The Sun’s supergranulation is a large-scale coherent pattern detected in the surface layers of the quiet
Sun. The impression given by observations is that it is simply superimposed on a stochastic, highly
nonlinear background smaller-scale flow pattern, the granulation. Characterising the supergranulation
velocity pattern requires monitoring solar surface flows over long times, over wide fields of views, or over a
large set of independent observations. The properties of the supergranulation velocity field can be
summarised as follows.
- The length scale of the supergranulation flow, as given by the kinetic energy power spectrum of
the horizontal component of solar surface flows, is in the range of 20 – 70 Mm with a preferred
scale of 36 Mm (Section 4.2.1). These results come from both Dopplergrams (Hathaway
et al., 2000) and from granule tracking in wide-field high resolution image series (Rieutord
et al., 2008). The size of the field must be sufficiently large to secure the statistical convergence
of the results.
- The typical size of supergranules, defined as coherent diverging flow cells at the solar surface,
is in the range 10 – 30 Mm (Hirzberger et al., 2008). The derived average size is sensitive to
the method used to identify supergranules.
- The kinetic energy excess associated with supergranulation in the power spectrum of solar
surface flows lies on the large-scale side of the injection range of photospheric turbulence located
at the granulation scale (Section 4.2.1 and Figure 4).
- The most recent estimate of the lifetime of supergranules, based on the largest sample of
supergranules collected so far, is 1.6 ± 0.7 d (Hirzberger et al., 2008). The dispersion in the
measurements of supergranules lifetimes is also fairly large (Section 4.2.2).
- Rms horizontal velocities at supergranulation scale are of the order of 350 m s–1, while rms
vertical velocities are around 30 m s–1(Hathaway et al., 2002). As velocities depend on the
scales considered, the relation between amplitude and scale, namely the power spectrum,
provides the most suitable observable to estimate the amplitude of the supergranulation velocity
field (Section 4.2.3).
- Local helioseismology indicates that supergranules are shallow structures (Section 4.4),
possibly not deeper than 5 Mm (Sekii et al., 2007). The mean vertical profile of the
supergranulation flow is not very well constrained at the moment. More precise determinations
are definitely called for.
Note that the foregoing determinations are not independent of each other, because velocity scales can be
derived from the combination of length and time scales. Namely, 30 Mm divided by 1.7 d gives 205 m s–1,
which is in reasonable agreement with direct measurements of supergranulation-scale velocities. In our view,
the computation of the power spectra of solar surface flows provides one of the most robust methods to
make progress on the determination of these various quantities in the future. Most notably, an accurate
determination of the vertical velocity spectrum of vertical velocities in the supergranulation range is still
Besides this set of typical scales associated with the supergranulation velocity pattern, several other
observational signatures and properties of supergranulation have been studied.
- Horizontal intensity fluctuations at supergranulation scales are very faint (Section 4.3). The
latest studies indicate that supergranules are slightly warmer at their centre. The temperature
drop is less than 3 K (Meunier et al., 2007b; Goldbaum et al., 2009).
- Supergranulation is affected by the global solar rotation (Section 4.5). Locally, supergranules
are anticyclonic structures (Gizon and Duvall Jr, 2003), their mean vertical vorticity is
negative in the northern hemisphere and positive in the southern one. The supergranulation
pattern has been observed to propagate anisotropically in the prograde direction (Gizon
et al., 2003).
- Supergranulation has dynamical interactions with the magnetic fields of the quiet Sun.
Most notably, supergranules are strongly correlated with the magnetic network (Section 4.6).
Correlations between the size of supergranules and the strength of network and internetwork
fields have been evidenced recently (Meunier et al., 2007a). The solar-cycle dependence of the
pattern remains uncertain though, as various papers have been giving contradicting results.
The bounds on intensity variations seem to be well established now. The proper rotation of
supergranules, as measured by their local mean vertical vorticity, is also well constrained by local
helioseismology. On the other hand, we believe that more work is required to quantitatively constrain the
interaction of supergranules with magnetic fields. A determination of the magnetic energy spectrum of the
quiet Sun over a wide range of scales would be extremely useful to put constraints on the physical processes
at the origin of the network and internetwork fields and on their interactions with supergranulation (see
Section 8 below).
Figure 14 is an attempt to depict the standard view of the supergranulation phenomenon, as
constrained by the observations summarised above.
Figure 14: A schematic view of the supergranulation phenomenon, as constrained by observations.
is the scale where the horizontal kinetic energy spectral density is maximum. d is the diameter
of “coherent structures” (supergranules). The red and blue patches depict the warm and cold regions
of the flow. I.N.B denotes the internetwork magnetic field. Note that the indicated internetwork and
network fields geometries roughly correspond to the standard historical picture of quiet Sun magnetic
fields and their relation to supergranulation (Section 4.6). As discussed in Sections 4.6.2 and 8, this
picture must be significantly nuanced in reality, as the dichotomy between network and internetwork
fields is probably not quite as clear as indicated in this drawing.