ELECTROSTATIC-WAVES NEAR THE FIELD REVERSAL REGION IN THE EARTHS MAGNETOTAIL

The general stability properties of ion distributions with velocity sp ace holes of the type expected in the Earth's central plasma sheet as a consequence of chaotic ion acceleration is investigated. It is shown that such distributions are unstable to electrostatic waves with wave vectors located in a narrow angular region about the Sun-Earth line. These waves can be generated in the magnetotail midplane even if the t ransition layer in velocity space between the background plasma and an empty hole region is of the order of the ion thermal velocity. Theref ore these electrostatic waves would form a robust and persistent compo nent of the electrostatic wave activity in the presence of the backgro und ion hole distributions. The solution of the quasi-linear equation shows that the saturation of the ion hole instability occurs because o f velocity space trapping of the low energy background ions and result s in a flattening of the reduced distribution function in the region o f the original velocity space hole. The energy level of the excited wa ves in the vicinity of the magnetotail field reversal region is obtain ed as a function of the ion hole parameters.