The convection power spectrum in the photosphere obtained from Doppler measurements peaks at granulation scales (), with a secondary peak at , corresponding to supergranulation (Hathaway, 1996b; Hathaway et al., 2000). At lower wavenumbers, the velocity spectrum appears to drop off nearly linearly: .
Evidence for a dramatically different giant cell structure has been presented by Lisle et al. (2004). They study the supergranulation pattern using correlation tracking and find a tendency for north-south alignment of supergranular cells. Such an alignment would be expected if the supergranulation were advected by larger-scale, latitudinally-elongated lanes of horizontal convergence such as those commonly seen in numerical simulations of solar convection (Section 6.2). Advection by such structures may also help to explain why the supergranulation pattern appears to rotate faster than the surrounding plasma (Lisle et al., 2004).
The most substantial recent advance in the search for large-scale non-axisymmetric motions in the solar envelope has been the mapping of horizontal flows by local helioseismology, as shown in Figure 3. After subtracting out the contributions from differential rotation and meridional circulation, the residual flow maps reveal intricate, evolving flows on a range of spatial scales (Haber et al., 2002; Zhao and Kosovichev, 2004; Komm et al., 2004; Hindman et al., 2004). Such flow patterns have become known as solar subsurface weather, SSW (Toomre, 2002).
The inferred SSW patterns show a high correlation with magnetic activity, becoming more complex at solar maximum. Near the surface, strong horizontal flows converge into active regions and swirl around them, generally in a cyclonic sense (counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere). Deeper down, roughly 10 Mm below the photosphere, the pattern reverses; here flows tend to diverge away from active regions (Zhao and Kosovichev, 2004).
The distribution and relative amplitude of horizontal divergence and vertical vorticity can provide insight into the nature of the flows and can be used to make contact with theoretical and numerical models. Komm et al. (2004) compute the divergence and vorticity fields from SSW flow maps along with other flow descriptors including the vertical velocity (obtained from the mass continuity equation) and vertical gradients of horizontal flows. The results again show a strong correlation with active regions which are associated with cyclonic vorticity, converging flows, and large velocity gradients.
Other flow diagnostics which can in principle be deduced from SSW flow maps include the horizontal Reynolds stress component (see Section 4.3). Although such quantities have not yet been investigated in detail with helioseismic measurements, they have been measured to some degree using sunspots as tracers. The results yield small but generally positive values, indicating equatorward angular momentum transport (Stix, 2002; Nesme-Ribes et al., 1997).
In addition to giant convective cells, large-scale, non-axisymmetric flow patterns may also arise from wave phenomena. A familiar example is the acoustic wave spectrum which forms the basis of helioseismology. There is also some evidence for the presence of Rossby wave modes or, more generally, inertial oscillations. Ulrich (2001) has interpreted long-lived features in photospheric Dopplergrams as a hierarchy of inertial oscillations with longitudinal wavelengths . Some hints of these patterns can also be seen in SSW flow maps (Haber et al., 2002). Further evidence for Rossby waves on the Sun has been reported by Kuhn et al. (2000) and Lou (2000). Gizon et al. (2003) have suggested that supergranulation patterns may also exhibit wavelike behavior although this has been disputed by Rast et al. (2004).
© Max Planck Society and the author(s)