Small-scale chaos heats solar wind

Spatial and temporal observations of solar wind have revealed small scale, rapid variations in magnetic fields. Recent measurements of solar wind by the European Space Agency reveal temperatures hotter than would be expected for simple channel flow.

“For the first time, we were able to obtain direct evidence for the existence of current sheets at these very small scales, where dissipation of magnetic energy into heat is thought to occur.” This statement comes from Melvyn Goldstein, project scientist for Cluster at NASA’s Goddard Space Flight Center. Goldstein is co-author of a paper regarding this work, published in the November 2012 issue of Physical Review Letters.

Areas of turbulence are termed ‘current sheets’, in which turbulent flow has developed and evolved into thin sheets of electrical current, which themselves separate regions of orientated magnetic field. Solar wind comprises electrically charged plasma made up of electrons and protons. These particles are ejected from the lower atmosphere of the Sun, and travel as far as the edge of our solar system. The Sun ejects these particles continuously at 250 miles per second and the plasma entrains the solar magnetic wind with it.

As the plasma travels through the solar system it expands and cools, although it is postulated that the observed turbulent flow adds heat to surroundings via energy dissipation from the magnetic field. There are also areas where the magnetic lines may break and reconnect. Further research by NASA is in the pipeline, with the aim of describing this process accurately.

The NASA study regarding solar wind used high-resolution Spatio Temporal Analysis Field Fluctuations (STAFF). This detects rapid magnetic field variations, so small-scale variations may be observed. Three spacecraft were used to gather these data and allowed current sheet structures to be observed as discrete, fast-moving and localised events within the solar wind. Although the solar wind appears to have a fairly regular large-scale trend, the small-scale turbulent structures demonstrate the release of energy which causes observable heating of the solar wind.


About Helen Ashcroft

Helen is studying for her DPhil in Earth Sciences.