Johannes Löhner-Böttcher and Rolf Schlichenmaier
Kiepenheuer-Institut für Sonnenphysik (KIS), 79102 Freiburg, Germany

As complex magnetic formations, sunspots strongly affect the photospheric dynamics of the granular convective pattern of the plasma in which they are embedded. They suppress the upward propagation of hot gas plasma in the magnetic core, seen as the cooler and therefore darker umbra, and they harbor the penumbral Evershed flow (EF) observed as a radially outwards-directed, mainly horizontal flow¹ in the photospheric layers with maximum velocities up to several km/s. The EF is linked to the magnetic field lines and seems to stop abruptly at the white-light boundary of the spot¹. The annular region surrounding the sunspot is largely nonmagnetic with single moving magnetic features and harbors the moat flow (MF), a radially outwards-oriented flow adjacent to the outer penumbral boundary. The existence of both the MF and the EF is coupled to the presence of a penumbra². With Doppler velocity measurements of HMI we were able to analyze the horizontal velocity components and extensions of these flows in and around sunspots and to elaborate on a possible link between the two.

Figure 1 | a) 720s intensity map of AR11084 located at 19S, 48W on 2010 July 6 at 01:36 UT with a FOV size of (150″)². An arrow is pointing to the disk center. The white-light boundary and the theoretical, circular spot shape (black ellipse) are displayed. b) Calibrated 720s velocity map corresponding to a), displaying velocities up to ±1 km/s. Foreshortened circles with rising radii surround the spot. c) Aligned time-average (3h) of AR11084 starting with b). The FOV size is (100″)². d) Same as c) with an applied spot masking highlighting the surrounding MF. e-h) Selection of analyzed sunspots³ displayed like d).

We calibrated³ HMI 720s Dopplergrams and used them to analyze 3 h time averages of 31 circular, stable, and fully developed sunspots at heliocentric angles of some 50° (see Figure 1). In this time period of three hours the MF turned out to be stable^3,4, the observing position was selected so that the component of the horizontal flow was significant, while the calibration uncertainties were still small. The selected sunspots had a radius of 9 Mm to 22 Mm and were observed between June 2010 and January 2012.

Assuming axially symmetrical flow fields following the direction of the penumbral filaments², we inferred the azimuthally averaged horizontal velocity component of the MF and EF by generating foreshortened circles due to the heliocentric angle and fitting the LOS-velocities for the circular angles by a sine function³ (see Figure 2). The amplitude of the sine curve was interpreted as the horizontal flow component. The velocities for the different distances to the spot center for AR11084 are shown in Figure 2.

Figure 2 | Left two panels: Angular LOS velocities (black, solid line) in m/s along the elliptical projection of circles surrounding the sunspot (AR11084). The fitting of the velocities by a sine curve (green, solid line) yields the horizontal flow component as the amplitude (red, dashed line) and the vertical flow component as the velocity offset (blue, dashed line). Right panel: Horizontal flow velocities in m/s from the sunspot center to its surrounding area in Mm. The dashed vertical lines divide the umbral (U), penumbral (PU), moat flow (MF) and quiet Sun (QS) regions.

As the left panel of Figure 3 shows, near the sunspot, the MF velocity with 1000 m/s is highest followed by a monotonic decrease to its half value after 4 Mm and to less than 200 m/s after 9 Mm. The average MF extension lies at 9.2 ± 5 Mm, where the velocity drops below 180 m/s. As the right panel of Figure 3 shows, neither the MF velocity nor its extension depend significantly on the sunspot size or EF velocity⁴. But, the EF velocity does show a tendency to be enhanced with sunspot size³. On a time scale of a week and a month for tracked long-lasting spots, we find a decreasing MF extension and a tendency for the MF velocity to increase for strongly decaying sunspots, whereas the changing EF velocity has no impact on the MF.

Figure 3 | Left panel: Horizontal flow velocities (in m/s) in the surrounding areas (in Mm from the spot boundary) of all 51 sunspots. The black, solid line displays the average of all flow profiles. Right two panels: The correlations of the sunspot sizes (in Mm) with the maximum moat flow velocities (in km/s) as well as with the moat flow extensions (in Mm) show only small positive values. The linear regressions (red, solid lines) indicate this slight increase for bigger sunspots.

We conclude that the physical origins of EF and MF have to be distinct and support the theory that the MF is driven by a surplus gas pressure beneath the penumbra. Therefore the MF would depend on the depth of the penumbra, but not on the spot size.

For more details of this work, please refer to our refereed paper ref[3].

References

[1] Schlichenmaier, R. & Schmidt, W., 2000, A&A, 358, 105
[2] Vargas Domínguez, S., Bonet, J. A., Martínez Pillet, V., et al., 2007, ApJ, 660, 165
[3] Löhner-Böttcher, J. & Schlichenmaier, R., 2013, A&A, 551, 105
[4] Sobotka, M. & Roudier, T., 2007, A&A, 472, 277