Constraining turbulent solar flare acceleration regions by connecting kinetic modeling and X-ray observations

Abstract

Spatially-resolved X-ray observations are the key to understanding electron acceleration in solar flares. Currently, the underlying processes that efficiently energize solar flare particles are poorly constrained. Abundant flare observations suggest that turbulence plays a crucial role in transferring energy between the magnetic field and energetic electrons. For the first time, we connect inhomogeneous acceleration from turbulence and hard X-ray spectroscopy and imaging observations with kinetic modeling to constrain the properties of flare acceleration. Observing three large flares with RHESSI, or Solar Orbiter/STIX, we extract X-ray imaging and spectroscopy observables. We compare with modeling results, mapping observables to electron acceleration and turbulent properties. We determine that extended regions of turbulence are required to match multiple X-ray observables, suggesting electrons are accelerated over a large fraction (~25%) of the flare loop; a property that is usually unconstrained from X-ray observations alone. Additionally, we determine acceleration timescales that vary between 7 and 22s by using fixed values for the turbulent scattering timescale and the velocity dependence of the acceleration diffusion coefficient. These fixed values are effectively unconstrained, but yield acceleration timescales that will help to restrict possible viable stochastic models.

Figures (12)

• Figure 1: Top row: SDO/AIA $171$ Å images of Flare 1 (left), Flare 2 (middle), and Flare 3 (right), respectively. Pink and blue contours show the 50% intensity levels of the $6-12 \, \rm{keV}$ and $50-100 \, \rm{keV}$ X-ray emission, respectively. Flare ribbons observed in $1700$ Å are shown by the green $50\%$ contours for Flare 3. Bottom Row: Corresponding light curves for each event. The grey shaded boxes highlight the time periods studied here for each flare.
• Figure 2: Left: SolO/STIX location during Flare 3 with respect to Earth and the Sun. The angle of separation between SolO and Earth was 83.4 degrees at the time of observation. Right: FSI/SolO 174Å image of Flare 3.
• Figure 3: Flare 1 RHESSI imaging. Columns left to right: RHESSI $6-12 \, \rm{keV}$, $20-30 \, \rm{keV}$, and $50-100 \, \rm{keV}$ images. Rows show different imaging algorithms, top to bottom: Pixon, CLEAN (CLEAN beam width = 2.2), VIS_FWDFIT, MEM_NJIT. Column 1 shows a white contour of the looptop, defined as the $50\%$ contour of the $6-12 \,\rm{keV}$ emission. The white contours in Columns 2 and 3 highlight footpoints, defined as the $50\%$ contour of the $50-100 \, \rm{keV}$ emission.
• Figure 4: Flare 2 RHESSI imaging. Columns left to right: RHESSI $6-12 \, \rm{keV}$, $20-30 \, \rm{keV}$, and $50-100 \, \rm{keV}$ images. Rows show different imaging algorithms, top to bottom: Pixon, CLEAN (CLEAN beam width = 2.5), VIS_FWDFIT, MEM_NJIT. Column 1 shows a white contour of the looptop, defined as the $50\%$ contour of the $6-12 \,\rm{keV}$ emission. The white contours in Columns 2 and 3 highlight footpoints, defined as the $50\%$ contour of the $50-100 \, \rm{keV}$ emission.
• Figure 5: Flare 3, STIX imaging. Columns left to right: STIX $6-12 \, \rm{keV}$, $20-30 \, \rm{keV}$, and $36-74 \, \rm{keV}$ images. Rows show different imaging algorithms, top to bottom: CLEAN (CLEAN beam width = 2.0), MEM_GE, EM, VIS_FWDFIT_PSO. Column 1 shows a white contour of the looptop, defined as the $50\%$ contour of the $6-12 \,\rm{keV}$ emission. The white contours in Columns 2 and 3 highlight footpoints, defined as the $50\%$ contour of the $36-74 \, \rm{keV}$ emission.