INFO 39-1997: Solar mystery nears solution with data from SOHO spacecraft
14 November 1997
A likely solution to one of the major mysteries of the Sun has emerged from recent observations with the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) mission. The new findings seem to account for a substantial part of the energy needed to cause the very high temperature of the corona, the outermost layer of the Sun's atmosphere, which becomes visible to the naked eye only during a total solar eclipse.Since the corona's temperature was first measured 55 years ago, scientists have lacked a satisfactory explanation for why that temperature is over one million degrees while the visible surface of the Sun is only about 6,000 degrees Celsius. According to the laws of physics, thermal energy cannot flow from the cooler surface to the much hotter corona, so the energy transfer has to be in the form of waves or magnetic energy, but no measurement to date had found adequate energy to account for the corona's high temperature.
"We now have direct evidence for the upward transfer of magnetic energy from the Sun's surface toward the corona above. There is more than enough energy coming up from the loops of the "magnetic carpet" to heat the corona to its known temperature", said Dr. Alan Title of the Stanford-Lockheed Institute for Space Research, Lockheed Martin Advanced Technology Center, Palo Alto, CA, who led the research. "Each one of these loops carries as much energy as a large hydroelectric plant, such as the Hoover dam, generates in about a million years !"
"We now appear to be closing in on an explanation as to why the solar corona is over 100 times hotter than the solar surface - the solution to a 55-year old puzzle," said Dr. George Withbroe, Director of the Sun-Earth Connection Programme at NASA Headquarters, Washington DC, "These results underline the importance of long-term study of the changing conditions on the Sun from the superior vantage point of space."
Energy flows from the loops when they interact, producing electric and magnetic "short circuits". The very strong electric currents in these short circuits are what heats the corona to a temperature of several million degrees. Images from the Extreme ultraviolet Imaging Telescope (EIT) and the Coronal Diagnostics Spectrometer (CDS) on SOHO show ever-changing brightenings of the hot gases in the corona in response - it is assumed -to the evolving magnetic fields rooted in the solar surface.
The observations with SOHO's Michelson Doppler Imager (MDI) provided long-duration, highly detailed, and well calibrated time-lapse movies of the magnetic fields on the visible surface or photosphere of the Sun. These revealed the rapidly changing properties of what Title calls "the Sun's Magnetic Carpet" a sprinkling of tens-of-thousands of magnetic concentrations; These concentrations have both north and south magnetic poles, which are the "foot points" of magnetic loops extending into the solar corona.
Like field biologists who study the populations and life cycles of animal herds, the SOHO researchers analysed the appearances and disappearances of large numbers of the small magnetic concentrations on the solar surface. "We find that after a typical small magnetic loop emerges, it fragments and drifts around and then disappears in only 40 hours," Title said, "It's very hard to understand how such a short-lived effect could be driven by the magnetic dynamo layer that is over 150 000 km beneath the surface of the Sun. This may be evidence that unknown processes are at work in or near the solar surface that continuously form these loops all over the Sun."
Professor Phillip Scherrer of Stanford University is the MDI Principal Investigator. MDI was built at the LM Technology Center and is a project of the Stanford-Lockheed Institute for Space Research. The EIT and CDS instruments were built by international consortia under the leadership of, respectively, Dr. Richard A. Harrison of Rutherford-Appleton Laboratory near Oxford (UK) and Dr. Jean-Pierre Delaboudiniére of the Institut d'Astrophysique spatiale in Orsay (F).
The new observations were made with these three instruments on SOHO, a spacecraft stationed 1.5 million Kilometres sunward of the Earth in interplanetary space, where it has an uninterrupted view of the Sun and of the solar wind particles blown from the Sun. The SOHO satellite developed by ESA and built by European industry is operated from a control center at NASA's Goddard Space Flight Center, Greenbelt, MD. SOHO was launched on 2 December 1995 aboard an Atlas-IIAS expendable launch vehicle from Kennedy Space Center.
