A Hot Cusp-Shaped Confined Solar Flare

Nugget
Number:	371
1st Author:	Aaron Hernandez-Perez
2nd Author:	(see Acknowledgments)
Published:	24 February 2020
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Previous Nugget:	The Temporal and Spatial Extension of Gamma-ray Emission from the Sun

Introduction

A growing cusp-shaped flare arcade is a typical feature in the standard model of eruptive flares, and is regarded as a signature of magnetic reconnection taking place at progressively larger coronal heights. This feature resulted from 2D approximations to the process, while an earlier Nugget described a generalization to 3D. As reconnection occurs at successively larger heights in the solar corona, the outer flare loops have systematically higher temperatures than the previously reconnected ones, located underneath. We report (Ref. 1) rare observations of a confined flare that exhibited an apparent cusp with a temperature distribution distinctly different from those reported in literature, exhibiting an increasing temperature with increasing height even in the absence of an eruption.

An Atypical Flare Loop

The flare SOL2014-01-13T21:51 appeared in the low corona at the location of substantial nonthermal precursor activity. Particle acceleration related to magnetic reconnection, subsequent plasma heating and chromospheric evaporation, revealed cusp-shaped flare loops extending almost 70 Mm into the corona (Figure 1).

Figure 1: Composite of AIA 1600 Å (red) + 304 Å (blue) + 131 Å (green) images showing the (E)UV flare emission during the impulsive phase. The 131 Â emission clearly outlines an overlying system of cusp-shaped loops toward solar west

After the impulsive phase, RHESSI X-ray emission, of a partly nonthermal nature, originated from the cusp-shaped structure. This indicates continued particle acceleration during the decay phase (Figure 2). Prolonged hot emission was further observed, consistent with ongoing energy release in the apparent cusp (Figure 3(b)).

Figure 2: Left: Composite of AIA 1600 Å (red) + 131 Å (green) images showing the (E)UV flare emission during the decay phase. The RHESSI sources for 3 - 8 keV (red) and 8 - 20 keV (blue) with contours at 60% and 80% of the maximum emission are over-plotted. Right: Corresponding X-ray spectrum. The X-ray spectrum of background-subtracted data (black solid line) is plotted together with the fitting results for the isothermal component (red dashed line), and the non-thermal component (blue dashed line). The background is represented by the grey solid line. The electron temperature, T, emission measure, EM, electron distribution spectral index δ, the cutoff energy, E_C, as well as the chi-squared of the fitting, Χ², are all listed in the figure legend.

Conclusion

Given the observational characteristics, the cusp-shaped appearance of the flare was most probably related to a kinked shape of the magnetic field (Figure 3(a)). The extended enhanced SXR and EUV emission during the late phase provides evidence of a weak process of ongoing energy release. This can also explain the increasing temperature distribution with height observed during the decay phase (Figure 3(b)).

Figure 3: (a) Diagram of the magnetic configuration of the flare. Positions L1 and L2 represent the locations of precursor activity. The grey lines represent the flare arcade, the black line is a loop connecting L1 and L2. The green lines represent the kinked flare loops. The blue arrow represents a jet-like feature that occurs during the impulsive phase. (b) EM-weighted temperature map during the decay phase of the M1.3 flare showing the increasing temperature distribution with height.

Acknowlegements

Co-authors

References

[1] "A Hot Cusp-shaped Confined Solar Flare"