Solar wind and solar energetic particles: Properties and interactions

This paper reviews findings since 1966 about the properties of the interplanetary magnetic field; the solar wind plasma; solar wind interactions with the earth, the moon, Mars, and Venus; and the properties and propagation characteristics of energetic solar flare particles. For most purposes the solar wind can be characterized as a fluid that retains some of the properties of its source as it flows outward and that contains an anisotropic temperature distribution and embedded magnetic field. The magnetic field has a sector structure which slowly varies in a manner that resembles the evolution of magnetic features on the solar surface. As the solar wind encounters planetary bodies in its path, the nature of its interaction depends on the size and magnetic properties of the bodies. The solar wind creates a bow shock similar to that at earth as it interacts with Venus, and probably also with Mars. At Venus the plasma is apparently not diverted by a magnetic field, but by the planetary, ionosphere. At Mars the plasma is diverted either by a weak magnetic field or by an ionosphere. Further studies near earth have led to an explanation of the earth's bow shock in terms of hydromagnetic and ion waves. Energetic electrons (>400 kev) have been found in the neutral sheet of the geomagnetotail. The absence of thermal plasma in the earth's outer magnetosphere, beyond the plasmapause, has been explained in terms of a large‐scale convection field. The solar wind interaction with the moon is of an entirely different character. No bow shock is found, and the interplanetary magnetic field appears to convect through the moon. A single‐particle model of the interaction appears adequate to explain the observed phenomena, including the region behind the moon that is void of plasma but contains an enhanced magnetic field. Energetic electrons (20 kev to several Mev), protons (500 kev to 200 Mev), and α particles produced in solar flares have been studied in conjunction with solar X rays and radio bursts. It is now believed that these particles are not necessarily produced in an impulsive event during the flash phase of the flare; proton precursors have been found in some events, and it appears that energetic particles continue to leave the vicinity of the flare for hours or even days. These particles propagate preferentially along interplanetary magnetic field lines. Thus, their velocity distribution is generally anisotropic as long as the injection continues.