[1] We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturn's magnetosphere over the interval from late 2004 to early 2011, spanning equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L ≤ 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the equinox interval, such that the beat period increases significantly from $20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a $10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.

We investigate magnetic data showing the presence of field-aligned magnetosphere-ionosphere coupling currents on 31 Cassini passes across Saturn's southern postmidnight auroral region. The currents are strongly modulated in magnitude, form, and position by the phase of the southern planetary period oscillations (PPOs). PPO-independent currents are separated from PPO-related currents using the antisymmetry of the latter with respect to PPO phase. PPO-independent downward currents~1.1 MA per radian of azimuth flow over the polar open field region indicative of significant plasma subcorotation are enhanced in an outer plasma sheet layer of elevated ionospheric conductivity carrying~0.8 MA rad À1and close principally in an upward directed current sheet at~17°-19°ionospheric colatitude carrying 2.3 MA rad À1 that maps to the outer hot plasma region in Saturn's magnetosphere (equatorial rangẽ 11-16 Saturn radii (R S )) colocated with the UV oval. Subsidiary downward and upward currents~0.5 MA rad À1 lie at~19°-20.5°colatitude mapping to the inner hot plasma region, but no comparable currents are detected at larger colatitudes mapping to the cool plasma regime inside~8 R S . PPO-related currents at~17.5°-20°colatitude overlap the main upward and subsidiary downward currents and carry comparable rotating upward and downward currents peaking at~1.7 MA rad À1 . The overall current layer colatitude is also modulated with 1°amplitude in the PPO cycle, maximum equatorward adjacent to the peak upward PPO current and maximum poleward adjacent to peak downward PPO current. This phasing requires the current system to be driven from the planetary atmosphere rather than directly from the magnetosphere.

We investigate the magnetic perturbations associated with field‐aligned currents observed on 34 Cassini passes over the premidnight northern auroral region during 2008. These are found to be significantly modulated not only by the northern planetary‐period oscillation (PPO) system, similar to the southern currents by the southern PPO system found previously, but also by the southern PPO system as well, thus providing the first clear evidence of PPO‐related interhemispheric current flow. The principal field‐aligned currents of the two PPO systems are found to be co‐located in northern ionospheric colatitude, together with the currents of the PPO‐independent (subcorotation) system, located between the vicinity of the open‐closed field boundary and field lines mapping to ~9 Saturn radius (Rs) in the equatorial plane. All three systems are of comparable magnitude, ~3 MA in each PPO half‐cycle. Smaller PPO‐related field‐aligned currents of opposite polarity also flow in the interior region, mapping between ~6 and ~9 Rs in the equatorial plane, carrying a current of ~ ±2 MA per half‐cycle, which significantly reduce the oscillation amplitudes in the interior region. Within this interior region the amplitudes of the northern and southern oscillations are found to fall continuously with distance along the field lines from the corresponding hemisphere, thus showing the presence of cross‐field currents, with the southern oscillations being dominant in the south, and modestly lower in amplitude than the northern oscillations in the north. As in previous studies, no oscillations related to the opposite hemisphere are found on open field lines in either hemisphere.

We examine magnetic field data obtained by the Cassini spacecraft on a sequence of high‐latitude orbits in Saturn's magnetosphere spanning October 2006 to May 2007 to determine whether planetary‐period oscillations are present on polar open field lines, such as have been found previously in near‐equatorial magnetic field data. Such oscillations are found generally to be present with amplitudes ∼0.5–1 nT, somewhat smaller than the few nT amplitudes typical of the quasi‐dipolar equatorial region. The polarization characteristics in the northern and southern polar regions are determined and found to differ significantly from those in the equatorial region. The phases of the oscillations in the northern and southern hemispheres are also determined relative to the equatorial oscillations, and hence relative to each other, requiring extension of the equatorial oscillation phase model to the end of 2007, spanning the interval of high‐latitude orbits. The results show that the overall pattern of field oscillations is not consistent with a rotating external current system that mimics a rotating transverse dipole in the outer regions. Rather, we suggest that the overall field perturbations are associated with a rotating partial ring current and its field‐aligned closure currents, the latter favoring the southern ionosphere during the southern summer conditions examined. A physical picture is presented that links together observed planetary‐period modulations in the middle and outer magnetospheric field, plasma, and radio emissions that may be subject to further test and makes predictions as to how these phenomena will evolve during future Saturn equinox and northern summer conditions.

[1] We examine the planetary-period oscillations in Saturn's magnetic field observed by the Cassini spacecraft on 23 near-equatorial periapsis passes in the inner magnetosphere spanning October 2004 to July 2006. Overall, we find that the phase of the magnetic oscillations is well organized by the long-timescale modulation phase of Saturn kilometric radiation (SKR) determined over the same interval by Kurth et al. (2007), suggesting that the slow period variation of the latter relates to inner magnetosphere processes. The relative phases of the oscillations in the spherical polar r and 8 magnetic field components imply the presence of a quasi-uniform equatorial field rotating near the SKR period, while the sense of the q component indicates that the perturbation field lines form loops with apices in the Northern Hemisphere. No consistent evidence is found for a sign reversal in any field component across the equatorial plane, within ±20°in latitude. The relative SKR phasing is such that the peak radio power occurs when the r and q component maxima lie at $0200 LT ± 2 hours. However, a slow drift of the magnetic phase relative to the SKR phase is also discerned, amounting to $75°over the study interval. This drift lies within the envelope of scatter in the SKR phase determinations, suggesting that it represents the refinement of a common periodicity. A revised magnetic phase or longitude model is derived that should form an improved organizational system for oscillatory phenomena observed during this interval of the Cassini mission. The magnetic oscillations are also found to exhibit pass-to-pass phase ''jitter'' about the long-term variation, of RMS amplitude $20°, with r and 8 strongly correlated, but not q. The relation with the solar wind-modulated short-timescale phase variations reported in SKR data by Zarka et al. (2007) remains to be investigated, though the latter are 5 times larger in magnitude.