vs. 1?km/s²). We confirmed the initial acceleration for two events for which CME measurements are available in the inner corona. The GLE particle release is delayed with respect to the onset of all electromagnetic signatures of the eruptions: type?II bursts, low frequency type?III bursts, soft X-ray flares and CMEs. The presence of metric type?II radio bursts some 17?min (median: 16?min; range: 3 to 48?min) before the GLE onset indicates shock formation well before the particle release. The release of GLE particles occurs when the CMEs reach an average height of ～3.09?R _s (median: 3.18?R _s ; range: 1.71 to 4.01?R _s ) for well-connected events (source longitude in the range W20–W90). For poorly connected events, the average CME height at GLE particle release is ～66?% larger (mean: 5.18?R _s ; median: 4.61?R _s ; range: 2.75–8.49?R _s ). The longitudinal dependence is consistent with shock accelerations because the shocks from poorly connected events need to expand more to cross the field lines connecting to an Earth observer. On the other hand, the CME height at metric type?II burst onset has no longitudinal dependence because electromagnetic signals do not require magnetic connectivity to the observer. For several events, the GLE particle release is very close to the time of first appearance of the CME in the coronagraphic field of view, so we independently confirmed the CME height at particle release. The CME height at metric type?II burst onset is in the narrow range 1.29 to 1.8?R _s , with mean and median values of 1.53 and 1.47?R _s . The CME heights at metric type?II burst onset and GLE particle release correspond to the minimum and maximum in the Alfvén speed profile. The increase in CME speed between these two heights suggests an increase in Alfvénic Mach number from?2 to?3. The CME heights at GLE particle release are in good agreement with those obtained from the velocity dispersion analysis (Reames in Astrophys. J. 693:812, 2009a; Astrophys. J. 706:844, 2009b) including the source longitude dependence. We also discuss the implications of the delay of GLE particle release with respect to complex type?III bursts by ～18?min (median: 16?in; range: 2 to 44?min) for the flare acceleration mechanism. A?similar analysis is also performed on the delay of particle release relative to the hard X-ray emission.     

Solar gamma rays

The theory of gamma-ray production in solar flares is treated in detail. Both lines and continuum are produced. The strongest line predicted at 2.225 MeV with a width of less than 100 eV and detected at 2.24±0.02 MeV, is due to neutron capture by protons in the photosphere. Its intensity is dependent on the photospheric ³He abundance. The neutrons are produced in nuclear reactions of flare accelerated particles which also produce positrons and prompt nuclear deexcitation lines. The strongest prompt lines are at 4.43 MeV from ¹²C and at 6.2 from ¹⁶O and ¹⁵N. These lines result from both direct excitation and spallation. The widths of individual prompt lines are determined by nuclear kinematics. The width of the 4.43 MeV line is 100 keV and that of the 6.2 MeV feature is 300 keV. Both these lines have been observed from a solar flare. Other potentially observable lines are predicted at 0.845 and 1.24 MeV from ⁵⁶Fe, at 1.63 MeV principally from ¹⁴N and ²⁰Ne, at 1.78 MeV from ²⁸Si, at 5.3 MeV from ¹⁵O and ¹⁵N, and at 7.12 MeV from ¹⁶O. The widths of the iron lines are only a few keV, while those of the other lines are about 100 keV. The only other observed line is at 0.511 MeV from positron annihilation. The width of this line is determined by the temperature, and its temporal variation depends on the density of the ambient medium in the annihilation region. Positrons can also annihilate from the ³ S state of positronium to produce a 3-photon continuum below 0.511 MeV. In addition, the lines of ⁷Li and ⁷Be at 0.478 keV and 0.431 keV, which have kinematical widths of 30 keV, blend into a strong feature just below the 0.511 MeV line.From the comparison of the observed and calculated intensities of the line at 4.4 MeV to that of the 2.2 MeV line it is possible to obtain information on the spectrum of accelerated nuclei in flares. Moreover, from the absolute intensities of these lines the total number of accelerated nuclei at the Sun and their heating of the flare region can be estimated. We find that about 10³³ protons of energies greater than 30 MeV were produced in the 1972, August 4 flare.The gamma-ray continuum, produced by electron bremsstrahlung, allows the determination of the spectrum and number of accelerated electrons in the MeV region. From the comparison of the line and continuum intensities we find a proton-to-electron ratio of about 10 to 10² at the same energy for the 1972, August 4 flare. For the same flare the protons above 2.5 MeV which are responsible for the gamma-ray emission produce a few percent of the heat generated by the electrons which make the hard X-rays above 20 keV.NAS-NRC Resident Research Associate.Research supported by the National Science Foundation under Grant GP 31620.     

