A statistical study of field-aligned electron beams associated with ion conics events

W Miyake,T Mukai,N Kaya

doi:10.1007/s00585-998-0940-x

W Miyake, T Mukai + Show 1 more

Open Access

https://doi.org/10.1007/s00585-998-0940-x

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Abstract

Abstract. A statistical study of field-aligned electron beams associated with upflowing ion conics is presented from Exos-D (Akebono) observations below 10 000 km. The electron beams are narrowly collimated along the field line and generally have energies of several tens of eV. They are divided in the analysis into three types: upflowing, downflowing, and counterstreaming. All the types of electron beams are almost equivalent in their energy and pitch angle characteristics and their association rate with upflowing ion events. About 50% of ion conics are found to be coincident with field-aligned electron beams. There is also a correlation in energy between the field-aligned electrons and ion conics. These show that the association is not a mere coincidence but rather that the field-aligned acceleration of electrons is related to the perpendicular energization of ions. The association rate of upflowing electrons is high on the nightside, while that of downflowing electrons is high on the dayside. The association rate of downflowing electrons is high at low altitudes, and the rates of the three types of electron beams become equivalent at high altitudes. Data indicate that the height of the electron acceleration region is lower on the nightside. It is suggested that the average height of the electron acceleration region is around the satellite apogee (–10000 km), and the average thickness of the region is about thousands km.Key words. Ionosphere-magnetosphere interactions · Particle acceleration · Auroral phenomena

Highlights

There have been several reports on ®eld-aligned electron beams over the auroral region. Sharp et al (1980) and Lin et al (1982) reported counterstreaming electronCorrespondence to: W
Various processes ranging from acceleration by a static electric ®eld to energization by waves have been proposed as a cause of perpendicular acceleration of ions
Ion conics have a good correlation with ®eld-aligned electron beams, but ion beams do not

Summary

Introduction

There have been several reports on ®eld-aligned electron beams over the auroral region. Sharp et al (1980) and Lin et al (1982) reported counterstreaming electron. The simple explanation for the generation is that they are the result of perpendicular acceleration of ions at low altitudes and the subsequent upward motion of the accelerated ions in the earth's mirror magnetic ®eld conserving the ®rst adiabatic invariant Another type of ion conic is called an elevated (or bi-modal) conic (Klumpar et al, 1984). Thelin and Lundin (1990) made a statistical study of the distributions in MLT, Inv. lat., and altitude of ®eld-aligned upowing electrons and upowing ions from Viking observations They found a correlation between ion conics and electron beams in the distributions in magnetic local time, invariant latitude, and altitude, and suggested that ion conics and electron beams emanate from the same physical process. The purpose of our study here is to obtain more detailed relations between the ®eld-aligned electrons and the ion conics through a statistical analysis

Data base

Basic properties of electron beams

Association rate

Findings

Discussion