Upstream hydromagnetic waves and their association with backstreaming ion populations: ISEE 1 and 2 observations

TLDR

The study investigates the nature of hydromagnetic waves associated with backstreaming ion populations from Earth’s bow shock using data from the Lepedea plasma instruments and flux gate magnetometers on ISEE 1 and 2. Measurements from the Lepedea plasma instruments and flux gate magnetometers on ISEE 1 and 2 were employed to analyze the wave–ion relationships. Reflected ions confined to a narrow energy and angular range generate small‑amplitude left‑handed 1 Hz waves, while diffuse backstreaming particles produce low‑frequency (~30 s) large‑amplitude waves that often steepen into mini‑shocks and break into right‑hand polarized whistler packets spanning frequencies below to above the proton gyrofrequency and are advected back toward Earth by the solar wind, and intermediate particles exhibit a mixture of these wave types.

Abstract

Measurements from the Lepedea plasma instruments and the flux gate magnetometers on ISEE 1 and 2 are used to examine the nature of the hydromagnetic waves associated with the various classes of ions backstreaming from the earth's bow shock. The reflected ions, which are confined to a narrow energy and angular range, are accompanied by small amplitude (≲½γ peak to peak) left‐handed waves at frequencies close to 1 Hz in the spacecraft frame. Diffuse backstreaming particles with a broad energy spectrum are associated with low frequency (∼ 30‐s period) large amplitude (∼5γ peak to peak) waves. Intermediate particles are associated with a mixture of these two wave types. Often the waves associated with the diffuse beams steepen as if they were mini shocks. The leading edge (trailing edge in the spacecraft frame) frequently appears to break up into a whistler mode wave packet. These discrete wave packets are right‐hand polarized and have frequencies from below the proton gyrofrequency to well above it in the plasma frame and are blown back towards the earth by the solar wind.