Reflected and diffuse ions backstreaming from the Earth's bow shock 1. Basic properties

Abstract

Plasma data supplied by the ISEE 2 solar wind experiment are used to perform the first extended statistical analysis of the basic moments of the ions backstreaming from the earth's bow shock. The analysis is based on 3253 ion spectra, corresponding to a total observation time of ≃87 hours. It turns out that the density and total energy density of the backstreaming ions are, on the average, equal to ≃1% and ≃10% of those of the solar wind, respectively. The distinction between the ‘reflected’ and ‘diffuse’ populations has been confirmed and put on a quantitative basis using the ratio A = V BP /W BP between the bulk velocity and the rms thermal speed of the ions. The reflected ions are characterized by a bulk velocity V BP of the order of 2 times the solar wind velocity and by a temperature of ∼7 × 10 6 K. In contrast, the diffuse ions have, on the average, a bulk velocity 1.2 times the solar wind velocity and a temperature of 40 × 10 6 K. Therefore the total energy density of the diffuse ions is ≃30% larger than that of the reflected ions. Finally, the kinetic and thermal energy densities are distributed quite differently in the two ion populations: in fact, ≃70% of the total energy density is kinetic for the reflected ions, while this percentage decreases to ≃20% for the diffuse ions.