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A possible model for electron circulation in Saturn's magnetosphere

Dr. Abigail Rymer (Johns Hopkins University, Applied Physics Laboratory, USA)

Unlike Jupiter, which has a relatively discrete source of cold electrons in the inner magnetosphere associated with the Io torus, Saturn has a distributed source of cold (<100 eV) electrons inside L ~ 12 associated with Saturn's satellites, rings, and extended neutral clouds.  Like Jupiter (and the Earth and presumably all planets with an appreciable magnetosphere) magnetic flux lost through cold plasma outflow is balanced by the injection of hot outer magnetospheric plasma inward.  Whilst direct observations of inward plasma injection into Saturn's magnetosphere are quite well documented, the outward counterpart to those injections has not been observed.  Hence, the mode of flux return to the middle magnetosphere, to balance the inward flux transport associated with injections, has been a mystery at both Jupiter and Saturn.  It has been suggested that broad-scale outward flow at Jupiter is the counterpart to the observed inward injection of flux.  Evidence that such a configuration exists at Saturn has recently been provided by Burch et al. [manuscript submitted to Nature 2006].  Specifically, these authors recently discovered 'butterfly' electron pitch angle distributions between 6 and 9 Saturn radii (RS) at energies of a few hundred to a few thousand eV.  They interpret these distributions as being a consequence of outward adiabatic transport.  The generation of particle distributions with enhancements in the field-aligned direction is a well-known consequence of outward adiabatic transport in a dipolar magnetic configuration.  Hence the configuration that the Burch et al. evidence supports comprises large-scale outward adiabatic "flow" balanced by small-scale inward injections.  We note that there is not an obvious mechanism to generate the internal source of 100-1000 eV electrons.  If the observed butterfly distributions evolve from outward transport as Burch et al. describe, we are left with a mystery: how are the source populations generated?  Here we propose a possible internal source in terms of electron circulation.

 

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