Characteristics of thermal and suprathermal ions associated with the dayside plasma trough as measured by the Dynamics Explorer Retarding Ion Mass Spectrometer

The retarding ion mass spectrometer (RIMS) instrument on Dynamics Explorer 1 has carried out high‐altitude measurements of the polar wind. During an outbound pass through the outer plasmasphere and plasma trough at 0900 LT the RIMS instrument measured the detailed characteristics of the thermal and suprathermal ions. Three distinct populations were observed: (1) thermal ions (H + and He + ) with densities ranging from > 500 ions/cm³ to < 1 ion/cm³, temperatures around 2 eV (20,000° K), and H + /He + density ratios of 2 to 4, (2) warm anisotropic plasma with densities ranging from 2 to 12 ions/cm³, temperatures between 10 and 15 eV, and H + /He + density ratios of 0.1 to 6, and (3) thermal field‐aligned H + and He + ion streams displaying both single stream and counterstreaming flow characteristics—the polar wind. These streaming ions display temperatures ≤ 1.5 eV and ≤ 2.0 eV for H + and He + , respectively, and respective parallel drift velocities of < 30 km/s and < 15 km/s with an H + /He + flux ratio of 3 to 10. Best‐fitting techniques for the H + and He + streaming distributions indicate a supersonic flow, although the data cannot be interpreted unambiguously. These comprehensive measurements of thermal and suprathermal plasma characteristics are interpreted in terms of plasmasphere refilling processes.