Hard X-ray emission in electron-heated solar flares — A comparison of nonthermal and thermal contributions

We calculate the spatial structure of hard X-ray emission during the impulsive phase of electron-heated solar flares. Both direct non-thermal bremsstrahlung and the thermal bremsstrahlung arising from the heated plasma are considered. Our results indicate that the spread of non-thermal emission into the upper parts of the loop, through evaporation of the chromospheric target, may be more important than the appearance of a hot thermal source in the corona. The effects of varying the viewing angle to the flare loop, and of finite-size resolution element, are also considered, and we compare our results with observations from the Solar Maximum Mission Hard X-Ray Imaging Spectrometer. We also contrast the predicted structures with those predicted by other models of flare energy release, and it is found that the electron-heated model provides the most satisfactory agreement with the observations.On leave from: Department of Physics and Astronomy, The University, Glasgow G12 8QQ, Scotland, U.K.Presidential Young Investigator.     

Hard X-ray emission processes in solar flares

Solar hard X-ray emission is one of the most direct diagnostics of accelerated particles during solar flares. In this review, the current understanding of hard X-ray emission processes is discussed: first the different emission mechanisms (in particular inverse Compton radiation, energetic ion or electron bremsstrahlung) are presented and the plausibility of each of these mechanisms is discussed. Then, different types of hard X-ray models (thermal or non-thermal, homogeneous or inhomogeneous emission regions) are presented together with the comparison of their predictions with X-ray observations (spectral, spatial and temporal informations - directivity and polarization).Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Radio and X-ray imaging observations of solar flares and coronal transients

We present a review of selected studies based upon simultaneous radio and X-ray observations of solar flares and coronal transients. We use primarily the observations made with large radio imaging instruments (VLA, BIMA, Nobeyama, and Nançay) along with Yohkoh/SXT and HXT and CGRO experiments. We review the recent work on millimeter imaging of solar flares, microwave and hard X-ray observations of footpoint emission from flaring loops, metric type IV continuum bursts, and coronal X-ray structures. We discuss the recent studies on thermal and nonthermal processes in coronal transients such as XBP flares, coronal X-ray jets, and active region transient brightenings.Dedicated to Cornelis de Jager     

Propagation and confinement of energetic electrons in solar flares

The propagation, cofinement and total energy of energetic (>25 keV) electrons in solar flares are examined through a brief review of the following hard X-ray measurements: (1) spatially resolved observations obtained by imaging instruments; (2) stereoscopic observations of partially occulted sources providing radial (vertical) spatial resolution; and (3) directivity of the emission measured through stereoscopic observations and the center-to-limb variation of the occurrence frequency of hard X-ray flares. The characteristics of the energetic electrons are found to be quite distinct in impulsive and gradual hard X-ray flares. In impulsive flares the non-thermal electron spectrum seems to extend down to 2 keV indicating that the total energy of non-thermal electrons is much larger than that assumed in the past.     

Optical-to-Radio Continua in Solar Flares

Spectral continua observed during solar flares may contain information about both thermal and non-thermal heating mechanisms. Using two semi-empirical flare models F2 and FLA, we synthesize the thermal continua from optical to mm–radio domains and compare their intensities with quiet-Sun values computed from a recent model C7. In this way, the far-infrared and sub-mm/mm continua are studied for the first time, and we present our results as a benchmark for further modeling and for planning new observations, especially with the ALMA instrument. Finally, we demonstrate how these continua are formed and show a close correspondence between their brightness temperature and the kinetic-temperature structure of the flaring atmosphere.     

A dominant role for protons at the onset of solar flares

The energetics of the onset of the impulsive phase of solar flares are examined on the premise that a single acceleration mechanism is operating in the corona. From considerations of recent observations of plasma turbulence and upflows, and nuclear gamma-rays it is concluded that a model where the bulk of the energy resides in a non-thermal electron beam with a low energy cut-off at 20–25 keV is incompatible with many of the observations. Conversely, a model where the bulk of the energy resides in non-thermal protons is consistent with the majority, if not all, of the observations. It is suggested that the bulk of the energy in the impulsive phase is initially transferred to 10²–10³ keV protons. Acceleration by a series of small shocks is an energy transfer mechanism which gives particles increments in velocity rather than energy and would naturally favour protons over electrons. An important consequence of this result is that the hard X-ray burst must be thermal. At this time the precise mechanism for thermal X-ray production is unclear; however recent theoretical plasma physics results have indicated promising avenues of research in this context.     

