Comparison of interball-2 spacecraft potential from IESP and KM-7 experiments in high-latitude regions of the magnetosphere at altitudes of 2–3 R E

Abstract

Measurements of Interball-2 spacecraft potential by two instruments, IESP and KM-7, are reviewed and simultaneous measurements are compared. Unacceptable discrepancy between results of spacecraft potential measurements, including opposite signs, was found. Actually, both experiments are methodically identical: they used the same type of sensor - spherical Langmuir probes operating in the “floating” mode, and they have measured the same parameter - the voltage difference between the probe and the satellite structure. The IESP instrument measured one value of this parameter at fixed bias current to the probe. The KM-7 measured the whole current-voltage characteristic (the probe current as a function of the probe potential), which consists of 11 IESP-type measurements at different values of bias current. The difference lies only in the way of technical implementation, as the probes were operating in different ambient conditions. The IESP probes were mounted at the ends of long booms and thus were affected by the solar UV emission, while the KM-7 probe was rather well protected against UV emission by proper mounting and screening the head of the sensor from both direct UV emission and those reflected from spacecraft elements. The comparison of two data sets and variations along the orbit has shown, that KM-7 correctly measures the spacecraft potential. In high-latitude inner magnetosphere (in auroral region and polar cap) at altitudes 2–3 R E the spacecraft potential was predominantly negative down to −10 V, increasing up to about +5 V in some locations at rather extended parts of the satellite orbit. Reasonably good agreement between two experiments was obtained only at spacecraft potential ≤ −2 V and after inversion the sign of available calibration curve of IESP experiment; in this case both experiments very accurately reflect variation of potential data even in the small details. The operation mode of IESP experiment was not optimal and requires further analysis. The wave results obtained with IESP may be considered as reliable only when both experiments show, after inversion of available IESP calibration curve, coincident data, but the phase relations, if any, will be inversed in any case. The method of spacecraft potential measurements adopted in IESP should be used only for positive potential; if realspacecraft potential is negative, the method gives wrong and misleading results.