Abstract
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.
Highlights
[5] The purpose of the present paper is to examine the evolution of the properties of the magnetic field oscillations over the interval from southern summer to post-equinox, from late 2004 to early 2011, both in phase and for the first time in amplitude, focusing on the combined northern- and southern-period oscillations within the quasi-dipolar core region
Using the phase jitter effect found by Provan et al [2011], it is shown that the phase and period of both southern and northern field oscillations can be independently retrieved from the data, together with measures of the amplitude of the two oscillations
We show that from conditions of southern-oscillation dominance during southern summer as indicated above, the amplitude of the northern oscillations in the core grows by a factor of two to become comparable with that of the southern oscillations during the equinox interval, with significant consequences for the properties of the oscillations observed from pass to pass
Summary
[2] Saturn’s internal planetary magnetic field has been found to be highly axisymmetric [Smith et al, 1980; Connerney et al, 1982], with an upper limit to the dipole tilt of $0.1 according to recent determinations [Burton et al, 2010], pronounced modulations of the magnetic field, plasma properties, plasma waves, and auroral radio and ultraviolet (UV) emissions near the $11 h planetary period have been found to be ubiquitous throughout Saturn’s magnetosphere [e.g., Warwick et al, 1981, 1982; Gurnett et al, 1981, 2005, 2007, 2010a; Desch and Kaiser, 1981; Sandel and Broadfoot, 1981; Sandel et al, 1982; Carbary and Krimigis, 1982; Espinosa. Starting near the beginning of 2009, ahead of vernal equinox in August 2009, the periods of the two SKR modulations have been observed to more rapidly converge, reaching nearcommon values of $10.7 h by early 2010 and either coalescing [Lamy, 2011] or crossing [Gurnett et al, 2010b, 2011] in the interval beyond. This behavior has been followed to mid-2010 in the magnetic oscillation data, but only in respect of the southern-period oscillations [Andrews et al, 2011]. It is shown that the periods of the two magnetic oscillations strongly converge during the equinox interval, but do not coalesce or cross during the interval examined to early 2011
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