ISEE-1 AND ISEE-2 OBSERVATIONS OF THE HIGH BETA-SHOCK

Measurements from the ISEE 1 and ISEE 2 spacecraft are used to examine the terrestrial bow shock under high beta conditions. On November 4, 1979, the large separation between the spacecraft, over 2500 km along the shock normal was such to allow simultaneous upstream and downstream measurements of the solar wind plasma for a period of about 8 min. These measurements are compared with and found to be in agreement with the predicted values of the Rankine-Hugoniot relations using the simple adiabatic approximation and a ratio of specific heats, gamma, of 5/3. No additional terms, such as the heat flux, are needed to bring the measured and predicted values into agreement. Large magnetic field and density fluctuations were observed, but average downstream plasma conditions well away from the shock were relatively steady, near the predicted Rankine-Hugoniot values. The magnetic disturbances persisted well downstream and a hot, dense ion beam was detected leaking from the downstream region of the shock. Both of these phenomena can be attributed to the excitation of the Alfven ion cyclotron instability, which acts quickly to isotropize the ions close to the ramp of the shock. Additional shock crossings under high beta conditions having smaller separations allow us to confirm many of these observations.