Disentangling Thermal and Abundance Characteristics in a Solar Flare Using SDO/AIA, Hinode/XRT, and MinXSS-1 Observations

TL;DR

This study uses coordinated Hinode/XRT and SDO/AIA observations, complemented by MinXSS-1 spectra, to dissect the thermal and compositional structure of the 2016-07-21 solar flare. Through DEM analysis with both coronal and photospheric abundance models and by incorporating XRT data, the authors reveal a hot, tenuous plasma cloud above flare loops that heats independently from the loop plasma, consistent with coronal heating via magnetic reconnection of eruptive filaments. The DEMs show substantial abundance-dependent differences, indicating spatially varying fractionation processes and underscoring the need for simultaneous spectral and spatial diagnostics to accurately diagnose heating mechanisms. MinXSS-1’s disk-integrated perspective, together with the cloud’s faint DEM signature, suggests that coronal low-FIP abundances at the flare peak may reside primarily in the flare loops rather than the tenuous cloud, though this remains subject to the instrument sensitivity and filling factors. The work highlights the value of multi-instrument, spatially resolved spectroscopy for disentangling thermal and chemical evolution in large solar eruptions and motivates future missions with simultaneous spectral-spatial capabilities.

Abstract

We investigate the thermal properties of a solar flare by the observations of soft X-ray Telescope (XRT) on board Hinode and the Atmospheric Image Assembly on board the Solar Dynamics Observatory (SDO/AIA). Our analysis reveals a tenuous but hot plasma cloud located above the flare loops. The flare loops and plasma cloud exhibit independent temperature profiles as a function of time, implying different heating mechanisms are present in these structures. We interpret the plasma cloud as an eruptive filament directly heated in the corona via magnetic reconnection that occurs when a rising filament interacts with this eruptive filament. Previous disk-integrated solar soft X-ray spectral measurements of this flare using the Miniature X-ray Solar Spectrometer CubeSat-1 (MinXSS-1) inferred coronal abundances at the flare peak. However, MinXSS synthetic spectra from AIA observations show that the plasma cloud is too tenuous to be detected in MinXSS-1 and thus the coronal abundances of low FIP elements must be in emission from the flare loops. Furthermore, we demonstrate the non-negligible differences in differential emission measures (DEMs) between coronal and photospheric abundance models. These results highlight the importance of instruments capable of simultaneous spectral and spatial diagnostics during large-scale solar events. Such coordinated measurements would enable more accurate thermal and compositional analyses and deeper insight into the underlying physical mechanisms.

Figures (15)

• Figure 1: Left: GOES flux around the flare; right: the evolution of Si during the flare (adapted from 2023ApJ...957...14S) where the purple bar indicates the photopheric value with FIP bias as 1. The dashed lines indicate the times with XRT observations, and the corresponding abundance models are labeled by different colors.
• Figure 2: Evolution of the flare and filament in XRT Be-thin (top row), AIA 131 Å (second row), AIA 304 Å (third row), and 304 Å overlaid with 131 Å (bottom row). Eruptive filament, pre-existing filament that crosses [470$^"$, 0] to [650$^{"}$, 250$^{"}$], and plasma cloud are indicated by F1, F2, and PC, respectively. The yellow arrows point out filament brightening where the eruptive filament F1 approaches the pre-existing filament F2. Curves C1, C2 indicate two paths for the position-vs.-time stackplots in Figures \ref{['aiastack']} and \ref{['stackplot']} where white arrows on the curves correspond to distance increment direction. The crosses marked in the curves correspond to dotted lines in the stackplots in Figures \ref{['aiastack']} and \ref{['stackplot']}. Note that the plotting ranges in XRT and AIA data are different. The corresponding animation from 01:00 UT to 02:27 UT is available online.
• Figure 3: The position-vs.-time stackplots of 131 Å, 304 Å, and 171 Å distribution along the paths of C1 (top row) and C2 (bottom row) indicated in Figure \ref{['filament']}. The distance increment corresponds to the arrow direction indicated on the paths C1 and C2. The dotted lines ca, cb, cc, and cd correspond to crosses indicated in Figure \ref{['filament']}. S1 indicates the ejected outflows from brightening intersection of eruptive filament F1 and pre-existing filament F2. The fitted speed of the outflows, indicated by solid lines, is 44 km/s. The minimum and maximum fitted speeds, shown by dashed lines 2015ApJ...807....7R, are 38 km/s and 45 km/s, respectively.
• Figure 4: SDO/AIA and Hinode/XRT response functions with coronal abundances (solid curves) and photospheric abundances (dotted lines, scaled by a factor of 3.98) for the date of 2016-07-21. Following the practice in 2017ApJ...844..132W, XRT response functions are further multiplied by a factor of 2.
• Figure 5: Evolution of the temperature (top row) and EM (bottom row) of the flare from AIA observations. Boxes at 01:48:07 UT marks the locations of plasma cloud (P), northern loops (NL), and southern loops (SL) for plotting temperature as a function of time in Figure \ref{['tcurve']}. Crosses mark the locations for checking DEM-temperature distributions at 01:55:19 in Figure. \ref{['peak_diffabun_curve']}.