Abstract
Generally, large solar flares accelerate electrons to high energies more efficiently than microflares. However, there are microflares, known as hard microflares (HMFs), that also produce high-energy electrons, as evidenced by their flat hard X-ray (HXR) spectra. These events are characterized by a footpoint rooted in or at the edge of a sunspot. The mechanisms underlying this efficient acceleration, and their connection to solar energetic electrons (SEEs), remain to be understood.
In this work, we compare for the first time the HXR spectra of HMFs with the in-situ electron spectra of the associated SEEs, using observations from the Solar Orbiter STIX and EPD instruments, respectively. We found eight HMFs (of GOES C class or lower), jointly observed by Solar Orbiter and Earth-bound observatories, associated with SEEs. This association was verified through timing and magnetic-connectivity analysis. Each event was analysed through STIX HXR spectroscopy, SEE velocity-dispersion analysis, and in-situ electron spectral analysis of EPD data.
Seven out of eight SEE events show consistent timing between the HXR emission from the flare and the estimated electron injection into interplanetary space, as well as good agreement between the flare location and the estimated magnetic connectivity to the Sun. When compared with the correlation between HXR photon and in-situ electron spectral indices established by previous studies, which include larger C- and M-class flares, our HMFs tend to populate the hard end of the distribution. This demonstrates, also through in-situ observations, that HMFs efficiently accelerate electrons to high energies, and that efficient acceleration does not necessarily require a large flare energy release. The magnetic morphology of HMFs, which involves open field lines from the sunspot, suggests that these events may be an important contributor to filling the heliosphere with energetic particles.
Authors :
Diane Mittaine, PhD student