Hard X-ray characteristics of solar flares using electron Monte-Carlo calculations and beamed thick target model

The experimental results of X-ray bursts with spectral characteristics, spatial distribution, fast time variations, polarization, and directivity measurements carried out with Intercosmos, PVO/ISEE-3 spacecrafts, imaging instrument observations of Hard X-ray (HXIS) and Hard X-ray Burst Spectrometers (HXRBS) during solar maximum mission have been reviewed. The observed results about the above characteristics are discussed in detail in terms of non-thermal and thermal models. It is shown that the results can be interpreted in terms of beamed thick-target model in which electrons stream down to the loop footpoints and produce hard X-rays through electron-ion bremsstrahlung.     

High-energy observations of solar flares

Extensive observations of solar flares made in high energy bands during the maximum of the present solar cycle are discussed with a special reference to the results from HINOTORI, and with attention to the relevant flare models. The hard X-ray (HXR) images from HINOTORI showed mostly coronal emission at 20–25 keV suggesting that the HXR is emitted from multiple coronal loops, consistent with the non-thermal electron beam model in a high density corona. The thermal HXR model seems to be inconsistent with some observations. Three types of flares which have been classified from the Hinotori results are described, along with newly discovered hot thermal component of 30–40 million K which contributes thermal HXR emission. A summary is given for the characteristics of the energy release in an impulsive burst; and an empirical model is described, which explains simultaneous energy releases in multiple loops and successive movements of the release site as suggested from the HXR morphology. The discovery of large blue-shifted hot plasma from the soft X-ray line spectrum leads to some quantitative arguments for the evaporating flare model. An electron-heated flare atmosphere appears to explain various observations consistently.Invited paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Hard X-ray Radiation from Solar Flares in the Second Half of 2001: Preliminary Results of the SPR-N Experiment Onboard the Coronas-F Satellite

The first results of the experiment with the SPR-N hard X-ray (20–100 keV) polarimeter onboard the Coronas-F observatory (the experiment started on August 15, 2001) are presented. Hard X-ray radiation was detected from several solar flares. The spectral and temporal parameters were determined and the polarization was estimated. Comparison with the GOES observations of thermal X-ray radiation shows that hard X-ray bursts occur at the growth phase of the thermal radiation and that they are associated with the bremsstrahlung of energetic electrons precipitating into the solar atmosphere.     

Imaging X-ray Polarimeter for Solar Flares (IXPS)

We describe the design of a balloon-borne Imaging X-ray Polarimeter for Solar flares (IXPS). This novel instrument, a Time Projection Chamber (TPC) for photoelectric polarimetry, will be capable of measuring polarization at the few percent level in the 20?C50 keV energy range during an M- or X-class flare, and will provide imaging information at the ??10 arcsec level. The primary objective of such observations is to determine the directivity of nonthermal high-energy electrons producing solar hard X-rays, and hence to learn about the particle acceleration and energy release processes in solar flares. Secondary objectives include the separation of the thermal and nonthermal components of the flare X-ray emissions and the separation of photospheric albedo fluxes from direct emissions.     

Hard X-ray emission from the solar corona

This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration.     

Thermal evolution of flare plasma

The evolution of hot thermal plasma in solar flares is analyzed by a single-temperature model applied to continuum emission in the 5 keV < E ? 13 keV spectral range. The general trend that the thermal plasma observed in soft X-rays is heated by the non-thermal electrons that emit as the hard X-ray bursts is confirmed by the observation of an electron temperature increase at the time interval of hard X-ray spikes and a quantitative comparison between thermal energy content and hard X-ray energy input. Non-thermal electrons of 10 keV < E < 30 keV energy may play an important role in pre- and post-burst phases.     

