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 compare the open flux content of the magnetosphere, quantified by measurements of the size of the northern ionospheric polar cap, with the radius of the magnetotail at X % À25 R E , deduced from observations made by the IMP-8 spacecraft. During an 8-hour period of observation we estimate that the proportion of terrestrial flux that is interconnected with the solar wind varies between 12 and 2.5%. This latter extreme, representing an almost closed magnetosphere, follows the incidence of a solar wind dynamic pressure step, the onset of a large substorm, and a 3-hour period of northward IMF. The deflated and compressed magnetotail is predicted to have a radius as small as 12 R E at this time. The magnetotail does not reinflate to more typical dimensions until some time after a southward turning of the IMF, leading to accumulation of open flux through low latitude reconnection. We compare our observations with estimates of the shape of the magnetopause from an empirical model. We also present a simple model of the varying length of the magnetotail, based on upstream solar wind conditions, and observations of the size of the polar cap.

[1] Presented here is a discussion of the results of a superposed epoch analysis of geomagnetic storms over the last solar cycle. Storms, identified by means of their characteristic SYM-H evolution, are separated by size into weak (−150 < SYM-H ≤ −80) nT, moderate (−300 < SYM-H ≤ −150) nT, and intense (SYM-H ≤ −300) nT categories. Where possible, the corresponding solar wind (SW) onset mechanisms were located by means of 1 min ACE OMNI data. Intense storms were observed to be driven solely by coronal mass ejections (CMEs); moderate storms were dominated by CME onset, while only weak storms were driven by both CMEs and corotating interaction regions (CIRs) at a ratio of ∼2:1, respectively. As might be expected, more intense storms resulted from the largest SW enhancements. Individual storm phase durations for different storm sizes were investigated, revealing that the duration of the main phase increases with storm size to a critical point, then decreases for more intense storms, contrary to the findings of a previous study by Yokoyama and Kamide (1997). Various SW-magnetosphere coupling functions were investigated for this data set in an attempt to estimate storm size from SW conditions.

[1] Cassini magnetic field observations show that few-nT oscillations near the planetary rotation period, first observed in Pioneer-11 and Voyager-1 and -2 fly-by data, are essentially ubiquitous in Saturn's magnetosphere, though their character differs between the quasi-dipolar ring current region and the dawn tail. Examination of data from the ringcurrent region shows, however, that the observed oscillation period is not fixed at the planetary period, but has smaller values on the inbound pass of the spacecraft, increasing to larger values at and beyond periapsis. These variations are shown to be consistent with the Doppler shifts due to spacecraft motion expected in a model in which the wave phase fronts rotate with the planet, and radiate outward at a speed comparable with the equatorial Alfvén speed.

Abstract. Resonance features of the Ionospheric AlfvénResonator (IAR) can be observed in pulsation magnetometer data from Sodankylä, Finland using dynamic spectra visualizations. IAR resonance features were identified on 13 of 30 days in October 1998, with resonance structures lasting for 3 or more hours over 10 intervals. The diurnal evolution of the harmonic features was quantified for these 10 intervals using a manual cursor-clicking technique. The resonance features displayed strong linear relationships between harmonic frequency and harmonic number for all of the time intervals studied, enabling a homogeneous cavity model for the IAR to be adopted to interpret the data. This enabled the diurnal variation of the effective size of the IAR to be obtained for each of the 10 time intervals. The average effective size was found to be 530 km, and to have an average variation of 32% over each time interval: small compared to the average variation in Alfvén velocity of 61%. Thus the diurnal variation of the harmonics is chiefly caused by the changing plasma density within the IAR due to changing insolation. This study confirms Odzimek (2004) that the dominating factor affecting the IAR eigenfrequencies is the variation in the Alfvén velocity at the F-layer ion-density peak, with the changing IAR size affecting the IAR eigenfrequencies to a smaller extent. Another IAR parameter was derived from the analysis of the IAR resonance features associated with the phase matching structure of the standing waves in the IAR. This parameter varied over the time intervals studied by 20% on average, possibly due to changing ionospheric conductivity.