The Sun’s Far Side as Seen via Martian Observations and GONG Helioseismic Mapping

Solar activity is now in the declining phase of Solar Cycle 25, leading to a marked reduction in both the number of active regions and the frequency of high‑energy events. Front‑side active regions continue to be monitored routinely from ground‑ and space‑based observatories, but the far side lacks continuous direct coverage. NASA’s STEREO spacecraft and ESA’s Solar Orbiter provide intermittent far‑side observations that have been essential for validating helioseismic techniques used to infer active regions on the hidden hemisphere. However, because the visibility of the far side depends on each spacecraft’s orbital position, none of these assets provide full‑hemisphere far‑side coverage at all times. For more information, please see an earlier post,

https://gongnisp.blogspot.com/2025/12/why-active-regions-appeared-displaced.html

Recent observations from NASA's Perseverance rover on Mars — currently positioned on the far side of the Sun relative to Earth —provide a valuable additional vantage point for viewing the Sun’s hidden hemisphere. In the image shown here, we compare the far‑side active regions inferred from GONG helioseismic mapping with those detected in the recently published Mars‑based observations reported by Spaceweather.com.

Martian observation (right bottom panel of the accompanied image) indicates that four active regions seen on March 6 in the northern hemisphere (left panel) remain on the solar disk, and these same regions are also captured in the GONG helioseismic maps (right top panel). One region highlighted in the GONG maps (marked with a red circle) does not appear in the Martian observations because of its position relative to Mars’s viewing geometry. The line marking this location shows clearly that the region lies outside Mars’s field of view. These differences arise primarily from the distinct image geometries and viewing angles of the two observing platforms.