Retrieving True Plasma Characteristics from Langmuir Probes Immersed in the Spacecraft Sheath: The Double Hemispherical Probe Technique

摘要

Langmuir probes have been widely used for in situ measurements of ambient plasma in space. However, in situations where the Debye sheath is relatively large (e.g., planetary magnetospheres and solar wind plasma), probes with a limited boom length have a risk of being engulfed by the sheath of the spacecraft (SC) as indicated in previous missions, causing errors in derived plasma parameters. Here we present a double hemispherical probe (DHP) technique which is able to identify whether the probe is in the SC sheath and retrieve true plasma parameters. The DHP consists of two hemispheres that are installed back-to-back and electrically isolated from each other. The two hemispheres are swept with the same bias voltage simultaneously. It is shown that the currents of the two hemispheres diverge when the probe is in the sheath. Through laboratory experiments, empirical relationships are established between the current ratio of the two hemispheres, measured parameters in the sheath, and true ambient plasma parameters. Using these relationships, the plasma parameters retrieved from the DHP measurements in the sheath show good agreement with the true ambient plasma parameters. The DHP technique significantly improves the measurement accuracy across a wide range of plasma environments.