Great Microwave Bursts and hard X-rays from solar flares

The microwave and hard X-ray characteristics of 13 solar flares that produced microwave fluxes greater than 500 solar flux units have been analyzed. These Great Microwave Bursts were observed in the frequency range from 3 to 35 GHz at Bern, and simultaneous hard X-ray observations were made in the energy range from 30 to 500 keV with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission spacecraft. The principal aim of this analysis is to determine whether or not the same distribution of energetic electrons can explain both emissions. The temporal and spectral behaviors of the microwaves as a function of frequency and the X-rays as a function of energy were tested for correlations, with results suggesting that optically thick microwave emission, at a frequency near the peak frequency, originates in the same electron population that produces the hard X-rays. The microwave emission at lower frequencies, however, is poorly correlated with emission at the frequency which appears to characterize this common source. A single-temperature and a multitemperature model were tested for consistency with the coincident X-ray and microwave spectra at microwave burst maximum. Four events are inconsistent with both of the models tested, and neither of the models attempts to explain the high-frequency part of the microwave spectrum. A source area derived on the basis of the single-temperature model agrees to within the uncertainties with the observed area of the one burst for which spatially resolved X-ray images are available.Swiss National Science Foundation Fellow from the University of Bern.Also Energy/Environmental Research Group, Incorporated, Tucson, Arizona, and Department of Physics and Astronomy, University of North Carolina, Chapel Hill. Present address: Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland.     

On the production of hard X-rays in solar flares

The problem of producing the hard X-ray burst at the onset of solar flares may be thought of in terms of the problem of producing the non-thermal electrons which emit the X-rays via bremsstrahlung. Electron acceleration to relativistic energies without similar ion acceleration is difficult to achieve, even in an ad hoc theoretical model. Yet from global energetic considerations, it is not feasible to accelerate the electrons as a minor constituent of the total energetic particle population. Therefore, it is necessary to invoke a more sophisticated process for the electron acceleration. In this paper we describe a mechanism for achieving this via an initial acceleration of a neutralized ion beam. When such a beam impacts the chromosphere, the electrons start to scatter while the ions continue downwards, rapidly setting up an electric field which is either cancelled by the inflow of background chromospheric electrons or results in the runaway acceleration of beam electrons. In the former case the result is simply heating, whereas in the latter case much of the ion kinetic energy is transferred into electron kinetic energy. The final electron energy may be similar to the typical energy of the ions. The electrons that are accelerated are those in the neutral beam that experience an electric field greater than the critical Dreicer field. Thus there will be a low-energy cut-off to the electron spectrum which overcomes the well-known energetics problem at low energies with certain other spectral forms.     

Evidence for or against a common origin of hard X-rays and microwaves from solar flares

The problem of whether hard X-rays and microwaves are emitted from the same electrons in common or closely separated sources is reviewed on direct and indirect observational evidence. Detailed analyses of time structure and peak flux suggest that hard X-rays and microwaves are emitted from nearly co-spatial sources due to electrons streaming down to the chromosphere. However this model has not been confirmed yet by direct imaging observations.

     

Evidence for or against a common origin of hard X-rays and microwaves from solar flares

The problem of whether hard X-rays and microwaves are emitted from the same electrons in common or closely separated sources is reviewed on direct and indirect observational evidence. Detailed analyses of time structure and peak flux suggest that hard X-rays and microwaves are emitted from nearly co-spatial sources due to electrons streaming down to the chromosphere. However this model has not been confirmed yet by direct imaging observations.     

Polarization results of solar X-rays from OSO-7

Polarization measurements of solar X-ray events that were obtained with an instrument on OSO-7 are presented. The results appear to be consistent with the results of Tindo et al. on the existence and magnitudes of polarization. A comparison with polarization predictions when X-rays are produced by radial beams of electrons gives two examples of deviations from such a model.     

Microwave and hard X-ray bursts from solar flares

We have applied detailed theories of gyro-synchrotron emission and absorption in a magnetoactive plasma, X-ray production by the bremsstrahlung of non-thermal electrons on ambient hydrogen, and electron relaxation in a partially ionized and magnetized gas to the solar flare burst phenomenon. The hard X-ray and microwave bursts are shown to be consistent with a single source of non-thermal electrons, where both emissions arise from electrons with energies < mc ². Further-more, the experimental X-ray and microwave data allow us to deduce the properties of the electron distribution, and the values of the ambient magnetic field, the hydrogen density, and the size of the emitting region. The proposed model, although derived mostly from observations of the 7 July 1966 flare, is shown to be representative of this type of event.NAS-NRC Resident Research Associate.     

