An Unusual Velocity Field in a Sunspot Penumbra

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-Pérot 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.

arXiv comment7 pages, 5 figures, accepted by Astronomy & Astrophysics

References in corpus (24)

• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Overview and Performance
• VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager
• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Optimization of the Spectral Line Inversion Code
• The European Solar Telescope
• Emergence of magnetic flux generated in a solar convective dynamo. I: Formation of Sunspots and Active regions, and Origin of Their Asymmetries
• Numerical simulations of sunspot decay: On the penumbra -- Evershed flow -- moat flow connection
• The GREGOR Fabry-Pérot Interferometer
• Horizontal flow fields observed in Hinode G-band images. I. Methods
• Formation of the penumbra and start of the Evershed flow
• Normal and counter Evershed flows in the photospheric penumbra of a sunspot. SPINOR 2D inversions of Hinode-SOT/ SP observations
• Unusual Filaments Inside the Umbra
• Temporal evolution of arch filaments as seen in He I 10830 Å
• Horizontal flow fields in and around a small active region-- The transition period between flux emergence and decay
• Evershed and counter-Evershed flows in sunspot MHD simulations
• Anomalous flows in a sunspot penumbra
• Properties of the Umbral Filament Observed in Active Region NOAA 12529
• Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope
• The formation of an atypical sunspot light bridge as a result of large-scale flux emergence
• Restoring process of sunspot penumbra
• Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots
• How rare are counter Evershed flows?
• Onset of penumbra formation
• Expulsion of counter Evershed flows from sunspot penumbrae
• Magnetic properties of orphan penumbrae