  Context. The electrostatic potential of a spacecraft, VS, is important for
the capabilities of in situ plasma measurements. Rosetta has been found to be
negatively charged during most of the comet mission and even more so in denser
plasmas. Aims. Our goal is to investigate how the negative VS correlates with
electron density and temperature and to understand the physics of the observed
correlation. Methods. We applied full mission comparative statistics of VS,
electron temperature, and electron density to establish VS dependence on cold
and warm plasma density and electron temperature. We also used
Spacecraft-Plasma Interaction System (SPIS) simulations and an analytical
vacuum model to investigate if positively biased elements covering a fraction
of the solar array surface can explain the observed correlations. Results.
Here, the VS was found to depend more on electron density, particularly with
regard to the cold part of the electrons, and less on electron temperature than
was expected for the high flux of thermal (cometary) ionospheric electrons.
This behaviour was reproduced by an analytical model which is consistent with
numerical simulations. Conclusions. Rosetta is negatively driven mainly by
positively biased elements on the borders of the front side of the solar panels
as these can efficiently collect cold plasma electrons. Biased elements
distributed elsewhere on the front side of the panels are less efficient at
collecting electrons apart from locally produced electrons (photoelectrons). To
avoid significant charging, future spacecraft may minimise the area of exposed
bias conductors or use a positive ground power system.