Calculation of anisotropy ratio of hard X-rays from solar flares — evidence for continuous electron injections/accelerations

Variation of electron energy and angular distributions has been studied as a function of column density by combining small-angle analytical treatment with large-angle Monte Carlo calculations. The distributions have been calculated for initial electron energy 300 keV and various incidence directions. Using these distributions and Sauter bremsstrahlung cross-section differential in photon energy and emission angle, we have calculated the X-ray energy and angular distributions for photon energies 10, 20, 50, 100, 150 and 200 keV. By taking the ratio of X-ray flux at 90 and 180°, we have computed the anisotropy ratio A as function of column density. Calculated anisotropy ratio compares well with ISEE-3 and PVO observations.     

The solar albedo of hard X-ray flares

The calculations of Compton backscattering from the solar surface of flare X-rays performed by Tomblin (1972) are extended to higher energies. It is shown that the effect is even more pronounced in the 40 keV region and that it can lead to substantial corrections to the observed X-ray spectra.     

Anisotropy of solar hard X-radiation during flares

The angular distribution of solar flare associated hard X-rays ( 10 keV) is calculated on the assumption that they originate as bremsstrahlung emission of energetic electrons with a power law spectrum. For the cross section the relativistic Sauter formula was used. Supposing the electrons to move in a fixed direction, the X-radiation is considerably anisotropic, especially at high photon energies. Taking into account a magnetic field, the anisotropy decreases with increasing pitch angles of the electrons. The anisotropic angular distribution of solar X-radiation seems to be connected with the centre-to-limb variation of hard X-ray bursts and with the correlation of shortwave fadeouts and geomagnetic crochets to H flares.     

Fast transients in hard X-ray solar flares

We present the results of a search for fast spikes in 5483 hard X-ray solar flares as observed with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission (SMM). Hundreds of fast spikes with durations of less than 1 second have been detected at time resolutions of 128 ms and 10 ms. Fast spikes have been detected with rise and decay times as short as 20 ms and with widths as short at 45 ms that represent the fastest hard X-ray variations yet seen from the Sun. The observations of such fast variations place new constraints on the physical nature of the source.     

Thermal bremsstrahlung hard X-rays and primary energy release in flares

Various methods are explored for obtaining regularized solutions of the severely ill-posed Laplace inversion problem involved in deriving plasma temperature (T) structure (differential emission measure(T)) from bremsstrahlung spectra. Inversions of simulated data show that zero-order regularisation (Tikhonov regularisation inL ² space) is very unsatisfactory even with weighting, while first-order regularisation (Tikhonov regularisation in Sobolev space) yields reasonable results.The method is applied to a high-resolution hard X-ray flare spectrum observed by Lin and Schwartz (1987) and yields a positive solution for(T) showing that a purely thermal interpretation is possible for that event. The form of(T) found has two broad features: one peaking at around 10⁷ K and falling off steeply toward 2 × 10⁸ K; a second spread around a peak near 4.5 × 10⁸ K. The interpretation of such(T) in terms of plasma heating and conductive flux is discussed in terms of plasma heat fluxes and heating rates. For 1-D geometry, the distribution of the plasma heating rateH(T) per unit volume is inferred from(T) in the limits of classical diffusive conduction and of saturated heat flux, the former being relevant atT below around 5 × 10⁷ K and the latter at much higherT. We find there exists a maximum inH(T) around 2 × 10⁸ K, a fact which may be important for energy release theories.     

A statistical analysis of hard X-Ray solar flares

In this study we perform a statistical study on, 8319 X-Ray solar flares observed with the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission satellite (SMM). The events are examined in terms of the durations, maximum intensities, and intensity profiles. It is concluded that there is no evidence for a correlation between flare intensity, flare duration, and flare asymmetry. However, we do find evidence for a rapid fall-of in the number of short-duration events.     

