Cluster observations of near-Earth magnetospheric lobe plasma densities – a statistical study

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 (n e >0.2 cm −3 ) seemed to decrease with the waning solar cycle. This points to a source region influenced by the diminishing solar UV/EUVintensity. 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.

Survey of cold ionospheric outflows in the magnetotail

“…This is also higher than measured in the study by Svenes et al (2008), who obtained plasma densities in the range 0.007-0.091 cm −3 for two thirds of the measurement points. It is natural that our values are higher, since we identify events with outflowing ions and estimate the density for those events, while Svenes et al (2008) estimate the mean of the density for all events during the observation time.…”

Electron bulk heating in magnetic reconnection at Earth's magnetopause: Dependence on the inflow Alfvén speed and magnetic shear

“…[30] Extrapolating our finding to the magnetotail plasma regime, where the typical Alfvén speed in the reconnection inflow (i.e., lobe) region is~2000-3000 km/s (based on a typical density of 0.05 cm À3 [Svenes et al, 2008] and magnetic field of 20-30 nT), the expected electron heating would be in the 700 eV to 1.5 keV range, roughly consistent with the strong electron heating typically seen in magnetotail exhausts. Thus, although only 1.7% of the available magnetic energy goes into electron heating, for sufficiently high inflow Alfvén speeds, reconnection substantially increases the electron temperature.…”

Plasma transport in the magnetotail lobes

“…The results are based on convection velocity measurements from the Electron Drift Instrument (EDI), combined with electron density measurements obtained from the Electric Field and Wave Experiment (EFW). The two data sets consist of data collected over a period of 7 years from the Cluster mission, and are very similar to the data sets described in Haaland et al (2008) and Svenes et al (2008), respectively.…”

“…The data sets and instrumentation are described in some detail in Haaland et al (2008) and Svenes et al (2008). For convenience, we here repeat some of this information, and point out some updates and changes in the data bases.…”

“…Similarly, the plasma sheet boundary layer (PSBL), consists of hot plasma with a density around 0.1-2 cm −3 . The lobes are characterized by a very low particle density, typically below 0.1 particles per cm −3 (e.g., Gosling et al, 1985;Svenes et al, 2008), and a strong and steady magnetic field. Typical magnetic field values range from approximately 30-50 nT, somewhat dependent on geomagnetic activity (e.g., Caan et al, 1975).…”