On the possible role of cusp/cleft precipitation in the formation of polar-cap patches

I. K. Walker,L. Kersley,D. A. Lorentzen,J. Moen

doi:10.1007/s00585-999-1298-4

I. K. Walker, L. Kersley + Show 2 more

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https://doi.org/10.1007/s00585-999-1298-4

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Abstract

Abstract. The work describes experimental observations of enhancements in the electron density of the ionospheric F-region created by cusp/cleft particle precipitation at the dayside entry to the polar-cap convection flow. Measurements by meridian scanning photometer and all-sky camera of optical red-line emissions from aurora are used to identify latitudinally narrow bands of soft-particle precipitation responsible for structured enhancements in electron density determined from images obtained by radio tomography. Two examples are discussed in which the electron density features with size scales and magnitudes commensurate with those of patches are shown to be formed by precipitation at the entry region to the anti-sunward flow. In one case the spectrum of the incoming particles results in ionisation being created, for the most part below 250 km, so that the patch will persist only for minutes after convecting away from the auroral source region. However in a second example, at a time when the plasma density of the solar wind was particularly high, a substantial part of the particle-induced enhancement formed above 250 km. It is suggested that, with the reduced recombination loss in the upper F-region, this structure will retain form as a patch during passage in the anti-sunward flow across the polar cap.Key words. Ionosphere (ionospheric irregularities; particle precipitation; polar ionosphere)

Highlights

Structures of enhanced F-region density at a variety of scale sizes are present in the polar-cap ionosphere
The present study has shown that particle precipitation in the cusp/cleft region can be responsible for the creation of structures in electron density with magnitude, scale and transport consistent with the known characteristics of polar-cap patches
The two examples presented here demonstrate that precipitation of soft particles into the cusp/ cleft region can play a potential role in the formation of structures in electron density with the characteristics of polar-cap patches

Summary

Introduction

Structures of enhanced F-region density at a variety of scale sizes are present in the polar-cap ionosphere. Buchau et al (1985) found that the electron densities in the patches were similar to those at dayside sub-auroral latitudes, leading to proposed production mechanisms whereby solar-produced plasma becomes entrained in the high-latitude convection system, feeding atongue-of-ionisation' into the polar cap. Walker et al.: On the possible role of cusp/cleft precipitation pattern segment the tongue of ionisation and generate patches Variations in both the Bz and By components of the IMF have been cited as being responsible for the plasma structuring. There is some evidence for ®ner structure within the main band of the 630 nm emission and rebrightening of the auroral features It must, be emphasised that the red-line observations studied here are of intense auroral emissions, in contrast to the weak airglow used for many studies of patches in the polar cap. Signi®cantly, the plasma density was about 33 cmA3, more than ®ve times larger than in the previous example

Findings

Discussion

Conclusion