Density and temperature of energetic electrons in the Earth's magnetotail derived from high-latitude GPS observations during the declining phase of the solar cycle

M H Denton,T E Cayton

doi:10.5194/angeo-29-1755-2011

M H Denton, T E Cayton

Open Access

https://doi.org/10.5194/angeo-29-1755-2011

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Abstract

Abstract. Single relativistic-Maxwellian fits are made to high-latitude GPS-satellite observations of energetic electrons for the period January 2006–November 2010; a constellation of 12 GPS space vehicles provides the observations. The derived fit parameters (for energies ~0.1–1.0 MeV), in combination with field-line mapping on the nightside of the magnetosphere, provide a survey of the energetic electron density and temperature distribution in the magnetotail between McIlwain L-values of L=6 and L=22. Analysis reveals the characteristics of the density-temperature distribution of energetic electrons and its variation as a function of solar wind speed and the Kp index. The density-temperature characteristics of the magnetotail energetic electrons are very similar to those found in the outer electron radiation belt as measured at geosynchronous orbit. The energetic electron density in the magnetotail is much greater during increased geomagnetic activity and during fast solar wind. The total electron density in the magnetotail is found to be strongly correlated with solar wind speed and is at least a factor of two greater for high-speed solar wind (VSW=500–1000 km s−1) compared to low-speed solar wind (VSW=100–400 km s−1). These results have important implications for understanding (a) how the solar wind may modulate entry into the magnetosphere during fast and slow solar wind, and (b) if the magnetotail is a source or a sink for the outer electron radiation belt.

Highlights

Numerous observations of particle populations in the magnetotail have been reported and surveys of these data are available in the literature
The downtail distance is calculated by mapping with the T89 model. This will be at best an approximation to the actual the field line on which the observations are made since the magnetic field is not static in time, even if the solar wind driving is steady
Are these electrons energised in situ or present due to transport from other regions? (ii) Since values of density and temperature of energetic electrons inferred in this study are so similar to those found in the outer electron radiation belt, is the more-distant magnetotail a significant source/sink for the radiation belt?

Summary

Introduction

Numerous observations of particle populations in the magnetotail have been reported and surveys of these data are available in the literature. The study emphasised energetic electrons (∼0.22–2.5 MeV) are present in the magnetotail much of the time ( in “bursts”), and that a positive correlation exists between geomagnetic activity (proxied by Kp) and “occurrence frequency” – the occurrence of flux exceeding a particular threshold. Magnetic field-line tracing from the satellite into the magnetotail is carried out using the T89 model (Tsyganenko, 1989) and the equatorial crossing of the field line is derived This enables a survey of the energetic electron density and temperature as a function of downtail distance. These periods are examined and we show that the density in the magnetotail is substantially greater during high-speed solar wind This result has potential implications for determining source/loss of energetic electrons to/from the outer electron radiation belt; i.e. the magnetotail may act as (a) a reservoir, or (b) a sink, of energetic electrons. These electrons may be signatures of acceleration processes elsewhere in the magnetosphere

Analysis: a density-temperature description of electrons in the magnetotail

Results and discussion

Summary and future work