Hot X-ray onset observations in solar flares with Solar Orbiter/STIX

(Solar Orbiter nugget #5 by Andrea Francesco Battaglia1,2, Hugh Hudson3,4, Säm Krucker1,4, Hannah Collier1,2, and the Solar Orbiter/STIX team)


Solar flares are a fascinating and mysterious phenomenon that have been puzzling scientists for decades. Despite our efforts to understand what causes the particles to accelerate and the plasma to heat up to millions of degrees Kelvin, we are still left with an incomplete picture. However, there is a widely accepted scenario that provides some insights into the process - the standard flare picture, also known as the CSHKP model (named after Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976). According to this theory, magnetic energy is suddenly released in the corona, generating kinetic energy in high-energy particles that travel along magnetic field lines. As they reach the chromosphere, the high-energy particles heat up the ambient plasma through Coulomb collisions, and produce bremsstrahlung emission observable in X-rays. Despite the evidence supporting this scenario, there are still mysteries to unravel, especially at the onset of flares, where the heated thermal plasma observable prior to the main energy release may not be exclusively heated by accelerated electrons.

The recent study by Hudson et al. (2021) has shed new light on this phenomenon. Their investigation of soft X-ray time profiles recorded by GOES revealed that flares generally have enhanced soft X-ray emissions before the onset of the impulsive phase (i.e., before the first clear occurrence of nonthermal emission due to accelerated electrons interacting with the chromosphere). These early elevated temperatures, ranging from 11 to 15 MK, were therefore termed "hot onsets". Because of the temperature of these hot onsets, STIX is the suitable instrument to study them in all their glory. With its stable background during flaring time-scales (Battaglia et al. 2021; Saqri et al. 2022; Battaglia et al. 2023), this instrument provides unprecedented diagnostic capabilities during times when counting statistics are limited, such as at the onset of the flare. Unlike GOES, STIX provides both spectral and imaging information simultaneously, enabling us to pinpoint the location of these phenomena.

Preliminary results reveal that hot onsets are clearly detected by STIX too. Figure 1 displays the time profiles of STIX and GOES, as well as the evolution of the temperature and emission measure. The left panel shows an increase in thermal emission (STIX 6-7 keV) before the onset of the main energy release (STIX 22-28 keV). The isothermal model parameters (middle and right panels) also reveal interesting information. Elevated temperatures between 11 to 16 MK are visible from the start, before the main energy release, while the emission measure steadily increases by about two orders of magnitude.



Figure 1. Time histories of Solar Orbiter/STIX and GOES/XRS fluxes as well as the isothermal parameters of temperature and emission measure. From left to right, the time evolution of STIX and GOES fluxes, time profiles of temperature and emission measure, and a correlation plot of temperature as a function of emission measure. The gray areas on the leftmost panels indicate the accumulation interval selected for producing the STIX images shown in Figure 2.


Figure 2 displays STIX images at two different times: at the onset of the flare (left) and around the nonthermal peak (middle). The standard flare configuration is shown at the nonthermal peak. It consists of two nonthermal footpoints (blue), which are co-located with the flare ribbons in the AIA 1600 Å image. In order to guide the eye, the nonthermal footpoints are connected by a semi-circle perpendicular to the solar surface. Additionally, there is a thermal source (red) that lies exactly on the drawn semi-circle, which is well in agreement with the standard cartoon. However, the location of the two sources that appear at the onset of the flare does not align with the standard picture (right). They do not correspond to the standard thermal source, and there are indications that they originate from lower altitudes (more details in Battaglia et al., in prep.).



Figure 2. Solar Orbiter/STIX and pre-flare subtracted SDO/AIA 1600 Å images, which have been reprojected to the Solar Orbiter vantage point. Left: STIX images reconstructed during the pre-flare interval. Middle: STIX thermal (red) and nonthermal (blue) images reconstructed around the first main nonthermal peak. In order to guide the eye, the nonthermal sources, which show the flare footpoints, are connected by a semi-circle perpendicular to the solar surface. Right: image comparing the location of the onset sources (gray) with the sources images around the nonthermal peak (blue and red).


In conclusion, STIX observations confirm the hot onset behavior previously reported by Hudson et al. (2021), with elevated temperatures ranging between 11 to 16 MK from the very beginning of the flare. Interestingly, the elevated temperature persists throughout the interval before the main energy release, while the emission measure steadily increases by about two orders of magnitude, which implies that continuous heating is needed. Therefore, based on the typical average temperatures of active regions, there must be a phase before the detection of these hot onsets that explains the temperature increase to over 10 MK. The lack of detected emission prior to these enhanced temperatures is likely due to the low emission measure, which suggests the potential existence of hot onset precursor events (HOPEs) prior to any X-ray detection.

The new results from STIX imaging is that these sources originate from lower altitudes with respect to the standard thermal flare loop-top source, possibly the chromosphere, which is contrary to the standard flare picture or the thermal conduction interpretation (for more details regarding thermal conduction, we refer to e.g. Acton et al. 1992; Dennis and Zarro 1993; Battaglia et al. 2009; Benz 2017). This may indicate that in the very early stages of the flare, an additional heating mechanism, such as Alfvén waves (Fletcher & Hudson 2008) or low-atmosphere turbulence (Jeffrey et al. 2018), may be at play before the electrons reach the chromosphere.

This hot onset behavior truly puts our understanding of the standard flare picture to the test, forcing us to investigate this further. Combining STIX observations with EUV images (e.g., SDO/AIA and Solar Orbiter/EUI) and magnetic field measurements (e.g., SDO/HMI and Solar Orbiter/PHI) will enable us to correlate the STIX hot onset sources with the flare and magnetic field topology during the early flare phase. This will help us to better understand them: exciting times are ahead with Solar Orbiter and STIX observations!



1Institute for Data Science, University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland

2Institute for Particle Physics and Astrophysics (IPA), Swiss Federal Institute of Technology in Zurich (ETHZ), Wolfgang-Pauli-Strasse 27, 8039 Zurich, Switzerland

3SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

4Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA



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