Temporal development of diffuse ion events upstream of the Earth's bow shock: The October 31, 1977, event

Abstract

We examine the origin of the intensity‐time profile characteristic of diffuse ion events upstream of the earth's bow shock. This profile is believed to result from a rotation of the interplanetary magnetic field which produces a systematic variation in the connection time of field lines with the bow shock. A plateau in the ion intensity is formed if the connection time exceeds the time needed to reach equilibrium between the shock acceleration and ion loss processes. This scenario is tested using the October 31, 1977, upstream diffuse ion event for which simultaneous magnetic field and ion intensity data have been published. We analyze this event using a two‐dimensional Gleeson‐Axford equation to describe the shock acceleration process and a model bow shock whose nose serves as a uniform source of ions injected into the acceleration process. Intensity‐time profiles are calculated for 30‐keV and 120‐keV protons for a range of diffusion coefficients, K∥ and K⊥, using connection times based upon the shock geometry and the magnetic field data. It is found that the calculated and observed profiles are in good general agreement during the growth phase but diverge during the decay. The best overall fit is achieved with diffusion coefficients, K∥(30 keV) ∼ 4×1017 cm² s−1, K∥(120 keV) ∼ 1.2×1018 cm² s−1, and K⊥(120 keV) ∼ 1016 cm² s−1. These values also produce cross‐field anisotropies at 0430 UT which are in satisfactory agreement with the observed anisotropies.