4.3 Giant cells
Reports on velocity field structures significantly larger than supergranulation were sporadic at best
(Cram et al., 1983; Hathaway et al., 1996; Ulrich, 1998) until SOHO/MDI made detection of such
long-lived and large scale features possible (Beck et al., 1998a,b). Data from SOHO/MDI also made a
spherical harmonic analyses possible; Figure 4 of Hathaway et al. (2000) demonstrates conclusively that
there is long-lived power at scales corresponding to spherical harmonic wave numbers ; i.e., on
scales larger than 100 Mm (cf. also Figure 22).
Using local helioseismic techniques, flows on scales larger than supergranulation are now
routinely detected and mapped (Hindman et al., 2004b; Featherstone et al., 2004; Hindman
et al., 2004a; Featherstone et al., 2006; Hindman et al., 2006).