Analysis of the X-ray/Hα flare of 1980 July 14

Using the X-ray data from the SMM Satellite and the optical data from the Yunnan Observatory, we analysed the Class 3B flare of 1980 July 14. We obtained the time variation of the X-ray spectrum, calculated the total number of electrons at the time of the flare and their mean energy and measured and compared the positions of the Hα flare and the X-ray burst source. The results show 1) that the hard X-ray burst was caused by high-energy non-thermal electron beam; 2) that the soft X-ray burst was basically generated by thermal bremsstrahlung of hot plasma, but the contribution by non-thermal electrons must also be included; 3) that the determined height of the X-ray burst source depends on the flare model and the magnetic field configuration of the active region. The results obtained support the newly emergent flux model of flares.     

Solar Impulsive Hard X-Ray Emission and Two-Stage Electron Acceleration

Heating and acceleration of electrons in solar impulsive hard X-ray (HXR) flares are studied according to the two-stage acceleration model developed by Zhang for solar 3He-rich events. It is shown that electrostatic H-cyclotron waves can be excited at a parallel phase velocity less than about the electron thermal velocity and thus can significantly heat the electrons (up to 40 MK) through Landau resonance. The preheated electrons with velocities above a threshold are further accelerated to high energies in the flare-acceleration process. The flare-produced electron spectrum is obtained and shown to be thermal at low energies and power law at high energies. In the non-thermal energy range, the spectrum can be double power law if the spectral power index is energy dependent or related. The electron energy spectrum obtained by this study agrees quantitatively with the result derived from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) HXR observations in the flare of 2002 July 23. The total flux and energy flux of electrons accelerated in the solar flare also agree with the measurements.     

The role of non-thermal electrons in the optical continuum of stellar flares

The continuum emission of stellar flares in UV and visible bands can be enhanced by two or even three orders of magnitude relative to the quiescent level and is usually characterized by a blue colour. It is difficult for thermal atmospheric models to reproduce all these spectral features. If the flaring process involves the acceleration of energetic electrons which then precipitate downwards to heat the lower atmosphere, collisional excitation and ionization of ambient hydrogen atoms by these non-thermal electrons could be important in powering the continuum emission. To explore such a possibility, we compute the continuum spectra from an atmospheric model for a dMe star, AD Leo, at its quiescent state, when considering the non-thermal effects by precipitating electron beams. The results show that if the electron beam has an energy flux large enough (for example, ℱ₁∼10¹² erg cm⁻² s⁻¹), the U -band brightening and, in particular, the U − B colour are roughly comparable with observed values for a typical large flare. Moreover, for electron beams with a moderate energy flux ℱ₁≲10¹¹ erg cm⁻² s⁻¹, a decrease of the emission at the Paschen continuum appears. This can explain at least partly the continuum dimming observed in some stellar flares. Adopting an atmospheric model for the flaring state can further raise the continuum flux, but it yields a spectral colour incomparable with observations. This implies that the non-thermal effects may play the chief role in powering the continuum emission in some stellar flares.     

Multi-Wavelength Analysis of High-Energy Electrons in Solar Flares: A Case Study of the August 20, 2002 Flare

A multi-wavelength spatial and temporal analysis of solar high-energy electrons is conducted using the August 20, 2002 flare of an unusually flat (γ₁ = 1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα, radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model-independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below ∼100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them.     

