Two-dimensional observations of the velocity field in and around sunspots

Abstract

Doppler spectroheliograms of sunspots and their surroundings have been obtained with a spatial resolution approaching one second of arc and a time resolution of 20 s per frame. Observations of 5 sunspots, located 18 ~ 45 ~ 56 ~ 60 ~ and 72 ~ from the disk center respectively, showed considerable long-lived fine structure and, in particular, indicated the following:(1) The Evershed outflow terminated in spoke-like structures that constitute the ragged outer boundary of the penumbra. Some of these spokes extended more than 8000 km beyond the average outer boundary.(2) Although there was considerable long-lived fine structure of both Doppler polarities in the extra-penumbral photosphere, the spatially-averaged horizontal flow was outward for roughly I0000 km beyond the outer boundary of the penumbra. This extra-penumbral velocity field was distinct from the Evershed flow, and, in particular, did not represent its extension beyond the end of the penumbral spokes.Although these results are based on observations of relatively few sunspots, they do suggest that if magnetic fields are carried outward from sunspots by material motions, then these motions are more like the supergranulation than an extension of the Evershed velocity.

The Sun’s Supergranulation

“…In contrast, further away from the sunspot umbra, larger-scale circulations characterised by a surface inflow and a deep (> 10 Mm) outflow are inferred from helioseismic inversions. Several authors studied the structure of the moat flow using Doppler signal (Sheeley Jr and Bhatnagar, 1971;Sheeley Jr, 1972), by tracking surface features, such as granules (Muller and Mena, 1987;Shine et al, 1987) or small-scale magnetic elements (Sheeley Jr, 1972;Harvey and Harvey, 1973;Hagenaar and Shine, 2005), or using helioseismology . One of the conclusions of these studies is that the outflow has properties similar to those of supergranulation (see notably Brickhouse and Labonte, 1988), albeit with a larger velocity ∼ 1 km s -1 .…”

Properties of sunspot moats derived from horizontal motions

“…He termed this region a moat. From Doppler spectroheliograms, Sheeley & Bhatnagar (1971) found radially oriented outflows in moats. Sheeley (1972) reported the Doppler speed of the outflows 0.5-1.0 km s −1 for distances 10-20 Mm beyond the outer boundary of the penumbra and suggested that these motions carry small-scale fragments of magnetic flux away from sunspots.…”

Phase diversity restoration of sunspot images

“…The magnetic flux is then carried away to the surrounding network throughout the moat (Sheeley 1969;Vrabec 1971;Harvey & Harvey 1973;Wallenhorst & Topka 1982;Muller & Mena 1987;Ryutova et al 1998) which is an annular cell of systematic horizontal outflows, 0.5 to 1 km s −1 , surrounding sunspots in late phases of evolution (Sheeley & Bhatnagar 1971;Sheeley 1972;Muller & Ména 1987). Soon after the discovery of this flow, which is very similar to that found in supergranules, Sheeley (1972) suggested that a decaying sunspot occupies the centre of a supergranular cell, and that small-scale fragments of magnetic flux are carried away from sunspots by the supergranular flows.…”