All microflares that accelerate electrons to high-energies are rooted in sunspots

(Solar Orbiter Nugget #39 by Andrea Francesco Battaglia¹, Säm Krucker^2,3 & Astrid Veronig^4,5)

Introduction

The acceleration of particles through the explosive release of magnetic energy is a ubiquitous phenomenon in plasmas throughout the Universe. Solar flares provide a unique laboratory for studying these processes. When 'free' magnetic energy is released, high-energy particles are accelerated in the low corona and these particles can either escape into interplanetary space or travel towards the solar surface. The latter, following magnetic field lines, deposit energy into the denser chromosphere and/or photosphere. This energy deposition leaves observable signatures, including the emission of hard X-rays (HXRs; for a review, see [1]). HXRs are considered the smoking guns of flare-accelerated electrons, serving as crucial diagnostic tools. They enable, among others, to assess the efficiency of the acceleration mechanisms in producing high-energy electrons, via the determination of the electron spectral index δ.

In order to study solar flares in the HXR wavelengths, Solar Orbiter carries onboard the Spectrometer/Telescope for Imaging X-rays (STIX), an instrument designed to observe a large range of solar flares [2], by performing imaging-spectroscopy in the energy range from 4 to 150 keV. Generally, large solar flares are more efficient at accelerating high-energy electrons than microflares [3]. Nonetheless, we sometimes observe microflares that accelerate electrons to high energies. Our study [4] focuses on statistically analyzing the location of 39 microflares (of GOES A and B class, after pre-flare subtraction) with strikingly hard spectra in the HXR range (δ from 2 to 5), which means that they are efficient in accelerating high-energy electrons. We refer to these events as "hard microflares." In our study, these events have been observed between January 2021 and May 2023. Figure 1 shows the time profiles of two different microflares: a hard microflare and a “standard” microflare. It is possible to see how the hard microflare produces more counts at higher energies, which is reflected in a hard spectrum. This is a clear signature for the efficiency of the acceleration mechanism, in the hard microflare case, in accelerating electrons to high energies.

Figure 1: STIX quicklook lightcurves of two microflares observed by STIX. On the left, a typical hard microflare, with counts observed at higher energies (> 25 keV). On the right, a standard microflare, where the counts are only observed at low energies (< 15 keV).

In Fig. 2, we compare the location of different types of microflares that occurred in AR12882. This figure clearly shows that hard microflares have one of their footpoints directly rooted in sunspots, as shown by the AIA 1600 Å contours. This is in agreement with what is reported in [5]. Instead, standard microflares are located away from the sunspots, such as in the plage regions surrounding the active region. Larger flares are more spatially extended and eventually cross sunspot areas during their time evolution. This is already known and reported in the literature (e.g., [6]).

Figure 2: Flare ribbon location of 2 hard microflares and 2 “standard” microflares that occurred within AR12882. The intensity map from SDO/HMI is plotted in the background and on top of it, we plot the contours of the flare ribbons identified in the AIA 1600 Å images. The orange contours refer to hard microflares, while the blue contours to standard microflares. In the legend, we report the GOES class and the electron spectral index δ.

The novelty here is that all 39 hard microflares in our statistical study are rooted in sunspots, with one of the footpoints rooted directly either in the umbra or the penumbra. In Fig. 3 we show four examples, in which we combined SDO/AIA and SDO/HMI observations with STIX. The statistical study allowed us to also obtain additional interesting information. First of all, for the events with the classic two-footpoints morphology, the absolute value of the mean line-of-sight magnetic field density at the footpoint rooted within the sunspot ranges from 600 to 1800 G, whereas the outer footpoint measures from 10 to 200 G. This means that the magnetic flux density at the footpoint directly rooted within the sunspot can be about 10 times stronger than the outer footpoint. In addition, despite the large difference of the magnetic field at the flare footpoints, approximately 78% of hard microflares, which exhibited two HXR footpoints, have similar or even stronger HXR flux from the footpoint rooted within the sunspot. Assuming a simple magnetic loop with similar densities on both sides, this is inconsistent with the magnetic mirroring scenario (e.g., [7], p. 30), as the HXR flux from the footpoint in the sunspot should be lower. Some potential explanations are the following: the assumption of similar densities on both sides of the loop may be inaccurate, the flare-accelerated electrons might have a beamed distribution, inhibiting mirroring effects, or the energy release site may be located close to the sunspot.

Figure 3: SDO/HMI, SDO/AIA and Solar Orbiter/STIX HXR images of four hard microflares. For each event, the left panel displays the SDO/HMI intensitygram and the SDO/AIA 1600 Å contours from the Earth's perspective. The right panel shows the SDO/AIA 1600 Å reprojected to the Solar Orbiter view, with the STIX images displayed as red (thermal emission) and blue (nonthermal emission) contours.

We conclude that all hard microflares are rooted in sunspots, which implies that the magnetic field strength plays a key role in efficiently accelerating high-energy electrons, with hard HXR spectra associated with strong fields. This key result will allow us to further constrain our understanding of the electron acceleration mechanisms in flares and space plasmas.

Affiliations

1 Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Università della Svizzera italiana, Locarno, Switzerland. E-mail: andrea.francesco.battaglia@irsol.usi.ch
2 University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Windisch, Switzerland
3 Space Sciences Laboratory, University of California, Berkeley, USA
4 Institute of Physics, University of Graz, Graz, Austria
5 Kanzelhöhe Observatory for Solar and Environmental Research, University of Graz, Treffen, Austria

References:

[1] Fletcher L., Dennis B. R., Hudson H. S., et al., 2011, SSRv, 159, 19. doi:10.1007/s11214-010-9701-8

[2] Krucker, S., Hurford, G. J., Grimm, O., et al. 2020, A&A, 642, A15. doi:10.1051/0004-6361/201937362

[3] Warmuth A., Mann G., 2016, A&A, 588, A115. doi:10.1051/0004-6361/201527474

[4] Battaglia, A. F., Krucker, S., Veronig, A. M., et al., 2024, accepted for publication in A&A; arxiv: https://arxiv.org/abs/2409.14466 

[5] Saqri J., Veronig A. M., Battaglia A. F., et al., 2024, A&A, 683, A41. doi:10.1051/0004-6361/202348295

[6] Kleint L., Heinzel P., Judge P., Krucker S., 2016, ApJ, 816, 88. doi:10.3847/0004-637X/816/2/88 [7] Benz A., 2002, ASSL, 279. doi:10.1007/0-306-47719-X