The Twin-Probe Method: Improving Langmuir Probe Measurements on Small Spacecraft

The Langmuir probe (LP) is generally accepted as an effective and relatively simple in situ space plasma instrument (plasma density, electron temperature, and spacecraft potential). As LPs transition to small spacecraft, their implementation encounters new technical challenges. For example, a negative charge is induced on the spacecraft while positively biasing an LP due to the small surface area ratios (spacecraft surface area to probe surface area MUCH LESS-THAN 1000). This results in a varying spacecraft potential that degrades the accuracy of electron temperature and electron density measurements, reducing the LP's effectiveness as a diagnostic tool. To mitigate the effects of this spacecraft charging, the twin-probe method (TPM) was developed. The TPM corrects LP measurements with tracked spacecraft potentials, measured by a separate high-impedance probe. By accounting for the changes in the spacecraft potential, the LP sweeps can be reconstructed to provide more accurate measurements of the ambient plasma's properties. Here, we detail the TPM and present laboratory experiments that study its effectiveness and summarize constraints. Through these experiments, we found that it is possible to correct for spacecraft charging effects that can cause deviations in temperature and density as large as 20% and 136%, respectively. Furthermore, we will demonstrate a clear correlation between area ratio, spacecraft charging, and the negative impact on temperature and density measurement accuracy.