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