Solar Wind

The conductivity of a hot ionized plasma is extremely high, and the coronal temperature decreases only as the 2/7 power of the distance from the Sun. Thus, the temperature of the interplanetary medium is still more than 200,000 K (i99,800°C, 360,000°F) near Earth. While the gravitational force of the Sun can hold the hot material near the surface, at a distance of 5-Rq the gravitational force is 25 times less, but the temperature is only 40 percent less. Therefore, except where hindered by magnetic fields, a continuous outflow of particles known as the solar wind occurs, except where hindered by magnetic fields.

The solar wind flows along a spiral path dictated by magnetic fields carried out from the Sun into the interplanetary medium. The velocity is typically 400 km/sec (250 miles/sec), with wide variations. Where magnetic fields are strong, the coronal material cannot flow outward and becomes trapped; thus the high density and temperature above active regions is due partly to trapping and partly to heating processes, mostly solar flares. Where the magnetic field is open, the hot material escapes, and a coronal hole results. Analysis of solar wind data shows that coronal holes at the equator are associated with high-velocity streams in the solar wind, and recurrent geomagnetic storms are associated with the return of these holes. The coronal material is thought to originate from spicules.

The solar wind drags magnetic field lines out from the surface. Traveling at a speed of 500 km/sec (300 miles/sec), particles will reach the orbit of Saturn in one solar rotation—27 days—but in that time period the source on the Sun will have gone completely around. In other words, the magnetic field lines emanating from the Sun describe a spiral. It takes four days for the solar wind to arrive at Earth, having originated from a point that has rotated about 50° west (13° per day) from its original position facing Earth. The magnetic field lines, which do not break, maintain this path, and the plasma moves along them. The solar-wind flow has a continual effect on the upper atmosphere of Earth. The total mass, magnetic field, and angular momentum carried away by the solar wind is insignificant, even over the lifetime of the Sun. A higher level of activity in the past, however, might have played a role in the Sun's evolution, and stars larger than the Sun are known to lose considerable mass through such processes.

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