Modeling Non‐Force‐Free and Deformed Flux Ropes in Titan's Ionosphere

C J Martin,C S Arridge,C J Barratt,S V Badman,D D Billett

doi:10.1029/2019ja027571

Abstract

AbstractPrevious work at Titan presented a set of 85 flux ropes detected during Cassini flybys of Titan from 2005 to 2017. In that study a force‐free model was used to determine the radii and axial magnetic field of the flux ropes. In this work we apply non‐force‐free models. The non‐force‐free model shows an improvement in the number of flux ropes that can be fitted with a model, along with improved uncertainties and χ2 values. A number of asymmetries and features in the magnetometer data cannot be reproduced by either model; therefore, we deform the force‐free model to show that small deformations can replicate these features. One such deformation is to use an elliptical cross section, which replicates a plateau in magnetic field strength along with asymmetries on either side of the center of the flux ropes. Additionally, we explore the properties of bending a flux rope, where we find that minimum variance analysis becomes increasingly degenerate with bending, along with a slight bend causing the switching of the axial field direction from intermediate to maximum variance direction. We conclude that the flux ropes at Titan show aspects of developing flux ropes, compared to other planetary bodies, which exhibit more agreement to the force‐free assumptions of mature flux ropes.

Highlights

Flux ropes are a magnetic phenomena found in most plasma and magnetic field regimes across the solar system and can be found in the solar wind (Burlaga et al, 1982), on the solar surface (e.g., Mouschovias & Poland, 1978), in the magnetospheres of Mercury, Earth and Saturn
An example of a modelled deformed and undeformed force-free flux rope is shown in figure 6, where it is shown that the asymmetry in the z-component of magnetic field is reproduced in Cartesian coordinates
When using minimum variance analysis (MVA) on a modelled force-free flux rope, we find that the component tangential to the flux rope is in the direction of maximum variance and the component along the axis of the flux rope is in the intermediate direction

Summary

Key Points:

Various models are fitted to 85 flux ropes that were detected in Titan’s ionosphere. A non-force-free model is a more suitable fit statistically compared to a force-free model, implying flux ropes are dynamic at Titan. Various deformations that can act on a flux rope are explored, and in some cases an elliptical cross-section is more appropriate than circular

Introduction

Method

Findings

Discussion

Summary