Planetary period modulations of Saturn's magnetotail current sheet during northern spring: Observations and modeling

• Published in: Journal of geophysical research. Space physics 2017-06, Vol.122 (6), p.6049-6077
• Main Authors: Cowley, S. W. H., Provan, G.
• Format: Article
• Language: English
• Subjects: Amplitudes; Asymmetry; Cassini mission; Magnetic fields; Magnetosphere; Magnetospheres; Mathematical models; Modulation; Oscillations; Phases; Planetary magnetic fields; plasma sheet; Saturn; Saturn magnetotail; Spring (season)
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2017JA023993

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). 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. Key Points We study dual PPO‐related modulation of Saturn's tail current sheet during 2015, when the northern PPO system was stronger than that of the southern Clear effects depending on the relative north‐south system phases are discernable for north/south amplitude ratios ~2 but not for >5 A simple mathematical model reproduces the main field modulation effects using a single set of parameters while varying the relative phase