Temporal development of composition, spectra, and anisotropies during upstream particle events

We present observations of the detailed temporal development of particle intensities, composition, and density gradient anisotropies for protons and heavier ions during several diffuse ion events observed upstream of the earth's bow shock. The measurements, covering the energy range ∼16 keV/Q to ∼130 keV/Q, were made with the ultra‐low energy charge analyzer sensor of the Max‐Planck‐Institut/University of Maryland experiment on ISEE 1. Composition ratios are found to be constant as a function of energy per charge not only during the equilibrium intensity levels of these events, but also during their onset and decay phases. We interpret our measurements in terms of temporal and spatial structures in the upstream region produced by a time dependent first‐order Fermi acceleration mechanism.