INTERPLANETARY GAS
Most interplanetary matter is in the form of an ionized gas comprising the solar wind. It consists of an almost continuous stream of particles, mostly protons and electrons, flowing out from the sun's corona. As the solar wind moves forward, it forms an expanding spiral pattern due to the sun's rotation, and its velocity increases until, several solar radii from the sun, it equals the speed of sound in the plasma. Its velocity continues to increase as it flows outward, much as rocket gases are accelerated to supersonic velocities in a rocket nozzle. Near the earth the solar wind reaches a velocity of about 400 kilometers per second. Beyond the earth its speed remains very nearly constant.
At earth's distance the wind's density is down to about five protons and five electrons per cubic centimeter on the average, but it can rise on occasion to 100 particles per cubic centimeter.
