Characterizing the limitations to the coupling between Saturn's ionosphere and middle magnetosphere

Abstract

[1] Observations of Saturn's ultraviolet and infrared aurora show structures that, when traced along the planetary magnetic field, map to the inner, middle, and outer magnetosphere. From low to high latitudes the structures seen in the UV are the Enceladus footprint, which maps to an equatorial radius of 4 R S (Saturn radii); a diffuse emission that maps to a broad equatorial region from 4-11 R S on the nightside; and a bright ring of emission that maps to $15 R S . With the exception of the Enceladus spot, the magnetospheric drivers for these auroral emissions are not yet fully understood. We apply a 1D spatial, 2D velocity space Vlasov solver to flux tubes mapping from equatorial radii of 4, 6, 9, and 13 R S to Saturn's southern atmosphere. The aim is to globally characterize the field-aligned potential structure and plasma density profiles. The ionospheric properties -the field-aligned current densities at the ionospheric boundary, energy intensity profiles and fluxes of the electrons precipitating into the ionosphere -are also determined. We then couple our results to an ionospheric model to calculate the Pedersen conductances at the foot of the relevant flux tubes. We find that for a zero net potential drop between the ionosphere and magnetosphere, there exists a sharp potential drop at $1.5 R S along the magnetic field line as measured from the planetary center. The strength of this potential drop is approximately equal to that of the ambipolar potential resulting from the centrifugal confinement of the heavy, cold magnetospheric ion population. We also find that the ionospheric properties respond to changes in the magnetospheric plasma population, which are reflected in the nature of the precipitating electron population. For the flux tube mapping to 9 R S (À70 ), the incident electron energy flux into the ionosphere resulting from a magnetospheric plasma population with a small fraction of hot electrons is an order of magnitude less than that inferred from observations, implying that significant high-latitude field-aligned potentials (up to 1.5 keV) may exist in the saturnian magnetosphere. Calculated ionospheric Pedersen conductances range from 3.0-18.9 mho, and are thus not expected to limit the currents flowing between the ionosphere and magnetosphere.Citation: Ray, L. C., M. Galand, L. E. Moore, and B. Fleshman (2012), Characterizing the limitations to the coupling between Saturn's ionosphere and middle magnetosphere,

Auroral Processes at the Giant Planets: Energy Deposition, Emission Mechanisms, Morphology and Spectra

“…Millward et al 2002;Hiraki and Tao 2008;Smith and Aylward 2009;Tao et al 2010) and to Saturn (e.g. Moore et al 2010;Galand et al 2011;Ray et al 2012a; or derived using Cassini/RSS electron density (Moore et al 2010). The Pedersen conductivity profiles are strongly peaked in altitude, as illustrated in Figure 8.…”

“…• latitude (Ray et al 2012a). Over the whole range of latitudes (which does not include the main ovals) Moore et al (2010) assessed Pedersen conductances from Cassini/RSS observations and obtained values as high as 8 mho when the full altitude profiles with sharp, narrow peaks in the low ionosphere are included.…”

Unusually short period in electrons at Saturn

“…One acceleration mechanism by which this would occur is, for example, a field‐aligned potential associated with the FAC. Such currents and potentials have, in fact, been suggested at Saturn [ Schippers et al , 2012; Ray et al , 2012]. In the region of the magnetosphere considered here, the energetic electrons are usually field‐aligned and magnetically connected to latitudes near the aurora where they stimulate auroral emissions through secondaries [ Mitchell et al , 2009; Carbary et al , 2011b].…”

Solar Wind and Internally Driven Dynamics: Influences on Magnetodiscs and Auroral Responses

“…The origin of the main emissions is related to the corotation breakdown, the large scale magnetosphere-ionosphere coupling and the associated field-aligned currents (e.g. Nichols 2011; Ray et al 2012 and reference therein). analysed far UV spectra from the STIS instrument on-board HST and studied the relationship between the brightness of the emissions and the methane absorption (through color ratios).…”