On Strengthening of the Solar f-Mode Prior to Active Region Emergence Using the Fourier-Hankel Analysis

Abstract

Recent results of Singh, Raichur, and Brandenburg (2016) show that the emergence of an active region (AR) can be seen in a strengthening of the f-mode power up to two days prior of the region’s formation. In the original work, ring diagram analysis was used to estimate the power evolution. In this study, we make use of the Fourier-Hankel method, essentially testing the aforementioned results with an independent method. The data are acquired from SDO/HMI, studying the ARs 11158, 11072, 11105, 11130, 11242 and 11768. Investigating the total power as a function of time, we find a similar behavior to the original work for four of the six listed ARs, which is an enhancement of f-mode power about one to three days prior to AR emergence. AR 11105 as well as AR 11242 also show detectable f-mode enhancement, although less significant than what was observed before. In addition, the analysis of the absorption coefficient $\alpha$, yielded by the Fourier-Hankel analysis, shows neither absorption ($\alpha > 0$) nor emission ($\alpha < 0$) of power during the enhancement. Finding no changes of the absorption coefficient (i.e., $\alpha = 0$) is an important result, as it narrows down the possible physical interpretation of the original f-mode power enhancement, showing that no directional dependence (in the sense of inward and outward moving waves) is present.