D. W. Datlowe

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

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 2/3 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 Hα 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.

Observations of solar X-ray bursts in the energy range 5-15 keV

Bursts of solar X-rays in the energy range 5–15 keV are associated with flares and are due to thermal emission from a hot coronal plasma. In this paper we present the results of the first study of a large sample of separate bursts, 197 events associated with subflares and a few importance 1 events. The observations were made by a proportional counter on the satellite OSO-7 from October 1971 to June 1972. In most cases the temperature characterizing the X-ray spectrum rises impulsively at the onset of the burst and then declines slowly throughout the remainder of the burst. The emission measure rises exponentially with a time scale of 30–100 s and then declines slowly, on a time scale of the order of 103 s. From these observations we show that the growth of the thermal energy in the flare plasma throughout the burst can be due to the heating of new cool material.

Evidence for thin-target X-ray emission in a small solar flare on 26 February 1972

We compare solar X-ray observations from the UCSD experiment aboard OSO-7 with high resolution energetic electron observations from the UCAL experiment on IMP-6 for a small solar flare on 26 February 1972. A proportional counter and NaI scintillator covered the X-ray energy range 5–300 keV, while a semiconductor detector telescope covered electrons from 18 to ∼ 400 keV. A series of four non-thermal X-ray spikes were observed from 1805 to 1814 UT with average spectrum dJ/d (hv) ∼ (hv)−4.0 over the 14–64 keV range. The energetic electrons were observed at 1 AU beginning 1840 UT with a spectrum dJ/dE ∼ E−3.1. If the electrons which produce the X-ray emission and those observed at 1 AU are assumed to originate in a common source, then these observations are consistent with thin target X-ray production at the Sun and inconsistent with thick target production. Under a model consistent with the observed soft X-ray emission, we obtain quantitative estimates of the total energy, total number, escape efficiency, and energy lost in collisions for the energetic electrons.

An upper limit to the anisotropy of solar hard X-ray emission

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 the emission. The X-ray data consist of 148 bursts observed by the University of California, San Diego, solar X-ray instrument on the OSO-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 40% 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-to-limb spectral variation is combined with the directivity result. Either the motions of the electrons which emit hard X-rays are predominantly random, or a variety of different emitting-region geometries commonly occur.

The role of the waveform in pulse pile-up

Abstract Pulse pile-up is the distortion of pulse-height distributions due to the overlap of detector responses to the arrival of two or more particles or photons within the detector resolving time. This paper presents a computational technique for simulating pile-up effects, which includes explicitly the dependence on the pulse-shape of the detector system. The basis of the technique is the manipulation of probability densities. The method is applicable to all types of linear pulse counting systems for nucleons, electrons, and photons, as long as the result is a pulse-height distribution. The algorithms are highly efficient in the amount of computing required for simulations, and internal checks for the numerical accuracy of the results are included. Studies of pile-up by monoenergetic pulses are used to determine the interrelationship between pulse shapes and spectral features; this information can be used to minimize pile-up. For broad spectra, the square wave approximation is compared with the present model including the correct waveform; introducing the pulse shape information smooths spectral features but does not qualitatively change the spectrum.

Spectral development of a solar X-ray burst observed on OSO-7.

The UCSD solar X-ray instrument on the OSO-7 satellite observes X-ray bursts in the 2–300 keV range with 10.24 s time resolution. Spectra obtained from the proportional counter and scintillation counter are analyzed for the event of November 16, 1971, at 0519 UT in terms of thermal (exponential spectrum) and non-thermal (power law) components. The energy content of the approximately 20 × 106K thermal plasma increased with the 60 s duration hard X-ray burst which entirely preceded the 5 keV soft X-ray maximum. If the hard X-rays arise by thick target bremsstrahlung, the nonthermal electrons above 10 keV have sufficient energy to heat the thermally emitting plasma. In the thin target case the collisional energy transfer from non-thermal electrons suffices if the power law electron spectrum is extrapolated below 10 keV, or if the ambient plasma density exceeds 4 × 1010 cm−3.

