A plasmapause‐like density boundary at high latitudes in Saturn's magnetosphere

Abstract

Here we report the discovery of a well‐defined plasma density boundary at high latitudes in Saturn's magnetosphere. The boundary separates a region of relatively high density at L less than about 8 to 15 from a region with densities nearly three orders of magnitude lower at higher L values. Magnetic field measurements show that strong field‐aligned currents, probably associated with the aurora, are located just inside the boundary. Analyses of the anisotropy of energetic electrons show that the magnetic field lines are usually closed inside the boundary and open outside the boundary, although exceptions sometimes occur. The location of the boundary is also modulated at the ∼10.6 to 10.8 hr rotational period of the planet. Many of these characteristics are similar to those predicted by Brice and Ioannidis for the plasmapause at a strongly magnetized, rapidly rotating planet such as Saturn.

The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals

“…Near the planet the boundary is typically located at dipole L values from about 8 to 15, although at greater radial distances larger L values are often observed, probably due to the distorting effects of magnetospheric currents. Anisotropy measurements from the Cassini MIMI-LEMMS instrument show that the density boundary is located near the boundary between "open" and "closed" magnetic field lines, see Gurnett et al [2010b]. The close relationship to the open/ closed field line boundary strongly suggests that the density boundary is aligned with the magnetic field.…”

“…The dawndusk oscillations were interpreted as being due to the eccentric rotation of the auroral ovals with amplitudes of about 1 to 2 degrees. In this paper we present a study of the rotation of the plasmapause-like density boundary discovered at high latitudes by Gurnett et al [2010b]. We will show that the density boundary rotates at different rates in the two hemispheres, and that the rotation period in a given hemisphere is the same as the SKR period in that hemisphere.…”

“…[4] Electron density measurements from the Langmuir probe on Cassini show that a well-defined rotating density boundary exists at high latitudes in Saturn's magnetosphere [Gurnett et al, 2010b]. The boundary is observationally very similar to Earth's plasmapause, and separates a region of very low densities, ∼10 −4 to 10 −3 cm −3 , at high latitudes from a region of much higher densities, 10 −2 to 10 0 cm −3 , at lower latitudes.…”

Planetary period oscillations in Saturn's magnetosphere: Examining the relationship between abrupt changes in behavior and solar wind‐induced magnetospheric compressions and expansions

“…Despite this, however, oscillations with a period near thẽ 11 h planetary period are observed throughout Saturn's magnetosphere. Such modulations are observed in the magnetic field, Saturnian kilometric radiation (SKR), plasma parameters, energetic particle fluxes, and associated neutral atom emissions, as well as auroral ultraviolet, infrared, and radio emissions [e.g., Warwick et al, 1981Warwick et al, , 1982Desch and Kaiser, 1981;Gurnett et al, 1981Gurnett et al, , 2007Gurnett et al, , 2010aSandel and Broadfoot, 1981;Sandel et al, 1982;Carbary and Krimigis, 1982;Espinosa and Dougherty, 2000;Krupp et al, 2005;Cowley et al, 2006;Kurth et al, 2007;Southwood and Kivelson, 2007;Carbary et al, 2007Carbary et al, , 2008aCarbary et al, , 2008bZarka et al, 2007;Andrews et al, 2008;Nichols et al, 2008Nichols et al, , 2010aNichols et al, , 2010bBurch et al, 2009;Provan et al, 2009aProvan et al, , 2009bClarke et al, 2006;Badman et al, 2012;Lamy et al, 2013]. Those oscillations were first observed in Voyager measurements of the SKR emissions, around the time of Saturn's northern hemisphere spring equinox [Kaiser et al, 1980].…”

Direct evidence of differences in magnetic rotation rate between Saturn's northern and southern polar regions

“…[39] Such a shear in the field or the sharp change in density reported in the same region by Gurnett et al [2010] can make a surface in the plasma which can be the seat of a surface wave, sometimes called an Alfvén surface wave [see, e.g., Uberoi, 1982]. The magnetic boundary appears very sharp.…”