Mesoscale structure of a morning sector ionospheric shear flow region determined by conjugate Cluster II and MIRACLE ground-based observations

O Amm,K Kauristie,M André,M Lester,P Janhunen,J.-M Bosqued,M Dunlop,K Mursula,M G G T Taylor,I Sillanpää,A Fazakerley,A Aikio,A Vontrat-Reberac

doi:10.5194/angeo-21-1737-2003

Abstract

Abstract. We analyse a conjunction event of the Cluster II spacecraft with the MIRACLE ground-based instrument net-work in northern Fennoscandia on 6 February 2001, between 23:00 and 00:00 UT. Shortly after the spacecraft were located at perigee, the Cluster II satellites’ magnetic footpoints move northwards over Scandinavia and Svalbard, almost perfectly aligned with the central chain of the IMAGE magnetometer network, and cross a morning sector ionospheric shear zone during this passage. In this study we focus on the mesoscale structure of the ionosphere. Ionospheric conductances, true horizontal currents, and field-aligned currents (FAC) are calculated from the ground-based measurements of the IMAGE magnetometers and the STARE coherent scatter radar, using the 1-D method of characteristics. An excellent agreement between these results and the FAC observed by Cluster II is reached after averaging the Cluster measurements to mesoscales, as well as between the location of the convection reversal boundary (CRB), as observed by STARE and by the Cluster II EFW instrument. A sheet of downward FAC is observed in the vicinity of the CRB, which is mainly caused by the positive divergence of the electric field there. This FAC sheet is detached by 0.5°–2° of latitude from a more equatorward downward FAC sheet at the poleward flank of the westward electrojet. This latter FAC sheet, as well as the upward FAC at the equatorward flank of the jet, are mainly caused by meridional gradients in the ionospheric conductances, which reach up to 25 S in the electrojet region, but only ~ 5 S poleward of it, with a minimum at the CRB. Particle measurements show that the major part of the downward FAC is carried by upward flowing electrons, and only a small part by downward flowing ions. The open-closed field line boundary is found to be located 3°–4° poleward of the CRB, implying significant errors if the latter is used as a proxy of the former.Key words. Ionosphere (electric fields and currents) – Magnetosphere physics (current systems; plasma convection)

Highlights

The antisunward convection in the outer parts of the magnetosphere and the sunward return flow in its inner parts lead to a convection reversal boundary that is projected to the ionosphere, and to the formation of the well-known ionospheric two-cell potential pattern for southward interplanetary magnetic field (IMF), or to more complex patterns for northward IMF
A detailed two-dimensional electrodynamical analysis of five shear flow events in the early morning sector, using the two-dimensional method of characteristics to derive the ionospheric currents, field-aligned currents (FAC), and conductances, was carried out by Amm et al (2000). They found that the downward FAC associated with the shear region consist of two parts: one part is caused by the divergence of the electric field at the convection reversal boundary” (CRB) itself, where the ionospheric conductances were found to show a minimum
The main results of our analysis of an early morning sector shear flow region on 6 February 2001, between 23:00 and 00:00 UT, using data of the Cluster II satellites, the groundbased multi-instrument array for ionospheremagnetosphere coupling studies (MIRACLE) network, and the CUTLASS radar can be summarised as follows: 1. The FAC structure along the shear flow region consists, from the westward electrojet on polewards, of three sheets: an upward and downward FAC sheet at the equatorward and poleward flank of the westward electrojet, mainly caused by meridional gradients of the ionospheric conductances, and another downward FAC sheet in the vicinity of the CRB, located 0.5◦–2◦ poleward of the previous one. This latter FAC sheet is mainly associated with the positive divergence of the electric field

Summary

Introduction

The antisunward convection in the outer parts of the magnetosphere and the sunward return flow in its inner parts lead to a convection reversal boundary that is projected to the ionosphere, and to the formation of the well-known ionospheric two-cell potential pattern for southward interplanetary magnetic field (IMF), or to more complex patterns for northward IMF. Newell et al, 1991; Ohtani et al, 1995a, b), surprisingly few studies analyse in detail the ionospheric electrodynamics of the shear flow region on the dawn or dusk flanks As these regions are colocated with the large-scale region 1 field-aligned currents (FAC), as defined by Iijima and Potemra (1978), they are key areas of ionosphere-magnetosphere coupling, as a considerable amount of interaction in terms of currents and energy flux takes place there A detailed two-dimensional electrodynamical analysis of five shear flow events in the early morning sector, using the two-dimensional method of characteristics to derive the ionospheric currents, FAC, and conductances, was carried out by Amm et al (2000) They found that the downward FAC associated with the shear region consist of two parts: one part is caused by the divergence of the electric field at the CRB itself, where the ionospheric conductances were found to show a minimum.

Instrumentation and analysis techniques

Data and analysis

Discussion

Summary