Abstract
Abstract. We have investigated statistically the electron density below 5 cm−3 in the magnetosphere of Saturn (7–80 RS, Saturn radii) using 44 orbits of the floating potential data from the RPWS Langmuir probe (LP) onboard Cassini. The density distribution shows a clear dependence on the distance from the Saturnian rotation axis (√X2+Y2) as well as on the distance from the equatorial plane (|Z|), indicating a disc-like structure. From the characteristics of the density distribution, we have identified three regions: the extension of the plasma disc, the magnetodisc region, and the lobe regions. The plasma disc region is at L<15, where L is the radial distance to the equatorial crossing of the dipole magnetic field line, and confined to |Z|<5 RS. The magnetodisc is located beyond L=15, and its density has a large variability. The variability has quasi-periodic characteristics with a periodicity corresponding to the planetary rotation. For Z>15 RS, the magnetospheric density distribution becomes constant in Z. However, the density still varies quasi-periodically with the planetary rotation also in this region. In fact, the quasi-periodic variation has been observed all over the magnetosphere beyond L=15. The region above Z=15 RS is identified as the lobe region. We also found that the magnetosphere can occasionally move latitudinally under the control of the density in the magnetosphere and the solar wind. From the empirical distributions of the electron densities obtained in this study, we have constructed an electron density model of the Saturnian nightside magnetosphere beyond 7 RS. The obtained model can well reproduce the observed density distribution, and can thus be useful for magnetospheric modelling studies.
Highlights
Observations during the flybys of Pioneer 11 and Voyagers 1 and 2 provided information on plasma conditions in Saturn’s magnetosphere
We have shown that the density estimated from the Using the floating potential of the probe (Ufloat) is consistent with the density independently measured by the electron detector (ELS) at another time interval, not used for the calibration (Fig. 2), and with the densities obtained from the RPWS upper hybrid wave emission observations
In the tenuous plasma of the magnetosphere, the spacecraft potential gives a good estimate of the electron number density
Summary
Observations during the flybys of Pioneer 11 and Voyagers 1 and 2 provided information on plasma conditions in Saturn’s magnetosphere. Most of the in-situ observations above were made by Cassini in the equatorial plane, while Krupp et al (2005) found a quasi-periodic modulation of the high-energy (28–49 keV) electrons when the orbit covered the Southern Hemisphere. They compared the electron flux with the magnetic field and concluded that the electron flux modulation was explained by the fact that the spacecraft repetitively entered and exited the plasma sheet and lobe regions due to the modulation of the plasmasheet thickness.
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