Coordinated optical and Yohkoh observations of 26 June 1992 flare loops

Optical spectra of large flare loops were detected by the Ondejov Multichannel Flare Spectrograph (MFS) during coordinated observations with MSDP at Pic du Midi (H) and the soft X-ray telescope (SXT) on Yohkoh. The CCD video images taken by the MFS slit-jaw camera document the time-development of the flare loops as seen through the H filter. Preliminary analysis of the MSDP images shows the intensity structure of the cool flare loops and their velocity fields. From the spectra we can clearly see the intensity variations along the cool loops. SXT images show the structure of hot X-ray loops similar to that of cool loops. Special attention is devoted to the bright tops, simultaneously observed in X-rays, H and other optical lines. Based on a preliminary analysis of the optical spectra, we speculate about possible mechanisms leading to an observed bright emission at the tops of cool loops. We suggest that direct soft X-ray irradiation of cool loops at their tops could be, at least partly, responsible for such a strong brightening.     

An EXOSAT observation of the morphology of the coronal X-ray emission from algol

The X-ray emission from Algol is thought to originate in a corona associated with the K star in this system. We report the results of a 35 hr continuous EXOSAT observation through secondary optical eclipse that was designed to measure the structure of the corona. No obvious X-ray eclipse was seen. The spectrum measured by the ME gives a temperature of 2.5 × 10⁷ K, consistent with the hard component previously seen by the Einstein SSS. The soft component previously reported by the SSS would only contribute at most 25% to the count rate seen in the LE (used with Al/P). The lack of a hard X-ray eclipse indicates the dimensions of the higher temperature emission region to be comparable to or greater than the size of the K star. An X-ray flare was detected with a peak luminosity of 1.4 × 10³¹ erg s^-1 and a total duration of 8 hours. The peak temperature was 5.0 keV with an emission measure of 9.4 × 10⁵³ cm^-3. The thermal nature of the flare is confirmed by the detection of an iron line with an EW of 2 keV. By equating the observed decay time of the flare to a known cooling law gives a dimension for the flaring loop of 0.3 stellar radii. This is much smaller than the dimensions of the hard component inferred from the lack of an eclipse. It seems probable that the flare occurred in one of the loops responsible for the lower temperature component seen by the SSS.     

The analysis of energy release in solar flares based on X-ray observations

Extended review of selected papers which deal with the problem of flare heating in solar coronal loops is presented. Discussed methods of the analysis of flare heating based on the X-ray observations have been worked out using the Palermo-Harvard hydrodynamic code. The case is presented when the assumption of the uniform heating across the loop is made. The existence of multiple elementary heating episodes is postulated as well. Next the possibility of the non-uniform heating across the loop is assumed and its manifestation in the X-ray observations is investigated. The application of proposed methods of the analysis to the observations of solar flares in X-rays is presented.     

SOHO contribution to the understanding of mass supply and flows in the solar corona

We expect a variety of dynamic phenomena in the quiescent non-flaring corona. Plasma flows, such as siphon flows or convective flows of chromospheric material evaporating into the corona, are expected whenever a pressure differences is established either between the footpoints or between the coronal and chromospheric segments of a coronal loop. Such flows can induce phenomena of spatial and temporal brightness variability of the corona. In particular, evaporation induces a net mass input into the corona and consequently coronal density enhancements. Flows are also expected in the regions where energy is released during magnetic reconnection. From the observational point of view the dynamics of the solar atmosphere has been investigated in great detail mostly in the lower transition region with the HRTS, and during flares with theSolar Maximum Mission andYohkoh. The high spectral, temporal and spatial resolution of theSOHO ultraviolet spectrometers should enable us in the near future to fill the gap providing a continuous coverage from the chromosphere to the corona, in the 10⁴–10⁶ K domain, and therefore to best study the dynamics throughout the solar atmosphere.     

