The Meridional Magnetic Field Lines of Saturn

Abstract

The magnetometer data for the Cassini mission (2004–2016) are used to derive a model of Saturn's meridional magnetic field lines. Using a bin map of the Bρ and Bz field components, the field lines can be traced from the equator to high latitudes in the inner magnetosphere. The traces reveal a magnetic field greatly compressed on the dayside and highly elongated on the nightside, which are presumably the effects of solar wind compression and viscous flow around the magnetosphere. A model of the traced field lines can accommodate this day‐night asymmetry and offer a way to connect various places in the magnetosphere. This model can be adjusted for magnetodisk warping by using solar latitude. The field line traces can be mapped down to the ionosphere using an offset dipole. The dayside magnetopause (L = 21) is mapped to colatitudes of ~13° in the north and ~16° in the south, while for L > 25 on the nightside, the mapped field lines approach asymptotic limits of ~16° in the north and ~18° in the south.

Local Time Variation in the Large‐Scale Structure of Saturn's Magnetosphere

“…Carbary () accounted for seasonal warping of the current sheet via a coordinate transformation; however, their model did not account for a change in external solar wind conditions, and so we have reproduced the same traces from Carbary () in the top and bottom panels. We can see that the overall magnetic field structures in our models are similar to those of the Carbary () model, in particular the expanded 27 R S dayside model, and the compressed 42 R S nightside model. Our expanded nightside model shows a magnetic field structure that is significantly more disk‐like than the Carbary () analytical model, suggesting that perhaps a magnetodisk radius of 54 R S is somewhat too extreme to accurately characterize the typical midnight magnetosphere.…”

“…Looking at the day‐night asymmetry in more detail, in Figure we show the magnetic field structure for our noonside and nightside magnetodisk models, for the compressed (top panel) and expanded (bottom panel) regimes in the range ρ =4−22 R S for the dayside and ρ =4−28 R S on the nightside, noting that our compressed dayside model only extends out to ρ =21 R S . For comparison, we include in gray field line traces based on empirical observations from a recent study by Carbary (). In that study the author binned magnetic field observations from almost the entire Cassini mission (2004–2016) into two local time sectors, dayside and nightside, and calculated traces using a Runge‐Kutta propagator (see Carbary, , and references therein for more details).…”

“…For comparison, we include in gray field line traces based on empirical observations from a recent study by Carbary (). In that study the author binned magnetic field observations from almost the entire Cassini mission (2004–2016) into two local time sectors, dayside and nightside, and calculated traces using a Runge‐Kutta propagator (see Carbary, , and references therein for more details). Carbary () accounted for seasonal warping of the current sheet via a coordinate transformation; however, their model did not account for a change in external solar wind conditions, and so we have reproduced the same traces from Carbary () in the top and bottom panels.…”

“…In that study the author binned magnetic field observations from almost the entire Cassini mission (2004–2016) into two local time sectors, dayside and nightside, and calculated traces using a Runge‐Kutta propagator (see Carbary, , and references therein for more details). Carbary () accounted for seasonal warping of the current sheet via a coordinate transformation; however, their model did not account for a change in external solar wind conditions, and so we have reproduced the same traces from Carbary () in the top and bottom panels. We can see that the overall magnetic field structures in our models are similar to those of the Carbary () model, in particular the expanded 27 R S dayside model, and the compressed 42 R S nightside model.…”

“…These factors will influence the magnetic and plasma configuration of the magnetosphere differently at different local times. In addition, a recent magnetic field model by Carbary () shows significant day‐night asymmetry in equatorial‐ionospheric magnetic mapping profiles, and local time asymmetries in the location of Saturn's aurora have been observed in studies such as Badman et al,(, ).…”