Hard X-ray sources from the high corona are an integral part of solar eruptions

¹ Institute for Data Science, University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210, Windisch, Switzerland
² Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, 464-8601, Japan
³ Space Sciences Laboratory, University of California, 7 Gauss Way, 94720, Berkeley, USA

^★ Corresponding author.

Received: 5 September 2025
Accepted: 9 January 2026

Abstract

Context. Hard X-ray bremsstrahlung emissions are strong diagnostics of electron acceleration during the magnetic energy release process that drives solar flares. Solar Orbiter’s unique vantage point away from the Earth-Sun line allows us to use limb occultation as a diagnostic tool to investigate faint hard X-ray source located above the main flare site that are generally lost in the dynamic range of hard X-ray telescopes.

Aims. We aim to highlight the feasibility and the diagnostic power of limb occultation with Solar Orbiter/STIX to quantify the intensity, spectral shape, and location of the occulted signals relative to the main flare emissions to investigate the energetic importance of nonthermal electrons in the high corona.

Methods. We report on a single event study of the GOES M6 flare on October 19, 2024, simultaneously observed by ASOS/HXI, Fermi/GBM, and Solar Orbiter/STIX. We used standard X-ray spectral fitting tools available through OSPEX to compare the photon spectra observed by the three different observatories. In a second step, HXI and STIX X-ray images were compared to UV and EUV images seen by SDO/AIA to investigate the location of the occulted sources within the flare geometry.

Results. From Earth’s perspective, the flare was seen on-disk near the west limb, while for STIX the flare is occulted by ∼19 degrees, allowing STIX to detect a much fainter thermal and nonthermal source in the high corona related to the coronal mass ejection (CME) associated with the flare. The nonthermal hard X-ray flux indicates that about 5% of flare-accelerated electrons are injected upward into the CME, assuming a thick target model, while the fraction increases for a partially thick target interpretation. The thermal source is hot (23.5 ± 0.7 MK) and expands together with the CME. Even though the thermal source from the high corona has a weak bremsstrahlung signal, due to its large volume, its thermal energy content is on the same order of magnitude as the energy content in the main flare loops. Hence, hard X-ray sources associated with the escaping CME are an important signature of the energy release process.

Conclusions. These observations highlight that high coronal sources are an integral part of solar eruptive events and are energetically important despite their intrinsically faint appearance. For future instrumentation, an emphasis should be put on high-dynamic-range imaging, such as that provided by hard X-ray focusing optics telescopes, to be able to simultaneously observe the flare and the high-coronal sources from a single vantage point observatory.