Environmental conditions during the reported charging anomalies of the two geosynchronous satellites: Telstar 401 and Galaxy 15

Abstract

Charging and subsequent electrostatic discharge is recognized as a serious operational threat to spacecraft. This paper concentrates on an analysis of the environmental conditions (geomagnetic measurables, but not the particle fluxes, that were associated with the geomagnetic disturbances) and the interplanetary signatures prior to the anomalies that affected the two geostationary satellites Telstar 401 and Galaxy 15, which were believed to have been caused by spacecraft charging/electrostatic discharge. In terms of geomagnetic conditions, the planetary Kp index was very different around the two events, despite Kp commonly being used to characterize periods that are hazardous for spacecraft charging. On the contrary, local geomagnetic records, both from GOES satellites and from ground records, reveal that each anomaly took place during the expansion phase of a triggered substorm. Combined with a large and south directed interplanetary magnetic field (IMF) Bz, northward turnings of negative IMF Bz and large fluctuations in IMF By played major roles in the magnetospheric responses of the substorms. The interplanetary features of the two events were also similar: the interplanetary counterpart of coronal mass ejections interacted with high‐speed streams, favoring the substorm triggers. Although there are several differences in the preconditioning and the inner conditions of the magnetosphere before the anomalies, in each case the respective spacecraft crossed the plasma sheet in the magnetotail during enhanced substorm activity, with a dipolarization event that took place close to the spacecraft location, indicating that each satellite was in the wrong place at a critical moment.