Transpolar arcs observed simultaneously in both hemispheres

J A Carter,R C Fear,Z A Harrison,M.‐T Walach,S E Milan,L J Paxton,B Hubert

doi:10.1002/2016ja023830

Abstract

AbstractTwo coexisting transpolar arcs are observed on 31 August 2005. We track the formation and motion of the arcs in both the Northern and Southern Hemispheres, using data from two independent satellites (Imager for Magnetopause to Aurora Global Exploration and a Defence Meteorological Satellite Program satellite). The observations are supported by supplementary ground‐based ionospheric convection data from the Super Dual Auroral Radar Network. The two arcs form during a period of northward interplanetary magnetic field. Following a change in the direction of the interplanetary magnetic field BY component from negative to positive, the dawnside arc traverses the polar cap to the duskside in the Northern Hemisphere. Over the same time period and in the Southern Hemisphere, the duskside arc traverses the polar cap to the dawnside. A complex magnetic field line topology resulting in the coexistence of two tongues of closed field lines protruding into the otherwise open polar cap is implied. We discuss these observations in terms of magnetic conjugacy and a model of transpolar arcs formation.

Highlights

Large-scale polar cap arcs, primarily observed via spacecraft-based auroral imaging, are a class of auroral features that protrude from the nightside auroral oval into the otherwise dark polar cap region
The observations are consistent with the hypothesis that, under the Milan et al [2005] model of Transpolar arcs (TPAs) formation, TPAs will initially form at mirror positions about the midnight meridian in opposite hemispheres
The Milan et al [2005] model requires a closed-field topology, so the observations present here provide supporting evidence for closed field line models for TPA formation, following the conclusions of statistical and individual TPA studies [Fear and Milan, 2012a; Fear et al, 2014]. These conclusions remain somewhat controversial within the community, and the data presented here provide a somewhat rare opportunity for an in-depth study using simultaneous observations from a range of spacecraft and ground-based sources to detect and monitor a set of TPAs

Summary

Introduction

Large-scale polar cap arcs, primarily observed via spacecraft-based auroral imaging, are a class of auroral features that protrude from the nightside auroral oval into the otherwise dark polar cap region. In one such model [Milan et al, 2005], the TPA forms as a tongue of closed magnetic flux, embedded in the otherwise open-flux polar cap region This can occur following magnetic reconnection in the tail, under northward IMF conditions with a significant BY component such that the tail is twisted [e.g., Grocott et al., 2003, 2004]. We utilize high-altitude global images obtained by the IMAGE far ultraviolet instrument, along with low-latitude images of swaths of auroral emissions from the Special Sensor Ultraviolet Spectrographic Imagers (SSUSI) [Paxton et al, 2002], which have flown on spacecraft of the DMSP program since 2005 This has provided near-simultaneous coverage of both hemispheres over part of the polar cap regions.

Observations

Discussion

Conclusions