He I 10830 observations of the 3N/M4.0 flare of 4 September, 1982

Hei 10830 Å spectroheliograms of a major 3N two-ribbon flare occurring in Boulder Region 3885/3886 early on 4 September, 1982 are discussed and compared with H and soft X-ray observations of the event. This flare, observed for more than 60 hr in Hei 10830, was associated with the eruption of a large filament in the active region complex, the formation of coronal holes, a long-duration soft X-ray event, and was the probable source of a earthward coronal mass ejection and the largest geomagnetic storm of this solar cycle. The results of this study suggest the Hei flare is a chromospheric manifestation of the X-ray coronal loop structures associated with flares.Visitor, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Observations of the 24 September 1997 Coronal Flare Waves

We report coincident observations of coronal and chromospheric flare wave transients in association with a flare, large-scale coronal dimming, metric radio activity and a coronal mass ejection. The two separate eruptions occurring on 24 September 1997 originate in the same active region and display similar morphological features. The first wave transient was observed in EUV and H data, corresponding to a wave disturbance in both the chromosphere and the solar corona, ranging from 250 to approaching 1000 km s^–1 at different times and locations along the wavefront. The sharp wavefront had a similar extent and location in both the EUV and H data. The data did not show clear evidence of a driver, however. Both events display a coronal EUV dimming which is typically used as an indicator of a coronal mass ejection in the inner corona. White-light coronagraph observations indicate that the first event was accompanied by an observable coronal mass ejection while the second event did not have clear evidence of a CME. Both eruptions were accompanied by metric type II radio bursts propagating at speeds in the range of 500–750 km s^–1, and neither had accompanying interplanetary type II activity. The timing and location of the flare waves appear to indicate an origin with the flaring region, but several signatures associated with coronal mass ejections indicate that the development of the CME may occur in concert with the development of the flare wave.     

The X-ray signature of solar coronal mass ejections

The coronal response to six solar X-ray flares has been investigated. At a time coincident with the projected onset of the white-light coronal mass ejection associated with each flare, there is a small, discrete soft X-ray enhancement. These enhancements (precursors) precede by typically 20 m the impulsive phase of the solar flare which is dominant by the time the coronal mass ejection has reached an altitude above 0.5 R . We identify motions of hot X-ray emitting plasma, during the precursors, which may well be a signature of the mass ejection onsets. Further investigations have also revealed a second class of X-ray coronal transient, during the main phase of the flare. These appear to be associated with magnetic reconnection above post-flare loop systems.NCAR is sponsored by the National Science Foundation.     

Heat flux saturation in hydrodynamic soft X-ray solar flare plasmas

The role of heat flux limitation in soft X-ray emitting solar flare plasmas is considered. Simple analytic arguments suggest that flux limitation is likely to be important during the explosive heating phase, even for relatively modest coronal energy fluxes (say 10⁹ erg cm^-2 s^-1). This conclusion is reinforced by a detailed flare loop simulation of the heating phase. Since flux saturation effectively bottles up the coronal heat flux, mass motions now assume a dominant role in transferring energy from the coronal flare source to the lower transition region. The mass-energy exchange between the corona and chromosphere produces dramatic changes in the thermal structure of the plasma which are reflected in the differential emission measure profile of the flaring loop.     

Active-Region Filaments and X-ray Sigmoids

We use Yohkoh soft X-ray telescope data and H full-disk observations to study the evolution of chromospheric filaments and coronal sigmoids in 6 active regions in association with coronal mass ejections (CMEs). In two cases, CMEs are directly observed by the SOHO/LASCO C2 coronagraph. In four cases, other observations (magnetic clouds, geomagnetic storms, sigmoid-arcade evolution) are used as CME indicators. Prior to eruption, each active region shows a bright coronal sigmoidal loop and underlying H filament. The sigmoid activates, erupts and gets replaced by a cusp, or an arcade. In contrast, the H filament shows no significant changes in association with sigmoid eruption and CME. We explain these observations in a framework of the classical two-ribbon flare model.     