10–100 keV electron acceleration and emission from solar flares

We present an analysis of spacecraft observations of non-thermal X-rays and escaping electrons for 5 selected small solar flares in 1967. OSO-3 multi-channel energetic X-ray measurements during the non-thermal component of the solar flare X-ray bursts are used to derive the parent electron spectrum and emission measure. IMP-4 and Explorer-35 observations of > 22 keV and > 45 keV electrons in the interplanetary medium after the flares provide a measure of the total number and spectrum of the escaping particles. The ratio of electron energy loss due to collisions with the ambient solar flare gas to the energy loss due to bremsstrahlung is derived. The total energy loss due to collisions is then computed from the integrated bremsstrahlung energy loss during the non-thermal X-ray burst. For > 22 keV flare electrons the total energy loss due to collisions is found to be 10⁴ times greater than the bremsstrahlung energy loss and 10² times greater than the energy loss due to escaping electrons. Therefore the escape of electrons into the interplanetary medium is a negligible energetic electron loss mechanism and cannot be a substantial factor in the observed decay of the non-thermal X-ray burst for these solar flares.We present a picture of electron acceleration, energy loss and escape consistent with previous observations of an inverse relationship between rise and decay times of the non-thermal X-ray burst and X-ray energy. In this picture the acceleration of electrons occurs throughout the 10–100 sec duration of the non-thermal X-ray burst and determines the time profile of the burst. The average energy of the accelerated electrons first rises and then falls through the burst. Collisions with the ambient gas provide the dominant energetic electron loss mechanism with a loss time of 1 sec. This picture is consistent with the ratio of the total number of energetic electrons accelerated in the flare to the maximum instantaneous number of electrons in the flare region. Typical values for the parameters derived from the X-ray and electron observations are: total energy in > 22 keV electrons total energy lost by collisions = 10^28–29 erg, total number of electrons accelerated above 22 keV = 10³⁶, total energy lost by non-thermal bremsstrahlung = 10²⁴erg, total energy lost in escaping > 22 keV electrons = 10²⁶erg, total number of > 22 keV electrons escaping = 10^33–34.The total energy in electrons accelerated above 22 keV is comparable to the energy in the optical or quasi-thermal flare, implying a flare mechanism with particle acceleration as one of the dominant modes of energy dissipation.The overall efficiency for electron escape into the interplanetary medium is 0.1–1% for these flares, and the spectrum of escaping electrons is found to be substantially harder than the X-ray producing electrons.Currently at Tokyo Astronomical Observatory, Mitaka, Tokyo, Japan.     

The temperature structure of chromospheric flares heated by non-thermal electrons

Heating of the deep chromosphere by a vertically descending beam of non-thermal electrons with power-law energy spectrum, in flares, is analysed. In lower regions of the flare, radiative losses can balance the energy input and the flare structure is described in terms of instantaneous quasi-steady temperature/depth profiles. Motion of the optical flare material is at constant pressure and is constrained to be purely vertical by a vertical magnetic field. The ionisation of hydrogen is determined by the same non-LTE processes as in the quiet chromosphere. Temperature profiles are obtained for a wide range of electron beam intensities and spectral indices and are discussed in terms of optical flare observations. Due to the steepness of the electron spectra, typical densities in the optical flare vary only over a narrow range, despite the diversity of beam intensities, in agreement with observation.Above a certain region, the flare material cannot attain a radiatively steady state against the electron input but evaluation of the level at which this occurs leads to an estimate of the mass of material involved in the high temperature flare plasma in this model. Results, which are again insensitive to the electron beam parameters, are found to be in satisfactory agreement with observations of the mass of flare ejecta and of soft X-ray flare emission measures.     

The Evaporation Regime in a Confined Flare

We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multi-wavelength observations that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition Region and Coronal Explorer (TRACE) together with Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and their topological changes revealing that we are dealing with an eruptive confined flare. RHESSI data do not record any detectable emission at energies ≥25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of the photon spectrum. A non-thermal electron flux F ≈ 5 × 10¹⁰ erg cm⁻² s⁻¹ is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of small loops that brightens successively. The analysis of the chromospheric spectra (Ca II K, He I D₃ and Hγ, acquired with a four-second temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s⁻¹) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He I 58.4 and O v 62.97 lines, we determine a velocity of ≈−40 km s⁻¹ while for the Fe XIX 59.22 line a velocity of ≈−80 km s⁻¹ is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations and they are consistent with the scenario outlined by Fisher (1989). No explosive evaporation is expected for a non-thermal electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It is suggested that the energy of non-thermal electrons can be dissipated to heat the high-density plasma, where possibly the reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive.     

Non-thermal processes during the ‘build-up’ phase of solar flares and in absence of flares

Hard X-ray and radio observations lead to the conclusion that production of non-thermal electrons is a common phenomenon of the active Sun. A preliminary analysis of three hard X-ray bursts observed with the OGO-5 satellite and the radio observations reported in the literature indicates that non-thermal particles are present in the flare region prior to the impulsive (flash) phase and also during the gradual rise and fall (GRF) bursts which are usually explained in terms of purely thermal radiation. The principal difference between the non-thermal electrons observed before the flash phase and during the flash phase appears to be in their total number rather than in the hardness of their energy spectrum. This indicates that the basic characteristics of the two acceleration processes are probably similar although the total energy converted into non-thermal electrons is considerably larger in the flash phase than in the build-up phase. Transient absorbing H features and filament activations are discussed in terms of their ability to produce energetic particle events and magnetic energy release.Presently at the Space Sciences Laboratory, University of California, Berkeley and Institute of Plasma Research, Stanford University, Stanford, California.     

