Hot X-ray Onsets of Solar Flares

TL;DR

This paper identifies a hot X-ray onset phase in solar flares occurring before the impulsive phase, characterized by elevated plasma temperatures detected via GOES and RHESSI, challenging existing flare heating models.

Contribution

It provides the first observational evidence of a hot X-ray onset interval prior to the main flare energy release, using multi-instrument data across different flare events.

Findings

Hot X-ray onsets occur before hard X-ray emission.

The hot onset is mainly observed in footpoints and low-lying loops.

This phenomenon challenges standard flare heating models.

Abstract

The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray onset interval of enhanced isothermal plasma temperatures in the range of 10-15~MK up to tens of seconds prior to the flare's impulsive phase. This `hot onset' interval occurs during the initial soft X-ray increase and prior to the detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite (GOES) X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), show no signs of gradual increase, and the `hot onset' phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events we identify this early hot onset soft X-ray emission mainly within footpoint and low-lying loops, rather than with coronal structures, based on images from the Atmospheric Imaging Assembly (AIA). We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence they challenge the standard flare heating modeling techniques.