Observations of loops and prominences

We review recent observations by the Yohkoh-SXT in collaboration with other spacecraft and ground-based observatories of coronal loops and prominences. These new results point to problems that SoHO will be able to address. With a unique combination of rapid-cadence digital imaging (32 s full-disk and 2 s partial-frame images), high spatial resolution (2.5 arcsec pixels), high sensitivity (EM 10⁴² cm^–3), a low-scatter mirror, and large dynamic range, SXT can observe a vast range of targets on the Sun. Over the first 21 months of Yohkoh operations, SXT has taken over one million images of the corona and so is building up an invaluable long-term database on the large-scale corona and loop geometry. The most striking thing about the SXT images is the range of loop sizes and shapes. The active regions are a bright tangle of magnetic field lines, surrounded by a network of large-scale quiet-Sun loops stretching over distances in excess of 10⁵ km. The cross-section of most loops seems to be constant. Loops displaying significant increase in the ratio of the footpoint to loop-top diameter () are the exception, not the rule, implying the presence of widespread currents in the corona.All magnetic structures show changes. Time scales range from seconds to months. The question of how these structures are formed, become filled with hot plasma, and are maintained is still open. While we see the propagation of brightenings along the length of active-region loops and in X-ray jets with velocities of several hundred km/s, much higher velocities are seen in the quiet Sun. In XBP flares, for example, velocities of over 1000 km/s are common. Active-region loops seem to be in constant motion, moving slowly outward, carrying plasma with them. During flares, loops often produce localized brightenings at the base and later at the apex of the loop. Quiescent filaments and prominences have been observed regularly. Their coronal manifestation seems to be an extended arcade of loops overlying the filament. Reliable alignment of the ground-based data with the X-ray images make it possible to make a detailed intercomparison of the hot and cold plasma structures over extended periods. Hence we are able to follow the long-term evolution of these structures and see how they become destabilized and erupt.     

LMC X-4: 13.5 s pulsations and an X-ray flare observed by EXOSAT

EXOSAT observed LMC X-4 on November 17/19, 1983 for one 1.4 day binary period during the high state of the 30.5 day cycle. An eclipse with sharp ingress and slow egress was detected with an eclipse angle of 27.1±1.0 dgr. In the medium energy experiment the source showed a hard power law spectrum. Outside eclipse the source was remarkably constant and only one flare was detected on November 17 at 19 UT lasting for about 1 h. The energy spectrum of the source softens considerably during that time and shows an emission line of cold iron. 13.5 sec pulsations are strongly present during the flare and have also been detected during the quiescent period and during several 1 min flares in another EXOSAT LMC X-4 observation on November 22, 1983. A pulse delay time analysis results in the determination of the pulse period (13.5019±0.0002) s and of the semimajor axis of the orbit of the X-ray star (26.0±0.6) It-sec. These results, together with other available information on LMC X-4, allowed to improve the binary parameters. The mass of the neutron star is found to be 1.34 _±0.44 ^0.48 M_o (95% confidence errors).     

Protons in flares

This work addresses the role of non-thermal protons as a means of transporting energy in stellar atmospheres. The most dramatic transient visible phenomena are flares, the best studied of which are from the Sun. It is believed that energetic particles take a fundamental part in flare development, but it is controversial as to whether protons or electrons play the dominant role. This review is aimed at helping resolve the controversy. We start by outlining acceleration mechanisms for energetic particles, on the premise that the acceleration site is in the corona. The propagation of a proton beam through the atmosphere is discussed, together with the radiation signatures it would produce. Chromospheric evaporation is expected as the beam reaches the dense part of the atmosphere. Direct observational evidence for energetic protons is reviewed, from gamma-ray production involving energies >30 MeV to H polarization, which is significant at energies 100 keV. Proton beams can be detected in the corona via slowly-drifting type III bursts, while they can be directly sampled by spacecraft and, at energies >1 GeV, by detectors on the Earth. A number of key flare observations and energy arguments are debated from the viewpoint of protons versus electrons. The conclusion is that primary non-thermal protons are much more important, in terms of total energy, than non-thermal electrons in flares, and that the bulk of the energetic electrons are secondary.