Observations of a Faint Nonthermal Onset before a GOES C-class Flare

TL;DR

The paper analyzes a GOES C1 flare using occulted NuSTAR data and on-disk STIX to characterize a faint nonthermal onset in the corona. It detects persistent coronal electron acceleration during onset, associated with hot plasma heating to ~10–11 MK and co-temporal type III radio bursts, implying continuous acceleration before the flare. Spectral fits show a consistent electron spectral index $\delta$ between the onset and the flare, with a higher low-energy cut-off $E_{c}$ at the footpoints, suggesting a shared acceleration mechanism under evolving plasma conditions. The multi-instrument results favor a behind-the-limb acceleration region and illustrate a link between preflare coronal acceleration and the main energy release, with implications for flare energy transport and heating processes.

Abstract

We present analysis of a GOES C1-class flare from 2022 September 6, which was jointly observed as occulted by Nuclear Spectroscopic Telescope ARray (NuSTAR) and on-disk by Spectrometer/Telescope for Imaging X-rays (STIX). NuSTAR observed faint coronal nonthermal emission as well as plasma heating > 10 MK, starting 7 minutes prior to the flare. This onset emission implies that during this time, there is a continuous electron acceleration in the corona which could also be responsible for the observed heating. The nonthermal model parameters remained consistent throughout the entire onset, indicating that the electron acceleration process persisted during this time. Furthermore, the onset coincided with a series of type III radio bursts observed by Long Wavelength Array-1, further supporting the presence of electron acceleration before the flare began. We also performed spectral analysis of the impulsive flare emission with STIX (thermal and footpoint emission). STIX footpoints and the onset coronal source were found to have similar electron distribution power-law indices, but with increased low-energy cut-off during the flare time. This could suggest that the nonthermal onset is an early signature of the acceleration mechanism that occurs during the main phase of the flare.

Figures (5)

• Figure 1: (Left) AIA, NuSTAR 4-7 keV (including 131 Å AIA lightcurve from the above panel but with y-aixs limited to 0.70 - 0.85 range), NuSTAR 7-10 keV and STIX (time corrected to 1 AU) lightcurves from the 2022 September 6 observation campaign. The time ranges used for integrating the spectra are highlighted in colour shaded regions. (Top right) The NuSTAR (Earth)--STIX--flare positioning for the observed events. The separation angle defines the approximate Earth--Sun--STIX separation. (Bottom right) EUI/FSI 174 Å image from the flare decay time. The red contours indicates the approximate flare location as seen by STIX and the black line indicates the limb seen from Earth.
• Figure 2: NuSTAR spectral fits and residuals from time ranges 0-3. The spectral fits show averaged FPMA and B spectra (with the mean pile-up model shown in red), however the fitting was done simultaneously to the separate FPM spectra and with the pile-up spectra as described in NuSTAR_STIX_fitting. The models fitted to FPMB spectra were scaled by a factor C$_{\text{FPMB}}$ to account for systematic differences between the two FPMs. The fitted model includes thermal (blue and orange lines) and thin-target nonthermal components (green). The black line indicates the mean MCMC-fitted model with several sample fits shown in yellow. The fitted energy range is shown in the solid green horizontal line.
• Figure 3: (Top row) NuSTAR 60, 80, 95 % contour levels (summed over the two focal plane modules) from TRs 1 (left), 2 (middle) and 3 (right) overlain on 131 Å AIA images. The yellow contours cover thermally dominated energy range and the red contour cover the nonthermally dominated energy range. A Gaussian filter with $\sigma$ of 2 pixels ($\approx 5"$) was used to smooth the NuSTAR images.(Bottom row) 94 Å AIA images (with 60 % contour levels from the corresponding 131 Å AIA images) from the same time ranges show a loop structure that was present before the flare onset.
• Figure 4:
• Figure 5: (Left panel) STIX spectral fit and residuals from time range 4 showing background subtracted spectra, with the backgrounds shown in grey. The fitted model includes thermal (orange lines), thick-target nonthermal (pink) models. The black line indicates the mean MCMC-fitted model parameters with several sample fits shown in yellow. The fitted energy range is shown in the solid green horizontal line. (Middle panel) STIX contours (60, 80 and 90 % levels) from TR4 overlain on an FSI image from 21:30. The red contours represent thermally dominated emission and the blue contours are nonthermally dominated emission. (Right panel) 131 Å AIA image from TR4 with 60% contours from 131 Å image from TR3.