The mutual effect of precipitated auroral electrons and the auroral electrojet

Abstract

It has been found that the perturbation in the geomagnetic field produced by the auroral electrojet also perturbs the pitch‐angle distribution of the precipitated electrons that are the dominant contributors to the auroral ionization within which it is assumed the auroral electrojet flows. To study this interaction in detail, a computer program has been developed for model calculations. This program computes: (1) the electron concentration profile in the ionosphere from a given pitch‐angle and energy distribution of the ionizing electrons, and (2) the changes in the pitch‐angle distribution created by the geomagnetic field deformations caused by the auroral electrojet current system itself. Results from preliminary computations are discussed. It is shown that the interaction can cause significant time variations in an initially time‐stationary electron precipitation event. Thus, for the first time it is seen that naturally occurring atmospheric ‘feed‐back’ effects can account for some of the dynamic auroral variations without requiring the auroral source itself to vary in time. The variations are governed by the pitch‐angle distribution and the total flux of precipitated electrons, as well as by the normal loss parameters in the upper atmosphere. For medium‐to‐strong electron precipitation events, the calculated periodicity of the variations lies in the range 50–200 seconds, which is in agreement with some of the direct observations of auroral electron periodicities.

Auroral pulsations and accompanying VLF emissions

“…In their interpretations the ionosphere plays the role of external factor, which can modulate the existing precipitating electron¯ux. Maehlum and O'Brien (1968) proposed an atmospheric`f eed-back'' mechanism, which could account for some of the dynamic auroral variations without requiring the auroral source to vary in time. The eect was caused by the changes in pitch angle distribution created by the geomagnetic ®eld deformations caused by the electrojet current system itself.…”

Two kinds of current-precipitation interaction.

“…Earlier a similar mechanism was studied by MAEHLUM anI O'BRIEN (1968). We consider the case of conservation of the adiabatic invariant when the magnetospheric plasma drifts from the magnetotail toward the Earth The instability criterion corresponding to (3) may be written as follows…”

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Instability in the magnetosphere-ionosphere system has been investigated by MAEHLUM and O'BRiEN (1968), SATO and HOLZER (1973), LYATSKAYA etal. (1976a, b) .

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Auroral pulsations from atmospheric waves

“…I.n particular , Heacock and Hunsucke r foun d that micropulsations appeared to be the effect rathe r than the cause of precip itation pulsations. A local or atmospheric mechanism related to the ionization caused by the particles was proposed by Maehlum and O'Brien (1968) but they found that exceptionally intense precipitation was required to sustain the self-gene rated pul sations . Observed pulsations , on the othe r hand , a re u sually associated with low intensity aurorae (Royrvik and Davis, 1977).…”