A time dependent relation between EUV solar flare light-curves from lines with differing formation temperatures

Edward M.B Thiemann,Francis G Eparvier,Thomas N Woods

doi:10.1051/swsc/2017037

Abstract

Extreme ultraviolet (EUV) solar flare emissions evolve in time as the emitting plasma heats and then cools. Although accurately modeling this evolution has been historically difficult, especially for empirical relationships, it is important for understanding processes at the Sun, as well as for their influence on planetary atmospheres. With a goal to improve empirical flare models, a new simple empirical expression is derived to predict how cool emissions evolve based on the evolution of a hotter emission. This technique is initially developed by studying 12 flares in detail observed by the EUV variability experiment (EVE) onboard the Solar Dynamics Observatory (SDO). Then, over 1100 flares observed by EVE are analyzed to validate these relationships. The Cargill and Enthalpy Based Thermal Evolution of Loops (EBTEL) flare cooling models are used to show that this empirical relationship implies the energy radiated by a population of hotter formed ions is approximately proportional to the energy exciting a population of cooler formed ions emitting when the peak formation temperatures of the two lines are up to 72% of each other and above 2 MK. These results have practical implications for improving flare irradiance empirical modeling and for identifying key emission lines for future monitoring of flares for space weather operations; and also provide insight into the cooling processes of flare plasma.

Highlights

Solar flare extreme ultraviolet (EUV, 10–121 nm) emissions have highly varying time histories or light curves
The time difference between the measured Fe XXIII and Fe XVIII peaks are used for the first Lumped Element Thermal Model (LETM) time-constant (t) and the time difference between the measured Fe XVIII and Fe XV are used for the second LETM time-constant (t)
L(Fe XVIII) predictions of Fe XV are shown with dotted blue curves

Summary

Introduction

Solar flare extreme ultraviolet (EUV, 10–121 nm) emissions have highly varying time histories or light curves. Solar flares begin with magnetic reconnection high in the solar corona that directly heats the magnetically confined plasma and accelerates particles along magnetic field lines away from the reconnection site. The downward moving particles are decelerated in the dense plasma of the lower solar atmosphere, resulting in rapid heating of this relatively cool plasma. This heated plasma subsequently flows upward along the field lines and cools by both conducting heat downward through the field line foot points and radiating energy into space. Aspects of flare evolution are evident in typical EUV flare light curves, where cooler-forming emission lines tend to show an early impulsive peak corresponding with the rapid heating of lower-atmosphere plasma, and hotter-forming emission

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Conclusion