The Growth of the Storm-Time Radiation Belt and the Magnetospheric Substorm

Abstract

Surrrmary Important features of the radiation belt (or the so-called ‘ ring current’) which grows in the magnetosphere during geomagnetic storms are summarized. The growth of the belt is studied in relation to a frequent occurrence of intense magnetospheric substorms (of which manifestations in the polar upper atmosphere are the auroral and polar magnetic substorms). It is concluded that during the magnetospheric substorm, the particles responsible for the storm-time belt are produced locally in the trapping region in the evening sector, but not in the midnight sector. In 1933, after completing their study of the impact of a fully ionized gac on a magnetic dipole, Chapman & Ferraro (1933) examined the formation of a ‘ring current’ around the Earth and its stability. Their study was intended to explain a large decrease of the horizontal component (H) of the Earth’s field during the main phase of geomagnetic storms. The results appeared in the last paper of the series ‘A new theory of magnetic storms’ with the subtitle ‘Part 11-The Main Phase’. Their toroidal ring current consisted ewentially of positive ions and electrons circulating around the Earth, in which their centripetal force due to the circular motion ic balanced by the Lorentz force. The stability of such a current system was, however, questioned by AlfvCn (1955) and others. It appears now that their study was an unsuccessful one on this particular subject, but it marked an epoch in geomagnetism, since they correctly located the ‘ring current’ inside the cavity carved in the solar wind, which is now called the magnetosphere; for some earlier concepts of the ‘ring currents’ see Chapman (1952, p. 213). In examining possibilities as to how the ring current is formed, Chapman & Ferraro (ibid., p. 80) thought that positive ions near the morning side of the magnetospheric boundary tend to leap across the cavity, but drift closer to the Earth. Further, they tend also to drag the electrons from the evening side of the magnetospheric boundary and may eventually form the ring current. Although the formation of the ‘ ring current’ does not seem to be as simple as they inferred in their paper, it has now become one of the most challenging problems in magnetospheric physics. The purpose of this paper is first to review both morphological and theoretical studies of the formation of the ‘ring current’ and associated phenomena during the main phase of geomagnetic storms, particularly the auroral substorm (Sections 2 and 3). In the second part (Section 4), we try to find the clues for the process responsible for the onset of the growth of the ‘ring current ’ and magnetospheric substorms.