OSO-7 observations of solar x-rays in the energy range 10?100 keV

Abstract

The solar X-ray experiment on the satellite OSO-7 has provided extensive observations of hard and soft X-ray bursts. We give a general description of the hard X-ray data here, in parallel with the description of the soft X-ray data already published (Datlowe et al., 1974). The data for this study consist of 123 hard X-ray bursts which occurred between 10 October 1971 and 6 June 1972. We examine the behavior of a typical event in terms of its spectral and flux variations. For the whole data sample, we find that ~-of the soft X-ray bursts have detectable hard X-ray emission. We present the distributions of frequency of occurrence of peak flux, spectral index, collisional energy loss, burst duration and the duration at half maximum of the flux profile. No correlation was found between the flux and the spectral slope of an individual data sample, nor was there a correlation between the peak flux and the full width at half maximum of a burst.We have utilized Ha flare identifications to study longitude variations. No statistically significant variation was found in the relative frequency of hard or soft X-ray bursts with longitude. However, the spectral slope does exhibit a center-to-limb variation, with limb spectra tending to be softer.We have compared the growth of energy in the hot flare plasma (soft X-ray source) with the collisional energy deposition (hard X-ray source) for the entire sample. This analysis shows that collision loss within the hot plasma is not the principal source of its heating.

X-Ray bursts from solar flares behind the limb

“…The most important result of the over-the-limb burst characteristics is that the fraction of the bursts which have a non-thermal component is 2/3, exactly the same as found for the 122 X-ray bursts studied in Datlowe et al (1974b). What is more, of the eight.bursts with expected locations with 0 greater than 1000, corresponding to minimum visible heights 4 5 from 10 to 10 km, five exhibit a detectable non-thermal component.…”

“…The height above which X-ray emission is observed can be expressed as In addition to those events, we have selected a set of 37 non-thermal 'limb' bursts identified from SGD as coming from flares located between 600 < 0 <900. As a reference for comparison to these, we used the same-set of 59 non-thermal events at 0O 0 <600 as used in Datlowe et al (1974b), hereafter identified as 'center' events.…”

“…For 50 events of 122 in Datlowe et al (1974b), the correlation between T and has been determined; the correlation coefmax ficient is -0.6. The corresponding coefficient of correlation for 39 limb and over-the-limb events is -0.4.…”

Self-Organized Braiding and the Structure of Coronal Loops

“…Observations show that the distribution of solar flare hard X-ray bursts is a power law in peak photon flux with logarithmic slope near 1.8 (Datlowe et al 1974;Dennis 1985;Lin et al 1984;Einaudi & Velli 1999;Charbonneau et al 2001). The powerlaw index of the distribution is independent of solar cycle.…”

Classification of probability densities on the basis of Pearson’s curves with application to coronal heating simulations

“…The results are power-laws, f (E) ∼ E −α , distributions with indexes α around 1.8 (Datlowe et al, 1974;Lin et al, 1984;Dennis, 1985;Crosby et al, 1993Crosby et al, , 1998Aschwanden et al, 2001). The later authors also present a substantial investigation of the total energy in the flare electrons observed in hard X-Ray bremsstrahlung, finding α = 1.5 ± 0.02.…”