Sean Quinn, Mihalis Mathioudakis, Christopher J. Nelson, Ryan O. Milligan, Aaron Reid, and David B. Jess
Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, Northern Ireland, UK
Sunquakes (SQs) have been routinely observed in the solar photosphere, but it is only recently that signatures of these events have been detected in the chromosphere. We investigate whether signatures of SQs are common in ultraviolet (UV) continua that sample the solar plasma several hundred kilometers above where SQs are typically detected. The photospheric perturbations that often accompany solar flares can generate helioseismic waves, commonly known as “sunquakes” (SQs). Recently, Quinn et al. detected a chromospheric responses to a SQ. This unusual observation led to this investigation, in order to determine if other flares have produced SQ signatures in the solar atmosphere.
The chromospheric SQ response was observed by the Swedish 1 m Solar Telescope (SST), in the Ca II 8542 Å and Hα 6563 Å lines, as well as the Atmospheric Imaging Assembly (AIA). Specifically, the 1600 Å and AIA 1700 Å filters. The main motivation behind this research was to investigate how common the signatures of SQs in the AIA 1600 and 1700 Å continua are. The starting point for this investigation is the statistical study of the SQs of Solar Cycle 24, recently presented in Ref .
The investigation in Ref  was based on three methods: (a) the movie method using running-differences; (b) the holography method, and (c) time–distance analysis. Using these methods, the authors determined that of the 507 flares, 181 had a photospheric perturbation, 114 of which resulted in the detection of at least one SQ. It is these 114 detections that form the basis for the sample studied in this work.
Figure 1| A 240″ × 240″ field-of-view (FOV) running-difference Dopplergram from the HMI LOS for the M5.2 flare from 2014 February 2, plotted at four time steps. The time of each panel is indicated in the title of each panel. The red arrows point to one of the propagating wavefronts of the SQ, but more can be seen in the bottom right of the flare ribbon.
These 114 flares were observed in the HMI line-of-sight Dopplergrams, as well as the AIA 1600 Å and 1700 Å lines. An example of an SQ in unfiltered running-difference images is shown in Figure 1. All observations were cropped to a 240′′ × 240′′ FOV, centered on the active region (AR) where the flare occurred.
In the quiet Sun, the main contribution of the intensity in AIA 1700Å comes from the photospheric continuum, while the intensity in AIA 1600Å comes from a combination of C IV 1550Å and photospheric continuum. However, in a recent paper, Simoes et al. concluded that during a flare, both the AIA 1600 Å and 1700 Å passbands are dominated by spectral lines that form in the chromosphere and transition region. We can say that these data sample a region of the solar atmosphere at least several hundred kilometers above the region observed by the HMI.
Figure 2| Unfiltered time–distance diagrams for the M1.3 flare from 2012 July 4 from HMI LOS velocity (A), temporally degraded AIA 1600 Å (B), and AIA 1700 Å (C) channels. The corresponding regression trends are overplotted as blue curves. This trend has been shifted vertically so as not to obscure the ridge. The red crosses indicate the points that were selected for the χ2 test to determine the accuracy of the fit. These points have also been offset by the same amount as the blue trend line.
The flares that displayed evidence of SQs ranged in strength from M1.0 to X9.3. We removed any SQs from our sample that had not been detected by eye, bringing the total number of events that we considered to 62, with 49 being M-Class and 13 X-Class. We created time-distance diagrams of the AIA 1600 Å and 1700 Å, as well as HMI LOS observations of each flare.
Several of the SQs present in the HMI observations appeared as ridges on these time–distance diagrams. The ridges were fitted with a regression trend of αx0.5, which SQs commonly follow. Figure 2 displays an example of such a ridge in the HMI LOS velocity time–distance diagram (A) together with AIA 1600 Å (B) and AIA 1700 Å (C) time– distance diagrams. The presence of multiple ridges indicates that multiple, sufficiently strong wavefronts are present (see Ref ).
Figure 3| A 60″ × 60″ FOV running difference covering the M1.3 flare from 2012 February 4 including: HMI LOS (top row), AIA 1600 Å (middle row), and AIA 1700 Å (bottom row). Images from each channel are plotted at three time steps that are as cotemporal as possible. The red arrows in each image indicate the wave front of the SQ in the SDO/AIA channels, as it evolves across the solar surface.
Of the 62 flares, all had an SQ observable in running-difference HMI LOS images. Of these, 25 SQs had some varying degree of visibility by eye in the AIA 1600 and 1700 Å channels. Five of these SQs had very clear wavefronts in running-difference images (see middle and bottom row of Figure 3 for an example). These responses were approximately co-spatial and co-temporal with the SQs in the SDO/HMI LOS velocity images. These events were classified as “Clear” responses. Additionally, 12 SQs had comparatively faint responses in the AIA 1600 and 1700 Å running- difference images. These are called “Faint.” Finally, eight data sets displayed a faint AIA 1700 Å response, but no AIA 1600Å response, and were labelled “Potential” responses.
If the SQ ridge was present in the AIA time–distance diagrams for the faint and potential responses, we classify that SQ to have had a “Visible” response. Our analysis revealed that four of the faint and potential responses in running-difference images had an SQ ridge present in both the SDO/AIA 1600 and 1700 Å time–distance diagrams, bringing the total number of SQs with obvious responses in the SDO/AIA UV filters to nine. This analysis shows us that SQs effecting the solar atmosphere is much more common than previously thought, and that this should be considered when analyzing the flaring atmosphere.
 Quinn, S., Reid, A., Mathioudakis, M., et al. 2019, ApJ, 881, 1
 Sharykin, I. N., & Kosovichev, A. G. 2020, ApJ, 895, 76
 Simoes, P. J. A., Reid, H. A. S., Milligan, R. O., & Fletcher, L. 2019, ApJ, 870, 114