Solar Hard X-ray Research Articles

The polarization and directivity of solar-flare hard X-ray bremsstrahlung from a thermal source

The polarization and directivity of hard X-ray bremsstrahlung from a thermal source consisting of a region in which a thermal flux drives a pair of steel collisionless conduction fronts were evaluated. The conduction fronts are symmetrically driven from a central region, heated by the flare energy dissipation process. By comparing results with similar calculations based on a nonthermal thick-target electron beam model of the source, it was aimed to determine the degree to which the observed polarization and directivity of solar flare hard X-rays favor either model. Results which exhibit significant polarization and directivity of the hard X-ray radiation emitted by the source are produced by using Maxwellian electron-phase-space distribution functions modified to take into account a directional heat flux and a steady direct current in the X-ray source, and a fully relativistic treatment of the bremsstrahlung emission process. The results are consistent with solar hard X-ray anisotropy and polarization observations to date, although these observations are too crude to be conclusive.

Production of a collisionless conduction front by rapid coronal heating and its role in solar hard X-ray bursts

The theoretical and observational evidence for and against an impulsively heated thermal bremsstrahlung source of solar hard X-ray bursts is briefly reviewed. In particular, it is noted that in a collision-dominated plasma of any reasonable density the collisional relaxation time would be much longer, and the conductive cooling time much shorter, than typical burst durations (cf. Kahler). It is then shown, however, that free expansion of impulsively heated electrons into a cool surrounding plasma will generate an ion-sound turbulent front which efficiently scatters the electrons, thus bottling up their heat flux. The conductive cooling time is thus increased by a factor (mt/me)112 over its free expansion limit, compatible with observations. The front thickness is less than about 1 km. Interaction with the turbulence permits the bulk of the electrons to reach a relaxed distribution, without the need for high densities, hence providing an efficient source of hard X-rays. Electrons of U > 2.6Ue, however, are not scattered by the ion-sound turbulence but do not necessarily escape freely either, because of the action of the thermoelectric field present. Finally, some brief remarks are directed to the mechanism of heating and to the compatibility of these conditions in the hard X-ray source with other flare observations. Subject headings: hydromagnetics - Sun: corona - Sun: X-rays

Effect of Asymmetry on a Trap Model for Solar Hard Xray Bursts

The basic model for the precipitation of trapped energetic particles from a magnetic flux tube is Kennel and Petschek’s (1966) model. Their model is symmetric, implying equal precipitation rates at the two feet of the flux tube. We have developed a model for precipitation in an asymmetric flux tube (Melrose and White 1979). Here we explore some of the consequences for the precipitation model of Melrose and Brown (1976) for solar hard X-ray bursts. In Melrose and Brown’s model roughly half the X-rays arise from precipitating electrons. With present instruments it is not possible to resolve the two feet of the flux tube. However, if the feet can be resolved, either directly by future X-ray telescopes, or indirectly through secondary optical, UV or radio observations, then, as we shall show, the additional information obtained could be used to derive information on processes in the magnetic trap.

Observations of the hard X-ray spectrum of the impulsive phase of solar flares

view Abstract Citations (35) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of the hard X-ray spectrum of the impulsive phase of solar flares. Elcan, M. J. Abstract The impulsive solar hard X-ray spectrum provides a direct diagnostic on the impulsive energy release of the flare. The data from the hard X-ray (14-342 keV) spectrometer on OSO 7 have been examined for events covering a wide energy range to determine the photon spectrum more precisely. This resulted in a sample of 38 events. The data have been fitted with either single- or double-power laws, and with an exponential spectrum, as from thermal bremsstrahlung. The chi-squared statistic between two-parameter fits strongly favors the exponential fit, particularly at peak intensity when the widest energy range is sampled. This result supports a single-temperature thermal source of the impulsive solar hard X-rays with temperatures in the range 4.7-31 keV and emission measures in the range from 8.4 x 10 to the 43rd to 6.0 x 10 to the 46th per cu cm. The temperature reaches a single maximum and then rapidly decays, but the emission measure usually increases throughout, even after temperature maximum, for data with 10-s resolution. Publication: The Astrophysical Journal Pub Date: December 1978 DOI: 10.1086/182840 Bibcode: 1978ApJ...226L..99E Keywords: Solar Flares; Solar Temperature; Solar X-Rays; X Ray Spectra; Bremsstrahlung; Oso-7; Plasma Diagnostics; Thermal Energy; Solar Physics; Solar Flares:X-Ray Spectra full text sources ADS |

