Spacecraft potential variations of the Swarm satellites at low Earth orbital altitudes

In this study, we provide the first detailed analysis of variations in the spacecraft potential (V-s) of the three Swarm satellites, which are flying at about 400-500 km. Unlike previous studies that have investigated extreme charging events, usually with spacecraft potentials as negative as -100 V, this study is focused on variations of Swarm Vs readings, which fall within a few negative volts. The Swarm observations show that spacecraft at low Earth orbital (LEO) altitudes are charged only slightly negatively, varying between -7 V and 0 V, with the majority of recorded potentials at these altitudes clustering close to -2 V. However, a second peak of V-s data is found at -5.5 V, though the event numbers for these more-negative observations are less, by an order of magnitude, than for incidents near the -2 V peak. These two distinct V-s peaks suggest two different causes. We have thus divided the Swarm spacecraft V-s data into two categories: less-negatively charged (-5 < V-s < 0 V) and more-negatively-charged (-6.5 < V-s < -5 V). These two V-s categories exhibit different spatial and temporal distributions. The Vs observations in the first category remain relatively closer to 0 V above the magnetic equator, but become much more negative at low and middle latitudes on the day side; at high latitudes, these first-category Vs readings are relatively more-negative during local summer. Second-category Vs events cluster into two bands at the middle latitudes (between +/- 20 degrees-50 degrees magnetic latitude), but with slightly more negative readings at the South Atlantic Anomaly (SAA) region; at high latitudes, these rarer but more-negative second-category Vs events exhibit relatively more-negative values during local winter, which is opposite to the seasonal pattern seen in the first category. By comparing Vs data to the distributions of background plasma density at Swarm altitudes, we find for the first category that more-negative Vs readings are recorded at regions with higher background plasma density, while for the second category the more-negative Vs data are observed at regions with lower background plasma density. This can be explained as follows: the electron and ion fluxes incident on Swarm surface, whose differences determine the potential of Swarm, are dominated by the background "cold" plasma (due to ionization) and "hot" plasma (due to precipitated particles from magnetosphere) for the two Vs categories, respectively.