High-wavenumber Solar f-mode strengthening prior to active region formation

by Harsha Raichur (Nordita)

Wednesday 9 Mar 2016, 13:30 → 14:30 Europe/Stockholm

122:026

We report a systematic strengthening of the local solar surface mode, i.e. the f -mode, 1–2 days prior to the emergence of an active region (AR) in the same location while no indication can yet be seen in the magnetograms. Our study is motivated by earlier numerical findings of Singh et al. (2014) which showed that, in the presence of a nonuniform magnetic field that is concentrated a few scale heights below the surface, the f -mode fans out in the diagnostic kω diagram at high wavenumbers. Here we exploit this property using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, and show for four ARs 11130, 11158, 11768, and, 12051, that at large latitudinal wavenumbers (corresponding to horizontal scales of around 3000 km), the f -mode displays strengthening about two days prior to AR formation and thus provides a new precursor for AR formation. The idea that the f -mode is perturbed days before any visible magnetic activity occurs on the surface can be important in constraining dynamo models aiming at understanding the global magnetic activity of the Sun.