Junwei Zhao1 & Ruizhu Chen2,1
1. W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085
2. Department of Physics, Stanford University, Stanford, CA 94305-4060
We report on the detection of waves of magnetic-field variations that were associated with flare-excited sunquake waves, a phenomenon that was not observed or reported before.
Sunquakes are detectable helioseismic waves excited by solar flares, exhibiting as expanding ripples in the photosphere. The GOES X-9.3 flare (SOL2017-09-06T11:53) that occurred in NOAA AR 12673 on September 9, 2017 generated a powerful sunquake event. Figure 1 shows a continuum image of this active region and the helioseismic power map reconstructed through a helioseismic holography technique by using the sunquake wave signals observed at a later time. At the initial stage of this flare, three sunquake sources are identified. More importantly, waves of magnetic-field variations are also observed accompanying the Doppler-velocity-observed sunquake waves in the green- and blue-line delimited regions. See the online movie below for the occurrence of the flare and the waves observed following the flare.
It is remarkable that all the SDO/HMI observables show evidences of the sunquake waves, including continuum intensity, line-core intensity, Doppler velocity, and magnetic field. Panels (a-d) of Figure 2 show the time–distance diagram of the sunquake wave observed in these four observables, with signals averaged from the green-line-delimited sectoral region in Figure 1b. Figure 2e-f shows the averaged oscillatory signals observed in the sunspot umbra and penumbra, respectively. Given the fact that waves in the umbra and penumbra show distinct properties, oscillatory signals in these two different areas are then analyzed separately.
Figure 1| (a) Map of helioseismic power at 11:56:15UT, reconstructed from the sunquake waves observed in Doppler velocity through a helioseismic holography method. `S1′, `S2′, and `S3′ represent the three distinct sunquake epicenters. (b) Continuum intensity map overlapped by white contours showing the locations of epicenters. Solid green lines delimit the area inside which observations are taken to calculate the time–distance diagrams shown in Figure 2, and the blue lines delimit the area where another magnetic wave was observed. Results for `umbra’ shown in Figure 3 are calculated from inside the green-line delimited area between the dashed arcs, and results for `penumbra’ are from the area between the right dashed arc and the green solid arc.
To better understand both the cause and the nature of the observed magnetic waves, we investigate the phase relations between the Doppler and magnetic signals, as well as the frequency-dependent power distribution of the oscillatory signals observed by the Doppler and magnetic field. In order to better understand these phase and power relations, it is useful to compare these results with background waves: obtained from the same areas but after the sunquake wave passed. We analyze the background waves in two different wave: one uses signals at each pixel and average results later; and one average signals in small patches first and then compute phase and power distributions. Figure 3 shows comparisons of all these results.
Figure 2| Time–distance diagrams for the flare-excited waves using the HMI data of (a) continuum intensity, (b) line-core intensity, (c) Doppler velocity, and (d) line-of-sight magnetic field. Signals within distances of 26.5-35.0 arcsec and of 35.0-47.5 arcsec in each panel are averaged to give relative amplitude variations for the umbra (e) and penumbra (f), respectively. The v0, B0, Ic0 and ILC0 are values averaged at the same locations from a period of 50 min before the flare. Time in all panels is relative to 11:52:30UT.
Left (right) panels in Figure 3 show results for the umbra (penumbra). It can be found that the phase relations (between the Doppler- and magnetic-signals) differ in sunquake waves and in background waves in both the umbra and penumbra. For the power distribution, in the umbra, the Doppler-observed sunquake wave shows a power much stronger than background waves, and the magnetic-wave shows double peaks near 3.2 and 5.6 mHz. In the penumbra, the Doppler-observed sunquake wave has a magnitude similar to the background wave, and the magnetic-wave still has double peaks. These comparisons demonstrate that sunquake-associated magnetic waves are likely fast magnetoacoustic waves, while the background waves in magnetic regions are not primarily fast magnetoacoustic waves. Comparing the phase relations shown in Figure 3(a-b) with the simulations in Ref., we conclude that the magnetic waves associated with the sunquake waves are likely due to the opacity changes in the areas where large-amplitude sunquake waves pass.
Figure 3| Comparison of the sunquake wave, background waves, and averaged background waves in the umbra and penumbra. Left (right) column shows results from the umbra (penumbra). Top panels show phase differences between the Doppler-velocity and magnetic-field signals, with the negative sign representing that Doppler signal leads magnetic signal. Middle panels show the oscillatory power measured from Doppler velcoity, and bottom panels show the oscillatory power measured from magnetic-field variations. Note that the vertical axis scales for the umbra and penumbra are different for both the velocity and magnetic power.
For more details, please refer to our recent publication: Zhao & Chen, 2018, ApJ Lett., 860, L29.
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