The Solar Wind

The sun gradually loses mass in the form of high speed protons and electrons leaking away from the sun's out layers. This flux of particles is called the solar wind. It can be thought of as a kind of "evaporation" of particles from the corona. The corona reaches a temperature of about a million Kelvin at a distance of 10,000 km above the photoshpere. Such a hot gas would have a thermal energy of about 130 electron volts, and the mean speed for hydrogen nuclei in such a gas if viewed as having a Maxwellian speed distribution is about 145 km/s. The escape velocity from the surface of the sun is about 618 km/s, so those hydrogen atoms with average speed would not escape. Considering the nature of the speed distribution would show that there will be a few with speed above the escape velocity. Chaisson & McMillan characterize the mass loss as being about a million tons of solar matter per second. They note that at this rate, less than 0.1% of the Sun has been lost through this mechanism in its 4.6 billion year lifetime.

If a planet has a magnetic field, it will interact with the solar wind to deflect the charged particles and form an elongated cavity in the solar wind. This cavity is called the magnetosphere of the planet.

In the vicinity of the earth, the particles of the solar wind are traveling about 400 km/s. They are slowed by the interaction with the earth to produce a bow shaped shock wave around the earth.

Inside a boundary called the magnetopause, the earth's magnetic field is dominant over the effects of the solar wind. The small fraction of the charged particles which do leak through the magnetopause are trapped in two large doughnut-shaped rings called the Van Allen radiation belts.

The solar wind was first detected directly by the spacecraft Mariner 2. It has been studied in more detail by the SOHO satellite.

Van Allen Belts

The earth satellite Explorer 1 carried a Geiger counter which detected bands of radiating particles surrounding the earth. James Van Allen headed the team of scientists who investigated these bands, and they were named the Van Allen belts. One motivation for naming the belts after Van Allen was that he was the one who insisted that the satellite carry a Geiger counter for particle detection. The two huge doughnut-shaped rings contain charged particles collected from the solar wind. The inner Van Allen belt extends over altitudes from about 2000 to 5000 kilometers and contains mainly protons. The outer Van Allen belt is about 6000 kilometers thick centered at about 16000 km from the earth. It contains mostly electrons. The outer belt was discovered by the Pioneer spacecraft.