By Ron Cowen
A spacecraft devoted to studies of the sun has spotted clouds of gas that seem to be headed the wrong way. The clouds are falling back toward the sun, against the rapidly outflowing streams of ionized gas known as the solar wind.
According to a new report, the Earth-orbiting Solar and Heliospheric Observatory (SOHO) has observed some 8,000 of these inflowing clouds. The researchers have determined that the sun’s gravity isn’t the primary force pulling the clouds back.
In the Nov. 20 Astrophysical Journal Letters, Neil R. Sheeley and Y.-M. Wang of the U.S. Naval Research Laboratory in Washington, D.C., suggest that it’s the sun’s magnetic field that drags the clouds of gas back toward the sun.
Magnetic fields carpet the region just above the sun’s surface, with some magnetic field lines looping out into space and others looping back in toward the sun. Explosive outpourings of gas, as well as the steady, outgoing solar wind, stretch some of these loops far into space. When the loops stretch out this way, adjacent magnetic field lines near the solar surface get closer together. These adjacent field lines point in opposite directions.
When the opposing fields meet, they combine to form new loops. These loops are less elongated and collapse back toward the sun, dragging with them some of the solar wind’s outflowing gas, Sheeley and Wang suggest.
They conclude that the continual restructuring of magnetic fields on the sun–from a highly stretched configuration to shorter, more compact loops–constitutes a regular route by which some gas high in the sun’s atmosphere returns to the sun. Although astronomers have for decades observed glowing arches of gas rise and fall in the sun’s lower atmosphere, the inflowing clouds begin their descent much farther up, at altitudes as high as about 3 million kilometers, twice the sun’s diameter.
The infalling gas clouds are the first visible sign that the stretched magnetic fields are closing back down, explains solar physicist Barbara J. Thompson of NASA’s Goddard Space Flight Center in Greenbelt, Md.
To view these wispy inflows at the sun’s periphery, Sheeley and Wang used a coronagraph, a device that blocks the bright light from the sun’s visible surface. Even with this device on SOHO, the researchers could discern the inflowing gas only by subtracting one electronic image from the next, a process that reveals subtle changes in gas movement.
