The Eternal Solar Wind

Just half a century ago, most people visualized our planet as a solitary sphere traveling in a cold, dark vacuum around the Sun. But we now know that the wide-open spaces in our Solar System are not empty, ttey are filled with tiny pieces of the Sun, in an ever-flowing solar wind.

tte notion that something is always being expelled from the Sun first arose from observations of comet tails. Comets appear unexpectedly almost anywhere in the sky, moving in every possible direction, but with tails that always point away from the Sun. A comet therefore travels headfirst when approaching the Sun and tail-first when departing from it. Ancient Chinese astronomers concluded that the Sun must have a chi, or "life force", that blows the comet tails away. And in the early 1600s, the German astronomer Johannes Kepler (1571-1630) proposed that the pressure of sunlight pushes the comet tails away from the Sun.

Modern scientists noticed that a comet could have two tails. One is a yellow tail of dust and dirt, which can litter the comet's curved path, tte dust is pushed away from the Sun by the pressure of sunlight, tte other tail is electric blue, shining in the light of ionized particles, tte ions in comet tails always stream along straight paths away from

FIG. 6.11 Magnetic fields near and far In the low solar corona, strong magnetic fields are tied to the Sun at both ends, trapping hot, dense electrified gas within magnetized loops. Far from the Sun, the magnetic fields are too weak to constrain the outward pressure of the hot gas, and the loops are vastly extended, allowing electrically charged particles to escape, forming the solar wind and carrying magnetic fields away. (Courtesy of Newton Magazine, the Kyoikusha Company.)

FIG. 6.11 Magnetic fields near and far In the low solar corona, strong magnetic fields are tied to the Sun at both ends, trapping hot, dense electrified gas within magnetized loops. Far from the Sun, the magnetic fields are too weak to constrain the outward pressure of the hot gas, and the loops are vastly extended, allowing electrically charged particles to escape, forming the solar wind and carrying magnetic fields away. (Courtesy of Newton Magazine, the Kyoikusha Company.)

the Sun with velocities many times higher than could be caused by the weak pressure of sunlight.

In the early 1950s, the German astrophysicist Ludwig Biermann (1907-1986) proposed that streams of electrically charged particles, called corpuscular radiation, poured out of the Sun at all times and in all directions to shape the comet ion tails. Summing up his work in 1957, Biermann concluded that:

"tte acceleration of the ion tails of comets has been recognized as being due to the interaction between the corpuscular radiation of the Sun and the tail plasma, "tte observations of comets indicate that there is practically always a sufficient intensity of solar corpuscular radiation to produce an acceleration of the tail ions of at least about twentytimes solar gravity.26

ttus, the ion tails of comets act like an interplanetary windsock, demonstrating the existence of a continuous, space-filling flow of charged particles from the Sun.

So the Sun is continuously blowing itself away, filling the Solar System with a perpetual flow of electrified matter called the solar wind. And every second the Sun blows away about a million tons, or a billion kilograms, of material that must be replaced from below, but this is a small amount compared with the enormous total mass of the Sun. At the present rate, it would take ten billion years for the Sun to lose only 0.01 percent of its mass by the solar wind, and the Sun will evolve into a giant star long before it blows away completely.

ttus, the space between the planets is not completely empty; it contains an eternal solar wind, a rarefied mixture of protons and electrons that stream out radially in all directions from the Sun. tte planets move through this wind as if they were ships at sea, and the wind wraps itself around the Earth. So we live inside the Sun.

tte solar gale brushes past the planets and engulfs them, carrying the Sun's corona out to interstellar space, tte radial, supersonic outflow thereby creates a huge bubble of plasma, with the Sun at the center and the planets inside, called the heliosphere, from helios the "God of the Sun" in Greek mythology (Fig. 6.12).

We also know that the million-degree corona is so hot that it cannot stand still. Indeed, the solar wind consists of an overflow corona, which is too hot to be entirely constrained by the Sun's inward gravitational pull, tte hot gas creates an outward pressure that tends to oppose the inward pull of the Sun's gravity; and at great distances, where the solar gravity weakens, the hot protons and electrons overcome the Sun's gravity and accelerate away to supersonic speed, like water overflowing a filled bathtub or a dam.

