We investigate how the photo-saturation current of the Radio and Plasma Waves instrument aboard the Solar Orbiter spacecraft responds to variations in solar illumination as well as to solar flares and energetic particle impacts, and model its remaining variations in order to predict its behaviour over the forthcoming years. Predicting the behaviour of this current is important for operating the instrument in an optimal fashion.
We normalize the time series of the photo-saturation current with respect to variations in solar illumination and find that, while the largest effect comes from distance to the Sun, it also exhibits a dependence on solar activity. Furthermore, both the overall amplitude for a specific level of activity and the slope of the proportionality decrease with time as the instrument reacts progressively less strongly to variations in illumination. We propose that this is due to degradation of the material of the probe. Modeling the material degradation as a combination of two exponentially decreasing outgassing processes, desorption and decomposition with time dependencies of around three weeks and three years respectively, predicts that the photo-saturation current will reach values about a fifth of its current amplitude by the beginning of the extended mission phase in five years time.
We conclude that while some decrease of the photo-saturation current may be mitigated by heightened solar illumination when solar maximum is reached towards the end of the nominal mission phase, by the next solar minimum, the photo-saturation current will have decreased to such an extent that electric field measurements with the Radio and Plasma Waves instrument become difficult. We find no correlation of variations in the photo-saturation current with solar flares, but due to a small sample size, we cannot rule a causal relationship out either. We also show that any effect from energetic particle impacts, if observed so far, must be far weaker than those for which we can establish variations of other parameters as the source.
Example bias currents sweeps from the three antennas of the RPW instrument on
Solar Orbiter, used to determine the photoelectron emission current.