Coronal scattering as a source of flare-associated polarized hard X-rays

We consider the scattering of flare-associated X-rays above 1 keV at coronal heights, particularly from regions of enhanced density. This includes a discussion of the polarization of the scattered X-rays. Although the scattered radiation would not be bright by comparison with the total hard X-ray flux from a flare, its detectability would be enhanced for events located a few degrees behind the limb for which the dominant `footpoint' hard X-ray sources are occulted. Thus we predict that major flares occurring beyond the solar limb may be detectable via scattering in density enhancements that happen to be visible above the limb, and that such sources may be strongly polarized. Since thin-target bremsstrahlung will generally greatly exceed the scattered thick-target flux in flare loops themselves, these considerations apply only to coronal structures that do not contain significant populations of non-thermal electrons.     

The Hα flare as a secondary product of a coronal instability

The assumption that the flare originates in the corona or transition layer, is confronted with the known properties of chromospheric flares. It is concluded that the basic mode of the energy transport into chromosphere is heat conduction. Only in some flares non-thermal particles contribute to the brightening in lower atmospheric layers: electrons with energy close to 100 keV produce chromospheric bright patches, and protons above 20 MeV cause the photospheric enhancements. The particle-produced brightenings are superposed on the basic quasi-thermal flare and involve only small areas as compared with the extensive regions heated through conduction.The most probable height of the flare origin appears to be close to the transition layer, between some 4000 and 7000 km above the photosphere. The non-thermal acceleration (when present) occurs probably higher than where the flare originates. There is no obvious reason why the high electron density in chromospheric flares could not be explained as simply due to increased ionization in the existing plasma, without any flare-induced mass condensations.Though there are several facts supporting the flare origin in the corona (or transition layer), one cannot exclude the alternative that the flare instability involves simultaneously a wide (and in different cases different) range of altitudes. Energy considerations give some support to such a supposition.Mitteilungen aus dem Fraunhofer Institut Nr. 121.Visiting scientist at the Fraunhofer Institute, grant of Stifterverband für die Deutsche Wissenschaft.     

Secondary electrons in aurora

The Bethe approximation is used with measured and theoretical values of ionization cross sections and measured values of differential oscillator strengths to derive the initial energy spectrum of auroral secondary electrons. The differential flux of the auroral secondaries is then calculated, using the approximation of continuous energy loss. The calculations are applied to a particular aurora for which rocket data have been published. There is substantial disagreement between theoretical and measured electron spectra. The theoretical spectra show structure at energies less than 20 eV, associated primarily with vibrational and electronic excitation of molecular nitrogen. This structure is largely absent in the measured spectrum. Substantially more high energy electrons were measured than theory predicts. In addition, there are disagreements in the altitude profiles of the total number of non-thermal secondary electrons.

Calculated values of OI green line photon emission rates which result from excitation by secondary electrons and dissociative recombination of O₂⁺ fall short of the measured values. The effect on the excitation rate of varying several parameters is investigated, and it is found that the results are particularly sensitive to competing inelastic processes in N₂.     

Flares in Sigmoidal Coronal Structures – a Case Study

We analyze radio observations, magnetograms and extrapolated field line maps, Hα filtergrams, and X-ray observations of two flare events (6 February 1992 in AR 7042 and 25 October 1994 in AR 7792) and study properties, evolution and energy release signatures of sigmoidal loop systems. During both events, the loop configuration seen in soft X-ray (SXR) images changes from a preflare sigmoidal shape to a relaxed post-flare loop system. The underlying magnetic field system consists of a quadrupolar configuration formed by a sheared arcade core and a remote field concentration. We demonstrate two possibilities: a sigmoidal SXR pattern can be due to a single continuous flux tube (the 1992 event). Alternatively, it can be due to a set of independent loops appearing like a sigmoid (the 1994 event). In both cases, the preflare and post-flare loops can be well reproduced by a linear force-free field and potential field, respectively, computed using preflare magnetograms. We find that thermal and non-thermal flare energy release indicators of both events become remarkably similar after applying spatial and temporal scale transformations. Using the spatial scaling between both events we estimated that the non-thermal energy release in the second event liberated about 1.7 times more energy per unit volume. A two-and-a-half times faster evolution indicates that the rate of the energy release per unit volume is more than four times higher in this event. A coronal type II burst reveals ignition and propagation of a coronal shock wave. In contrast, the first event, which was larger and released about a 10 times more energy during the non-thermal phase, was associated with a CME, but no type II burst was recorded. During both events, in addition to the two-ribbon flare process an interaction was observed between the flaring arcade and an emerging magnetic flux region of opposite polarity next to the dominant leading sunspot. The arcade flare seems to stimulate the reconnection process in an `emerging flux-type' configuration, which significantly contributes to the energy release. This regime is characterized by the quasiperiodic injection of electron beams into the surrounding extended field line systems. The repeated beam injections excite pulsating broadband radio emission in the decimetric-metric wavelength range. Each radio pulse is due to a new electron beam injection. The pulsation period (seconds) reflects the spatial scale of the emerging flux-type field configuration. Since broadband decimetric-metric radio pulsations are a frequent radio flare phenomenon, we speculate that opposite-polarity small-scale flux intrusions located in the vicinity of strong field regions may be an essential component of the energy release process in dynamic flares.     

