Plasma waves observed by the IRM and UKS spacecraft during the AMPTE solar wind lithium releases: Overview

Abstract

The two September 1984 solar wind lithium releases produced a rich variety of plasma waves which have been measured in situ by the plasma wave instrumentation on board the Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM and UKS spacecraft. Reflection of the natural galactic and terrestrial electromagnetic radiation from the dense Li plasma caused a cutoff in the high‐frequency electric field intensities from which the temporal and spatial variation of the plasma density can be determined. Inside the diamagnetic cavity the electron plasma frequency and also temporarily the Li plasma frequency have been excited. The emission at the electron plasma frequency is near the thermal fluctuation level. In addition, weak low‐frequency ion acoustic waves were observed. The boundary between the diamagnetic cavity and the external magnetic field was found to be surprisingly stable and contained extremely low levels of wave activity. In the transition region from the diamagnetic cavity to the solar wind, high wave activity at the medium and very low frequencies propagating mainly in the ion acoustic and electrostatic cyclotron harmonic modes was encountered. No wave magnetic fields were detected in this region. The upstream edge of the transition region was characterized by a steep decay in magnetic field strength and density and by a sudden increase in the quasi‐static electric field. At this time the ELF/MF rms wave amplitude explosively increased to a value of 50 mV/m and remained at an enhanced level for more than 1 min. The spectrum of this wave activity is similar to the electrostatic noise observed in collisionless shocks. Data from UKS indicate that during the releases, UKS was in the magnetic transition zone. The wave activity at UKS was distinctly different from that encountered by IRM. The intense emission at the electrostatic shocklike transition was weaker than that on IRM and for the second release appeared at a different time. This can be related to the different positions of the two spacecraft with respect to the interaction regions. Despite the high wave intensities the estimated wave energy densities are, however, too low by orders of magnitude to drive significant magnetic field diffusion during the in situ observation times. Some differences in the wave excitations for the two releases can be traced back to the different solar wind conditions.