DISCRETE RANDOM HEATING EVENTS IN CORONAL LOOPS

The response of the coronal plasma in a magnetic loop to the release of discrete, random amounts of energy quanta over fixed time intervals is investigated. Nanoflare heating (10²⁴ erg per event) with event lifetimes on a scale of 1–20 s are shown to be able to maintain a coronal loop at typical coronal temperatures, 2 x 10⁶ K (Parker, 1988; Kopp and Poletto, 1993). Microflare events (10²⁷ erg) observed by Porter et al. (1995) with a lifetime of approximately 1 min are also investigated and it is found that the loop apex temperature varies by at most 40% from its initial static condition. However, larger energy events of the order of 10²⁸ erg (Schmieder et al., 1994) occur too infrequently and the plasma cools to chromospheric values. The implications of time-dependent heating of the corona upon observations are also discussed.     

Simultaneous radio and white light observations of the 1984 June 27 coronal mass ejection event

We present the two-dimensional imaging observations of radio bursts in the frequency range 25–50 MHz made with the Clark Lake multifrequency radioheliograph during a coronal mass ejection event (CME) observed on 1984, June 27 by the SMM Coronagraph/Polarimeter and Mauna Loa K-coronameter. The event was spatially and temporally associated with precursors in the form of meter-decameter type III bursts, soft X-ray emission and a H flare spray. The observed type IV emission in association with the CME (and the H spray) could be interpreted as gyrosynchrotron emission from a plasmoid containing a magnetic field of 2.5 G and nonthermal electrons with a number density of 10⁵ cm^–3 and energy 350 keV.On leave from Indian Institute of Astrophysics, Kodaikanal, India.     

Multiwavelength Flare Observations: Temporal Evolution of the 20 August 1992 Flare

The 20 August 1992 flare around 14:28 UT was observed in H, H and Ca ii H with the imaging spectrographs at Locarno-Monti, Switzerland, with the radiotelescopes in Bern, and in soft and hard X-rays by the Yohkoh satellite. In this paper we discuss the analysis of the temporal and spatial evolution of this flare, well observed at chromospheric and coronal layers. We find that the chromospheric electron density shows well-correlated rises with the hard X-rays emphasizing the direct response of the chromosphere to the energy deposition. Although both footpoints of the loops show simultaneous rises of the electron density, non-thermal electron injection is only observed in one of the footpoints, while an additional heating mechanism, like thermal conduction, must be assumed for the other footpoint. However, it is puzzling that all the chromospheric observations in both footpoints are delayed by 3 s compared to the hard X-ray light curve. Although this would be compatible with the thermal heating of one footpoint, it is in contradiction to the non-thermal heating of the other one. Finally, we observed evidence that during the first part of the flare a thermal conduction front propagates at a speed of 2000 km s^-1 into a second loop, in which the energy release occurs in the second part of the flare.     

Energetics of a compact flare

We study the spatial and spectral characteristics of the 3.5 to 30.0 keV emission in a solar flare of 9 May, 1980. We find that: (a) A classical thick target interpretation of the hard X-ray burst at energies E 10 keV implies that approximately all the electrons contained within the flare loop(s) have to be accelerated per second. (b) A thermal model interpretation does not fit the data, unless its characteristics are such that it does not represent an efficient alternative to the acceleration model. We thus conclude that: (c) Acceleration does take place during the early phase of the impulsive hard X-ray event, but substantial amount of the emission at low (<20 keV) energies is of thermal origin. (d) We show the evolution of the energy content in the flare volume, and find that the energy input requirements are such that 10² erg cm^-3 s^-1 have to be released within the flare structure(s), for a period of time comparable to that of the hard X-ray burst emission. We also point out that although the main flare component ( 90% of the soft X-ray emission) was confined to a compact magnetic kernel, there are evidences of interaction of this structure with a larger field structure connecting towards the leading portion of the active region, where secondary H brightenings were observed.     

