Abstract
AbstractWe study Cassini magnetic field observations at Saturn on a sequence of passes through the near‐equatorial magnetotail during 2015, focusing on dual modulation of the plasma/current sheet associated with northern and southern planetary period oscillations (PPOs). Previous study of inner magnetosphere PPOs during this northern spring interval showed that the southern system amplitude was generally half that of the northern during the first part of the year to late August, after which the southern amplitude weakened to less than one‐fifth that of the northern. We examine four sequential tail passes in the earlier interval, during which prominent PPO‐related tail field modulations were observed, with relative (beat) phases of the two PPO systems being near in phase, antiphase, and two opposite near‐quadrature conditions. We find that the radial field displayed opposite “sawtooth” asymmetry modulations under opposite near‐quadrature conditions, related to previous findings under equinoctial conditions with near‐equal northern and southern PPO amplitudes, while modulations were near symmetric for in‐phase and antiphase conditions, but with larger radial field modulations for in‐phase and larger colatitudinal field modulations for antiphase. A simple physical mathematical model of dual modulation is developed, which provides reasonable correspondence with these data using one set of current sheet parameters while varying only the relative PPO phases, thus demonstrating that dual modulation can be discerned and modeled even when the northern and southern amplitudes differ by a factor of ~ 2. No such effects were consistently discerned during the later interval when the amplitude ratio was >5.
Highlights
Despite the near axisymmetry of Saturn’s planetary magnetic field [Burton et al, 2010], Cassini observations have shown that modulations near the planetary rotation period of ~10.6 h are ubiquitously present in essentially all magnetospheric parameters
We study Cassini magnetic field observations at Saturn on a sequence of passes through the near-equatorial magnetotail during 2015, focusing on dual modulation of the plasma/current sheet associated with northern and southern planetary period oscillations (PPOs)
This interval corresponds to Saturn’s northern spring, during which the amplitude of the northern PPO system was initially about twice that of the southern system to late August of 2015, after which the amplitude of the southern system declined further to become less than one fifth of that of the northern [Provan et al, 2016]
Summary
Despite the near axisymmetry of Saturn’s planetary magnetic field [Burton et al, 2010], Cassini observations have shown that modulations near the planetary rotation period of ~10.6 h are ubiquitously present in essentially all magnetospheric parameters (see, e.g., the review by Carbary and Mitchell [2013, and references therein]). A third and final interval of near-equatorial Cassini tail passes took place during 2015 after period reversal [see Cowley et al, 2017, Figure 1], for which the north/south amplitude ratio was k ≈ 2.3 during the first part of the year to late August, increasing after that to greater than ~5, as indicated above We examine these data to determine whether beat phase asymmetry effects can be discerned under such near-limiting conditions and show that they are present during the earlier interval of 2015, though being less distinct than in the transequinoctial 2010 data, but are not discerned in the later interval. Consideration of these data is supported by physical modeling calculations using an augmented version of the mathematical model introduced by Cowley et al [2017]
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