[1] For many years it has been known that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies by up to one percent on a time scale of years. Recent radio observations from the Cassini spacecraft have revealed the appearance of a second component, with a rotation period of about 10.6 hours, significantly less than the period of the previously known component, which is currently about 10.8 hours. In this paper we show that the first component originates from the southern auroral region, and that the second component originates from the northern auroral region. This north-south asymmetry in the rotation period has potentially important implications on how angular momentum is transferred from the interior to the magnetosphere. Citation:

a b s t r a c tWe demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturn's rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15-20 Rs range, probably associated with reconnection and plasmoid formation in Saturn's magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes.

The low energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, and spatial distributions of ions (30 keV ≲ E ≲ 150 MeV) and electrons (22 keV ≲ E ≲ 20 MeV) during encounters with the Saturnian magnetosphere in November 1980 and August 1981, respectively. Detailed analysis of the data has revealed the following: (1) Energetic ions are present in the interplanetary medium both upstream (to ∼200 Rs) and off the dawn bow shock (to ∼400 Rs) of the magnetosphere, with maximum energies ∼100 keV. (2) Low‐energy (≳22 keV) electrons are generally depleted inward of L ∼ 10 Rs, while low‐energy (≳30 keV) ions are greatly enhanced in the same region. (3) The composition of low‐energy ions is most likely dominated by protons in the outer magnetosphere but is consistent with oxygen in the inner (L ≲ 9) magnetosphere. (4) The ion spectrum is described well by the κ distribution with characteristic temperatures kTH ranging from ∼15 to ∼55 keV; the hot plasma region is generally confined between the L shells of Tethys and Rhea but exhibits substantial variability. (5) The electron energy spectrum at L ≲ 10 develops a secondary peak at E ≳ 200 keV that shifts to higher (∼1 MeV) energies inside the orbits of Enceladus and Mimas, indicative of electron resonance interactions with the planetary satellites. (6) There is a noon‐dawn asymmetry in ion and electron intensities with peak fluxes near the Rhea‐Dione L shells at local morning; this is the region in local time where Saturn kilometric radiation is modulated by the presence of Dione. (7) The ion energy density (≳30 keV) represents a significant fraction of the field energy density in the outer magnetosphere of the planet (L ≳ 13 Rs), with values of β ranging from 0.1 up to ∼4, when projected to the equator. (8) Comparison of electron and ion intensities measured by Voyagers 1 and 2 in the inner (L ≲ 6) magnetosphere at common points in B, L space shows that the radiation belts are substantially stable over periods of ∼9 months; both ion and electron intensities compared well with Pioneer 11 observations in 1979. It is evident from the results that the inner satellites of Saturn play a dominant role in the determination of intensity and spectral features of energetic particles at L ≲ 10. These aspects of the data are discussed in the context of proposed physical mechanisms expected to be operating within the magnetosphere of Saturn.

Ion conics accompanied by electron beams are observed regularly in Saturn's magnetosphere. The beams and conics are seen throughout the outer magnetosphere, on field lines that nominally map from well into the polar cap (Ldipole > 50) to well into the closed field region (Ldipole < 10). The electron beams and ion conics are often observed together but also sometimes separately. Typically, the ion conics are prominent at energies between about 30 keV and 200 keV. The electron beams extend from below the ∼20 keV lower energy threshold for the instrument to sometimes as high as 1 MeV. The electrons may be either unidirectional (upward) or bidirectional; the ions are exclusively unidirectional upward. The ion conics are usually seen in conjunction with enhanced broadband electromagnetic noise in the 10 Hz to few kHz frequency range. Most of the wave energy appears below the local electron cyclotron frequency, hence, is propagating in the whistler mode, although some extension to higher frequencies is sometimes observed, suggesting an electrostatic mode. Sometimes the particle phenomena and the broadband noise occur in pulses of roughly 5 min duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. The ions are almost exclusively light ions (H, H2, H3, and/or He) with only occasional hints of oxygen or other heavier species, suggesting an ionospheric source. Taken together, the observations are strikingly similar to those made at Earth in association with auroral zone downward sheet currents, except that in the case of Saturn the particle energies are 20 to 100 times higher.

The low‐energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, angular distributions, and composition of ions (30 keV ≲E ≲150 MeV) and electrons (14 keV ≲E ≲10 MeV) during encounters with the Jovian magnetosphere in 1979. Detailed analysis of the multicomponent (H, He, O, S) low‐energy (∼30 keV to ∼4 MeV) ion population reveals the Jovian environment to be dominated by magnetospheric ions to distances ≳200 RJ upstream and ≳350 RJ downstream from the planet. Inside the magnetosphere, ions move generally in the sense of corotation to the dayside magnetopause, and on the nightside to distances of ∼130–150 Rj, beyond this distance, but inside the magnetopause, ion flow abruptly changes to an antisunward, anti‐Jupiter direction and continues to large (>350 RJ) radial distances outside the magnetosphere. The ion particle spectrum is characterized by a nonthermal power law (E−γ) component for E ≳200 keV, and a convected Maxwellian for E ≲200 with characteristic temperatures (kT) of ∼20‐45 keV. Temperature maxima generally coincide with crossings of the Jovian plasma sheet, while at higher energies spectra become softer at the equator. The ion spectra and composition are affected strongly by convective flows in all parts of the magnetosphere. By using the observed spectra and angular distributions, density and pressure profiles are produced for ions measured above the lowest LECP detector threshold (E ≳30 keV) and are compared with reported ambient total electron densities and magnetic field pressures. The particle pressures are found to be comparable to magnetic field pressures to at least ∼10 RJ, i.e., Jovian magnetosphere dynamics are determined by pressure variations in a high β plasma. Energetic ion densities are found to be comparable with the total electron densities in the outer (≳40 RJ) dayside magnetosphere but are generally lower at smaller radial distances and exhibit substantial variability. We interpret the hot plasma outflow on the nightside of Jupiter as a ‘magnetospheric wind' and estimate the mass and energy loss through this region at ∼2 × 1027 ions/s and ∼2 × 1020 ergs/s, respectively. We find the plasma source from the active volcanoes on Io to be adequate for supplying the mass outflow; only the rotational kinetic energy of Jupiter is sufficient to provide the energy in the flow, although energy from the solar wind interaction can perhaps contribute a significant fraction. A phenomenological model of Jupiter's magnetosphere is presented which accounts for the observations.