Using average temperature and density values between the lower corona and the solar wind measured at the earth, the difference in pressure is of the order of a factor of a million, causing outward flow. Since the electrical conductivity of the wind material is very high, the plasma being nearly fully ionized, the solar magnetic field lines are frozen into the material, and since the material flows outward from a rotating star, the flow and field patterns take on the general form of a spiral. The solar wind, in fact, is the medium which connects the magnetic variation of the Sun out through the heliosphere, the volume of interplanetary space which is influenced by solar magnetic fields, to various bodies found in the solar system. The wide range of physical conditions found in the solar wind and its interactions with magnetized and non-magnetized planets, dust, cosmic rays, comets, and spacecraft allow the investigation of physical mechanisms not easily duplicated in the terrestrial laboratory. Many contributions to the basic understanding of plasma processes have been identified and understood in the context of the solar wind.
Numerous problems await further investigation and resolution.
In the space between the orbits of Mercury and Jupiter, research in the solar wind was dominated by data obtained from instruments carried on spacecraft restricted to near the ecliptic plane. In 1990 the joint ESA-NASA Ulysses mission was launched with the intent of conducting a systematic experimental investigation of the solar wind as a function of solar latitude. The spacecraft was launched toward Jupiter and after a gravity assist around the planet, the resulting plane of the orbit is highly inclined allowing the spacecraft to pass over the polar regions of the Sun. The first polar passage occurred in 1994 during which time the Ulysses sampled the plasma originating from regions near the south solar pole. Ulysses operations are aimed at the collection of a complete observational picture of the solar wind from the polar regions.