Michelson Doppler Imager (MDI) Main Page.


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 http://www.gsfc.nasa.gov/topstory/2003/0102wave.html

The ESA press release (including movies and images) is at http://sci.esa.int/

See also SOHO Hot Shot.

Media coverage

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 ESA and NASA.
  Solar and Heliospheric Observatory (SOHO) Main Page.

The following images and movies help understand the results.

TopicSample ImageImage or Video links Notes
MDI/SOHO movie of solar supergranulation  MPEG
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 400 m/s.
Animation showing travelling-wave convection  GIF
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.