An electric-current description of solar flares

The presently prevailing theories of solar flares rely on the hypothetical presence of magnetic flux tubes beneath the photosphere and the two subsequent hypotheses, their emergence above the photosphere and explosive magnetic reconnection, converting magnetic energy carried by the flux tubes to solar flare energy. In this paper, we discuss solar flares from an entirely different point of view, namely in terms of power supply by a dynamo process in the photosphere. By this process, electric currents flowing along the magnetic field lines are generated and the familiar ‘force-free’ fields or the ‘sheared’ magnetic fields are produced. Upward field-aligned currents thus generated are carried by downward streaming electrons; these electrons can excite hydrogen atoms in the chromosphere, causing the optical Hα flares or ‘low temperature flares’. It is thus argued that as the ‘force-free’ fields are being built up for the magnetic energy storage, a flare must already be in progress.     

A multiwavelength study of an M-class flare and the origin of an associated eruption from NOAA AR 11045

In this paper, we study multiwavelength observations of an M6.4 flare in Active Region NOAA 11045 on 7 February 2010. The space- and ground-based observations from STEREO, SoHO/MDI, EIT, and Nobeyama Radioheliograph were used for the study. This active region rapidly appeared at the north-eastern limb with an unusual emergence of a magnetic field. We find a unique observational signature of the magnetic field configuration at the flare site. Our observations show a change from dipolar to quadrapolar topology. This change in the magnetic field configuration results in its complexity and a build-up of the flare energy. We did not find any signature of magnetic flux cancellation during this process. We interpret the change in the magnetic field configuration as a consequence of the flux emergence and photospheric flows that have opposite vortices around the pair of opposite polarity spots. The negative-polarity spot rotating counterclockwise breaks the positive-polarity spot into two parts. The STEREO-A 195 Å and STEREO-B 171 Å coronal images during the flare reveal that a twisted flux tube expands and erupts resulting in a coronal mass ejection (CME). The formation of co-spatial bipolar radio contours at the same location also reveals the ongoing reconnection process above the flare site and thus the acceleration of non-thermal particles. The reconnection may also be responsible for the detachment of a ring-shaped twisted flux tube that further causes a CME eruption with a maximum speed of 446 km/s in the outer corona.     

Interpretation of time characteristics of solar X-ray bursts referring to associated microwave bursts

It has been controversial whether the flare-associated hard X-ray bursts are thermal emission or non-thermal emission. Another controversial point is whether or not the associated microwave impulsive burst originates from the common electrons emitting the hard X-ray burst.It is shown in this paper that both the thermal and non-thermal bremsstrahlung should be taken into account in the quantitative explanation of the time characteristics of the hard X-ray bursts observed so far in the photon energy range of 10–150 keV. It is emphasized that the non-thermal electrons emitting the hard X-rays and those emitting the microwave impulsive burst are not common. The model is as follows, which is also consistent with the radio observations.At the explosive phase of the flare a hot coronal condensation is made, its temperature is generally 10⁷ to 10⁸K, the number density is about 10¹⁰ cm^–3 and the total volume is of the order of 10²⁹ cm³. A small fraction, 10^–3–10^–4, of the thermal electrons is accelerated to have power law distribution. Both the non-thermal and thermal electrons in the sporadic condensation contribute to the X-ray bursts above 10 keV as the bremsstrahlung. Fast decay of the harder X-rays (say, above 20 keV) for a few minutes is attributed to the decay of non-thermal electrons due to collisions with thermal electrons in the hot condensation. Slower decay of the softer X-rays including around 10 keV is attributed to the contribution of thermal component.The summary of this paper was presented at the Symposium on Solar Flares and Space Research, COSPAR, Tokyo, May, 1968.     

