Hard X-ray and microwave pulsations: a signature of the flare energy release process

(Solar Orbiter Nugget #28 by H. Collier^1,2, L. A. Hayes³, S. Yu⁴, A. F. Battaglia^1,2, W. Ashfield^5,6, V. Polito⁶, L. K. Harra^2,7, S. Krucker^1,8)

Introduction

Rapid amplitude variations on timescales of seconds to tens of seconds are frequently observed in the emission from solar flares [1,2]. These variations are present in all wavelengths of flare emission. In particular, rapid variations (sometimes classified as quasi-periodic pulsations or ‘QPPs’) are often clearly present in hard X-ray emission which is a consequence of accelerated electrons interacting with the dense chromosphere to produce non-thermal bremsstrahlung emission. Furthermore, microwave observations often exhibit oscillatory signatures of similar amplitude and phase as in hard X-ray since microwave observations probe the population of electrons trapped in the flare loop. Several models have been proposed to explain this phenomenon which typically involve direct modulation of the plasma by magnetohydrodynamic (MHD) waves, MHD wave driven reconnection and magnetic reconnection occurring on an intrinsically periodic timescale e.g. [3]. Constraints on the potential driver of QPPs can be obtained by identifying the spatial structure of QPP emission at various wavelengths [3]. Here, we leverage new observations obtained by the Spectrometer/Telescope for Imaging X-rays (STIX) onboard Solar Orbiter [4,5] combined with microwave observations from the ground-based Expanded Owens Valley Solar Array (EOVSA) [6] to study the spatial evolution of QPPs in time and energy. Further we assess the spectral evolution in time. From this, the suite of possible drivers is significantly constrained. 

Event Overview

A large solar flare (X1.3 GOES-class flare, SOL2022-03-30T17:21:00) was observed whilst Solar Orbiter was at a distance of 0.33 AU with an angular separation to the Sun-Earth line of 95°, as shown in Figure 1. The flare was observed on-disk from Earth and towards the East limb, as viewed from Solar Orbiter, the flare is highlighted by the black box from both points of view in Figure 1.

Figure 1: Overview of the field of view of Earth observatories and Solar Orbiter on March 30th 2022.

The event displayed remarkable rapid variations in both the hard X-ray emission from STIX and microwave flare emission from the Earth-based EOVSA radio telescope on timescales which grew from 7s in the first phase to 35s in the third phase, as classified by Collier et al. 2023. This evolution is shown in Figure 2. 

Figure 2: Event time profiles from STIX, EOVSA and GOES showing pulsations growing from 7-35 s. The dashed grey lines in the second panel indicate the centre of the imaging interval used to produce the hard X-ray images shown in Fig. 3.  
 

Hard X-ray and Microwave Imaging

By combining STIX and EOVSA imaging capabilities, we constructed images of the emission at high cadence. Figure 3 shows the hard X-ray and microwave emission for each peak in the first phase of pulsations denoted by the dashed grey lines in Figure 2. The integration time used for hard X-ray imaging was the Full Width at Half Maximum (FWHM) of each gaussian peak fit to the time profile in Collier et al. 2023. Interestingly, the hard X-ray emission originates from all along the UV ribbons and the exact location of maximum brightness evolves in time in an unordered fashion. The optically thick microwave emission originates from all along the arcade, whereas the optically thin microwave emission originates from a localised region towards the northern part of the flare arcade, where the magnetic field strength is high.  

Figure 3: AIA 1600 Å maps with 20-76 keV STIX Clean map 40-90% contours overlaid and 60 - 90% EOVSA microwave contours observed at frequencies ranging from 4 to 18 GHz. Each image corresponds to a single hard X-ray peak denoted by the grey dashed line in Fig. 2.

Spectral Analysis

We also investigated the hard X-ray and microwave spectral properties during the peaks of the flare emission. In particular, the hard X-ray spectrum was fitted with a thermal and a thick-target bremsstrahlung model. We found that the electron spectral index, inferred from the thick-target fit, was found to be anti-correlated with the pulsations. In particular, the spectra displayed a soft-hard-soft evolution with each sub-peak. This indicates that the rapid variations are associated with acceleration and/or injection of an electron population in the flaring loops.  
 

Figure 4: Electron spectral index evolution compared to hard X-ray flux. The electron spectral index is anti-correlated with the hard X-ray flux and displays the “soft-hard-soft” behaviour for each pulse.
 

