Large-scale dynamics of Saturn's magnetopause: Observations by Cassini

摘要

The long-term statistical behavior of the large-scale structure of Saturn's magnetosphere has been investigated. Established statistical techniques for Jupiter have been applied to the kronian system, employing Cassini magnetometer data and a new empirical shape model of the magnetopause based on these data. The resulting distribution of standoff distance R MP for Saturn, covering a time interval of ～400 days, is well-described by a “dual” or “bimodal” model—the sum of two normal distributions with different means at ～22 and ～27 planetary radii. We have made a comparison between the dual model's prediction for the probability distribution of solar wind dynamic pressure at Saturn with a sequence of observations from the Cassini Plasma Spectrometer (CAPS) instrument. Although the solar wind dynamic pressure observations are limited to a smaller time interval than the magnetometer data, we find that their overall range is in broad agreement with the that of the modeled pressures. However, the bimodal structure exhibited by the model is not apparent in the solar wind data for the corresponding range of dynamic pressures (～0.008 – 0.06 nPa), which suggests that other mechanisms at Saturn also influence the size distribution of the magnetopause. Considering internal processes at Saturn and their influence on magnetopause size, we conclude that the effect of internal mass loading and loss from the magnetospheric disk is plausibly able to explain the observed bimodal distribution in magnetopause standoff distance.