Statistical characteristics of field-aligned currents in Saturn's nightside magnetosphere

D. L. Talboys,S. W. H. Cowley,M. K. Dougherty,E. J. Bunce,A. J. Coates,C. S. Arridge

doi:10.1029/2010ja016102

Abstract

During 2008, the Cassini spacecraft traversed Saturn's high-latitude field-aligned current systems on sequential north-south periapsis passes in the nightside magnetosphere. Two types of current systems have previously been identified, associated with antisymmetric azimuthal field signatures in the northern and southern hemispheres. The first exhibits exclusively lagging field morphology, while the second also includes an equatorward interval of leading field. Here we report the statistical characteristics of these currents, their strength, ionospheric location, and relationship to plasma boundaries. From high to low latitude, the first type comprises a downward current followed by an upward current, whose strengths are similar to 0.5-3.5 MA per radian of azimuth. The downward current maps to ionospheric colatitudes of similar to 13.5 degrees and similar to 16 degrees in the north and south, respectively, usually centered in the outer magnetosphere, while the upward current maps to similar to 16.5 degrees and similar to 19 degrees in the north and south, located within the ring current region. The second type comprises a distributed downward current of similar to 1-2 MA rad(-1) flowing in the open field and outer magnetosphere regions, followed by an upward current of similar to 2.5-5 MA rad(-1) mapping to similar to 15.5 degrees and similar to 18 degrees in the north and south, corresponding to the outer magnetosphere and outer ring current, and a further downward current of similar to 1-2.5 MA rad(-1) mapping to similar to 17.5 degrees and similar to 20 degrees in the north and south, corresponding to the inner ring current.

Highlights

[2] The first global study of the magnetic field perturbations associated with field‐aligned (Birkeland) currents at the Earth was performed using the polar orbiting U.S Navy TRIAD satellite [Iijima and Potemra, 1976a, 1976b]
For the usual case in which the magnetospheric plasma flow subcorotates relative to the planet’s neutral atmosphere, this current system generates an azimuthally directed perturbation field which is negative in the northern hemisphere and positive in the southern, producing a “lagging” field configuration in the magnetosphere in which the field lines are “swept back” out of meridian planes relative to the sense of planetary rotation
[25] Considering first the Type I field‐aligned current strengths, and taking the northern and southern hemispheres together, we find that the downward currents have a mean strength of 1.7 ± 1.3 MA rad−1, while the upward current has a mean strength of 2.4 ± 1.0 MA rad−1 (∼1–3.5 MA rad−1)

Summary

Introduction

[2] The first global study of the magnetic field perturbations associated with field‐aligned (Birkeland) currents at the Earth was performed using the polar orbiting U.S Navy TRIAD satellite [Iijima and Potemra, 1976a, 1976b]. [4] More quantitatively (following previous discussion, e.g., by Bunce et al [2008b] and Talboys et al [2009a]), azimuthal flow of the coupled magnetospheric and ionospheric plasma relative to the neutral atmosphere results in the appearance of an electric field in the rest frame of the neutrals, that is directed equatorward in both hemispheres for subcorotating flow and poleward in both hemispheres for super‐corotating flow

Objectives

Findings

Conclusion