Analysis of bremsstrahlung source spectra in terms of integral moments

The problems of deriving temperature structure for thermal hard X-ray sources and electron spectra for nonthermal sources, from their bremsstrahlung continuum spectra, are briefly reviewed and the dangers of model-fitting are reiterated. A more satisfactory approach is developed in terms of the evaluation of integral moments of the distribution function.Using thermal analysis of the impulsive solar hard X-ray burst of 1970 March 1 as an illustration, the integral moment method is shown to give a rapid assessment of the real information content of a bremsstrahlung spectrum. In particular it is shown that due to limitations of bandwidth and, to a lesser extent, of spectral resolution, current hard X-ray spectrometry alone is incapable of distinguishing isothermal, multithermal, and nonthermal sources. Criteria are established for the spectrometer needed to define a thermal source distribution to within some specified accuracy.

Determination of the source height and anisotropy of solar hard X-rays by measurements with good time resolution

When emitted at the same time, solar hard X-rays reflected by the photosphere arrive at an observer at later times than primary hard X-rays coming directly from the source. This time lag of albedo photons, therefore, should be taken into account in interpreting fine-scale hard X-ray time profiles. If hard X-ray bursts consist of succession of short-lived ‘elementary bursts’, under favorable conditions reflected hard X-rays can be resolved from primary hard X-rays with good time resolution. If so, from the time lag and the ratio of the albedo flux to the primary flux, one can determine the source height and anisotropy of solar hard X-rays.

Diagnostics of solar flare hard X-ray sources

The dynamics of hard X-ray producing electron beams in solar flares can be strongly affected by the occurrence of a reverse current. The parameter diagram for a beam can be divided into three regimes, one of which is the usual thick target case, the two others being due to two different possible consequences of the reverse current. The use of this parameter diagram as a possible diagnostic tool for solar flare hard X-ray sources is discussed, together with the necessary observations and their interpretation.

Pulsations in solar hard X-ray bursts

We have analyzed the hard X-ray emission from 28 large solar events, searching for pulsations in intensity profiles. Periodicity occurred in 26 events, usually soon after the onset, with periods in the range 10–100 s. Pulsations occurring at common frequencies in different energy bands are observed to be closely in phase. Periodic behavior in hard X-ray emission is related to that at microwave and decametric wavelength. We discuss our observations briefly in terms of two models: that of McClean et al. (1971), applied to X-ray emission, and that of Brown and Hoyng (1975). As periodicity is normal in extended hard X-ray bursts and occurs through a broad energy band, it is probably directly related to a principal flare acceleration mechanism. Our observations constrain possible mechanisms of flare acceleration and physical properties of the acceleration region.

Backscatter, anisotropy, and polarization of solar hard X-rays

The problems of anisotropy, polarization, center-to-limb variation of the X-ray spectrum, and Compton backscatter are investigated in a study of solar hard X-rays. Effect of backscatter are found particularly important for anisotropic sources which emit hard X-rays predominantly toward the photosphere; for such anisotropic primary X-ray sources, the observed X-ray flux near 30 keV does not depend significantly on the position of the flare. In addition, the degree of polarization of the sum of the primary and reflected X-rays with energies in the 15 to 30 keV range may be as high as 30%. Determination of the height and anisotropy of the primary X-ray sources from study of the albedo patch is also discussed.

Collective plasma effects and the electron number problem in solar hard X-ray bursts

Due to the relatively high stream densities involved, collective interactions with the ambient plasma are likely to be important for the electrons producing solar hard X-ray bursts. In thick- and thin-target bremsstrahlung models the most relevant process is limitation of the invoked electron beams by ion sound wave generation in the neutralizing reverse current established in the atmosphere. For the thick target model it is shown that typical electron fluxes are near the maximum permitted by stability of the reverse current so that ion-sound wave generation may be the process which limits the electron injection rate. On the other hand the chromospheric reverse current is sufficient to supply the large total number of electrons which have to be accelerated in the corona. For the thin target the low density of the corona severely limits the possible reverse current so that the maximum upward flux of fast electrons is probably much too small to explain X-ray bursts but compatible with observations of interplanetary electrons.

