Resolving Two Distinct Thermal X-ray Components in A compound Solar Flare

TL;DR

This study presents a detailed analysis of a long-duration solar flare revealing two distinct thermal X-ray components, with superhot plasma located above hot flare loops, providing new insights into flare heating mechanisms.

Contribution

It offers the first stereoscopic observation confirming the coexistence and spatial separation of superhot and hot X-ray emitting plasmas in a solar flare.

Findings

Superhot plasma (~30 MK) is located above hot plasma (~12 MK) in the flare.

Two flare loop systems are associated with different CME speeds.

The superhot source is positioned above the hot source, confirmed by multiple viewing angles.

Abstract

X-ray emission provides the most direct diagnostics of the energy-release process in solar flares. Occasionally, a superhot X-ray source is found to be above hot flare loops of ~10 MK temperature. While the origin of the superhot plasma is still elusive, it has conjured up an intriguing image of in-situ plasma heating near the reconnection site high above the flare loops, in contrast to the conventional picture of chromospheric evaporation. Here we investigate an extremely long-duration solar flare, in which EUV images show two distinct flare loop systems that appear successively along a Gamma-shaped polarity inversion line (PIL). When both flare loop systems are present, the HXR spectrum is found to be well fitted by combining a hot component (Te ~12 MK) and a superhot component (Te ~30 MK). Associated with a fast CME, the superhot X-ray source is located at top of the flare arcade that appears earlier, straddling and extending along the long "arm" of the Gamma-shaped PIL. Associated with a slow CME, the hot X-ray source is located at the top of the flare arcade that appears later and sits astride the short "arm" of the Gamma-shaped PIL. Aided by observations from a different viewing angle, we are able to verify that the superhot X-ray source is above the hot one in projection, but the two sources belong to different flare loop systems. Thus, this case study provides a stereoscopic observation explaining the co-existence of superhot and hot X-ray emitting plasmas in solar flares.