Imaging the Sun's Far-side Active Regions by Applying Multiple Measurement Schemes on Multiskip Acoustic Waves

Being able to image active regions (ARs) on the Sun's far side is useful for modeling the global-scale magnetic field around the Sun and for predicting the arrival of major ARs that rotate around the limb onto the near side. Helioseismic methods have already been developed to image the Sun's far-side ARs using near-side high-cadence Doppler-velocity observations; however, the existing methods primarily explore the 3-, 4-, and 5-skip helioseismic waves, leaving room for further improvement in the imaging quality by including waves with more multiskip waves. Taking advantage of the fact that 6-skip waves have the same target-annuli geometry as 3- and 4-skip waves and that 8-skip waves have the same target-annuli geometry as 4-skip waves, we further develop a time-distance helioseismic code to include a total of 14 sets of measurement schemes. We then apply the new code on the Solar Dynamics Observatory/Helioseismic and Magnetic Imager observed Dopplergrams and find that the new code provides substantial improvements over the existing codes in mapping newly emerged ARs and ARs near both far-side limbs. Comparing 3 months of far-side helioseismic images with the Solar TErrestrial RElations Observatory/Extreme UltraViolet Imager observed 304 A images, we find that 97.3% of the helioseismically detected far-side ARs that are larger than a certain size correspond to an observed region with strong EUV brightening. The high reliability of the new imaging tool will potentially allow us to further calibrate the far-side helioseismic images into maps of magnetic flux.