Waves of Magnetic-field Variations Observed in a Flare-excited Sunquake Event

We report on the detection of waves of magnetic-field variations that were associated with flare-excited sunquake waves. An X-9.3 flare that occurred on 2017 September 6 excited strong sunquakes, and the sunquake waves were observed sweeping across the flare's host active region. This rare event gives us an unprecedented opportunity to study responses of magnetic field to passing sunquake waves. A wave of magnetic-field variations was observed in each of the two sunspots that the sunquake waves swept through, and the time-distance relations for the waves observed in magnetic field and Doppler velocity are similar. The phase relations measured between, as well as the oscillatory power distributions calculated from, the Doppler velocity variations and magnetic-field variations associated with the sunquake waves are compared with those obtained from the background waves in the same areas of the sunspot umbra and penumbra separately. The phase relations seem to favor the theory that the waves of magnetic variations are owing to opacity changes associated with the passing sunquake waves. The comparisons of phases and power distributions indicate that the background magnetic variations observed in sunspots are a combination of various wave modes, and fast magnetoacoustic waves only account for a fraction of those magnetic variations.