6. Perpetual Change
Getting up to speed
By the time that the solar wind has reached the Earth, it is moving along at supersonic velocities of hundreds of thousands of meters per second. What forces propel it to such high velocities? The expansion of the hot corona is responsible for some of it.
The corona’s expansion will begin slowly near the Sun, where the solar gravity is the strongest, and then continuously accelerate out into space as it breaks away from the Sun, gaining speed with distance and reaching supersonic velocities. Since there is a limit to the amount of energy being pumped into it, the solar wind will eventually reach a limiting asymptotic or terminal velocity, and then cruise along at a roughly constant speed.
The slow wind naturally reaches terminal velocities of a few hundred thousand meters per second as the million-degree corona expands away from the Sun. Additional energy must be deposited in the low corona to give the fast wind an extra boost and double its speed. In technical terms, the fast wind has a velocity and mass flux density that are too high to be explained by heat transport and classical thermal conduction alone.
You have to look down into the bottom of the corona to investigate the regions where the corona is heated and the solar wind is accelerated. Yohkoh X-ray observations show that the coronal electrons become fully heated at a height of between 0.2 and 0.5 solar radii, or between 140 and 348 million meters, above the photosphere (Fig. 6.24). In addition, the electron temperatures in coronal holes are several hundred thousand degrees cooler than the temperatures of electrons in coronal streamers at the same height. Ulysses measurements of ion temperatures also indicate that the fast polar wind originates in a relatively low-temperature region in the corona.
Images from SOHO’s Large Angle Spectrometric Coronagraph, or LASCO, suggest that the slow wind takes a long time to get up to speed (Fig. 6.25). Blobs moving along the stalks of helmet streamers have to move out to 20 or 30 solar radii from Sun center to accelerate to speeds of 300 or 400 thousand meters per second. In contrast, the high-speed wind is accelerated relatively close to the Sun. Radio scintillation measurements indicate that the polar wind reaches terminal speeds of 750 thousand meters per second within just 10 solar radii or less (Fig. 6.26). Thus, the fast wind accelerates quickly, like a racing horse breaking away from a starting gate.
Another SOHO coronagraph, known as UVCS for UltraViolet Coronagraph Spectrometer, has measured temperatures and velocities within the source regions of the solar wind from 1.2 to 10 solar radii from Sun center. It has used the Doppler shifts of ultraviolet spectral lines to show that the high-speed solar wind, emerging from coronal holes, accelerates to supersonic velocity within just 2.5 solar radii from Sun center.
SOHO’S UVCS has additionally demonstrated that heavier particles in polar coronal holes move faster than light particles in coronal holes. Above two solar radii from the Sun’s center, oxygen ions have the higher outflow velocity, approaching 500 thousand meters per second in the holes, while hydrogen moves at about half this speed (Fig. 6.27). In contrast, within equatorial regions where the slow-speed wind begins, the lighter hydrogen moves faster than the oxygen, as one would expect for a gas with thermal equilibrium among different types of particles.
The amazing thing is that the heavier oxygen ions move faster than the lighter hydrogen in coronal holes. That violates common sense. It would be something like watching people jogging around a race track, with heavier adults running much more rapidly than lighter, slimmer youngsters. Something is unexpectedly and preferentially energizing the heavier particles in coronal holes.
Magnetic waves might preferentially accelerate the heavier ions by pumping up their gyrations around the open magnetic fields. More massive ions gyrate with lower frequencies where the magnetic waves are most intense, thereby absorbing more magnetic-wave energy and becoming accelerated to higher speeds.
The ponderous magnetic waves remind us of the waves in a stormy ocean that push heavy logs to shore. The lighter shells twist and spiral about in the pounding surf, rarely reaching the beach. That is why the heaviest debris is sometimes found left on the beach after high tide.
The magnetic waves probably block high-energy cosmic rays coming into the Sun’s polar regions, repelling them back into outer space (Fig. 6.28). The incoming cosmic rays meet an opposing force, like a swimmer entering the surf on a distant shore or one trying to swim upstream against the current of a powerful river. To put it in more scientific language, the Alfvén waves are very long, so they can resonate with the energetic cosmic rays and oppose their entry into the polar regions.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University