Observations from the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) are used in an in-depth investigation of the occurrence and location of reconnection at Saturn's magnetopause. Heated, streaming electrons parallel and/or antiparallel to the magnetic field in the magnetosheath adjacent to the magnetopause indicate that reconnection is occurring somewhere on the boundary. In these instances, the Cassini spacecraft is connected to open magnetic field lines that thread the magnetopause boundary. A survey of 99 crossings with sufficient pitch angle coverage from CAPS/ELS indicates that 65% of the crossings had this evidence of reconnection. Specific crossings from this survey are used to demonstrate that there are times when reconnection at Saturn's low-latitude magnetopause may be suppressed.
Magnetospheric Multiscale (MMS) observations during an extended crossing of the Earth's dayside magnetopause show evidence of multiple reconnections at the boundary. This crossing occurred when the Interplanetary Magnetic Field (IMF) was southward and had a significant BY component. Approximately 2 hr after this crossing, the Twin Rocket Investigation of Cusp Electrodynamics‐2 rockets were launched into the northern hemisphere cusp and observed overlapping cusp ion injections. These overlapping injections are also evidence of multiple reconnections at the magnetopause. Observations more than 2 hr apart do not constitute conjunction between the spacecraft and the rockets. However, the IMF conditions during the magnetopause crossing and the cusp traversal were very similar. Therefore, had the magnetopause crossings and cusp traversals occurred at the same time, the observations would have been similar. Thus, these two events illustrate the link between multiple reconnections at the magnetopause and overlapping cusp ion injections.
At Saturn, solar wind driving of the magnetosphere is predicted to be reduced due to the extreme rotationally driven interior. As a result, the rotationally driven plasma corotates with the planet, possibly out to the magnetopause. Modeling suggests that this corotation should set up conditions that are unfavorable for the onset of magnetic reconnection on the dawnside magnetopause due to the generation of a significant flow shear. The flow shear model being tested makes use of idealized steady state models and a dominant east-west interplanetary magnetic field orientation with a slight northward component. When the flow shear is larger than the Alfvèn speed, the model indicates that reconnection is suppressed. In this study, we employ observations from Cassini, when the interplanetary magnetic field has a northward component, to test this flow shear suppression model and whether reconnection is suppressed by diamagnetic drifts. Observations from the electron spectrometer on the Cassini Plasma Spectrometer suite of instruments show that reconnection does occur at the dayside magnetopause of Saturn, even in locations where modeling suggests that conditions are unfavorable for its onset. The inconsistency between modeling and observations may be caused by the conditions and assumptions within the model itself. Furthermore, the occurrence of reconnection on the dawnside magnetopause in general may hint at additional complexities in the global structure of Saturn's magnetosphere.
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