Implications for electron acceleration and transport from non-thermal electron rates at looptop and footpoint sources in solar flares

Abstract

The interrelation of hard X-ray (HXR) emitting sources and the underlying physics of electron acceleration and transport presents one of the major questions in high-energy solar flare physics. Spatially resolved observations of solar flares often demonstrate the presence of well-separated sources of bremsstrahlung emission, so-called coronal and footpoint sources. Using spatially resolved X-ray observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and recently improved imaging techniques, we investigate in detail the spatially resolved electron distributions in a few well-observed solar flares. The selected flares can be interpreted as having a standard geometry with chromospheric HXR footpoint sources related to thick-target X-ray emission and the coronal sources characterised by a combination of thermal and thin-target bremsstrahlung. Using imaging spectroscopy techniques, we deduce the characteristic electron rates and spectral indices required to explain the coronal and footpoint X-ray sources. We found that, during the impulsive phase, the electron rate at the looptop is several times (a factor of 1.7−8) higher than at the footpoints. The results suggest that a sufficient number of electrons accelerated in the looptop explain the precipitation into the footpoints and imply that electrons accumulate in the looptop. We discuss these results in terms of magnetic trapping, pitch-angle scattering, and injection properties. Our conclusion is that the accelerated electrons must be subject to magnetic trapping and/or pitch-angle scattering, keeping a fraction of the population trapped inside the coronal loops. These findings put strong constraints on the particle transport in the coronal source and provide quantitative limits on deka-keV electron trapping/scattering in the coronal source.

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

“…Despite the simplifications in the thick target electron beam model employed here, we conclude that flare heating with a high flux electron beam can be used as a powerful tool to interpret spectral phenomena and to understand important radiative-hydrodynamic processes in the brightest flaring footpoints. The consistencies between the 5F11 model predictions and the IRIS observations will serve as a benchmark for models with additional physical processes that address observational challenges (Battaglia & Benz 2006;Krucker et al 2011;Martínez Oliveros et al 2012;Dickson & Kontar 2013;Simões & Kontar 2013) to the standard electron beam model for the brightest hard X-ray flare footpoints. Measurements of the Balmer jump ratio as a constraint on optical depth, the broadening of the hydrogen lines as a constraint on electron density, and modeling lines such as Si I (Judge et al 2014) for constraints on the heating of the upper photospheric layers will complement future comparisons of IRIS flare spectra and RHD models.…”

Simultaneous Iris and Hinode/Eis Observations and Modeling of the 2014 October 27 X2.0 CLASS Flare

“…Figures 3(a) and (b) show HXR sources associated with the positive-and negative-polarity ribbons (PR and NR1 respectively), as well as with the main flaring region. Near the maximum of the HXR emission (Figures 3(c) and (d)) the emission is dominated by the main flaring region, with the northern and eastern sources probably associated with the ribbons, while the southern source is possibly a loop-top source (e.g., Simões & Kontar 2013), given the lack of chromospheric features in IRIS SJI 1330 Å(see Figure 5) and AIA 1600 Å. We believe that the co-alignment of AIA and RHESSI images is very good-see for instance the good agreement of the outermost HXR sources and the 1600 Åribbons in Figure 3.…”

Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind (Invited Review)

“…Reproduced with permission. Karlický, 2009;Simões and Kontar, 2013), in particular in the above-the-looptop source (e.g. Krucker et al, 2010;Ishikawa et al, 2011;Chen and Petrosian, 2012;Oka et al, 2013Oka et al, , 2015Krucker and Battaglia, 2014).…”

“…A more recent quantitative analysis indicated that there is a sufficient number of accelerated electrons in the coronal source to suggest not only precipitation into footpoint sources but also a fraction of the population being trapped within the source (Simões and Kontar, 2013). In fact, the observations of the above-thelooptop source over a time period longer than the electron transit time across the source already indicates that electrons are trapped within the source region.…”