Sean Quinn¹, Aaron Reid¹, Mihalis Mathioudakis¹, Christopher Nelson¹, S. Krishna Prasad¹ and Sergei Zharkov²
1. Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, Northern Ireland, UK
2. E.A. Milne Centre for Astrophysics, School of Mathematics and Physical Sciences, Hull University, Kingston upon Hull, HU6 7RX, England, UK

Active region NOAA 12673 was extremely volatile in September 2017, producing many solar flares, including the largest of solar cycle 24, an X9.3 flare on 6 September 2017. This flare produced a number of Sunquakes (SQs)^[1] along the flare ribbon^[2,3]. Using co-temporal and co-spatial Helioseismic and Magnetic Imager (HMI) line-of-sight (LOS) and Swedish 1-m Solar Telescope (SST) observations, we have been able to display how the chromosphere responds to such SQs. Using a combination of SST and HMI observations, we created a time-distance diagram for one of these SQs. We applied the NICOLE inversion algorithm^[4] on the SST data to find that the responses were created by an upflow of plasma, which had a LOS velocity of approximately 3.2 km s^-1. We believe this is the first reported chromospheric signature of a flare induced SQ. Based on the previous reports of photospheric SQs being created by the X9.3 flare, the ripples observed in the Ca II 8542Å moving away from the flare ribbon, depicted as a running difference in the bottom left of Figure 1, were thought to be of the same nature as these photospheric responses. The Ca II 8542Å data were aligned co-temporally and co-spatially to the HMI LOS observations using R. Rutten’s alignment routines ( http://www.staff.science.uu.nl/~rutte101/rridl/sdolib/ ).

Figure 1| Difference images of the two SST wavelengths: Ca 8542Å (bottom left) and Hα 6563Å (bottom right). The top left is the AIA 1700Å channel observing the X9.3 flare, and the top right is the HMI LOS Doppler-velocity channel. The red dotted box on the HMI LOS data is the FOV of the SST data. The green arrows indicate the SQ. The red cross in the Ca II image is the approximate epicenter of the SQ response.

We were able to create a single dataset of the HMI LOS Doppler-velocity data with Ca II 8542Å ‘overplotted’ and this is displayed in Figure 2. Using the single dataset, we followed the ripples from the Ca II 8542Å field-of-view moving into the HMI LOS field-of-view. The ripples travel in the same direction, with the same apparent transverse velocity. One such ripple was detected in both the SST and HMI LOS dataset, both co-spatially and co-temporally, leading us to believe that they were created by the same mechanism.

Figure 2| The Ca II 8542Å running difference (over-laid panel) aligned to the HMI LOS running difference (background panel) showing the progression of the wavefront across the two datasets. The Ca II image was obtained at 12:18:37UT and the HMI LOS at 12:22:57UT. The blue and red arrows indicate the seismic response in the chromosphere and photosphere, respectively.

To confirm this, we created a single time-distance diagram of the ‘overplotted’ dataset to determine if these waves form a continuous ridge from one observation to the other (Figure 3).

Using the fit from Figure 3 we were able to confirm that the ridge was continuous, and the chromospheric response was a consequence of the photospheric SQ. Using the NICOLE inversion algorithm, we created a LOS velocity map in the Ca II 8542Å and concluded that these ripples were created by upflows with a velocity of approximately 3.2 km s^-1.

Figure 3| A time-distance diagram created from the ‘overplotted’ dataset. Multiple ridges can be seen in the Ca II part of the diagram, which correlate to the multiple wavefronts of the HMI data. One of these ridges extends out of the SST FOV and matches well to the ridge in the HMI LOS. The red crosses on this image are points selected by hand and used to perform a χ² analysis. The blue curve is the fit that most accurately matches these points. Both the red points and the blue curve have been shifted vertically to ease the visibility of the ridge.

For completeness, we also examined the 1600Å and 1700Å AIA channels and found evidence for the presence of the SQ. This provides further evidence that the response propagated upwards from the photosphere into the chromosphere, as displayed in Figure 2. We also found the presence of wavefronts in the SST observations of the Hα 6563Å line.

Our result highlights the importance of combining the high-spatial resolution ground-based observations with the large field-of-view afforded by space-borne facilities to investigate the SQ signatures in the solar atmosphere. For further information, please refer to Quinn et. al 2019^[5].

References

[1] Kosovichev, A. G., & Zharkova, V. V. 1998, Nature, 383, 317
[2] Sharykin, I. N., & Kosovichev, A. G. 2018, ApJ, 864, 86
[3] Zhao, J., & Chen, R. 2018, ApJ Lett., 860, L29
[4] Socas-Navarro, H., Trujillo Bueno, J., & Ruiz Cobo, B. 2000, ApJ, 530, 977
[5] Quinn, S., Reid, A., Mathioudakis, M., Nelson, C., Prasad, S. K., & Zharkov, S. 2019, ApJ, 881, 82