So, the solar corona is really the visible, inner base of the solar wind, and the solar wind is just the hot corona expanding into cold, vacuous interstellar space. In 1957, geo-physicist Sydney Chapman (1888-1970) demonstrated mathematically that the Sun's million-degree corona conducts heat so well that its temperature will stay high far out into space, and that its electrons and protons must extend beyond the Earth's orbit, even if it is gravitationally held to the Sun. In the following year, Eugene Parker (1927- ), a young astrophysicist at the University of Chicago, showed that the extended corona will not only extend to the Earth, it will expand and flow out there. So Biermann's continual bombardment by solar corpuscles, which propelled comet ion tails, could be attributed to the outward expansion of Chapman's extended corona.

FIG. 6.12 The heliosphere With its solar wind going out in all directions, the Sun blows a huge bubble in space called the heliosphere. The heliopause is the name for the boundary between the heliosphere and the interstellar gas outside the Solar System. Interstellar winds mold the heliosphere into a non-spherical shape, creating a bow shock where they first encounter it. The orbits of the planets are shown near the center of the drawing.

Parker showed from hydrodynamics that the corona must expand rapidly outward because it is extremely hot, and that as the outer corona disperses, gases welling up from below will replenish it. tte hydrodynamics shows that the expansion would begin slowly near the Sun, where the solar gravity is the strongest, and then continuously accelerate outward into space, gaining speed with distance and reaching the supersonic velocities needed to account for the acceleration of comet tails, ttis would create a strong, persistent, solar wind, forever blowing at speeds of hundreds of kilometers per second throughout the Solar System.

Any doubts about the existence of the solar wind were removed by in situ (Latin for "in original place", or literally "in the same place") measurements made by instruments on board the Soviet Lunik 2 spacecraft on the way to the Moon in 1959 and by those aboard NASA's Mariner II spacecraft during its trip to Venus in 1962. tte solar wind has now been sampled for nearly half a century, and it has never stopped blowing.

Measurements from spacecraft indicate that the solar wind has a fast and slow component, tte fast, uniform wind blows at about 750 kilometers per second, and the variable, gusty slow one moves about half as fast. Both winds are supersonic, moving at least ten times faster than the sound speed in the solar wind, tte density and temperature of the slow wind are about twice those of the fast one, but both components are much more tenuous, hotter and faster than any wind on Earth.

tte solar wind has been diluted to rarefied plasma by the time it reaches the Earth. Near the Earth's orbit, there are about 5 million electrons and 5 million protons per cubic meter of the solar wind. Space probes have also shown that the magnetism entrained in the solar wind has been dragged, stretched, and enormously weakened by the time it reaches the Earth's orbit.

Since the electrified wind material is an excellent conductor of heat, the temperature falls off only gradually with distance from the Sun, reaching about 100,000 kelvin at the Earth's distance, tte Sun's wind also rushes on with little reduction in speed, for there is almost nothing out there to slow it down.

tte expansion of the solar wind combined with the Sun's rotation determines the magnetic structure of interplanetary space, and thereby establishes the magnetic pathways for energetic particles leaving the Sun. While one end of the solar magnetic field remains firmly rooted in the solar photosphere and below, the other end is extended and stretched out into space by the solar wind. As the wind streams radially outward, the Sun's rotation bends the radial pattern into a spiral shape within the plane of the Sun's equator, coiling the magnetism up (Fig. 6.13).

tte spiral magnetic field creates an interplanetary highway that can connect the site of a solar eruption, or flare, to the Earth. Energetic charged particles, that are fewer in total number and much greater in energy than those in the solar wind, are hurled out from the Sun during these brief eruptions, creating powerful gusts in the solar wind. If they occur in just the right place, near the west limb and the solar equator, the energized material will connect to the interplanetary spiral and travel along it to the Earth in about half an hour, threatening astronauts or satellites, tte spiral magnetic pattern

FIG. 6.13 Spiral path ofinterplanetary electrons The trajectory of flare electrons in interplanetary space as viewed from above the Sun's polar regions using the Ulysses spacecraft. As the high-speed electrons move out from the Sun, they excite radiation at successively lower plasma frequencies; the numbers denote the observed frequency in kiloHertz, or kHz. Since the flaring electrons are forced to follow the interplanetary magnetic field, they do not move in a straight line, but instead move along the spiral pattern of the interplanetary magnetic field, shown by the solid curved lines. The squares and crosses indicate Ulysses radio measurements of type III radio bursts on 25 and 30 October 1994. The approximate locations of the orbits of Mercury, Venus and the Earth are shown as circles. (Courtesy of Michael J. Reiner. Ulysses is a project of international collaboration between ESA and NASA.)

has, in fact, been confirmed by tracking the radio emission of charged particles thrown out during such solar flares, as well as by spacecraft that have sampled the interplanetary magnetism near the Earth.

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