Authors: John T. Stefan (New Jersey Institute of Technology) & Alexander G. Kosovichev (New Jersey Institute of Technology)

With the increasing focus on space weather prediction, it is becoming more important to improve our understanding of the origins of this activity: solar active regions. Previous studies have found evidence for pre-emergence signatures in deep measurements (below 40 Mm) associated with sound speed changes and in more shallow measurements (above 15 Mm) associated with horizontal flows. While these measurements—the acoustic wave travel times—carry information about the subsurface state along the wave path. The travel times are inverted to obtain, for example, the horizontal flow velocities and relative sound speed variations in physical units. The inversion process also has the advantage of spatially localizing these features, though a degree of smoothing is maintained which can make distinguishing weak signatures from the background state difficult. In this work, we develop a time-distance helioseismology pipeline for tracking areas of active regions before and after their emergence using SDO/HMI Dopplergrams. Using this pipeline, we obtain the travel time maps for acoustic waves traveling to the depth of 30 Mm, and by inverting the travel times we obtain maps of subsurface flows and perturbations of the sound speed. Then, we evaluate the performance of the travel times vs inverted measurements for forecasting active region emergence. We pay particular attention to the depth localization and temporal coherence of these features during the pre-emergence process. The overall effectiveness of each feature is assessed from its magnitude relative to the quiet Sun background and the lead time of its maximum relative to the active region’s emergence.