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

Abstract

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 3and 4-skip waves, and that 8-skip waves have the same target-annuli geometry as 4skip 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/HMIobserved 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 304Å 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.

Performance of solar far-side active region neural detection

“…As a proxy for the location of active regions, we used 304 Å Carrington maps from the Solar Museum Server of NASA 4 (Liewer et al 2014), composed of observations from the Extreme Ultraviolet Imager (EUVI, Wuelser et al 2004) onboard STEREO and the instrument Atmospheric Imaging Assembly (AIA, Lemen et al 2012) onboard Solar Dynamics Observatory (SDO, Pesnell et al 2012). STEREO 304 Å data have been commonly used as an indication of far-side magnetic activity by several works analysing seismic detections of far-side active regions (Liewer et al 2012;Liewer et al 2014;Zhao et al 2019) Here, we have performed a more statistically significant study by extending the analysis to a larger amount of cases. We use data from the whole temporal period when there was a good STEREO far-side coverage, spanning from April 2011 to May 2016.…”

“…Far-side imaging relies on multiple-skip waves, using waves that are reflected on the surface at least once on their propagation from the focus point to the pupil or vice versa. Recently, Zhao et al (2019) have developed new schemes to perform time-distance far-side analyses including waves whose paths follow a higher number of skips.…”

A Multi-purpose Heliophysics L4 Mission

“…Helioseismic holography (Gizon et al, 2018;Liewer et al, 2014;Lindsey & Braun, 2000;Yang, 2018) and time-distance helioseismology (Duvall et al, 1993;Zhao et al, 2019) could be a further interesting application of magnetograph data from the Lagrange points L5 and particularly L4. Both techniques enable the detection and specification of active regions on the far side of the Sun.…”

Structure of the Solar Atmosphere: A Radio Perspective

“…Impressive results have been obtained on the subphotospheric structure of sunspots (Gizon et al, 2009;Zhao et al, 2010) and supergranular flows (Jackiewicz et al, 2008). We can even detect active regions on the far side of the solar disk (see the recent article by Zhao et al, 2019), with data routinely available at http://jsoc.stanford.edu/data/farside/.…”