Discussion & Conclusions

This interesting flare displays intriguing pulsations on timescales which evolve from 7-35s. Thanks to the enhanced capabilities of STIX onboard Solar Orbiter and its unique orbit which reaches a distance of just 0.3 AU at perihelion, the spatial origin of the pulsations was determined. The hard X-ray emission originated from all along the ultraviolet flare ribbons. Further, the emission was shown to be highly dynamic, evolving in time in an unordered fashion. Microwave imaging provided important context observations of this phenomenon. This indicates that the mechanism driving the observed pulsations must be dynamic and 3D in nature. Mechanisms involving wave propagation along the polarity inversion line were thus excluded. The insight provided by hard X-ray spectral analysis enabled us to hone in on models capable of producing significant variation in the electron spectral index, thereby excluding sausage mode oscillations, for example. 

In summary we found that the driver of the observed pulsations must be able to satisfy to following criteria:

• Simultaneous modulation of hard X-ray and microwave emission.
• Electron precipitation in multiple locations along the flare ribbons which changes in time.
• Modulation of the electron spectral index such that it is anti-correlated with the hard X-ray flux (soft-hard-soft evolution with each pulse).

Consideration of the constraints listed above lead us to a driving mechanism which involves spontaneous quasi-periodic energy release, for the flare. Possible mechanisms of this kind include particle acceleration from contracting magnetic islands in the flare current sheet [8,9,10] and oscillatory reconnection - although this is expected to produce a damped signal [11].

We have demonstrated that simultaneous microwave and hard X-ray observations of solar flares enable us to probe the feasibility of proposed models of QPPs in flare emission. Future coordinated observations between hard X-ray imagers including STIX, ASO-S/HXI [12] Aditya-L1/HELIOS [13] and EOVSA will greatly advance our understanding of rapid variation in flaring emission.
 

This study can be found in the publication A&A 684 A215 (2024), DOI: 10.1051/0004-6361/202348652

Affiliations

[1] University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland
[2] ETH Zürich, Rämistrasse 101, 8092 Zürich Switzerland
[3] European Space Agency, ESTEC,Keplerlaan 1 - 2201 AZ, Noordwijk, The Netherlands
[4] New Jersey Institute of Technology, Newark, USA
[5] Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA, 94035, USA
[6] Lockheed Martin Solar & Astrophysics Laboratory, 3251 Hanover Street, Palo Alto, CA, 94304, USA
[7] PMOD/WRC, Dorfstrasse 33, CH-7260 Davos Dorf, Switzerland
[8] Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA

References

[1] Inglis, A. R., Ireland, J., Dennis, B. R., Hayes, L., & Gallagher, P. 2016, The Astrophysical Journal, 833, 284

[2] Hayes, L. A., Inglis, A. R., Christe, S., Dennis, B., & Gallagher, P. T. 2020, The Astrophysical Journal, 895, 50

[3] Zimovets, I. V., McLaughlin, J. A., Srivastava, A. K., et al. 2021, Space Sci. Rev., 217, 66

[4] Krucker, S., Hurford, G. J., Grimm, O., et al. 2020, A&A, 642, A15

[5] Müller, D., St. Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1

[6] Gary, D. E., Chen, B., Dennis, B. R., et al. 2018, The Astrophysical Journal, 863, 83

[7] Collier, H., Hayes, L. A., Battaglia, A. F., Harra, L. K., & Krucker, S. 2023, A&A, 671, A79

[8] Guidoni, S. E., DeVore, C. R., Karpen, J. T., & Lynch, B. J. 2016, The Astrophysical Journal, 820, 60

[9] Guidoni, S. E., Karpen, J. T. & DeVore, C. R.  2022, The Astrophysical Journal, 925, 191

[10] Guidoni, S. E., DeVore, C. R., Karpen, J. T., & Alaoui, M. J. 2024, The Astrophysical Journal, 965, 6

[11] McLaughlin, J. A., Moortel, I. D., Hood, A. W., & Brady, C. S. 2008, Astronomy & Astrophysics, 493, 227

[12] Zhang, Z., Chen, D.-Y., Wu, J., et al. 2019, Research in Astronomy and Astrophysics, 19, 160

[13] Seetha, S. & Megala, S. 2017, Current Science, 113, 610