Understanding the Duration of Solar and Stellar Flares at Various Wavelengths

Abstract

Recent irradiance measurements from numerous heliophysics and astrophysics missions including Solar Dynamics Observatory (SDO), GOES, Kepler, TESS, Chandra, the X-ray Multi-Mirror Mission, and NICER have provided critical input into understanding the physics of the most powerful transient events on the Sun and magnetically active stars:solar and stellar flares. The light curves of flare events from the Sun and stars show remarkably similar shapes, typically with a sharp rise and protracted decay phase. The duration of solar and stellar flares has been found to be correlated with the intensity of the event in some wavelengths, such as white light, but not in other wavelengths, such as soft X-rays, but it is not evident why this is the case. In this study, we use a radiative hydrodynamics code to examine factors affecting the duration of flare emission at various wavelengths. The duration of a light curve depends on the temperature of the plasma, the height in the atmosphere at which the emission forms, and the relative importance of cooling due to radiation, thermal conduction, and enthalpy flux. We find that there is a clear distinction between emission that forms low in the atmosphere and responds directly to heating, and emission that forms in the corona, indirectly responding to heating-induced chromospheric evaporation, a facet of the Neupert effect. We discuss the implications of our results for a wide range of flare energies.

Generalized Coronal Loop Scaling Laws and Their Implication for Turbulence in Solar Active Region Loops

“…Description of the abbreviations in model names: CL is for a wholly collisional loop, TL for a strongly turbulent loop, UH for uniform heating, FH for footpointconcentrated heating, UC for a uniform loop cross section, and EC for an expanding cross section with height. b Values outside (inside) parentheses denote results derived using the piecewise power-law (CHIANTI) radiative loss function.the evaporative flow to the corona(Reep et al 2022), while in the second-stage radiative cooling it prevents the evaporated materials from draining back to the chromosphere until a catastrophic cooling occurs(Cargill et al 2022;Reep & Airapetian 2023). Both processes (that cause the reduction of both conductive and radiative losses) can account for a longerthan-expected cooling time observed in some solar flares(Ryan et al 2013) without the need for additional heating.…”