6. Perpetual Change
Where do the Sun's winds come from?
The solar wind has never stopped blowing during the more than three decades that it has been observed with spacecraft. Two winds are always detected Ė a fast, uniform wind blowing at about 750 thousand meters per second, and a variable, gusty slow wind, moving at about half that speed.
One method of studying the velocities and origin of the solar wind is called radio scintillation. This technique uses two or more radio telescopes to observe very distant, cosmic radio sources that fluctuate, or scintillate, when their radio waves pass through the solar wind. Optically visible stars similarly twinkle when seen through the Earthís wind-blown atmosphere. The velocity of the solar wind can be inferred from the time it takes the fluctuating radio signal to move between two telescopes. You could similarly determined the speed of a wind-blown cloud by seeing how long it takes for its shadow to move along the ground.
Yet, until recently, spacecraft measurements were always made near the ecliptic, and the radio scintillation data only hinted at what the flows looked like above the Sunís poles. Then, in 1994-95 the Ulysses spacecraft made measurements all around the Sun, at a distance comparable to that of the Earth and near a minimum in the Sunís 11-year activity cycle. Ulyssesí velocity data conclusively proved that a relatively uniform, fast wind pours out at high latitudes near the solar poles, and that a capricious, gusty, slow wind emanates from the Sunís equatorial regions (Fig. 6.20).
As the winds blow away, they must be replaced by hot gases welling up from somewhere on the Sun. However, since Ulysses never passed closer to the Sun than the Earth does, simultaneous observations with other satellites were required to tell exactly where the winds come from. Fortunately, the Ulysses data were obtained near activity minimum with a particularly simple corona characterized by marked symmetry and stability. There were pronounced coronal holes at the Sunís north and south poles, and its equator was encircled by coronal streamers.
Comparisons of Ulyssesí high-latitude passes with Yohkoh soft X-ray images showed that coronal holes were then present at the poles of the Sun, as they usually are during activity minimum. Much, if not all, of the high-speed solar wind therefore seems to come from the open magnetic fields in coronal holes, at least during the minimum in the 11-year cycle of magnetic activity. The coronal holes have comparatively weak and open magnetic fields that stretch radially outward with little divergence, providing a fast lane for the electrified wind. The high-speed solar wind squirts out of the nozzle-like coronal holes, like water out of a fire hose.
The detailed structure a polar coronal hole has only recently been investigated by the SUMER instrument on SOHO. It showed that the high-speed outflow is concentrated at the boundaries of the magnetic network formed by underlying supergranular convection cells (Fig. 6.21). These edges are places where the magnetic fields are concentrated into inverted magnetic funnels that open up into the overlying corona. The strongest high-speed flows apparently gush out of the crack-like edges of the network, like grass or weeds growing in the dirt where paving stones meet. Thus, SOHO has for the first time discovered one of the exact sources of the fastest winds.
It was once thought that polar plumes might be the main source of the high-speed wind. Since the long, narrow features are the brightest things around in the dark coronal holes, something had to be energizing them (Fig. 6.22). However, careful SOHO measurements indicate that the fast winds are pouring out of the entire coronal hole, with no substantial difference between the narrow plumes and the inter-plume regions. The honeycomb-shaped boundaries of the magnetic network are apparently the main localized source of the high-speed wind in coronal holes.
Comparisons of Ulysses data with coronagraph images pinpointed the equatorial streamers as the birth place of the slow and sporadic wind during the minimum in the 11-year activity cycle. Hot gas is bottled up in the closed coronal loops at the bottom of the helmet streamers. The capricious slow wind can therefore only leak out along elongated, stretched-out streamer stalks. The part that manages to escape seems varies in strength as the result of the effort.
Coronal loops are found down at the very bottom of streamers, and the expansion of these magnetized loops may provide the energy and mass of the slow component of the solar wind. Sequential soft X-ray images, taken from the Yohkoh spacecraft, have shown that magnetic loops expand out into space, perhaps contributing to the slow wind. When viewed at the Sunís apparent edge, near the photosphere, coronal loops are seen rising upwards at speeds of some tens of thousands of meters per second (Fig. 6.23).
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Copyright 2010, Professor Kenneth R. Lang, Tufts University