Correlation between solar microwave bursts and hard X-ray flares

We compared the microwave bursts with short timescale fine structure observed at 2.84 GHZ at Beijing Astronomical Observatory with the hard X-ry bursts (HXB) observed by the YOHKOH satellite during the period 1991 Oct–1992 Dec, and found that of the 20 microwave events, 12 had HXB counterparts. For the typical event of 1992-06-07, we analyzed the common quasi-period oscillations on the order of 10² s and calculated the parameters of the source region, together with a brief discussion.     

The interpretation of hard X-ray polarization measurements in solar flares

We review recent observations of polarization of moderately hard X-rays in solar flares and compare them with the predictions of recent detailed modeling of hard X-ray bremsstrahlung production by non-thermal electrons. We find that the recent advances in the complexity of the modeling lead to substantially lower predicted polarizations than in earlier models and more fully highlight how various parameters play a role in determining the polarization of the radiation field. The new predicted polarizations are comparable to those predicted by thermal modeling of solar flare hard X-ray production, and both are in agreement with the observations. In the light of these results, we propose new polarization observations with current generation instruments which could be used to discriminate between non-thermal and thermal models of hard X-ray production in solar flares.     

Relative timing of hard X-rays and radio emissions during the different phases of solar flares: Consequences for the electron acceleration

Observations relevant to the relative timing of hard X-ray, microwave and lower frequency radio bursts in different phases of flare are reviewed. It is shown that such timing comparisons give important information concerning the electron acceleration/injection process, the magnetic field topology at the acceleration site and the flare development itself. In particular it is shown that acceleration begins before the flash phase of flares and that it keeps going on continuously during the entire duration of a flare. Moreover, despite their wide separation in altitude, hard X-ray, microwave and lower frequency sources appear to arise from a common injection of electrons going on continuously through the different phases of flare. In situ acceleration by shock waves giving rise to type II radio emission is briefly discussed. As an alternative interactions between small and large scale magnetic structures is proposed.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

Contribution of electron-electron bremsstrahlung to solar hard X-radiation during flares

The bremsstrahlung produced in collisions of energetic electrons with thermal electrons and protons of a hydrogen plasma is calculated. The importance of electron-electron bremsstrahlung to the X-radiation in solar flares is discussed.     

Relativistic electrons from solar flares

Observations of interplanetary relativistic electrons from several solar-flare events monitored through 1964 to mid-1967 are presented. These are the first direct spectral measurements and time histories, made outside the magnetosphere, of solar-flare electrons having relativistic velocities. The 3- to 12-MeV electrons detected have kinetic energies about two orders of magnitude higher than those solar electrons previously studied in space, and measurements of both the time histories and energy spectra for a number of events in the present solar cycle were carried out. These measurements of interplanetary electrons are also directly compared with solar X-ray data and with measurements of related interplanetary solar protons.The time histories of at least four electron events show fits to the typical diffusion picture. A demonstrated similarity between the electron and the medium-energy proton fits for the event of 7 July, in particular, indicates that at these electron energies, but over several orders of magnitude of rigidity, whatever diffusion does take place is very nearly on a velocity, rather than a rigidity or an energy, basis. Diffusion-fit time histories varied as a function of T ₀ also indicate that the electrons in certain flare events originate at times near the X-ray and microwave burst, establishing their likely identity as the same electrons which cause the impulsive radiations. Also, the energy spectra and total numbers of the interplanetary electrons, compared with those of the flare-site electrons calculated from X-ray and microwave measurements, indicate that probably a small fraction of flare electrons escape into interplanetary space.     

X-ray emission from solar flares

Solar X-ray Spectrometer (SOXS), the first space-borne solar astronomy experiment of India was designed to improve our current understanding of X-ray emission from the Sun in general and solar flares in particular. SOXS mission is composed of two solid state detectors, viz., Si and CZT semiconductors capable of observing the full disk Sun in X-ray energy range of 4–56 keV. The X-ray spectra of solar flares obtained by the Si detector in the 4–25 keV range show evidence of Fe and Fe/Ni line emission and multi-thermal plasma. The evolution of the break energy point that separates the thermal and non-thermal processes reveals increase with increasing flare plasma temperature. Small scale flare activities observed by both the detectors are found to be suitable to heat the active region corona; however their location appears to be in the transition region.