Solar flare X-ray source motion as a response to electron spectral hardening

Abstract

Context. Solar flare hard X-rays (HXRs) are thought to be produced by nonthermal coronal electrons stopping in the chromosphere or remaining trapped in the corona. The collisional thick target model (CTTM) predicts that more energetic electrons penetrate to greater column depths along the flare loop. This requires that sources produced by harder power-law injection spectra should appear further down the legs or footpoints of a flareloop. Therefore, the frequently observed hardening of the injected power-law electron spectrum during flare onset should be concurrent with a descending hard X-ray source. Aims. We test this implication of the CTTM by comparing its predicted HXR source locations with those derived from observations of a solar flare which exhibits a nonthermally-dominated spectrum before the peak in HXRs, known as an early impulsive event.Methods. The HXR images and spectra of an early impulsive C-class flare were obtained using the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). Images were reconstructed to produce HXR source height evolutions for three energy bands. Spatially integrated spectral analysis was performed to isolate nonthermal emission and to determine the power-law index of the electron injection spectrum. The observed height-time evolutions were then fitted with CTTM-based simulated heights for each energy, using the electron spectral indices derived from the RHESSI spectra. Results. The flare emission was found to be dominantly nonthermal above ∼7 keV, with emission of thermal and nonthermal X-rays likely to be simultaneously observable below that energy. The density structure required for a good match between model and observed source heights agreed with previous studies of flare loop densities. Conclusions. The CTTM has been used to produce a descent of model HXR source heights that compares well with observations of this event. Based on this interpretation, downward motion of nonthermal sources should occur in any flare where there is spectral hardening in the electron distribution during a flare. However, this is often masked by thermal emission associated with flare plasma preheating. To date, flare models that predict transfer of energy from the corona to the chromosphere by means other than a flux of nonthermal electrons do not predict this observed source descent. Therefore, flares such as this will be key in explaining this elusive energy transfer process.

X-Ray Source Heights in a Solar Flare: Thick-Target Versus Thermal Conduction Front Heating

“…Note that the observed flare, after background subtraction, had a peak GOES flux of B3.8 (1-8Å) and A4.8 (0.5-4Å). The cutoff energy was varied between [1, 3, 5, 7] keV, since the observational value was uncertain (O'Flannagain et al 2013 note that the non-thermal emission extends down to RHESSI's observational limit of 3 keV). Recall that the spectral index δ was time-dependent, so that the electron beam changes in time (i.e.…”

INVESTIGATING THE DIFFERENTIAL EMISSION MEASURE AND ENERGETICS OF MICROFLARES WITH COMBINEDSDO/AIA ANDRHESSIOBSERVATIONS

“…They occur in active regions and their frequency distribution is similar to large flares. Yet in one important aspect they differ significantly; their spectra above 10 keV (usually interpreted as non-thermal emission) are generally steep compared to large flares; interpreted as a power-law the spectral index of microflares is generally between −5 and −8 (see Benz & Grigis 2002;Krucker et al 2002;Christe et al 2008;Hannah et al 2008a), while for large flares it typically ranges from −2.5 to −4, (Saint-Hilaire et al 2008), although there are some notable exceptions (Hannah et al 2008b;O'Flannagain et al 2013). A recent full review of microflare properties may be found in Hannah et al (2011).…”

Amended Results for Hard X-Ray Emission by Non-Thermal Thick Target Recombination in Solar Flares

“…It is important to note that thermal emissions are weak early in the impulsive phase in some small events due to a small emission measure (e.g. Hannah et al 2008a;O'Flannagain et al 2013), and that they are weak in so-called cold flares (e.g. Fleishman et al 2011;Masuda et al 2013;Fleishman et al 2016), so that there are times when thermal emissions are relatively less important compared to non-thermal processes.…”