MIST

By Cristian Radulescu (Mullard Space Science Laboratory, UCL)

Pick‐up ions (PUIs) are charged particles that have been accelerated by electric and magnetic fields beyond the speed of the bulk plasma and then proceed to thermalize through wave‐particle interactions. In this paper we investigate the distribution of PUIs in Saturn’s magnetosphere. We specifically look at water group PUIs as these are expected to be the most prevalent due to the majority of plasma in the inner to middle magnetosphere of Saturn coming from the moon Enceladus. Data from the Cassini spacecraft’s CAPS instrument is used, with the bulk plasma population and penetrating radiation subtracted, spanning radial distances from 3 to 10 Saturn radii (R_S) and the time period between 2005 and 2012. Ten-minute intervals of this data are plotted on radial v_perp vs v_parallel plots. The width in pitch angle (PA) and velocity are measured and then distributed into 0.2 R_S by 2° bins on global maps of the Saturnian magnetosphere. We find that the PUI distributions broaden with increasing latitude creating a “Pacman”-like signature. This pattern is approximately anti-correlated with the amplitude of ion cyclotron waves (ICWs). The explanation for this phenomenon lies in cyclotron resonance between the PUIs and ICWs. ICWs get driven/damped by the PUIs resulting in energy transfer between them and scattering of the PUIs in PA. The strongest resonance, and hence scattering,  is expected in areas of high ICW amplitude, which is close to the equator. We find that this is not the case here because the PA scattering time is much longer than the PUI bounce period meaning ions reach high latitudes by the time they get scattered by the equatorial ICWs. In contrast, the half-widths of the velocity distributions remain mostly constant, increasing only past the orbit of Rhea, likely because of elevated plasma temperatures.

Global maps of the Saturn magnetosphere with top-down (left) and side (right) views. a)-b) show the PA distributions shown as the full width at half maximum (FWHM) of
the distribution at the injection velocity at the middle of the measured interval. c)-d) show the velocity distributions shown as half width at half maximum (HWHM) of the distribution. e)-f) show the average amplitude of ion cyclotron waves (reproduced from Long et al. (2022)). The grey bins in f) are bins where measurements were made, but no waves were detected. In the top down view, the inner black circle is the orbit of Enceladus, the white circle is the orbit of Dione and the outer black circle is the orbit of Rhea.

See publication for details:
Radulescu, C. R., Coates, A. J., Simon, S.,Verscharen, D., & Jones, G. H. (2025).Pick‐Up ion distributions in the inner andmiddle saturnian magnetosphere. Journalof Geophysical Research: Space Physics,130, e2024JA033390. https://doi.org/10.1029/2024JA033390