Ivan N. Sharykin1,2,3 and Alexander G. Kosovichev1,2
1. Center for Computational Heliophysics, New Jersey Institute of Technology, Newark, NJ 07102, USA
2. Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102, USA
3. Department of Space Plasma Physics, Space Research Institute of RAS, Moscow, 117997, Russia
The X9.3 flare of September 6, 2017, was the most powerful flare of Solar Cycle 24. It generated strong white-light emission and multiple helioseismic waves (sunquakes). By using data from Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) as well as hard X-ray data from KONUS instrument onboard WIND spacecraft, we investigate spatio-temporal dynamics of photospheric emission sources, identify sources of helioseismic waves, and compare the flare photospheric dynamics with the hard X-ray (HXR) temporal profiles.
The flare generated strong white light emission and helioseismic waves traveling from a large scale photospheric disturbance well seen in all the HMI observables[1]. The helioseismic response of this flare was first detected by Kosovichev[2], who noted an unusual feature: excitation of several sunquakes, probably, by different mechanisms. The flare Dopplergram disturbance, sunquake wave, and time-distance (TD) diagrams for this wave are shown in Figure 1. We present two time-distance diagrams: (a) calculated from the nonfiltered and (b) filtered around 6 mHz time differences of Dopplergrams. The first plot is noisier but allows us to better estimate the sunquake initiation time without temporal uncertainty connected with the finite frequency range of the bandpass filter. The theoretical time-distance relation calculated from the ray approximation using a standard solar interior model is marked by a dashed curve. The position of the wave ripples in the TD diagram matches the theoretical model. Thus, the observed wave was generated in the source corresponding to the Dopplergram disturbance.
Figure 1| (a-b): The sunquake time-distance diagrams calculated {from nonfiltered (a) and filtered around 6~mHz (b) HMI data} along the red lines plotted in panel~c and d. (c-d): The time differences of Dopplergrams projected onto the Heliographic coordinates and filtered with a Gaussian frequency filter centered around 6 mHz for two moments of time showing the photospheric impact (c), and the helioseismic wave front (d).
To reconstruct the two-dimensional structure of the seismic sources, we used the helioseismic holography method[3]. The egression acoustic power map made in the frequency range of 5-7 mHz is shown in Figure 2 by red contours. It shows a complex distribution of helioseismic sources which were located in the close vicinity to the PIL. There are two regions of generation of the helioseismic waves: northern and southern groups shown by two yellow circles in Figure 2a. Several sources (northern group) are located in the same place as the initial perturbations observed during the pre-impulsive phase and the first HXR peak. Possibly, the helioseismic waves were generated in the late pre-impulsive phase at the start of the first HXR pulse. The southern helioseismic sources were located in the place of the photospheric perturbations observed around the second and third HXR peaks. Thus, one can conclude that the helioseismic waves were generated during the whole impulsive phase and, probably, even in the pre-impulsive phase.
Figure 2| (a-d): Comparison of the total acoustic egression power map (red contours with 30 and 50% levels of maximal pixel value) with the flare Dopplergrams (black-white background images) for different time moments. Blue and cyan contours correspond to positive and negative Doppler velocities with levels of 3 km/s. White line marks the Polarity Inversion Line (PIL) deduced from the HMI vector magnetogram for time moment 12:12:04 UT; (e) The GOES lightcurve in band of 1-8 Å (thick black line), the KONUS/WIND count rate in the energy range of 0.4-30 MeV (thin black line), time profiles of the total egression power (background grey histogram), egression power for the Northern (magenta histogram) and Southern (blue histogram) acoustic sources.
The temporal profiles of the egression acoustic power (Figure 2e) are calculated for entire flare region (grey background histogram), northern (black histogram) and southern (blue histogram) groups of acoustic sources. The temporal profiles reveal a maximum of acoustic power for the entire flare region and the southern acoustic sources two minute after the third HXR peak. In the case of the northern sunquake sources, the acoustic emission reached its maximum between the time moments of the two most intense HXR peaks. The power started to increase in the pre-impulsive phase. However, we can conclude that the most efficient generation of helioseismic waves was definitely during the precipitation of nonthermal electrons into the lower layers of solar atmosphere.
Figure 3| Panels (a) and (c): The flare emission sources (gray-scale features) from the HMI filtergrams for two moments of time during the impulsive phase. The PIL is marked by blue color. Panels (b) and (d): The photospheric emission light-curves (black for HMI Camera 1 and red for Camera 2) at four different points marked by red crosses in panels a and c, and the KONUS/WIND count rate (blue) in the energy range of 0.4-30 MeV measured from the whole Sun.
To identify the impact sources in the photosphere, we used the high-cadence HMI level-1 filtergrams. In Figure 3, we compare the filtergram light-curves from four characteristic flare points (shown by red crosses in panels a and c) with the HXR time profiles. These points were selected to characterize the dynamics of continuum emission in the vicinity of the photospheric impacts observed during the pre-impulsive (points 1 and 2, panel b) and impulsive (points 3 and 4, panel d) phases. The filtergrams corresponding to the pre-impulsive and the impulsive phase are shown in panels a and c, respectively. Comparison with the KONUS/WIND HXR time profile (blue line) in the energy range of 70-300 keV reveals that the photospheric emission during the first and second HXR peaks were associated with points 1 and 2, and initially appeared during the pre-impulsive phase. Points 3 and 4 generated emission during the last HXR pulse. The flare continuum emission was sequentially generated along the PIL starting from the places of strong horizontal magnetic field in the pre-impulsive phase and finishing in sunspot areas of predominantly vertical magnetic field. This suggests that the flare energy release was developed in the form of sequential involvement of compact low-lying magnetic loops that were sheared along the PIL.
References
[1] Zhao, J., & Chen, R. 2018, ApJ Lett, 860, L29
[2] Kosovichev, A. G. 2017, HMI Science Nuggets, 73
[3] Lindsey, C., & Braun, D. C. 1997, ApJ, 485, 895