Observations of steady anomalous magnetic heating in thin current sheets

A faint steadily emitting loop-like structure has been observed by HXIS in its low energy channels (3.5–8.0 keV) on November 5/6, 1980. These HXIS observations have permitted us to follow the thermal evolution of this loop for a period of about 15 hr and from this study we conclude that only a fraction of 0.1% of the volume of the loop is steadily heated at the rather large rate of 0.6 erg cm^-3 s^-1. We interpret this heating as the dissipation of magnetic fields in thin current sheets and we find that the dissipation with classical resistivity is very unlikely, while ion-kinetic tearing, as proposed by Galeev et al. (1981), suits the observations very well. The enhancement of the resistivity over the classical resistivity then turns out to be a factor 4 × 10⁴. Dissipation in extremely thin sheets via the ion-acoustic instability (Duijveman et al., 1981) cannot be completely excluded when the cross-field heat conductivity is anomalously enhanced by a factor 400.We identify the source of the X-ray emission in this paper with the H filament in the same region. The hot X-ray emitting plasma and the cool plasma radiating in H are thermally separated by the strong magnetic field.The main conclusion of the paper is that for the first time direct evidence is found for the steady dissipation of coronal magnetic fields via enhanced resistivity in thin current sheets.     

Supra-arcade Downflows in Long-Duration Solar Flare Events

This report seeks to introduce the reader to a set of observations made during the decay phase of long-duration event (LDE) flares on the Sun. In a number of events, the soft X-ray images from Yohkoh SXT indicate a downward-directed flow field in the region immediately above the flare arcade. The prototypical event is the M5 flare of 20 January 1999, presented by McKenzie and Hudson (1999); since the time that paper was completed, eleven more events, all related to coronal mass ejections, have been found in the interval 1 January 1998 to 8 May 1999. As in the 20 January event, the speeds of downflow are 45–500 km s^–1, lower than both the free fall speed and the typically assumed Alfvén speed. A comparison is drawn between the SXT observations and the EUV and H data, where such are available. Important additions to our knowledge since the first paper are: (i) the motion is evidenced by both dark and bright (i.e., X-ray emitting) features, some of which may have a loop-like morphology; (ii) no cool counterparts have been detected in H or EUV observations. The data are compared to the expected appearance according to two likely (and opposing) interpretations: an `above-the-arcade coronal rain' interpretation and a `shrinking magnetic flux tube' interpretation. In the current standard explanation of the formation of post-CME arcades, motion of the latter type may be related to outflow from a reconnection site. Movies depicting some of the flare events are on the accompanying CD-ROM.     

X-ray observations of filament eruption in the 1980 May 21 flare

X-ray and H observations of an erupting filament, discussed herein, and other observations of the associated flare on 1980 May 21, suggest that an erupting filament played a major role in the X-ray flare. While Antonucci et al. (1985) analyzed the May 21 flare as one of the best cases of chromospheric evaporation, the possible contribution from X-ray emitting erupting plasma has been ignored. We show that pre-heated plasma existed and may have contributed part of the blue-shifted X-ray emission observed in the Caxix line, which was formerly attributed solely to chromospheric evaporation. Thus it remains an open question - in two-ribbon flares in particular - just how important chromospheric evaporation is in flare dynamics.     

Lyman continuum observations of solar flares

A study is made of Lyman continuum observations of solar flares, using data obtained by the Harvard College Observatory EUV spectroheliometer on the Apollo Telescope Mount. We find that there are two main types of flare regions: an overall mean flare coincident with the H flare region, and transient Lyman continuum kernels which can be identified with the H and X-ray kernels observed by other authors. It is found that the ground level hydrogen population in flares is closer to LTE than in the quiet Sun and active regions, and that the level of Lyman continuum formation is lowered in the atmosphere from a mass column density m 5/sx 10^–6 g cm^–2 in the quiet Sun to m 3/sx 10^–4 g cm^–2 in the mean flare, and to m 10^–3g cm^–2 in kernels. From these results we derive the amount of chromospheric material evaporated into the high temperature region, which is found to be - 10¹⁵g, in agreement with observations of X-ray emission measures. A comparison is made between kernel observations and the theoretical predictions made by model heating calculations, available in the literature; significant discrepancies are found between observation and current particle-heating models.     

