Calibration of Seismic Signatures of Active Regions on the Far Side of the Sun

Abstract

Synoptic maps of the far hemisphere of the Sun calculated from seismic holography have proven to be very reliable in localizing large active regions before they rotate onto the visible hemisphere. We show here the first results toward a calibration of the far-side signatures of active regions in terms of active region size and magnetic field strength. We compare helioseismic maps of large active regions on the far side of the Sun, calculated from Global Oscillation Network Group (GONG) Doppler observations, with magnetic and visible-continuum images of the same active regions on the visible hemisphere before and after their far-side passage. The far-side seismic signature is expressed as a phase shift that a far-side active region introduces to waves from the near hemisphere as they are reflected into the solar interior on their way back to the near hemisphere. There is a significant correlation between this far-side signature and both the total area of the active region, as viewed on the near hemisphere, and the area of the sunspots contained in the active region. We have studied the relationship between the magnetic field strength and the phase signature for six of the larger, more stable active regions. We find an approximately logarithmic increase in the seismic phase signature with increasing magnetic field strengths above a critical field of ~10 G. This is roughly consistent with similar helioseismic signatures measured on the near solar hemisphere concurrent with associated magnetic fields.