6. Perpetual Change
The fullness of space
The space between the planets, once thought to be a tranquil, empty void, is swarming with hot, charged invisible pieces of the Sun. They expand and flow away from the Sun, forming a perpetual solar wind. The relentless wind was inferred from comet tails, suggested by theoretical considerations, and fully confirmed by direct in situ measurements from spacecraft in the early 1960s.
The reason that space looks so empty is that the Sunís wind is exceedingly tenuous, even at its origin near the visible Sun. By the time that it reaches the Earthís orbit, the solar wind has been further diluted by expanding into the increasing volume of space. There are about 5 million electrons and 5 million protons per cubic meter in the solar wind near the Earth. By way of comparison, there are 25 million, billion, billion (2.5 x 1025) molecules in every cubic meter of our transparent air at sea level. The density of the solar wind is so low that if we could go out into space and put our hands on it, we would not be able to feel it.
The Sunís continuous wind moves at supersonic speeds near the Earth. It travels with two main velocities, like an automobile with one high gear and one low gear. There is a fast component moving at about 750 thousand meters per second, and a slow one with about half that speed.
At large distances from the Sun, the charged particles in the solar wind drag the Sunís magnetic fields with them. While one end of the interplanetary magnetic field remains firmly rooted in the photosphere and below, the other end is extended and stretched out by the radial expansion of the solar wind. The Sunís rotation bends this radial pattern into an interplanetary spiral shape within the plane of the Sunís equator, coiling the magnetism up like a tightly-wound spring. This spiral pattern has been confirmed by tracking the radio emission of high-energy electrons emitted during solar flares (Fig. 6.17), as well as by spacecraft that have directly measured the interplanetary magnetism in the ecliptic. The ecliptic plane is the plane of the Earthís orbit around the Sun, and it is nearly coincident with the Sunís equatorial plane.
The shape of the interplanetary magnetic field depends on the Sunís 11-year cycle of magnetic activity. Near activity minimum, the large-scale, global magnetism of the Sun can be described as a simple magnet with north and south poles where large, unipolar coronal holes are located. The northern hole is of one magnetic polarity, or direction, and the southern one of opposite polarity. The negative and positive field lines meet near the solar equator, where a magnetically neutral layer, called a current sheet, is dragged out into space by the out-flowing wind (Fig. 6.18). Near the Sun, the current sheet coincides with a belt of coronal streamers that seem to meander across the star like the seam of a baseball.
Because the Sunís magnetic dipole axis is tilted with respect to its rotation axis, spacecraft near Earth detect a warped current sheet (Fig. 6.19). As the Sun rotates, the current sheet wobbles up and down, like the folds in the skirt of a whirling dervish, sweeping regions of opposite magnetic polarity past the Earth.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University