Michal Švanda1,2, Michal Sobotka1, Tomáš Bárta2
1. Astronomical Institute, Academy of Sciences of the Czech Republic (v. v. i.), Ondřejov, Czech Republic
2. Astronomical Institute of Charles University, Prague, Czech Republic
Sunspots are probably the most intensively studied topic of solar physics. The strong magnetic field, which is responsible for the appearance of sunspots and their evolution, significantly affects the pattern of convection and plasma flows in the upper layers of the solar convection zone. Sunspots mainly suppress an upward propagation of the heated plasma in their cores (umbrae) and harbor strong radial outflows in their penumbrae, usually termed the Evershed flow. Even though not directly seen, penumbrae of evolved sunspots are usually surrounded by an additional outflow region called a moat. This intriguing region around sunspots seems to be present mostly around evolved and decaying spots and plays a role in transporting flux away from the spot, hence contributing to its decay. The amplitude of the moat flow is usually about 500 m s-1. The moat seems to be present only on the side of the spots where the penumbra exists. The connection of the Evershed flow to the moat flow in unclear.
Attempts to explain the observed properties of the moat flow were seen in the literature shortly after the discovery, when the observers pointed to the resemblance of the moat flow and supergranular flows. Supergranulation is a mid-scale convection-like velocity pattern with cells having a typical size of ~30 Mm and a lifetime of around one day (recently e.g., Ref ) covering all the solar surface. Indeed, the predominantly horizontal outflow velocity field within supergranules with a peak flow of around 500 m s-1 is reminiscent of the outflow in the moat. Studies showed that the moat flow was usually faster than an average supergranular flow and moats also lived longer (for several days) than the ordinary supergranules. Already in 1970s scientists suggested that the moat flow was essentially a supergranular flow with a sunspot in the middle of the cell.
Recently, we compared the structure of the vector flow in the sunspot moats with the flow in a typical supergranular cell. The time-distance helioseismology applied to a set of HMI Dopplergrams was a principal method used to measure all three components of the flow vectors in the near-subsurface layers (roughly scanning depths of 0 – 3 Mm) of the solar convection zone. We obtained a large amount of daily vector flow maps covering the central-meridian patch over three years of observations, which contained the flow systems belonging to the quiet-Sun supergranules, and also flows around sunspots.
For a detailed comparison of the flow systems we constructed the typical representatives of both flow phenomena by employing an ensemble averaging scheme. In the maps we detected more than 220,000 individual supergranules using an automatic recognition code based on a watershed algorithm. We further isolated an ensemble of 104 individual sunspots of McIntosh type H, which were at least 10 heliographic degrees far from other magnetic field concentrations. The type H represents the evolved monopolar axisymmetric (“round”) sunspots with symmetrical penumbra, where we assume also the moat to be symmetrical. All sunspots were normalized to the penumbra radius of 10 Mm by a shift of the radial coordinate. The flows of all the representatives of both phenomena were averaged. The ensemble averaging serves mainly to suppress the independent realizations of the random noise. The level of random noise decreases with a square-root of the number of representatives, thus allowing to have the signal-to-noise ratio of all flow components (including the weak vertical one) well above unity.
Figure 1 |Comparison of the flow components around an average H-type sunspot (two concentric black circles, inner for umbra, outer for penumbra) and an average supergranule for reference. Maps of (a) radial, (b) tangential, and (c) vertical flow components around an average H-type spot and corresponding maps of (d) radial, (e) tangential, and (f) vertical flow components in the average supergranule show a clear distortion of the horizontal moat outflow due to the westward proper motion of the sunspot with respect to the local frame of rest (a). The moat shows up clearly as a downflow region (c). The tangential component is positive in the counter-clockwise direction. The sector-like structure of the tangential component of the horizontal flow in the average supergranule (e) is an artifact caused by the slightly elliptical shape of the inversion averaging kernel in the horizontal domain.
From our comparison of statistically significant samples, it turned out that the moat flows around symmetric H-type spots and the outflows within the supergranular cells are similar. There are, however, two principal differences.
1.While the outflow region within the average supergranular cell is very symmetric about the center of the cell, the moat outflow region displays a clear asymmetry in the east-west direction. Such an asymmetry was already found in Ref . The moat outflow is distorted, due to a viscosity of the gas, by the proper motion of sunspots to the west. The mass conservation in both the distorted radial outflow and a nearly symmetrical downflow is kept due to the asymmetry in the tangential component of the flow. The radial outflow is redirected (by the sunspot’s proper motion) around the spot, first to the north and south and then eastward.
2.The vertical components of both flow phenomena are different. Within the supergranular cell, there is an upflow near its center which turns into a downflow around 60% cell radius from the cell center. The moat is a pure downflow region extending from the penumbral boundary by about 12 Mm where it is adjacent to downflows at the borders of neighboring supergranules.
The average distance of the surrounding supergranules from the spot center is 40 Mm, which is only slightly larger than the average distance between centers of neighboring supergranular cells (38 Mm). This would favor the hypothesis that an isolated medium-sized symmetrical sunspot and the flow system around it (the moat flow) act, on average, as a larger supergranular cell.
To summarize, the moat is an intriguing structure closely related to the formation and evolution of sunspots. Measurements such as ours provide firm constraints to numerical models of sunspots, which aim to study and explain the magnetic activity of our Sun.
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