Non‐E × B ordered ion beams upstream of the Earth's bow shock

Abstract

The unexpected appearance of spin modulations in our fixed voltage electrostatic analyzer detectors on ISEE 1 and 2 has given us the opportunity to investigate the plasma properties of the upstream ions in high time resolution. The detectors are narrowly collimated and point southward, antiparallel to the satellite spin axis covering a region in phase space not viewed by the other plasma experiments on the satellites. The count rate modulations (up to three orders of magnitude) are due to the convolution of the anisotropic detector response function with the local plasma distribution function and reflect the existence of strong velocity gradients in the local distribution function. By assuming a form for this distribution function, we can simulate the output of any of our detectors. Comparing the simulated and observed count rates, we can then estimate several of the local plasma parameters. This of course is dependent upon the distribution function chosen. Using an isotropic ‘flowing’ Maxwellian distribution (which should be thought of as a ‘local’ approximation to the actual distribution near the region in phase space occupied by our detector response functions), we have obtained estimates of the local plasma temperature and three‐dimensional flow velocity of the upstream ion population we observe. We find that in almost all of our observations of upstream particles there exist beam‐like ions with temperatures in the range 5–80 eV. They are extremely dynamic in velocity with changes of 200–300 km/s occurring in 3–6 s being common. Most significantly, their ‘flow’ velocities cannot be ordered by E × B. This last point separates these observations from the previously reported reflected and diffuse populations of upstream ions. Mechanisms that can gyrophase bunch the ions reflected at the bow shock are discussed as a method of explaining the data.