Relating field-aligned beams to inverted-V structures and visible auroras

E Lee,S Y Fu,M Fillingim,Y B Cui,I Dandouras,G K Parks,H Rème,J Hong

doi:10.5194/angeo-33-1263-2015

Abstract

Abstract. The ion composition experiment on Cluster measures 3-D distributions in one spin of the spacecraft (4 s). These distributions often measure field-aligned ion beams (H+, He+ and O+) accelerated out of the ionosphere. The standard model of these beams relies on a quasi-static U-shaped potential model. The beams contain important information about the structure and distribution of the U-shaped potential structures. For example, a simple beam with a narrow velocity range tells us that the particles are accelerated going through a quasi-static U-shaped potential structure localized in space. A more complex beam with a large range of velocities varying smoothly (a few tens of kilometers per second to > 100 km s−1) tells us that the potential structure is extended and distributed along the magnetic field. The Cluster experiment has now revealed new features about the beams. Some beams are broken into many individual structures each with their own velocity. The U-shaped potential model would interpret the new features in terms of particles accelerated by narrow isolated potential structures maintained over an extended region of the magnetic field. Another interpretation is that these features arise as Cluster traverses toward the center of a small-scale U-shaped potential region detecting particles accelerated on different equipotential contours. The estimate of the distance of the adjacent contours is ~ 590–610 m at a Cluster height of ~ 3.5 RE. The observed dimensions map to ~ 295–305 m in the ionosphere, suggesting Cluster has measured the potential structure of an auroral arc.

Highlights

The electric field parallel to magnetic field direction (E||) is fundamentally important for accelerating laboratory, space and astrophysical plasmas
The first evidence of E|| was obtained by satellite observations of auroral electron and ion beams that were accelerated in opposite directions along the magnetic field (Shelley et al, 1972)
We present a new feature of the upward flowing ionospheric ion beams in the auroral region observed by the Cluster ion spectrometry (CIS) experiment (Escoubet et al, 2001; Rème et al, 2001)

Summary

Introduction

The electric field parallel to magnetic field direction (E||) is fundamentally important for accelerating laboratory, space and astrophysical plasmas. From Freja and Cluster observations, the scale sizes of auroral electric field structures were reported to be ∼ 1– 5 km (Karlsson and Marklund, 1996; Johansson et al, 2007) These dimensions are much larger than the dimensions of a typical aurora. We suggest that the multiple discrete beams are accelerated on the different equipotential contours, which have small spatial scales, when Cluster crossed an auroral potential structure at a height of 3.5 RE. This new measurement will resolve why the dimension of inverted-V structures obtained by satellites and rockets is different from the dimensions of typical auroras. 104 (a) 103 102 104 (b) 103 102 104 (c) 103 102 180 (d) pitch angle(o) pitch angle(o) pitch angle(o)

Observations

Discussion