Geomagnetic storms, auroras and associated effects

Abstract

Our knowledge of the interplanetary medium is outlined and its frictionless interaction with the geomagnetic cavity, first discussed by Chapman and Ferraro, is described. An important feature of this interaction is the interplanetary field which is compressed and may possibly lead to the formation of a shock wave. The possibility of frictional interaction between the solar wind and the cavity is discussed; an effect which appears to cause friction is the instability of interpenetrating ion-electron streams. This effect will also cause strong heating and trapping of ions and the generation of electromagnetic waves. The theory of propagation of geomagnetic disturbances in the magnetosphere and ionosphere is reviewed, first in general terms and than for some of the various components of a geomagnetic storm. Sea-level disturbances are divided into stormtime (Dst) and other (DS) components and also into different phases and the experimental data is reviewed. Theories of Dst, including the ringcurrent theory and magnetic tail theory are discussed and compared. Attempts to explain the complex DS field comprise the “magnetospheric dynamo” theory and the asymmetrical ring-current theory; these are compared in the light of experimental evidence. Motions of plasma and field lines in the magnetosphere are discussed in general terms: there are motions which deform the field and there are interchange motions. The former are opposed by Earth currents; the latter are not. The two types of motion are coupled through ionospheric Hall conductivity. Theories of the DS field in terms of the two types of motion are described; in particular motions caused by frictional interaction with the solar wind are discussed. These motions cause a helical twist in the field lines which propagates into the polar ionosphere as a hydromagnetic wave. In the ionosphere the motions of the field lines drive currents (“moving-field dynamo”) which cause the DS field. Drifts of neutral ionization in the lower ionosphere lead to localized accumulations which play a vital part in storm and auroral theory: they cause polarization fields which change the DS current system; they react on the magnetospheric motions to cause particle acceleration and precipitation. Auroral morphology and theories are briefly reviewed; the solar wind friction theory, although far from complete may provide a start. Further development should take the form of determining ionospheric drifts, polarization electric fields and consequent magnetospheric effects. A brief discussion is given of some associated effects: growth and decay of belts of geomagnetically trapped corpuscules; increase in ionospheric absorption of radio waves and lower-level X-ray production, ionospheric “storm” and high-latitude irregularities, micropulsations, VLF and ELF radio emissions from the magnetosphere, atmospheric heating and wave generation.