Thermal electrons runaway from a hot plasma during a flare in the reverse-current model and their X-ray bremsstrahlung

The behaviour of the thermal electrons escaping from a hot plasma to a cold one during a solar flare is investigated. We suppose that the direct current of fast electrons is compensated by the reverse current of the thermal electrons in ambient plasma. It is shown that the direct current strength is determined only by the regular energy losses due to Coulomb collisions. The reverse-current electric field and the distribution function of fast electrons are found in the form of an approximate analytical solution to the self-consistent kinetic problem of the dynamics of a beam of escaping thermal electrons and its associated reverse current.The reverse-current electric field in solar flares leads to a significant reduction of the convective heat flux carried by fast electrons escaping from the high-temperature plasma to the cold one. The spectrum and polarization of hard X-ray bremsstrahlung, and its spatial distribution along flare loops are calculated and can be used for diagnostics of flare plasmas and escaping electrons.Send offprint requests to B. V. Somov.     

Yohkoh observations of the creation of high-temperature plasma in the flare of 16 December 1991

Yohkoh observations of an impulsive solar flare which occurred on 16 December, 1991 are presented. This flare was a GOES M2.7 class event with a simple morphology indicative of a single flaring loop. X-ray images were taken with the Hard X-ray Telescope (HXT) and soft X-ray spectra were obtained with the Bragg Crystal Spectrometer (BCS) on board the satellite. The spectrometer observations were made at high sensivity from the earliest stages of the flare, are continued throughout the rise and decay phases, and indicate extremely strong blueshifts, which account for the majority of emission in Caxix during the initial phase of the flare. The data are compared with observations from other space and ground-based instruments. A balance calculation is performed which indicates that the energy contained in non-thermal electrons is sufficient to explain the high temperature plasma which fills the loop. The cooling of this plasma by thermal conduction is independently verified in a manner which indicates that the loop filling factor is close to 100%. The production of superhot plasma in impulsive events is shown to differ in detail from the morphology and mechanisms appropriate for more gradual events.     

Distribution of temperature and emission measure in a steadily heated solar atmosphere

The distribution of temperature and emission measure in the stationary heated solar atmosphere was found for the limiting cases of slow and fast heating, when either the gas pressure or the concentration are constant through the layer depth. Results are relevant to the conditions when the energy injected by waves or by non-thermal particles or in some different way quickly transforms into a thermal flux. Under these conditions the temperature distribution with depth is determined by radiation loss and thermal conductivity, and at any values of energy flux and plasma concentration it is characterized by two universal functions. One of them gives the relation between the energy flux and temperature at the region boundary: the other - the temperature run with the depth. This run is such that a considerable part of the energy is radiated by a thin transition region with a very large temperature gradient.The results may be applied for calculation of the temperature and the emission measure both for the high temperature region of a flare, and for the quiet corona. The dimensionless structure of the transition region is the same for any value of the energy flux. These results concerning solar flares can help to explain the identity of optical spectra for flares of different types, the emission in a wide temperature interval from nearly the same region of space and the very small thickness of the region emitting optical lines. The latter is due to the shell structure of the flare as opposed to the usually assumed filamentary one.     

Evolution of a flaring loop after injection of energetic electrons

For the November 5, 1980 flare it is investigated how the plasma in a large flaring loop responds to the injection of energetic electrons. Observations are compared with the results of a one-dimensional numerical simulation. For the simulation it is assumed that at the time the injection is started, the plasma is in an equilibrium state with a constant pressure along the loop and conductive heating compensated by radiative losses. Especially important for the evolution of the impulsively heated plasma is the penetration depth of the fast electrons compared to the depth of the transition layer. Both parameters are known from the observations. The injected energy is 2.6 × 10¹¹ ergs cm ^?2 in 30 s (as derived from the hard X-ray observations) and computations show that the high temperature plasma of the loop responds to it with upward motions of about 50 km s^?1, i.e. with velocities much smaller than the ion sound speed (≈ 500km s^?1). The heating of the plasma due to the absorption of beam energy can be understood using a constant density approximation. After the heating phase the plasma returns in about 5 min to its initial state by conductive cooling. The downward conducted energy is radiated away in the transition zone. The numerical simulation shows that impulsive heating by non-thermal electrons only does not explain the observed large increase in the density of the loop during the flare. It is therefore required that continuous energy and/or mass input occur after the impulsive phase.     

Thick target X-ray bremsstrahlung from partially ionised targets in solar flares

The effect of partial ionisation of a thick target bremsstrahlung source on the emitted X-ray intensity is analysed. It is shown that a totally ionised target produces an X-ray burst only about one third as intense as that from an unionised target.In the case of a solar flare plasma target, the ionisation decreases with increasing depth in the flare. Thus, in an X-ray flare model in which electrons are continuously accelerated down into the chromosphere, high energy photons are produced with increased efficiency in the deeper layers of the flare plasma with consequent hardening of the X-ray spectrum. As a result, the spectra of nonthermal electrons in flares, inferred from X-ray spectra, are steepened and their total energy correspondingly increased.