Validation of the Coronal Thick Target Source Model

Abstract

We present detailed 3D modeling of a dense, coronal thick-target X-ray flare using the GX Simulator tool, photospheric magnetic measurements, and microwave imaging and spectroscopy data. The developed model offers a remarkable agreement between the synthesized and observed spectra and images in both X-ray and microwave domains, which validates the entire model. The flaring loop parameters are chosen to reproduce the emission measure, temperature, and the nonthermal electron distribution at low energies derived from the X-ray spectral fit, while the remaining parameters, unconstrained by the X-ray data, are selected such as to match the microwave images and total power spectra. The modeling suggests that the accelerated electrons are trapped in the coronal part of the flaring loop, but away from where the magnetic field is minimal, and, thus, demonstrates that the data are clearly inconsistent with electron magnetic trapping in the weak diffusion regime mediated by the Coulomb collisions. Thus, the modeling supports the interpretation of the coronal thick-target sources as sites of electron acceleration in flares and supplies us with a realistic 3D model with physical parameters of the acceleration region and flaring loop.

Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare

“…NuSTAR does not have fine enough angular resolution to resolve much spatial detail of the flare source shape at high energies, but the 6-9 keV RHESSI source map in Figure 2 shows a loop matching the location, elongation, and orientation of the loop observed by AIA. Therefore, the nonthermal emission observed by NuSTAR and RHESSI emanates from the flare loop, not from its footpoints, akin to the "coronal thicktarget" flares studied by Veronig & Brown (2004), Veronig et al (2005), and Fleishman et al (2016). This differs from the standard thick-target flare model in which energy is transported from the corona to the chromosphere primarily by accelerated electron beams, which emit strong bremsstrahlung radiation at their thermalization locations in the chromosphere and much fainter HXR emission from collisions in the tenuous corona.…”

Evolution of Flare-Accelerated Electrons Quantified by Spatially Resolved Analysis

“…As a result of the topological diversity of flares, the HXR and MW emissions display a variety of appearances and relationships. In some flares, both emissions are produced by a single population of non-thermal electrons in a single flaring loop (e.g., Fleishman et al, 2016b). In other cases, there could be different populations of the non-thermal electrons in distinct flaring loops, thus, different populations can dominate HXR and MW emissions.…”

“…Closed flaring flux tubes can represent rather large reservoirs of highenergy electrons located either nearby (Kuroda et al, 2018) or far away from Fleishman et al (Fleishman et al, 2017) the main flare acceleration sites, possibly providing the seed population for solar energetic particles (SEPs). Fleishman et al (2011Fleishman et al ( , 2013Fleishman et al ( , 2016a probed the acceleration sites using MW observations and concluded that the acceleration regime was consistent with stochastic acceleration, while Fleishman et al (2018b) extended in time the studies of Fleishman et al (2016b) and Kuroda et al (2018) to quantify the acceleration and transport of the nonthermal electrons in the 3D domain. In all of these studies, broadband MW spectroscopy and imaging have been crucial.…”

“…The MWs are dominated by the gyrosynchrotron (GS) emission due to non-thermal electrons gyrating in the coronal magnetic field with a contribution from free-free emission. As a result of these distinct emission mechanisms, even a single population of non-thermal electrons distributed over a single (but possibly magnetically-asymmetric) flaring loop yields spatially-displaced HXR and MW emissions (e.g., Fleishman et al, 2016b). While most of the HXR spectrum is formed by non-thermal electrons with energies from a few to a (few) hundred keV, the spectrum of the GS-emitting electrons may extend to much higher energies including the MeV range (Nitta and Kosugi, 1986;Kundu et al, 1994).…”

Electron Power-Law Spectra in Solar and Space Plasmas

“…Based on analysis and modeling of the source properties (such as the source size and number density distributions), it was argued that the coronal thick-target sources can be interpreted as sites of electron acceleration (e.g. Xu et al 2008;Fleishman et al 2016).…”