An Upper Limit to the Anisotropy of Solar Hard X-Ray Emission

view Abstract Citations (41) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Upper Limit to the Anisotropy of Solar Hard X-Ray Emission Datlowe, D. W. ; O'Dell, S. L. ; Peterson, L. E. ; Elcan, M. J. Abstract A statistical study of center-to-limb variations in the frequency of occurrence of solar hard X-ray bursts is used to look for directivity in sthe emission. The X-ray data consist of 148 bursts observed by the University of California, San Diego, solar X-ray instrument on the 0S0-7 satellite. No center-to-limb variation in hard X-ray burst occurrence was found. The upper limit on any limb darkening or limb brightening of 20 keV X-ray emission is 407o at the 95 confidence level. This result rules out downward-streaming thick-target models for the motion of the X-ray emitting electrons. Furthermore, strong east-west streaming is also ruled out when the observed center-tolimb spectral variation is combined with the directivity result. Either the motions of the selectrons which emit hard X-rays are predominantly random, or a variety of different emitting-region geometries commonly occur. Subject headings: Sun: flares - Sun: X-rays Publication: The Astrophysical Journal Pub Date: March 1977 DOI: 10.1086/155075 Bibcode: 1977ApJ...212..561D full text sources ADS |

Spectral features in solar hard X-ray and radio events and particle acceleration

view Abstract Citations (79) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectral features in solar hard X-ray and radio events and particle acceleration. Benz, A. O. Abstract Hard X-ray and radio intensities of two major solar outbursts are found to anticorrelate in time with their spectral indices, which, furthermore, are in satisfactory correlation with each other. The radio emission must be synchrotron radiation from the same electron population that causes the X-ray bremsstrahlung. A delay of temporal features, increasing with energy, is clearly observed in one of the X-ray events. The observations are interpreted with a simple model, which assumes ion-acoustic instability in a current sheet, transformation of turbulence energy into Langmuir waves, and their escape into the ambient plasma where they accelerate particles. A power-law index of the electron energy distribution is derived for acceleration being balanced by escape of particles. Anticorrelation of intensity and spectral index then follows naturally from variations of the spectral wave energy density or the dimension of the acceleration region. The finite collision time in the acceleration region and its increase with energy introduce a perturbation which can explain the observed drift of temporal features. Publication: The Astrophysical Journal Pub Date: January 1977 DOI: 10.1086/154928 Bibcode: 1977ApJ...211..270B Keywords: Particle Acceleration; Solar Radio Emission; Solar Wind; Solar X-Rays; X Ray Spectra; Bremsstrahlung; Electron Energy; Ion Acoustic Waves; Solar Electrons; Synchrotron Radiation; Solar Physics full text sources ADS |

Precipitation in Trap Models for Solar Hard X-ray Bursts

Precipitation of electrons due to collisions necessarily occurs in trap models for hard X-ray bursts and the thick-target emission from the precipitating electrons produces an X-ray spectrum similar in intensity and in spectral shape to that from the trapped electrons. Such a trap-plus-precipitation model combines attractive features and removes some of the difficulties of thick-target and simple trap models. The evolution of the trap-plus-precipitation model is amenable to an analytic description, which is presented, including inversion, to find the injection electron spectrum from the X-ray spectrum. Streaming instabilities are unlikely to be important and resonant scattering is also probably not important for the electrons |$(E\tilde{\lt}\,100\,\text{keV})$| which emit most of the X-rays but may well be important for the higher energy electrons which generate microwave bursts.

High time resolution analysis of solar hard X-ray flares observed on board the ESRO TD-1A satellite

The Utrecht solar hard X-ray spectrometer S-100 on board the ESRO TD-1A satellite covers the energy range above 25 keV with 12 logarithmically spaced channels. Continuous sun-pointing is combined with high time resolution: 1.2 s for the four low energy channels (25–90 keV) and 4.8 s for the others. It is emphasized that the instrument design and calibration yield data virtually free of pile-up and other instrumental defects. A complete set of observations is presented for all well-observed flares during the period March 12, 1972 to October 1, 1973, including four from the highly active period August 1–8, 1972. Photon spectra are computed every 1.2 s for each event by deconvolution through the instrument response, rather than by fitting techniques. Using these actual photon spectra, the index γ for the best fitting single power law and the minimum (thick target) injection rate of electrons above 25 keV, F 25, are calculated. Results for γ and F 25 at 1.2 s intervals are presented for each event. Examination of all these results tentatively suggests a real distinction between events of a purely impulsive nature and prolonged events. Techniques of time series analysis are applied to the burst time profiles. Specifically: A search is made for correlations between instantaneous values of inferred parameters (e.g. F 25, γ and the time scales). Most results are negative, but in the August 4 and 7, 1972 events a very well defined path was followed through the (F 25, γ)-plane, giving insight into the electron acceleration process. Finally some general conclusions are drawn concerning the implications of our analysis for the physics of particle acceleration, including the possibility of two classes of event. Specifically, the severe problems posed by the large electron fluxes (equivalent current ∼1017 A) demanded by the data are discussed in relation to flare theories. Some possibilities for getting around these problems, such as by reacceleration in a confinement region, are briefly considered.

