Simultaneous Field Aligned Current and Charged Particle Measurements in the Cleft

Abstract

The clefts are major permanent features of the magnetospheric topology which, nevertheless, are still poorly understood. Their feet form the dayside part of the auroral ovals which extend through all MLT as morphologically continuous features (Lui et al., 1975). The ovals map out from the northern and southern hemispheres to important boundary regions of the magnetosphere — the magnetopause via the clefts on the dayside and the plasma sheet-polar cap interface on the night side. (Heikkila, 1972; Vasyliunas, 1974). The mechanisms of energy and momentum transport across these boundary surfaces and energy conversion processes in the adjacent regions are fundamental to our understanding of the solar wind coupling to the magnetosphere. Although the ovals are continuous, they are by no means uniform in their average characteristics as a function of MLT, nor are the particle fluxes which cause them (Frank and Ackerson, 1972; Riedler, 1972; McDiarmid et al, 1975). The red (6300 Å) enhanced auroras in the dayside ovals (Eather and Mende, 1972, 1973) result from low energy (~ 100 eV) precipitating electrons which, one infers, have been recently transported from the magnetosheath into the clefts. (Heikkila and Winningham, 1971). However, the transport processes are complex so that the clefts can be considered ‘direct entry’ regions only in the simplest approximation. The present challenge in studying the clefts is to deduce the hierarchy of physical processes which is modifying the ‘direct entry’ approximation by observing both the average characteristics and detailed structure. Advances in this area will also lead to a better understanding of the dawn and dusk transition regions between the cleft and the plasma sheet.