Interdependencies Between the Actively Controlled Cluster Spacecraft Potential, Ambient Plasma, and Electric Field Measurements

The payload of the four Cluster spacecraft dedicated to multipoint measurements in the earth's magnetosphere includes ion emitters that can emit currents up to a few tens of microamperes to partially compensate for the photoemission current originating at the spacecraft surfaces and thereby achieve a reduction in the spacecraft potential from values often as high as several tens of volts positive to values well below 10 V. This effect is highly desirable to improve on-board particle measurements, especially of electrons, by reducing the photoelectron flux into the sensors and by reducing the distortions of particle trajectories in the spacecraft sheath. On the other hand, a perfectly stable spacecraft potential precludes the utilization of the spacecraft as a plasma probe, including the useful technique to estimate ambient plasma density from the spacecraft potential. This paper shows that the small residual variations of the potential still allow the determination of ambient plasma density, albeit at reduced accuracy. We present an outline of the applied method with examples. In spring 2002, in the cusp region, the Cluster interspacecraft distances were well below 1000 km. For selected intervals, the potentials of controlled and uncontrolled spacecraft are compared, taking advantage of the fact that the ion emitters were not operating on all spacecraft. Special considerations regarding the secondary electron production and its effect on the controlled spacecraft potential were found to be necessary, and examples of this aspect are presented as well. Finally, the effect of the potential control on the double-probe electric field measurements is assessed. It is found that the normal sun-aligned offset of the electric field caused by the asymmetric spatial distribution of photoelectrons around the spacecraft can be affected by the potential control. Nonetheless, the information about plasma density is not lost when active spacecraft potential control (ASPOC) is applied. This is also relevant for future missions with ASPOC, particularly the NASA Magnetospheric Multiscale mission launched in March 2015. Regular intercalibration of controlled and uncontrolled potentials is nonetheless advisable to increase the reliability of the method.