Surface charging is one of the most common causes of spacecraft
anomalies. When and to what potential the spacecraft is charged are two
important questions in space weather. Here, for a Chinese geosynchronous
navigation satellite, we infer the extreme negative surface charging
potentials from the ion differential fluxes measured by a low-energy ion
spectrometer. Without the solar eclipse effect away from the midnight,
the charging potentials are found to have a negative limit which is
determined by the maximum SuperMAG electrojet index in the preceding 2
hr. Such an empirical relation can be reasonably explained by the
dependence of 1--50 keV electron fluxes on substorm strength. Similar
relations may also exist for other inner magnetospheric spacecraft in
the non-eclipse region, which would be useful for spacecraft engineering
and space weather alerts.