The effect of the Earth׳s optical shadow on thermal plasma measurements in the plasmasphere

• Data processing technique is presented for measurements of rarified space plasma.
• Deduced spacecraft potential rapidly drops in the Earth' shadow.
• Plasma density and temperature do not undergo abrupt variation at shadow boundary.
• Gradual temperature decrease is connected to the shading of magnetic field line feet.

The data processing technique was developed for thermal plasma measurements by wide-angle plasma analyzers, which was successfully used for the Interball mission instruments. This technique considers the effect of spacecraft potential on plasma measurements. When the spacecraft enters the optical shadow of the Earth, the evaluated spacecraft potential suddenly drops, but no abrupt changes of plasma density or temperature are observed. Often observed decrease in temperature of protons in the Earth's shadow is actually associated with shading of ionospheric feet of magnetic field line passing through the spacecraft. This suggests that ionospheric photoelectrons are an important heat source for the plasmasphere. Besides, the Interball 1 data suggest that photoelectrons coming from the nearest ionosphere are more effective in plasmaspheric ion heating than photoelectrons from the conjugate hemisphere.