Abstract
A new time–distance far-side imaging technique was recently developed by utilizing multiple multi-skip acoustic waves. The measurement procedure is applied to 11 years of Doppler observations from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, and over 8000 far-side images of the Sun have been obtained with a 12-hour temporal cadence. The mean travel-time shifts in these images unsurprisingly vary with the solar cycle. However, the temporal variation does not show good correlations with the magnetic activity in their respective northern or southern hemisphere, but show very good anti-correlation with the global-scale magnetic activity. We investigate four possible causes of this travel-time variation. Our analysis demonstrates that the acoustic waves that are used for mapping the Sun’s far side experience surface reflections around the globe, where they may interact with surface or near-surface magnetic field and carry travel-time deficits with them. The mean far-side travel-time shifts from these acoustic waves therefore vary in phase with the Sun’s magnetic activity.
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Data Availability
The far-side images used in this analysis are made by the SDO/HMI time–distance far-side imaging pipeline and can be downloaded from the webpage jsoc.stanford.edu/data/timed/. The near-side magnetograms are from the SDO/HMI observations and can be downloaded from the webpage hmi.stanford.edu/magnetic/.
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We thank an anonymous reviewer for their valuable comments and suggestions that help improve the quality of this article.
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This work was partly funded by the NOAA grant NA18NWS4680082.
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This article belongs to the Topical Collection:
Celebrating a Solar Cycle of Discovery with SDO
Guest Editors: Dean Pesnell, Ryan Milligan and Shin Toriumi
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Zhao, J., Jiang, G.Y. & Chen, R. Long-Term Variation of Helioseismic Far-Side Images and What Causes It. Sol Phys 296, 186 (2021). https://doi.org/10.1007/s11207-021-01937-8
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DOI: https://doi.org/10.1007/s11207-021-01937-8