Global MHD simulations of Neptune's magnetosphere

Abstract

A global magnetohydrodynamic (MHD) simulation has been performed in order to investigate the outer boundaries of Neptune's magnetosphere at the time of Voyager 2's flyby in 1989 and to better understand the dynamics of magnetospheres formed by highly inclined planetary dipoles. Using the MHD code Gorgon, we have implemented a precessing dipole to mimic Neptune's tilted magnetic field and rotation axes. By using the solar wind parameters measured by Voyager 2, the simulation is verified by finding good agreement with Voyager 2 magnetometer observations. Overall, there is a large‐scale reconfiguration of magnetic topology and plasma distribution. During the “pole‐on” magnetospheric configuration, there only exists one tail current sheet, contained between a rarefied lobe region which extends outward from the dayside cusp, and a lobe region attached to the nightside cusp. It is found that the tail current always closes to the magnetopause current system, rather than closing in on itself, as suggested by other models. The bow shock position and shape is found to be dependent on Neptune's daily rotation, with maximum standoff being during the pole‐on case. Reconnection is found on the magnetopause but is highly modulated by the interplanetary magnetic field (IMF) and time of day, turning “off” and “on” when the magnetic shear between the IMF and planetary fields is large enough. The simulation shows that the most likely location for reconnection to occur during Voyager 2's flyby was far from the spacecraft trajectory, which may explain the relative lack of associated signatures in the observations.

Dipole Tilt Effect on Magnetopause Reconnection and the Steady‐State Magnetosphere‐Ionosphere System: Global MHD Simulations

“…In particular, the dipole axes of Uranus and Neptune are offset by large angles to their rotation axes (by 60°and 47°, respectively) (Russell & Dougherty, 2010). Coupled with their severe obliquity to the ecliptic plane, these magnetospheres undergo significant reconfiguration both diurnally and seasonally as shown in both observations (e.g., Cowley, 2013) and simulations (e.g., Cao & Paty, 2017;Mejnertsen et al, 2016). More exotically, cases of tidally locked exoplanets and direct evidence of exoplanet magnetic fields suggest the possibility of magnetospheres that, as well as being highly inclined to the stellar wind, are locked-in to this extreme tilted configuration-though such a slow rotation rate might not support a strong planetary dynamo (Grießmeier et al, 2004).…”

A More Viscous‐Like Solar Wind Interaction With All the Giant Planets

“…The giant magnetospheres of Uranus and Neptune are better described by our toy magnetosphere due to the absence of clear evidence for similarly strong internal plasma sources (Belcher et al, ; Bridge et al, ). The relatively large angles between planetary rotation and magnetic dipole axes and large planetary obliquities at Uranus and Neptune will lead to strong diurnal and seasonal variations in the interaction between each magnetosphere and the solar wind (e.g., Cao & Paty, ; Mejnertsen et al, ). The location of these systems at the largest distances from the Sun suggests considerably more viscous‐like solar wind interactions than at Earth (see Figure ), which may be the dominant driver of magnetospheric dynamics at each of these outermost giant planets.…”

Global MHD Simulations of the Earth's Bow Shock Shape and Motion Under Variable Solar Wind Conditions

“…Gorgon is a 3-D magnetohydrodynamic code, initially developed for studying high energy, collisional plasma interactions such as Z-pinches (Chittenden et al, 2004;Jennings, 2006;Jennings et al, 2010), laser-plasma interactions (Smith et al, 2007), and magnetic tower jets (Ciardi et al, 2007). It has recently been adapted to simulate planetary magnetospheres and their interaction with the solar wind (Mejnertsen et al, 2016).…”

“…In this work, global MHD simulations of the Earth's magnetosphere under variable solar wind are performed using the Gorgon code (Chittenden et al, 2004;Ciardi et al, 2007;Mejnertsen et al, 2016). This work examines the motion of the bow shock.…”