Betatron acceleration in a large solar hard X-ray burst

The problem of diagnosing flare particle acceleration mechanisms from hard X-ray bursts is discussed, and it is argued that the electron trap model of bursts is more amenable to observational investigation at present than models of thick-target type. It is then shown that data for the large X-ray burst of 1972 August 4 are consistent with the source electrons being trapped in a very large vibrating coronal magnetic bottle. Furthermore, the observations show that the burst time profile is not dominated by collisional losses. (AIP)

Solar Flare X-Ray Measurements and Their Relation to Microwave Bursts

This review discusses the available observational material of solar hard X-ray bursts, their interpretation in terms of a model of the source region and their relation with other flash-phase phenomena, in particular the impulsive microwave bursts.

Criteria for plasma-wave losses for electrons streaming from an acceleration region

view Abstract Citations (12) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Criteria for plasma-wave losses for electrons streaming from an acceleration region. Smith, D. F. Abstract Consistent criteria are derived for determining when losses due to collective excitation of electron plasma waves will be important for electrons streaming from an acceleration region. The analysis is nonrelativistic and one-dimensional, but includes the effect of Coulomb collisions. These criteria are applied to source models for solar hard X-ray bursts, and it is shown that these losses will almost never be important for the observations obtained thus far. The case in which the acceleration region and the X-ray source partially overlap is considered briefly. Publication: The Astrophysical Journal Pub Date: October 1975 DOI: 10.1086/153914 Bibcode: 1975ApJ...201..521S Keywords: Astronomical Models; Electron Plasma; Particle Acceleration; Plasma Waves; Solar X-Rays; Bursts; Collisional Plasmas; Criteria; Energy Dissipation; Nonrelativistic Mechanics; Plasma-Particle Interactions; Solar Physics full text sources ADS |

The Interpretation of Spectra, Polarization, and Directivity of Solar Hard X-Rays

The importance of interpretation of hard X-ray burst spectra, polarisation and directivity to the flare process as a whole is emphasised. After critically reviewing observations of these and related burst characteristics, the problems of analytic and numerical inversion of the X-ray spectrum to give the flare electron spectrum are discussed and it is concluded that electron spectra cannot be accurately and unambiguously inferred from their bremsstrahlung emission. Consideration of directional, albedo, and model-dependent effects, on the other hand, shows that none of the X-ray data are at present inconsistent with a power-law electron acceleration spectrum.Characteristics of thick-target, thin-target and electron-trap models of hard X-ray sources are discussed quantitatively and their ability to fit the observations is examined. Selection of a satisfactory model is precluded by lack of both sufficient observations and of adequate theoretical description of models. Nevertheless, it is suggested that redistribution of the flaring atmosphere and the effects of collective energy losses may reconcile even behind-the-limb burst observations and interplanetary electron spectra with a thick-target description (which fits other data well). This is attractive since a thick-target X-ray source makes the minimal demand on flare energy. Even a thick-target, however, requires an embarrassingly large number and energy of fast electrons. Therefore the review is completed by discussing how these requirements might be reduced if thermal emission extended to hard X-ray energies or if multiple reacceleration of electrons occurred.

A simulation of the directivity effect to be expected in hard X-ray flares

A Monte Carlo technique has been used to predict the relative visibility of solar hard X-ray flares as a function of solar longitude assuming the model of Takakura and Kai to be realistic. Comparison is made with previous statistical studies of observations. A discernable longitudinal variation in the relative visibility of flares is shown to be expected but the probability of flares being visible towards the limb is shown to be higher than had previously been evident.

The directivity and polarisation of thick target X-ray bremsstrahlung from solar flares

The directivity and polarisation of solar hard X-ray bursts is discussed in terms of two bremsstrahlung source models. These involve continuous and impulsive injection of electrons respectively, as described widely in the literature.