Toward Waveform Heliotomography: Observing Interactions of Helioseismic Waves with a Sunspot

Abstract

We investigate how helioseismic waves that originate from effective point sources interact with a sunspot. These waves are reconstructed from observed stochastic wavefields on the Sun by cross-correlating photospheric Doppler-velocity signals. We select the wave sources at different locations relative to the sunspot, and investigate the p-and f -mode waves separately. The results reveal a complicated picture of waveform perturbations caused by the wave interaction with the sunspot. In particular, it is found that for waves originating from outside of the sunspot, p-mode waves travel across the sunspot with a small amplitude reduction and slightly higher speed, and wave amplitude and phase get mostly restored to the quiet-Sun values after passing the sunspot. The f -mode wave experiences some amplitude reduction passing through the sunspot, and the reduced amplitude is not recovered after that. The wave-propagation speed does not change before encountering the sunspot and inside the sunspot, but the wavefront becomes faster than the reference wave after passing through the sunspot. For waves originating from inside the sunspot umbra, both f -and p-mode waves show significant amplitude reductions and faster speed for all propagation paths. A comparison of positive and negative time lags of cross-correlation functions shows an apparent asymmetry in the waveform changes for both the f -and p-mode waves. We suggest that the waveform variations of the helioseismic waves interacting with a sunspot found in this article can be used for developing a method of waveform heliotomography, similar to the waveform tomography of the Earth.

Multiscale Seismic Tomography

“…They have used the method to map the source zone of the great 2011 Tohoku-oki earthquake (Mw 9.0) beneath the Pacific Ocean (for details, see Zhao et al 2011;Huang and Zhao 2013, and Chap. proposed an off-network tomography method to study the 3-D velocity structure beneath oceanic areas adjacent to a land-based seismic network.…”

“…Instead of the raw travel-time residuals used in LET, relative travel-time residuals are used in 1.2 Classification of Seismic Tomography TET (see Chap. In the past two decades, however, solar physicists have been actively investigating the 3-D sound velocity structure of the solar interior using helioseismology methods (e.g., Duvall et al 1993Duvall et al , 1996Zhao et al 2011). Thus, the effects of hypocentral mislocations of the teleseismic events and structural heterogeneities outside of the study area can be greatly reduced.…”

“…Its western and eastern blocks are two major Archean continental nuclei which are separated by a major lithospheric boundary in its central block (e.g., Zhao et al 2001). Several major geologic events have affected the NCC reactivation since the late Mesozoic, and compound geological processes, rather than just a single event, caused the NCC reactivation (Menzies et al 1993;Wu et al 2005;Yang et al 2008;Tian et al 2009;Zhao et al 2009Zhao et al , 2011Zhao 2011, 2013). Several major geologic events have affected the NCC reactivation since the late Mesozoic, and compound geological processes, rather than just a single event, caused the NCC reactivation (Menzies et al 1993;Wu et al 2005;Yang et al 2008;Tian et al 2009;Zhao et al 2009Zhao et al , 2011Zhao 2011, 2013).…”

Detection of Fast-Moving Waves Propagating Outward Along Sunspots’ Radial Direction in the Photosphere

“…This method has been widely used in various research fields, including seismology (e.g., Snieder & Wapenaar 2010;de Ridder 2014) and helioseismology (e.g., Cameron et al 2008;Zhao et al 2011b). The analysis procedure used in this study is the same as described by Zhao et al (2011b), but the virtual wave sources are chosen inside the sunspot. The analysis scheme is illustrated in Figure 1(a): all waves originating from a virtual source "A" can be reconstructed by cross-correlating the time series observed at "A" with the time series observed at all other locations inside the field of view.…”

“…In this Letter, employing a procedure similar to reconstruct waves traveling from virtual point sources (Zhao et al 2011b), we report on a detection of fast-moving waves at the photospheric level, propagating radially from the sunspot umbra through the penumbra to the outside. The observed properties of this wave are consistent with a fast magnetoacoustic wave excited beneath the sunspot's surface, the wavefront of which sweeps across the photosphere.…”