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

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.

References in corpus (5)

• High-resolution observations of flare precursors in the low solar atmosphere
• The existence of hot X-ray onsets in solar flares
• Statistical analysis of the onset temperature of solar flares in 2010-2011
• First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX
• Energetic particle contamination in STIX during Solar Orbiter's passage through Earth's radiation belts and an interplanetary shock