J. R. Burrows

Simultaneous Field Aligned Current and Charged Particle Measurements in the Cleft

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 A) 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.

Trapped energetic ions in Jupiter's inner magnetosphere

Ion measurements made with the high-flux telescope at ∼1 MeV/nucleon during the passage of the Ulysses spacecraft through Jupiters inner magnetosphere are presented. Stable pancake-shaped pitch angle distributions were observed for protons inside ∼17 RJ. They are fitted by a time-independent model in which an assumed directional flux at the magnetic equator is transformed to the spacecrafts position using Liouvilles equation and the O6CS magnetic field model. Nongyrotropic features are fitted with a flow in the corotation direction. The derived equatorial pitch angle distribution is approximately independent of radial distance and has broad shoulders. However, it is not as isotropic as that predicted for strong diffusion. From a flux-composition fitting analysis, we find (1) the equatorial proton omnidirectional flux decreases approximately exponentially with magnetic equatorial distance RM and is nearly longitudinally symmetric, (2) the equal energy per nucleon abundance ratios for O/H, S/H, and S/O decrease with RM, (3) the ion spectral index softens linearly with RM, (4) the phase space densities of protons and oxygen at a constant magnetic moment increase rapidly with RM, and (5) the sulfur phase space density turns up inside 10 RJ. The data are best fitted with similar spectral indices for oxygen and sulfur. Thus there is likely a source of energetic sulfur in the Io torus. Particle losses throughout the region are required to fit the radial variation of the phase space densities with a physically reasonable radial diffusion coefficient. Further work is needed to determine whether these losses are consistent with weak equatorial pitch angle diffusion.