Three‐Dimensional Currents in Saturn's Magnetosphere

Abstract

Observations of Saturn's magnetic field from July 2004 to December 2016 have been combined to allow a numerical calculation of ∇ × B throughout the magnetosphere, thus permitting the first global mapping of Saturn's currents in three dimensions. The azimuthal ring current Jϕ dominates the magnetospheric currents, peaking near the orbit of the moon Rhea and having a total magnitude of over 10 mega‐Amperes integrated within the L‐shell of Titan. In meridional cross section, the ring current has a teardrop shape rather than a block shape. Jϕ also has a noticeable local time asymmetry, being the largest in the midnight and dawn sectors. The meridional currents Jρ and Jz are smaller than the ring current by an order of magnitude; they flow radially outward near the equator and return at higher latitudes in a looping pattern reminiscent of Hadley cell atmospheric convection. There are two such loops: one in the northern hemisphere and one in the south, flowing in opposing directions. In the noon sector, a strong Jz current may indicate the dayside cusp.

A New Ring Current Model for Saturn

“…The ring current is concentrated between 5 and 15 R S and has a peak strength of ~0.25 MA/R S 2 ≈ 70 pA/m 2 , which is crudely similar to the block model. These results are in general agreement with previous estimates of the location and strength of the ring current (Bunce et al, ; Carbary, ; Kellett et al, , , ; Sergis et al, , , ).…”

“…Figure shows the azimuthal current maps in four local time sectors. These are essentially the same maps shown in Carbary () except the binning has been extended down to ~2 R S . A number of bins had to be filled in (red crosses) even though magnetic field data from several seasons were used.…”

“…Enough data is available, with some minor gap-fills, that J ϕ can be estimated in three dimensions throughout the magnetosphere. Indeed, this has already been accomplished in a previous paper (Carbary, 2019). After a review of the binning and numerical differentiation used to find the azimuthal current, that current is then fitted to a simple equation involving a lognormal in ρ and a Gaussian in z.…”

“…A third method of determining Saturn's ring current employs direct use of the magnetic field data (Carbary, ; Carbary et al, ). The magnetic field components B ρ and B z are first binned in cylindrical coordinates to obtain a continuous estimate of the magnetic field, and then, the azimuthal current is obtained from the Maxwell equation J ϕ = 1/ μ 0 (∇ × B ) ϕ .…”

“…The magnetic field components B ρ and B z are first binned in cylindrical coordinates to obtain a continuous estimate of the magnetic field, and then, the azimuthal current is obtained from the Maxwell equation J ϕ = 1/ μ 0 (∇ × B ) ϕ . This exercise readily yields the ring current, although some gaps in the binning may have to be filled (Carbary, ). The total ring current strengths and locations from this method lie close to those obtained by the pressure‐balance method and the block method.…”