Evidence of Significant Energy Input in the Late Phase of a Solar Flare from NuSTAR X-Ray Observations

Abstract

We present observations of the occulted active region AR12222 during the third {\em NuSTAR} solar campaign on 2014 December 11, with concurrent {\em SDO/}AIA and {\em FOXSI-2} sounding rocket observations. The active region produced a medium size solar flare one day before the observations, at $\sim 18$UT on 2014 December 10, with the post-flare loops still visible at the time of {\em NuSTAR} observations. The time evolution of the source emission in the {\em SDO/}AIA $335\text{A˚}$ channel reveals the characteristics of an extreme-ultraviolet late phase event, caused by the continuous formation of new post-flare loops that arch higher and higher in the solar corona. The spectral fitting of {\em NuSTAR} observations yields an isothermal source, with temperature $3.8-4.6$ MK, emission measure $0.3-1.8\times 10^{46}{\text{ cm}}^{-3}$, and density estimated at $2.5-6.0\times 10^8{\text{ cm}}^{-3}$. The observed AIA fluxes are consistent with the derived {\em NuSTAR} temperature range, favoring temperature values in the range $4.0-4.3$ MK. By examining the post-flare loops' cooling times and energy content, we estimate that at least 12 sets of post-flare loops were formed and subsequently cooled between the onset of the flare and {\em NuSTAR} observations, with their total thermal energy content an order of magnitude larger than the energy content at flare peak time. This indicates that the standard approach of using only the flare peak time to derive the total thermal energy content of a flare can lead to a large underestimation of its value.

Published in ApJ, 8 pages

References in corpus (5)

• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• Soft X-ray Fluxes of Major Flares Far Behind the Limb as Estimated Using STEREO EUV Images
• On the origin of the Extreme-Ultraviolet late phase of solar flares
• SWAP Observations of Post-flare Giant Arches in the Long-Duration 14 October 2014 Solar Eruption
• Post-flare evolution of AR 10923 with Hinode/XRT

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