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Abstract
Abstract. The Cluster-mission has enabled a study of the near-Earth magnetospheric lobes throughout the waning part of solar cycle 23. During the first seven years of the mission the satellites crossed this region of space regularly from about July to October. We have obtained new and more accurate plasma densities in this region based on spacecraft potential measurements from the EFW-instrument. The plasma density measurements are found by converting the potential measurements using a functional relationship between these two parameters. Our observations have shown that throughout this period a full two thirds of the measurements were contained in the range 0.007–0.092 cm−3 irrespective of solar wind conditions or geomagnetic activity. In fact, the most probable density encountered was 0.047 cm−3, staying roughly constant throughout the entire observation period. The plasma population in this region seems to reflect an equilibrium situation in which the density is independent of the solar wind condition or geomagnetic activity. However, the high density tail of the population (ne>0.2 cm−3) seemed to decrease with the waning solar cycle. This points to a source region influenced by the diminishing solar UV/EUV-intensity. Noting that the quiet time polar wind has just such a development and that it is magnetically coupled to the lobes, it seems likely to assume that this is a prominent source for the lobe plasma.
Highlights
The four Cluster spacecraft have been in orbit for more than seven years, providing a wealth of measurements on various aspects of the interaction between the solar wind and the magnetosphere
Our observations have shown that throughout this period a full two thirds of the measurements were contained in the range 0.007–0.092 cm−3 irrespective of solar wind conditions or geomagnetic activity
The plasma population in this region seems to reflect an equilibrium situation in which the density is independent of the solar wind condition or geomagnetic activity
Summary
The four Cluster spacecraft (see Escoubet et al, 1997a) have been in orbit for more than seven years, providing a wealth of measurements on various aspects of the interaction between the solar wind and the magnetosphere. It should be noted that this time period coincides with the waning phase of solar cycle 23 providing measurements over a range of magnetospheric conditions. In the present paper we will discuss plasma density measurements obtained during traversals of the magnetospheric lobes. These are the regions of space tailward of the polar cap delimited by the mantle and the plasmasheet. The Cluster orbit transverse these regions during several hours each orbit from July to October
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