X-RAY BURSTS FROM SOLAR FLARES BEHIND THE LIMB

From the UCSD OSO-7 X-ray experiment data, we have identified 54 X-ray bursts with 5.1–6.6 keV flux greater than 103 photon cm−2 keV−1 which were not accompanied by visible Hα flare on the solar disk. By studying OSO-5 X-ray spectroheliograms, Hα activity at the limb and the emergence and disappearance of sunspot groups at the limb, we found 17 active centers as likely seats of the X-ray bursts beyond the limb. We present the analysis of 37 X-ray bursts and their physical parameters. We compare our results with those published by Datlowe et al. (1974a, b) for disk events.The distributions of maximum temperature, maximum emission measure, and characteristic cooling time of the over-the-limb events do not significantly differ from those of disk events. We show that of conduction and radiation, the former is the dominant cooling mechanism for the hot flare plasma. Since the disk and over-the-limb bursts are similar, we conclude that the scale height for X-ray emission in the 5–10 keV range is large and is consistent with that of Catalano and Van Allen (1973), 11000 km, for primarily 1–3 keV emission.Twenty-five or about 2/3 of the over-the-limb events had a non-thermal component. The distribution of peak 20 keV flux is not significantly different from that of disk events. However, the spectral index at the time of maximum flux is significantly different for events over the limb and for events near the center of the disk; the spectral index for over-the-limb events is larger by about δγ = 3/4. If hard X-ray emission came only from localized sources low in the chromosphere we would expect that hard X-ray emission, would be occulted over the limb; on the contrary, the observation show that the fraction of soft X-ray bursts which have a nonthermal component is the same on and off of the disk. Thus hard X-ray emission over extended regions is indicated.

Heating and cooling of the thermal X-ray plasma in solar flares

Characteristic times for heating and cooling of the thermal X-ray plasma in solar flares are estimated from the time profile of the thermal X-ray burst and from the temperature, emission measure and over-all length scale of the flare-heated plasma at thermal X-ray maximum. The heating is assumed to be due to magnetic field reconnection, and the cooling is assumed to be due to heat conduction and radiation.Temperatures and emission measures derived from UCSD OSO-7 X-ray flare observations are used, and length scales are obtained from Big Bear large-scale Hα filtergrams for 17 small (subflare to Class 1) flares. The empirical values obtained for the characteristic times imply (1) that flares are produced by magnetic field reconnection, (2) that conduction cooling of the thermal X-ray plasma dominates radiative cooling and (3) that reconnection heating and conduction cooling of the thermal X-ray plasma are approximately in balance at thermal X-ray maximum. This model in combination with the data gives estimates for the electron number density (1010–1011 cm−3) and the magnetic field strength (10–100 G) in the thermal X-ray plasma and for the total thermal energy generated in a subflare (≈ 1030 erg for an Hα area ≈ 1 square degree) which agree with previous observational and theoretical estimates obtained by others.

Pulse pile-up in nuclear particle detection systems with rapidly varying counting rates

Abstract Pulse pile-up in nuclear particle detection systems is the distortion of the measured pulse height distribution which occurs when there is a significant probability that more than one particle will arrive within the detector resolving time. This paper treats the problem in cases where the probability of pile-up varies on a time scale comparable to the rise time of the detector system electronics. These variations introduce structure into the pulse height distributions which cannot occur for a time-independent pile-up probability. Three classes of problems which exemplify these effects are as follows: 1. 1) Pile-up rejection circuits. 2. 2) Cascaded nuclear decays, in which the lifetime for emission of a second X-ray is comparable to the detector rise time. 3. 3) Bursts of particles where the intensity is modulated on a time scale comparable to the detector rise time. These problems are solved computationally by an extension of a numerical technique previously developed.