Current to the ionosphere following a lightning stroke

Abstract

Satellite¹ and rocket² observations of high-energy electrons precipitated from the magnetosphere associated with lightning have focused new attention on mechanisms for coupling energy to the ionosphere and magnetosphere, particularly because such events are observed in the daytime when the propagation of radio waves through the ionosphere is very difficult because of losses in the ionospheric D-region due to enhanced electrical conductivity. Electric field measurements^3,4 have indicated electrical pulses in the ionosphere following lightning strokes which are much longer in duration than predicted by electromagnetic radiation or 'relaxation time' theories. These fields can be studied theoretically using the decay of charge 'monopoles' injected into the atmospheric conductivity profile by the lightning currents. We derive here a simple analytical expression for calculating the total current waveform to the ionosphere after a lightning stroke. The validity of this expression is demonstrated by comparison with a more rigorous computer solution of Maxwell's equations. The analytic model demonstrates that the temporal variation of the current induced in the ionosphere and global circuit and the corresponding return current in the earth depends on the conductivity profile at intervening altitudes in the middle atmosphere. A conclusion is that capaci-tive coupling may provide tighter coupling between the lower atmosphere and the ionosphere than usually considered, in both directions, which may help to explain observations which seem to indicate that magnetospheric phenomena may in some instances trigger lightning^5,17.