An Unusual Velocity Field in a Sunspot Penumbra

TL;DR

This study documents an unusual penumbral velocity field in a rapidly evolving active region (NOAA 12146) where ongoing flux emergence near a pre-existing spot produces opposite Doppler streams of up to $\pm 2\ \mathrm{km\ s^{-1}}$ that cross the PIL and extend beyond the outer penumbral boundary. High-resolution spectroscopic data from the GFPI/BIC on the GREGOR telescope, combined with SDO/HMI context, reveal that the flows follow magnetic field lines with nearly horizontal fields ($\sim 90^{\circ}$) and that horizontal proper motions are modest (\lesssim $0.6\ \mathrm{km\ s^{-1}}$). The blueshifted component constitutes a counter Evershed flow associated with emergent flux, while the redshifted component aligns with the regular EF; no supersonic speeds are observed and the phenomenon persists over at least $16$ minutes, likely reflecting dynamic reconfiguration of adjacent flux tubes during flux emergence. These findings highlight complex penumbral dynamics in developing active regions and motivate further multi-instrument, high-cadence observations to disentangle overturning convection from siphon-flow processes in such environments.

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.

Figures (5)

• Figure 1: White-light image of the active region obtained with HMI at 09:48 UT on August 24, 2014. The red box marks the field of view of GFPI. The black arrow points to the solar disk center.
• Figure 2: HMI-data at four time steps. Each of these time steps is displayed in a row. Left column: intensity, column 2: HMI-velocities, column 3: total magnetic field strength, and right column: magnetic inclination. Velocities are scaled in the range $\pm$2.5 km s$^{-1}$, the magnetic field strength is clipped at 3000 G, and the range of inclination is 0 -- 180$^\circ$. The bar in the upper right corner of the intensity images indicates a length of 10 Mm. An animation of this figure from 00:00 UT on August 23 till 23:48 UT on August 24 is available online.
• Figure 3: Upper: Vertical component of the magnetic field recorded by HMI at 09:48 UT on August 24. Black and white contours indicate the boundaries of umbra and penumbra according to Fig. \ref{['FigHMI_Int']}. Values are suppressed where the total magnetic field strength is below 150 G. Lower: Corresponding Doppler-velocity from HMI.
• Figure 4: a: Speckle restored blue continuum image of the investigated region obtained at GREGOR, restricted to the area recorded with the GFPI. The red contours mark the zero-velocity lines and the yellow line the PIL. Black contours outline the outer penumbral boundary and white the umbral-penumbral boundary. The arrow points to disk center (note image rotation compared to Fig. \ref{['FigHMI_Int']}). b: horizontal velocity map obtained by LCT. Color and length of the arrows indicate the velocity in a spectral scale from black to red according to the color bar. The average blue continuum image serves as background. c and d: Velocity maps, derived from polynomial fits (c) and from the Fourier-method (d) applied to GFPI-data. Red and black contours mark the umbra and penumbra according to the GFPI intensity map. The yellow line indicates the PIL. Here, trajectories of high velocities shown in Fig. \ref{['Fig_graph']} are marked in light blue. e: Total magnetic field strength. f: Inclination of the magnetic field in the local frame of reference, taken from HMI. Black and white contours indicate the boundaries of umbra and penumbra in the GFPI-data. Values are set to 90$^\circ$ where the total magnetic field strength is below 150 G. The violet line marks the PIL. The trajectories are marked in panel f in black. An animation of panels c and d is available online.
• Figure 5: Left panel: Parameters along the line with maximum blueshifts. Solid blue: Doppler velocities from the polynomial fit, dashed blue: Doppler velocities from the Fourier-method, green: magnetic inclination, violet: total magnetic field strength, and orange: horizontal magnetic field strength. Horizontal lines mark the zero reference for velocities (blue) and 90$^\circ$ for the magnetic inclination (green). Right panel: Parameters for the maximum redshift. Here, the velocities are displayed in red. An animation of this figure is available online.