Alfv�nic Wave Resonances in the Kronian and Terrestrial Magnetospheres: Modeling and Observations
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Alfv�nic Wave Resonances in the Kronian and Terrestrial Magnetospheres: Modeling and Observations


Ultra-low frequency (ULF) waves have been commonly detected in inner and outer Solar System planetary magnetospheres. ULF waves with periods that are of the order of the Alfv�n wave transit time in a planetary magnetosphere are often associated with resonant field lines that can be excited by either internal or external triggers. In this dissertation, we present a comparative study of standing Alfv�n waves in realistic magnetic field and plasma density models for the Kronian and Terrestrial magnetospheres.At Saturn, we carried out the first calculation of standing wave resonances in a realistic model of the Kronian magnetosphere. The resulting eigenperiods of the 4th harmonic vary little with radial distance from 5 to 20RS, matching the quasi-periodic 60-minute (QP60) waves that have been reported in observations at a wide range of local times and radial distances. We have used 13 years of the Cassini magnetometer data and identified quasi-periodic fluctuations with periodicities of around 30 minutes (QP30), 60 minutes (QP60), and 120 minutes (QP120) that reoccur at the period of planetary period oscillations (PPO) of roughly 10.7 h. We suggest that these correspond to even-mode harmonics of Saturn’s magnetic field lines that are excited by the periodic vertical flapping of Saturn’s magnetotail. At Earth, we evaluated the field line resonances for several plasma density models and investigated the effects of geomagnetic activity on the field line eigenperiods up to L = 10 at all magnetic local times. We find that the dipole-field and the time-of-flight approximations used to estimate the fundamental eigenperiods of standing waves lead to significantly different eigenperiods, especially during active times. Additionally, the eigenperiods are shown to be more sensitive to the magnetic field configuration and equatorial plasma densities than the distribution of the mass density along the field lines. Saturn’s and Earth’s magnetospheres have large differences in scale, rotational speed, and plasma distribution. Despite these differences in the parameter regimes, field line resonances are important at both magnetospheres. The QP30, QP60, and QP120 waves at Saturn reoccur consistently at a PPO period, with the highest transverse to parallel magnetic perturbation power in the post-dusk sector, suggesting mainly an internal driver such as the vertical flapping of the magnetotail. At Earth, the field line resonances that are associated with Pc4 to Pc5 waves do not demonstrate similar periodicity in reoccurrence as at Saturn and are likely to be driven externally by processes such as the solar wind dynamic pressure variations.

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