An unusual velocity field in a sunspot penumbra

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
² Institut für Physik und Astronomie, Universität Potsdam, D-14476 Potsdam, Germany
³ Institut für Sonnenphysik (KIS), George-Köhler-Allee 401a, D-79110 Freiburg, Germany
⁴ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁶ Hansa-Gymnasium, Fährwall 19, D-18439 Stralsund, Germany
⁷ Udaipur Solar Observatory, Physical Research Laboratory, Udaipur, India
⁸ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 September 2021
Accepted: 30 October 2025

Abstract

Context. 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.

Aims. We studied the velocity field in a special case of an active region with two mature sunspots, the lesser of which formed several days after the main one. Flux emergence between the two spots is still ongoing, influencing the velocity pattern.

Methods. We observed the active region NOAA 12146 on August 24, 2014, with the GREGOR Fabry-Pérot Interferometer and the Blue Imaging Channel of the GREGOR solar telescope at Observatorio del Teide on Tenerife. Context data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory complement the high-resolution data.

Results. In the penumbra of a newly formed spot, we observe opposite Doppler velocity streams of up to ±2 km s⁻¹ very close to each other. These velocities extend beyond the outer penumbral boundary and cross 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 high transversal velocities in horizontal flow maps obtained via the local correlation technique.

Conclusions. The ongoing emergence of magnetic flux in an active region causes flows of opposite directions that penetrate the penumbra of a preexisting sunspot.