Non-thermal processes in large solar flares

We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:

1. The ～10–10² keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
2. The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
3. The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
4. Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～10³¹ ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
5. High energy protons are accelerated later than the 10–10² keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
6. The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×10³⁰ ergs in >20 keV electrons is required for detectable white-light emission.

The highly efficient electron energization required in these flares suggests that the flare mechanism consists of rapid dissipation of chromospheric and coronal field-aligned or sheet currents, due to the onset of current-driven Buneman anomalous resistivity. Large proton flares then result when the energy input from accelerated electrons is sufficient to form a shock wave.     

The structure of high-temperature solar flare plasma in non-thermal flare models

We solve the energy equation for the high-temperature (coronal) component of flare plasma for two models of energy input: (i) direct collisional heating by a beam of suprathermal electrons, and (ii) ohmic heating by the beam-neutralizing reverse current. We discuss the regimes where each case is applicable, and solve for the differential emission measure distribution of the coronal plasma in each case. Scaling laws between loop temperatures and injected electron fluxes are derived for both models; these are testable observationally through coordinated soft X-ray and hard X-ray observations, thus providing a method of discriminating between the two cases. We also readdress the question of the energetic importance of a return current which is below the instability threshold for generation of ion-acoustic plasma turbulence. We find that unless the ambient coronal density is very low ( 10⁹ cm ^–3), collisional heating will always dominate there, in agreement with the findings of previous authors. However, in the chromosphere/corona transition region, the relatively low temperature and correspondingly high plasma resistivity imply that reverse current ohmic heating can predominate the flare energetics, by up to an order of magnitude.Presidential Young Investigator.     

The analysis and interpretation of solar X-ray photographs

This paper discusses the methods used to analyse and interpret X-ray filtergrams obtained by solar soft X-ray telescopes such as the S-056 Skylab instrument. First, an appropriate definition of the line-of-sight emission measure _L(T) is developed, and it is shown how the X-ray data may be analysed to obtain an approximation to _L(T). The accuracy of this approximation is severely limited by the mathematical ill-conditioning of the problem, and additional constraints on the solutions must be imposed through the use of a specific model of the coronal region under study. Such a model is also required for the proper interpretation of the results in terms of coronal plasma processes. Examples of such models are provided and the forms of _L(T) derived from them compared with other, semi-empirical forms.The filter ratio method (a simplified form of analysis in which the region under study is assumed isothermal) is discussed. It is shown that in the presence of line-of-sight temperature gradients, the values of effective temperature and emission measure yielded by this method cannot be directly related to the physical state of the plasma and so are of little utility in the study of coronal processes.Now at: Institute for Plasma Research, Stanford University, Via Crespi, Stanford, Calif. 94305, U.S.A.     

EUV emission,filament activation and magnetic fields in a slow-rise flare

The evolution of coronal and chromospheric structures is examined together with magnetograms for the 1B flare of January 19, 1972. Soft X-ray and EUV studies are based on the OSO-7 data. The H filtergrams and magnetograms came from the Sacramento Peak Observatory. Theoretical force-free magnetic field configurations are compared with structures seen in the soft X-ray, EUV and H images. Until the flare, two prominent spots were connected by a continuous dark filament and their overlying coronal structure underwent an expansion at the sunspot separation rate of 0.1 km s^–1. On January 19, the flare occurred as new magnetic fields emerged at 10¹⁹ Mx h^–1 beneath the filament, which untwisted and erupted as the flare began. The pre-flare coronal emissions remained unchanged during the flare except for the temporary addition of a localized enhancement that started 5 min after flare onset. EUV lines normally emitted in the upper transition region displayed a sudden enhancement coinciding in time and location with a bright H point, which is believed to be near the flare trigger or onset point. The EUV flash and the initial H brightening, both of which occurred near the center of the activated filament, were followed by a second EUV enhancement at the end of the filament. The complete disruption of the filament was accompanied by a third EUV enhancement and a rapid rise in the soft X-ray emission spatially coincident with the disappearing filament. From the change of magnetic field inferred from H filtergrams and from force-free field calculations, the energy available for the flare is estimated at approximately 10³¹ erg. Apparently, changes in the overlying coronal magnetic field were not required to provide the flare energy. Rather, it is suggested that the flare actually started in the twisted filament where it was compressed by emerging fields. Clearly, the flare started below the corona, and it appears that it derived its energy from the magnetic fields in or near the filament.NCAR is sponsored by NSF.     

