Formation of the penumbra and start of the Evershed flow

TL;DR

This study investigates the rapid formation of the penumbra and the onset of the Evershed flow around a solar pore, revealing magnetic field changes and flow dynamics during the process.

Contribution

It provides new observational evidence on the timing and magnetic field evolution during penumbra formation and proposes a magnetic flux sinking scenario for the Evershed flow onset.

Findings

Evershed flow begins within 1-3 hours of penumbra formation.

Magnetic field inclination changes significantly, with the vertical component reversing near the penumbral edge.

Magnetic field strength increases by about 400 G during penumbra formation.

Abstract

We studied the variations of line-of-sight photospheric plasma flows during the formation phase of the penumbra around a pore in Active Region NOAA 11490. We used a high spatial, spectral, and temporal resolution data set acquired by the Interferometric BIdimensional Spectrometer (IBIS) operating at the NSO/Dunn Solar Telescope as well as data taken by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory satellite (SDO/HMI). Before the penumbra formed we observed a redshift of the spectral line in the inner part of the annular zone surrounding the pore as well as a blueshift of material associated with opposite magnetic polarity further away from the pore. We found that the onset of the classical Evershed flow occurs in a very short time scale -- 1-3 hours -- while the penumbra is forming. During the same time interval we found changes in the magnetic field inclination in the penumbra, with the vertical field actually changing sign near the penumbral edge, while the total magnetic field showed a significant increase, about 400 G. To explain these and other observations related to the formation of the penumbra and the onset of the Evershed flow we propose a scenario in which the penumbra is formed by magnetic flux dragged down from the canopy surrounding the initial pore. The Evershed flow starts when the sinking magnetic field dips below the solar surface and magnetoconvection sets in.