Electron Bremsstrahlung Hard X-Ray Spectra, Electron Distributions, and Energetics in the 2002 July 23 Solar Flare

Abstract

We present and analyze the first high-resolution hard X-ray spectra from a solar flare observed in both X-ray/ g-ray continuum and g-ray lines. Spatially integrated photon flux spectra obtained by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are well fitted between 10 and 300 keV by the combination of an isothermal component and a double power law. The flare plasma temperature peaks at 40 MK around the time of peak hard X-ray emission and remains above 20 MK 37 minutes later. We derive the nonthermal mean electron flux distribution in one time interval by directly fitting the RHESSI X-ray spectrum with the thin-target bremsstrahlung from a double-power-law electron distribution with a low-energy cutoff. We find that relativistic effects significantly impact the bremsstrahlung spectrum above 100 keV and, therefore, the deduced mean electron flux distribution. We derive the evolution of the injected electron flux distribution on the assumption that the emission is thick-target bremsstrahlung. The injected nonthermal electrons are well described throughout the flare by a double-power-law distribution with a low-energy cutoff that is typically between 20 and 40 keV. We find that the power in nonthermal electrons peaks before the impulsive rise of the hard X-ray and g-ray emissions. We compare the energy contained in the nonthermal electrons with the energy content of the thermal flare plasma observed by RHESSI and GOES. The minimum total energy deposited into the flare plasma by nonthermal electrons, ergs, is on the order of the energy in the thermal plasma. 31 2.6 # 10 Subject headings: Sun: flares -Sun: X-rays, gamma raysThe time history of the flare emission in three energy bands is shown in Figure 1a. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) uses two sets of aluminum attenuators, known as thin shutters and thick shutters, to avoid saturating the detectors during large flares. The July 23 flare was observed in two attenuator states. The instrument was primarily in the A3 state, with both sets of attenuators in place. Early in the flare, before 00:26:08 UT, and late in the flare, after 00:59:21 UT, the instrument was in the A1 state, with only the thin shutters in place. There were also four brief periods during which the instrument switched from A3 to A1 and back to A3. These transitions in attenuator state are apparent in the time history of the lowest energy band in Figure 1a. The flux calibration is currently uncertain during these four brief periods, so these time periods appear as gaps in subsequent results derived from the data.We corrected the observed counts for pulse pileup and decimation (see Smith et al. 2002). Pulse pileup occurs at high count rates, with multiple photons recorded as a single photon with an energy equal to the sum of the energies of the individual photons. Decimation conserves onboard memory by recording only a fraction of the incident photons. Background counts were determined from the data by linearly interpolating between the background level...

The Ulysses Catalog of Solar Hard X-Ray Flares

“…High-resolution observations with Ge detectors (Lin et al, 1981), now carried out systematically with the Reuven Ramaty High-Energy Solar Spectrometric Imager spacecraft (RHESSI) (e.g., Holman et al, 2003), make it clear that this artefact complicates all solar hard X-ray catalogs derived from scintillation counters. Interestingly, it also makes it very difficult to recognize the analogous thermal/non-thermal components of stellar hard X-ray spectra because of their still higher thermal temperatures (Osten et al, 2007).…”

“…A very thorough Monte Carlo program was written for the Ulysses detector, and was used to infer the influence of pulse pile-up on the spectra of major flares (Kane et al, 1995). We have now used this program to estimate the effects of pile-up using RHESSI observations for the input spectrum, specifically those described by Holman et al (2003). The thermal component of this flare had an effective temperature of 3.2 keV at maximum.…”

Diagnostics of non-thermal distributions in solar flare spectra observed by RESIK and RHESSI

“…This can be achieved with a modelindependent approach, such as regularised techniques (Piana et al 2003;Kontar et al 2004). Also, the forward-fitting method is often used after assuming that the spectrum of the electrons emitting in this energy range is the sum of an isothermal component and a non-thermal power-law distribution: e.g., Brown (1971) and Holman et al (2003). This approach is adopted here.…”

Kappa distribution and hard X-ray emission of solar flares

“…A widely used method to determine the electron distribution functions in solar flares is the forwardfitting of X-ray spectra in which the low-energy part usually is described by isothermal bremsstrahlung while for higher energies, electron power-law distributions with a low-energy cutoff are assumed (e.g. Holman et al 2003). On the other hand, energetic electrons observed in situ in space have a distribution with an enhanced number of particles in the high-energy tail.…”