Abstract
Using measurements from the Cassini spacecraft in Saturn's magnetosphere, we propose a 3D physical picture of a corotating reconnection site, which can only be driven by an internally generated source. Our results demonstrate that the corotating magnetic reconnection can drive an expansion of the current sheet in Saturn's magnetosphere and, consequently, can produce Fermi acceleration of electrons. This reconnection site lasted for longer than one of Saturn's rotation period. The long-lasting and corotating natures of the magnetic reconnection site at Saturn suggest fundamentally different roles of magnetic reconnection in driving magnetospheric dynamics (e.g., the auroral precipitation) from the Earth. Our corotating reconnection picture could also potentially shed light on the fast rotating magnetized plasma environments in the solar system and beyond.
Highlights
The terrestrial magnetospheres receive energy from the Sun via dayside magnetopause reconnection and occasionally explosively release this energy, causing strong perturbations within their nightside magnetospheres, ionospheres, and atmospheres
The agreement between our corotating reconnection picture and the Cassini in situ measurements is striking, indicating that the reversal of Bq cannot be fully explained by a 2D magnetic reconnection retreat picture, which can either be driven by the solar wind or an internal source
We must emphasize that if this long-lasting reconnection is driven by the solar wind, the reconnection site should not corotate with the planet
Summary
The terrestrial magnetospheres receive energy from the Sun via dayside magnetopause reconnection and occasionally explosively release this energy, causing strong perturbations within their nightside magnetospheres, ionospheres, and atmospheres. The magnetospheric currents can from time to time divert into the ionosphere and cause auroral intensifications (Mitchell et al 2005) During this process, the magnetic field in the magnetosphere would naturally become more dipolar. In situ evidence directly linking the Vasyliūnas cycle to global magnetospheric dynamics is rare, the quasi-periodicity of the particle and magnetic field perturbations at Jupiter implies that the planet’s rotation may participate in planetary energy reloading (Kronberg et al 2005). Statistical studies of reconnection events at Jupiter (Vogt et al 2014) and Saturn (Jackman et al 2014) suggest that the reconnection line orientation may oppose that predicted by simulations and theoretical models of the Vasyliūnas cycle It is still unclear how the Vasyliūnas cycle drives the dynamics of giant magnetospheres. Regarding the different environments of Saturn and Earth, we hereby define an Earth substorm-like EER as a process that includes current sheet north–south expansion (i.e., thickening), particle acceleration, and magnetic dipolarization
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