Inductive electric fields in the magnetotail and their relation to auroral and substorm phenomena

The paper reviews the importance of inductive electric fields in explaining different magnetospheric and auroral phenomena during moderately and highly disturbed conditions. Quiet-time particle energization and temporal development of the tail structure during the substorm growth phase are explained by the presence of a large-scale electrostatic field directed from dawn to dusk over the magnetotail. Conservation of the first adiabatic invariant in the neutral sheet with a small value of the gradient in the magnetic field implies that the longitudinal energy increases at each crossing of the neutral sheet. At a certain moment, this may result in a rapid local growth of the current and in an instability that triggers the onset. During the growth phase energy is stored mainly in the magnetic field, since the energy density in the electric field is negligible compared to that of the magnetic field (ratio 1: 10⁷). An analytical model is described in which the characteristic observations of a substorm onset are taken into account. One major feature is that the triggering is confined to a small local time sector. During moderate disturbances, the induction fields in the magnetotail are stronger by at least one order of magnitude than the average cross-tail field. Temporal development of the disturbed area results in X- and O-type neutral lines. Particles near to these neutral lines are energized to over 1 MeV energies within a few seconds, due to an effective combination of linear and betatron acceleration. The rotational property of the induction field promotes energization in a restricted area with dimensions equivalent to a few Earth's radii. The model also predicts the existence of highly localized cable-type field-aligned currents appearing on the eastern and western edges of the expanding auroral bulge. It is shown that the predictions agree with satellite observations and with the data obtained from the two-dimensional instrument networks operated in Northern Europe during the International Magnetospheric Study (IMS, 1976–79).