Measuring Subsurface Magnetic Fields and Detecting Emerging Active Regions Using Helioseismic Travel Times

While two and a half decades of nearly constant observation by the Solar and Heliospheric Observatory (SOHO) and the Solar Dynamics Observatory (SDO) spacecraft have yielded key insights into the structure and dynamics of active regions, there remain many open questions regarding what properties of active regions are detectable through helioseismology. In particular, can active regions be identified before emerging on the solar surface and, once emerged, can the subsurface structure of an active region’s magnetic field be measured.

To answer these questions, we complete a survey of 47 large active regions and their associated helioseismic signatures. Here, we use deviations of the mean phase travel time of acoustic waves to detect the rise of magnetic flux from the solar interior. These deviations are associated with perturbations to the wave speed; in particular, we detect deviations associated with an increase to the wave speed, possibly caused by the contribution of magnetic pressure. We first confirm the results from a past study (Ilonidis et al., 2013), where mean phase travel time deviations are detected prior to the emergence of several active regions, and provide some calibration and testing of the technique outlined using two independent simulations of submerged sound speed perturbations. We find that a majority of the active regions—at best 34 of the 47 studied here—have mean phase travel time deviations that are most well-correlated with the surface magnetic flux prior to emergence.

Additionally, we develop a novel technique for the study of existing active region magnetic fields. By combining the travel time of acoustic waves traveling in varying directions, we are able to isolate perturbations caused by subsurface horizontal magnetic fields from those produced by other factors, such as flows and sound speed changes. The resulting measurements are used to provide a proxy for the magnitude of the horizontal magnetic field as well as a direct measure of the field’s orientation. We first validate the technique using a simulation containing a uniform background magnetic field, where the orientation of the subsurface magnetic field is accurately measured by the helioseismic technique (± 3 degrees). Additional validation is performed using a realistic sunspot simulation for which the subsurface state is known, which provides important context for measurements derived from HMI observations. The measurement scheme is then used to investigate the subsurface magnetic structure of several sunspots, where we also find some evidence for subsurface connections in sunspots which have a nearby flux patch.