Filamentary Alfvénic structures excited at the edges of equatorial plasma bubbles

R Pottelette,M Parrot,J J Berthelier,M Malingre,E Seran

doi:10.5194/angeo-25-2159-2007

Abstract

Abstract. Recent observations performed by the French DEMETER satellite at altitudes of about 710 km suggest that the generation of equatorial plasma bubbles correlates with the presence of filamentary structures of field aligned currents carried by Alfvén waves. These localized structures are located at the bubble edges. We study the dynamics of the equatorial plasma bubbles, taking into account that their motion is dictated by gravity driven and displacement currents. Ion-polarization currents appear to be crucial for the accurate description of the evolution of plasma bubbles in the high altitude ionosphere. During their eastward/westward motion the bubbles intersect gravity driven currents flowing transversely with respect to the background magnetic field. The circulation of these currents is prohibited by large density depressions located at the bubble edges acting as perfect insulators. As a result, in these localized regions the transverse currents have to be locally closed by field aligned currents. Such a physical process generates kinetic Alfvén waves which appear to be stationary in the plasma bubble reference frame. Using a two-dimensional model and "in situ" wave measurements on board the DEMETER spacecraft, we give estimates for the magnitude of the field aligned currents and the associated Alfvén fields.

Highlights

After sunset, the absence of sunlight in the low-latitude ionosphere leads to a much faster recombination at lower than at higher altitudes
We show wave data recorded by the French DEMETER (Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions) polar satellite at altitudes of ∼700 km
We have shown that during these disturbed time-periods, the edges of equatorial densitydepressions are characterized by the presence of filamentary structures with a typical kilometre transverse width

Summary

Introduction

The absence of sunlight in the low-latitude ionosphere leads to a much faster recombination at lower than at higher altitudes. A steep density gradient is generated between the depleted bottom side ionosphere and higher density in the upper F region. The present paper is focused on the interpretation of sporadic electromagnetic emissions detected at the edges of the bubbles in the ELF frequency range. These electromagnetic fluctuations occur in the regions where steep, large-scale (1–10 km) density gradients are recorded; they. R. Pottelette et al.: Filamentary Alfvenic structures excited at the edges of equatorial plasma bubbles are interpreted in the present paper in terms of Kinetic Alfven waves.

Observations

Excitation of kinetic Alfven waves

Conclusions