Abstract
AbstractWe present the discovery of seven new flux transfer events (FTEs) at Saturn's dayside magnetopause by the Cassini spacecraft and analyze the observations of all eight known FTEs. We investigate how FTEs may differ at Saturn where the magnetopause conditions are likely to diamagnetically suppress magnetic reconnection from occurring. The measured ion‐scale FTEs have diameters close to or above the ion inertial length di∼1–27 (median and mean values of 5 and 8), considerably lower than typical FTEs found at Earth. The FTEs magnetic flux contents are 4–461 kWb (median and mean values of 16 and 77 kWb), considerably smaller (<0.1%) than average flux opened during magnetopause compression events at Saturn. This is in contrast to Earth and Mercury where FTEs contribute significantly to magnetospheric flux transfer. FTEs therefore represent a negligible proportion of the amount of open magnetic flux transferred at Saturn. Due to the likely suppression of the two main growth‐mechanisms for FTEs (continuous multiple x‐line reconnection and FTE coalescence), we conclude that adiabatic expansion is the likely (if any) candidate to grow the size of FTEs at Saturn. Electron energization is observed inside the FTEs, due to either Fermi acceleration or parallel electric fields. Due to diamagnetic suppression of reconnection at Saturn's magnetopause, we suggest that the typical size of FTEs at Saturn is most likely very small, and that there may be more di∼1 FTEs present in the Cassini magnetometer data that have not been identified due to their brief and unremarkable magnetic signatures.
Highlights
Magnetic reconnection is a phenomenon that rearranges the topology of magnetic fields resulting in magnetic energy being transferred to particles
Due to the likely suppression of the two main growth-mechanisms for flux transfer events (FTEs), we conclude that adiabatic expansion is the likely candidate to grow the size of FTEs at Saturn
Taking the largest magnetic flux content estimated for the 02-Feb-2007b FTE of 0.46 MWb and comparing it to the average change in open flux calculation of ∼7 GWb during a Dungey cycle from Badman et al (2014), this would mean that FTEs account for less than 0.1% of the opening of flux
Summary
Magnetic reconnection is a phenomenon that rearranges the topology of magnetic fields resulting in magnetic energy being transferred to particles. Flux transfer events (FTEs) are transient magnetic structures that are a result of reconnection occurring at the dayside magnetopause of a magnetosphere As the solar wind expands radially outwards throughout the solar system, the solar wind magnetosonic Mach number increases with radial distance from the Sun, resulting in higher plasma-β magnetosheaths at the outer planet magnetospheres (where plasma-β is the plasma to magnetic field pressure ratio). This leads to a large difference in plasma-β across the magnetopause, which acts to suppress magnetic reconnection from occurring unless the magnetic shear across the boundary is very high
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