The formation of flare loops by magnetic reconnection and chromospheric ablation

Slow-mode shocks produced by reconnection in the corona can provide the thermal energy necessary to sustain flare loops for many hours. These slow shocks have a complex structure because strong thermal conduction along field lines dissociates the shocks into conduction fronts and isothermal subshocks. Heat conducted along field lines mapping from the subshocks to the chromosphere ablates chromospheric plasma and thereby creates the hot flare loops and associated flare ribbons. Here we combine a non-coplanar compressible reconnection theory with simple scaling arguments for ablation and radiative cooling, and predict average properties of hot and cool flare loops as a function of the coronal vector magnetic field. For a coronal field strength of 100 G the temperature of the hot flare loops decreases from 1.2 × 10⁷ K to 4.0 × 10⁶ K as the component of the coronal magnetic field perpendicular to the plane of the loops increases from 0% to 86% of the total field. When the perpendicular component exceeds 86% of the total field or when the altitude of the reconnection site exceeds 10⁶km, flare loops no longer occur. Shock enhanced radiative cooling triggers the formation of cool H flare loops with predicted densities of 10¹³ cm^–3, and a small gap of 10³ km is predicted to exist between the footpoints of the cool flare loops and the inner edges of the flare ribbons.     

Impulsive phenomena in a small active region

The temporal and spatial variations of EUV emission from a small growing active region were investigated. Frequent localized short term ( few minutes) fluctuations in EUV emission were observed throughout the 7.2 hr interval when the most continuous observations were acquired. Approximately 20% of the 5 x 5 pixels had intensity variations exceeding a factor of 1.3 for the chromospheric L line, a factor of 1.5 for lines formed in the chromospheric-coronal transition region and a factor of 1.4 for the coronal Mg x line. A subflare in the region produced the largest intensity enhancements, ranging from a factor of 2.3 for the chromospheric L line to 8 for the transition region and coronal lines. The EUV fluctuations in this small active region are similar to those observed in coronal bright points, suggesting that impulsive heating is an important, perhaps dominant form of heating the upper chromospheric and lower coronal plasmas in small magnetic bipolar regions. The responsible mechanism most likely involves the rapid release of magnetic energy, possibly associated with the emergence of magnetic flux from lower levels into the chromosphere and corona.     

The Origin of Sequential Chromospheric Brightenings

Sequential chromospheric brightenings (SCBs) are often observed in the immediate vicinity of erupting flares and are associated with coronal mass ejections. Since their initial discovery in 2005, there have been several subsequent investigations of SCBs. These studies have used differing detection and analysis techniques, making it difficult to compare results between studies. This work employs the automated detection algorithm of Kirk et al. (Solar Phys. 283, 97, 2013) to extract the physical characteristics of SCBs in 11 flares of varying size and intensity. We demonstrate that the magnetic substructure within the SCB appears to have a significantly smaller area than the corresponding \(\mbox{H}\upalpha\) emission. We conclude that SCBs originate in the lower corona around \(0.1~R_{\odot}\) above the photosphere, propagate away from the flare center at speeds of \(35\,\mbox{--}\,85~\mbox{km}\,\mbox{s}^{-1}\), and have peak photosphere magnetic intensities of \(148\pm2.9~\mbox{G}\). In light of these measurements, we infer SCBs to be distinctive chromospheric signatures of erupting coronal mass ejections.