Monday

Spacecraft Charging (SCh) is a condition whereby a spacecraft accumulates excess electrons as a result of being immersed in a plasma. For spacecraft carrying on-board instrumentation, such as mass spectrometers, this results in a shift of the energy distribution of the particles to be measured. This shift leads to inaccuracies when calculating parameters such as particle density, bulk speed and temperature. From a mission perspective, SCh can adversely affect the overall science return.

The Ion Neutral Mass Spectrometer (INMS) is a miniaturised analyser capable of measuring ions and neutral particles in the ram direction of a spacecraft. We use data from the INMS aboard the Phoenix-CubeSat, which launched in 2017 into a 52° orbit and returned data from an altitude of 360 – 380 km. INMS datum is compared with the International Reference Ionosphere (IRI-2016) model and the Spacecraft Plasma Interaction (SPIS) software to examine the impact of SCh on measurements.

The study reveals a notable diurnal variation in SCh. During the day spacecraft potentials may reach up to -0.8V, whereas at night they do not exceed -0.25V. This is because SCh is directly proportional to electron temperature, which is much higher during the day, 2300K, vs 890K at night. Without correcting for SCh, certain ion densities were 59% higher and temperatures were 16% lower. These effects were less dramatic at night because surface potentials were smaller. The nightside data also reveals the existence of a second particle species which could be NO+ or N2+. This was unexpected at this altitude. We also see evidence of latitude-dependent ion-drift variations. Both require further investigation.