Post-flare UV light curves explained with thermal instability of loop plasma

TL;DR

This study explains the observed UV light curves during a solar flare by modeling thermal instability in the cooling plasma of a flaring loop, showing how different emission lines evolve in time.

Contribution

It introduces a model that accounts for simultaneous emission of lines with different formation temperatures due to thermal instability, improving the understanding of flare plasma cooling.

Findings

Thermal instability causes simultaneous emission of lines at different temperatures.

A simple loop model reproduces the observed light curve timing.

Ensemble of subloops explains slower line evolution.

Abstract

In the present work we study the C8 flare occurred on September 26, 2000 at 19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of the impulsive phase to well beyond the disappearance in the X-rays. The emission first decayed progressively through equilibrium states until the plasma reached 2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Si iii (formed in the temperature range log T = 4.3 - 6.3 under equilibrium conditions), are emitted at the same time and all evolve in a similar way. Here we show that the simultaneous emission of lines with such a different formation temperature is due to thermal instability occurring in the flaring plasma as soon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relative start time of the light curves of each line in the correct order with a simple (and standard) model of a single flaring loop. The agreement with the observed light curves is greatly improved, and a slower evolution of the line emission is predicted, if we assume that the model loop consists of an ensemble of subloops or strands heated at slightly different times. Our analysis can be useful for flare observations with SDO/EVE.