An Unusual Velocity Field in a Sunspot Penumbra

TL;DR

This study investigates unusual opposite velocity flows in a sunspot penumbra caused by ongoing magnetic flux emergence, revealing complex flow patterns that challenge typical models of the Evershed flow.

Contribution

It provides high-resolution observations of opposite Doppler velocity streams in a sunspot penumbra linked to flux emergence, highlighting complex magnetic and flow interactions.

Findings

Opposite velocity streams of up to ±2 km/s observed near the penumbra.

Flows extend beyond the penumbral boundary crossing the polarity-inversion line.

Magnetic field properties remain consistent despite velocity differences.

Abstract

The photospheric Evershed flow is normally oriented radially outward, yet sometimes opposite velocities are observed not only in the chromosphere but also in the photospheric layers of the penumbra. We study the velocity field in a special case of an active region with two mature sunspots, where one of them formed several days later than the main one. Between the two spots, flux emergence is still ongoing influencing the velocity pattern. We observed the active region NOAA 12146 on August 24, 2014, with the GREGOR Fabry-P\'erot Interferometer (GFPI) and the Blue Imaging Channel (BIC) of the GREGOR solar telescope at Observatorio del Teide on Tenerife. Context data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) complement the high-resolution data. In the penumbra of a newly formed spot, we observe opposite Doppler velocity streams of up to +-2 km/s very close to each other. These velocities extend beyond the outer penumbral boundary and cross also the polarity-inversion line. The properties of the magnetic field do not change significantly between these two streams. Although the magnetic field is almost horizontal, we do not detect large transversal velocities in horizontal flow maps obtained with the local correlation technique. The ongoing emergence of magnetic flux in an active region causes flows of opposite directions intruding the penumbra of a pre-existing sunspot.