Surface magnetic field effects in local helioseismology

TL;DR

This paper demonstrates that surface magnetic fields in sunspots significantly influence helioseismic measurements, affecting phase and travel-time signals, which has important implications for local helioseismic analysis of active regions.

Contribution

It provides the first strong evidence that magnetic field orientation impacts helioseismic phase measurements within sunspots, highlighting a key factor for accurate local helioseismology.

Findings

Helioseismic phase depends on line-of-sight angle in sunspots.

Magnetic field inclination significantly affects helioseismic signals.

Surface magnetic effects are crucial for analyzing active regions.

Abstract

Using helioseismic holography strong evidence is presented that the phase (or equivalent travel-time) of helioseismic signatures in Dopplergrams within sunspots depend upon the line-of-sight angle in the plane containing the magnetic field and vertical directions. This is shown for the velocity signal in the penumbrae of two sunspots at 3, 4 and 5 mHz. Phase-sensitive holography demonstrates that they are significantly affected in a strong, moderately inclined magnetic field. This research indicates that the effects of the surface magnetic field are potentially very significant for local helioseismic analysis of active regions.