HIGH-WAVENUMBER SOLAR f-MODE STRENGTHENING PRIOR TO ACTIVE REGION FORMATION

Abstract

We report a systematic strengthening of the local solar surface or fundamental f -mode 1-2 days prior to the emergence of an active region (AR) in the same (corotating) location. Except for a possibly related increase in the kurtosis of the magnetic field, no indication can be seen in the magnetograms at that time. 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 explore this possibility using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and show for six isolated ARs, 11130, 11158, 11242, 11105, 11072, and 11768, 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. Furthermore, we study two ARs, 12051 and 11678, apart from a magnetically quiet patch lying next to AR 12529, to demonstrate the challenges in extracting such a precursor signal when a newly forming AR emerges in a patch that lies in close proximity of one or several already existing ARs which are expected to pollute neighboring patches. We then discuss plausible procedures for extracting precursor signals from regions with crowded environments. The idea that the f -mode is perturbed days before any visible magnetic activity occurs at the surface can be important in constraining dynamo models aimed at understanding the global magnetic activity of the Sun.

Intermittency spectra of current helicity in solar active regions

“…In all three cases, we observed an abrupt dip in the current helicity flatness exponent κ H C time profile approximately 12 hours prior to the emergence onset. This result resembles the findings by Singh, Raichur, and Brandenburg (2016), who analysed the distribution of line-of-sight magnetic field within patches that enclosed emerging AR. They found that the kurtosis, i.e.…”

Magnetic concentrations in stratified turbulence: the negative effective magnetic pressure instability

“…However, the helioseismic signatures of such a scenario [20] have not been detected [21]. Observations are more consistent with a gradual build-up of an active region on the timescale of one to two days [22]. An entirely different kinematic process that can form magnetic concentrations is flux expulsion, by which magnetic fields are expelled from regions of rapid motion.…”

f-mode strengthening from a localised bipolar subsurface magnetic field

“…These numerical studies and their predictions led to an observational case study of f -modes in relation to the emergence of about half a dozen ARs, observed with HMI, and the resulting line-of-sight Dopplergrams and magnetograms (Singh et al 2016). This study reported strengthening of the local f -mode about two days before the emergence of ARs at the same corotating patch.…”

“…It is known that the sunspots or ARs absorb the f -mode power, thus causing its damping (Cally and Bogdan 1997), and therefore it is expected to be harder to extract the precursor signal associated with a newly forming AR in a "crowded" environment with many existing ARs. A possible cleaning procedure to still extract the signal was discussed in Singh et al (2016). Although this is yet to be confirmed for larger data sets and with independent observational techniques, the potential significance of such f -mode related precursors cannot be understated.…”