An Observational Study on Coronal Hard X-ray Sources in Flares

Observational studies on solar ?ares with footpoints partially occulted by the solar limb provide an important method for diagnostics of coronal hard X-ray emissions. The statistics of hard X-ray sources in 71 such ?ares observed by RHESSI (Reuven Ramaty High-Energy Solar Spectroscopic Imager) show that the two kinds of hard X-ray sources proposed in previous studies (i.e., the sources with respectively a smaller and larger spatial separations between the thermal and non-thermal sources of coronal hard X-ray emissions) have no evident difference in the aspects of their photon spectra, images, light curves, GOES durations, etc. The area of the radiation region, the ?are's total thermal energy and GOES duration are well correlated with the distance of separation. These results support some uni?ed models of solar ?ares proposed in recent years, and indicate that the Masuda ?are is only a kind of special event, which does not possess the general features of coronal hard X-ray emissions     

太阳耀斑研究的新进展(英)

近年来对太阳耀斑的研究取得了重要的进展。一些新的发现主要来自高分辨率的观测，特别是来自"阳光"卫星的结果。综述的范围包括太阳耀斑中磁重联的新证据、硬X射线源（包括所谓的超热源）的分类、X射线喷流的发现、环－环相互作用的证据以及对耀斑大气动力学过程的新认识等。基于这些新的知识，讨论了有关耀斑模型的一些问题。     

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.     

Evidence of Magnetic Reconnection in Solar Flares and a Unified Model of Flares

The solar X-ray observing satellite Yohkoh has discovered various new dynamic features in solar flares and corona, e.g., cusp-shaped flare loops, above-the-loop-top hard X-ray sources, X-ray plasmoid ejections from impulsive flares, transient brightenings (spatially resolved microflares), X-ray jets, large scale arcade formation associated with filament eruption or coronal mass ejections, and so on. It has soon become clear that many of these features are closely related to magnetic reconnection. We can now say that Yohkoh established (at least phenomenologically) the magnetic reconnection model of flares. In this paper, we review various evidence of magnetic reconnection in solar flares and corona, and present unified model of flares on the basis of these new Yohkoh observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the origin of solar flare X-rays

The origin of X-ray solar bursts is investigated on the basis of the theoretical model developed by Syrovatskii. According to this model (i) one of the most important manifestations of flares is the acceleration of charged particles (mainly of electrons) to subrelativistic and relativistic energies, and (ii) the two flare phases: stationary (soft) and nonstationary (hard) should be distinguished. The first phase is accompanied by the generation of the soft (2–8 Å) thermal X-rays and the second one by the generation of hard thermal and nonthermal X-rays in the 10 keV range. The thermal X-rays arise in both phases due to the heating of the ambient gas by accelerated particles. The possible mechanisms of non-thermal X-rays are investigated. Simple models of the emitting region are considered, taking into account the simultaneous observations in different regions of the electromagnetic spectrum.     

Solar flare hard X-ray observations

Recent hard X-ray observations of solar flares are reviewed with emphasis on results obtained with instruments on the Solar Maximum Mission satellite. Flares with three different sets of characteristics, designated as Type A, Type B, and Type C, are discussed and hard X-ray temporal, spatial, spectral, and polarization measurements are reviewed in this framework. Coincident observations are reviewed at other wavelengths including the UV, microwaves, and soft X-rays, with discussions of their interpretations. In conclusion, a brief outline is presented of the potential of future hard X-ray observations with sub-second time resolution, arcsecond spatial resolution, and keV energy resolution, and polarization measurements at the few percent level up to 100 keV.     

The high-frequency characteristics of solar radio bursts

We compare the millimeter, microwave, and soft X-ray emission from a number of solar flares in order to determine the properties of the high-frequency radio emission of flares. The millimeter observations use a sensitive interferometer at 86 GHz which offers much better sensitivity and spatial resolution than most previous high-frequency observations. We find a number of important results for these flares: (i) the 86 GHz emission onset appears often to be delayed with respect to the microwave onset; (ii) even in large flares the millimeter-wavelength emission can arise in sources of only a few arc sec dimension; (iii) the millimeter emission in the impulsive phase does not correlate with the soft X-ray emission, and thus is unlikely to contain any significant thermal bremsstrahlung component; and (iv) the electron energy distributions implied by the millimeter observations are much flatter (spectral indices of 2.5 to 3.6) than is usual for microwave or hard X-ray observations.     

Heating of the solar flare plasma by high energy electrons

Heating of the ambient plasma by high energy electrons in solar flares is discussed. It is shown that for large flares the heating is enough to produce a thermal plasma of a temperature up to a few times of 10⁷K rapidly in the initial phase of the flares. Thus thermal bremsstrahlung in addition to non-thermal bremsstrahlung should be considered for the X-ray emission of solar flares in the initial phase.NAS-NRC Resident Research Associate.