2007-12-08:

Publication in Science: HINODE findings explain forces that power the solar wind

Oslo, December 7, 2007 - In a paper published today in the journal Science, researchers from the Institute of Theoretical Astrophysics, University of Oslo, along with colleagues at other institutions in California, Colorado, and Japan have described new observations from the Solar Optical Telescope (SOT) on the Japanese Hinode satellite that provide new insight to the mechanisms that generate the solar wind.

For years, many solar physicists have believed that Alfvén waves - created from convective motions and acoustic energy jostling solar magnetic fields - were most likely the force powering the solar wind. But before the SOT on Hinode, the resolution of solar instruments was not sufficient to resolve the mystery.

"The high spatial and temporal resolution of SOT - imaging solar structure just 150 km across every five seconds - enabled us to resolve, for the first time, the predicted amplitudes of the Alfvén waves, as well as some of the dominant spatial and temporal scales of the chromosphere - the region sandwiched between the solar surface and the sun's atmosphere or corona," said Dr. Bart De Pontieu, a solar physicist at Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, California, who led the research team that produced the results reported today in Science. "We found that the chromosphere is permeated with Alfvén waves that are energetic enough to accelerate the solar wind," continued De Pontieu.

It was the behavior of spicules in the sun's chromosphere - waving from side to side like a field of wheat in a gentle breeze - which heralded the presence of the elusive Alfvén waves in the SOT images. Spicules are jets of gas or plasma propelled upwards from the surface, shooting into the atmosphere at supersonic speeds of 100,000 miles per hour, and reaching heights of 5,000 miles above the solar surface in less than five minutes. Spicules outline the direction of the sun's magnetic field, so that their oscillations indicate the passage of Alfvénic wave motions. "Most of the Alfvén waves we observe have periods of several minutes, much longer than many theoretical models have assumed in the past," said Dr. Scott McIntosh, a solar physicist at Southwest Research Institute in Boulder, Colorado.

Additionally, the team used high fidelity computer simulations performed on the Norwegian national High-performance computing infrastructure (NOTUR) to investigate how the waves are generated. "Our simulations imply that it is the bumping and jostling of the magnetic fields in the lower atmosphere that is driving these Alfvén waves, which then impart their energy to the solar wind," said Prof. Mats Carlsson of the Institute of Theoretical Astrophysics at the University of Oslo, Norway.

These findings, along with several other reports, appear in the December 7 issue of Science. This special issue features the latest scientific findings from Hinode, an international mission led by Japan to study the sun's magnetic field and how its explosive energy propagates through the different layers of the solar atmosphere. Hinode is a Japanese mission developed and launched by the Japanese Space agency with NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with the European Space Agency and the Norwegian Space Center.

Links:
Institute of Theoretical Astrophysics, University of Oslo
Science Magazine