Combined Modeling of Acceleration, Transport, and Hydrodynamic Response in Solar Flares. II. Inclusion of Radiative Transfer with RADYN

TL;DR

This study integrates particle acceleration, hydrodynamics, and radiative transfer in solar flare simulations, revealing that even low-energy flux models can induce explosive chromospheric evaporation and produce observable spectral signatures.

Contribution

It introduces a coupled simulation framework combining particle kinetics with detailed radiative transfer, advancing the modeling of solar flare atmospheric responses.

Findings

Stochastic acceleration models can cause explosive evaporation at low energy flux.

Simulated emission lines match observed spectral features, constraining electron properties.

HeII 304 Å formation site reaches unusually high temperatures during flares.

Abstract

Solar flares involve complex processes that are coupled and span a wide range of temporal, spatial, and energy scales. Modeling such processes self-consistently has been a challenge in the past. Here we present results from simulations that couple particle kinetics with hydrodynamics of the atmospheric plasma. We combine the Stanford unified Fokker-Planck code that models particle acceleration and transport with the RADYN hydrodynamic code that models the atmospheric response to collisional heating by accelerated electrons through detailed radiative transfer calculations. We perform simulations using two different electron spectra, one an {\it ad hoc} power law and the other predicted by the model of stochastic acceleration by turbulence or plasma waves. Surprisingly, the later model, even with energy flux $\ll 10^{10}$ erg s$^{-1}$ cm$^{-2}$, can cause "explosive" chromospheric evaporation and drive stronger up- and downflows (and hydrodynamic shocks). This is partly because our acceleration model, like many others, produces a spectrum consisting of a quasi-thermal component plus a power-law tail. We synthesize emission line profiles covering different heights in the lower atmosphere, including H$\alpha$ 6563 \AA, HeII 304 \AA, CaII K 3934 \AA\ and SiIV 1393 \AA. One interesting result is the unusual high temperature (up to a few $10^5$ K) of the formation site of HeII 304 \AA, which is expected due to photoionization-recombination under flare conditions, compared to those in the quiet Sun dominated by collisional excitation. When compared with observations, our results can constrain the properties of non-thermal electrons and thus the poorly understood particle acceleration mechanism.