  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.

Solar and Stellar Astrophysics