Stanford University Department of Physics - Reversing Our Ideas on the Sun's Corona

Reversing Our Ideas on the Sun's Corona

Extending outward from the visible surface of the Sun is a tenuous, high-temperature atmosphere known as the corona. How the Sun maintains a million-degree gas above its cool (6000 K) surface has remained a puzzle to solar physicists since the recognition of this fact in the late 1930's. To date, no consensus has emerged concerning the coronal heating problem, although it is accepted that the source of the energy must be turbulent motions below the photosphere, and that a fraction of this energy must be transported, in non-thermal form, to the corona where it is dissipated. Competing theories include wave transport, heating by steady currents, and heating by many small scale, flare-like events, but all coronal heating models have shared a common perspective. It has been the standard view that the deposition of nonthermal energy occurs low in the corona, in a region called the "inner corona," and that this region in turn supplies heat to the upper corona and to the "solar wind," a term used to represent the continuous expansion of the corona into interplanetary space. This standard picture may need revision, in view of results of recent studies by Dr Michael Wheatland and Professor Peter Sturrock at Stanford University and Professor Loren Acton at Montana State University. Their research indicates the possibility that the solar wind may supply heat to the inner corona rather than the other way around.

Wheatland, Sturrock and Acton have analyzed soft X-ray images of the Sun's corona obtained from the Japanese Yohkoh spacecraft using the Soft X-ray Telescope (SXT) that was designed and constructed at the Lockheed-Martin Palo Alto Research Laboratory under the direction of Acton and Dr. James Lemen. Since the launch of Yohkoh in August 1991, the SXT has produced a record of the Sun's corona of unprecedented beauty and detail. Working with this database, Wheatland, Sturrock and Acton have examined two regions of diffuse corona imaged at the Sun's limb, which are visible out to 0.7 and 0.95 solar radii above the limb. By modelling the radial variation of temperature in these regions, they have tried to identify where the non-thermal energy that heats these regions is deposited. Their results have been surprising: in both regions the temperature increases steadily with height, indicating that non-thermal energy is deposited beyond the observed range of heights and is then conducted back down along magnetic field lines, in the form of heat, to the inner corona. There is no evidence of nonthermal heating in either the observed regions or in the inner corona.

These results favor a coronal heating mechanism involving the deposition of nonthermal energy at great heights in the corona. The regions under examination are believed to consist of gas constrained in extended magnetic loops. It appears that the energy deposition occurs at or near the tops of these loops. Sturrock has recently produced a theoretical model in which coronal heating is due to "sudden magnetic relaxation," in which magnetic field lines suddenly "snap," like the string of a bow when it is released. According to this model, most heating will occur at the tops of loops or in the solar wind itself. These new results, from data analysis and from analytical theory, indicate the need to re-think the standard picture of the corona-solar-wind system.

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