Magnetic field emergence in quiet Sun granules

TL;DR

This study reports a new small-scale magnetic flux emergence process in the quiet Sun, where vertical magnetic fields rise from the interior to the photosphere, associated with granular convection and lasting about 20 minutes.

Contribution

It provides observational evidence of vertical magnetic flux emergence in quiet Sun granules using high-cadence spectropolarimetric data from Hinode, a phenomenon not previously well characterized.

Findings

Magnetic signals emerge at the centers of granular cells.

Emerging flux patches grow and then fade over approximately 20 minutes.

No significant chromospheric response observed during emergence.

Abstract

We describe a new form of small-scale magnetic flux emergence in the quiet Sun. This process seems to take vertical magnetic fields from the solar interior to the photosphere, where they appear above granular convection cells. High-cadence time series of spectropolarimetric measurements obtained by Hinode in a quiet region near disk center are analyzed. We extract line parameters from the observed Stokes profiles and study their evolution with time. The circular polarization maps derived from the observed \ion{Fe}{i} 630 nm lines show clear magnetic signals emerging at the center of granular cells. We do not find any evidence for linear polarization signals associated with these events. The magnetic flux patches grow with time, occupying a significant fraction of the granular area. The signals then fade until they disappear completely. The typical lifetime of these events is of the order of 20 minutes. No significant changes in the chromosphere seem to occur in response to the emergence, as revealed by co-spatial \ion{Ca}{ii} H filtergrams. The Stokes I and V profiles measured in the emerging flux concentrations show strong asymmetries and Doppler shifts. The origin of these events is unclear at present, but we suggest that they may represent the emergence of vertical fields lines from the bottom of the photosphere, possibly dragged by the convective upflows of granules. Preliminary inversions of the Stokes spectra indicate that this scenario is compatible with the observations. The emergence of vertical field lines is not free from conceptual problems, though.