Double-Probe Measurements in Cold Tenuous Space Plasma Flows

Cold flowing tenuous plasmas are common in the terrestrial magnetosphere, particularly in the polar cap and tail lobe regions, which are filled by the supersonic plasma flow known as the polar wind. Electric field measurements with double-probe instruments in these regions suffer mainly from two error sources: 1) an apparent sunward electric field due to photoemission asymmetries in the probe-boom system and 2) an enhanced negatively charged wake forming behind the spacecraft, which will affect the probe measurements. The authors investigate these effects experimentally by Fourier analysis of the spin signature from the double-probe instrument Electric Fields and Waves (EFW) on the Cluster spacecraft. They show that while the signature due to photoemission asymmetry is very close to sinusoidal, the wake effect is characterized by a spectrum of spin harmonics. The Fourier decomposition can therefore be used for identifying wake effects in the data. As a spin-off, the analysis has also given information on the cold flowing ion population.