Imaging the Sun's Far-Side Active Regions by Applying Multiple Measurement Schemes on Multi-Skip Acoustic Waves
Being able to image active regions 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 active regions that rotate around the limb onto the near side. Helioseismic methods have already been developed to image the Sun's far-side active regions 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 multi-skip waves. Taking advantage of the facts 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 SDO/HMI-observed Dopplergrams, and find that the new code provides substantial improvements over the existing codes in mapping newly-emerged active regions and active regions near both far-side limbs. Comparing 3 months of far-side helioseismic images with the STEREO/EUVI-observed 304A images, we find that 97.3% of the helioseismically detected far-side active regions 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.