Michelson Doppler Imager (MDI) Main Page.
THE SUN DOES THE WAVE:
Wave-like Properties of Solar Supergranulation
Supergranulation on the surface of the Sun is a pattern of horizontal outflows with a distinct scale of 30,000 km and a lifetime of about 2 days, outlined by a network of small magnetic features. It is believed that supergranulation corresponds to a preferred cellular scale of thermal convection, and that rising magnetic fields are dragged by the flows and concentrate into ropes at the cell boundaries. However, the solar convection zone is so highly turbulent and stratified that numerical modeling of supergranulation has remained elusive. The dynamics of the supergranulation is not understood and there was no explanation for the observation that the pattern appears to rotate faster around the Sun than the magnetic features.
Using data from the MDI experiment onboard the SOHO satellite,
Stanford scientists report that supergranulation undergoes oscillations and supports waves with periods of 6-9 days. Waves are predominantly prograde, which explains the super-rotation of the pattern.
Thus, supergranulation appears to be travelling wave convection.
Solar scientists are hopeful that these clues about
supergranulation behavior will also clarify the mysterious nature of
supergranules themselves. Supergranules remain mysterious
because there is no explanation for their characteristic size of
30,000 kilometers. The depth of the supergranulation layer is also
unknown. A good understanding of
supergranulation would help understand how magnetic fields are
transported and dispersed near the solar surface. Understanding
the dynamics of solar magnetism is important because scientists
believe rapid changes in solar magnetic fields power violent solar
activity, like flares and coronal mass ejections.
The results are reported by Laurent Gizon, Tom Duvall and Jesper Schou
in the paper "Wave-like Properties of Solar Supergranulation"
published in the January 2 issue of the journal Nature (vol. 421, pp. 43-44, 2003).
Erratum: the units on the x-axis of Fig. 1 should be microHz.
You may download a preprint of the paper in postscript
or pdf format. The paper includes figures.
The NASA press release (including movies and images) is at
The ESA press release (including movies and images) is at
SOHO Hot Shot.
Additional documents can be downloaded at
http://soi.stanford.edu/press/Nature_2Jan03 (this page).
Past MDI press releases can be found at
SOHO is a mission of international cooperation between
Solar and Heliospheric Observatory (SOHO) Main Page.
The following images and movies help understand the results.
|Topic||Sample Image||Image or Video links|| Notes
| MDI/SOHO movie of solar supergranulation
| Supergranules are large convection cells.
The "smooth" area near the center of the images is where the supergranules
do not contribute to the signal: their motion is mostly horizontal and
MDI measures only the component of motion directed towards or away from SOHO.
Supergranules flow outwards from their centers, with a typical velocity of
| Animation showing travelling-wave convection
Although there is no net plasma flow in any direction,
the pattern appears to propagate to the right of the picture.
This animation illustrates how the super-rotation
of superganules is really just an illusion.