Ann. Geophys., 26, 2845–2852, 2008 www.ann-geophys.net/26/2845/2008/ Annales © European Geosciences Union 2008 Geophysicae Cluster observations of near-Earth magnetospheric lobe plasma densities – a statistical study K. R. Svenes1, B. Lybekk2, A. Pedersen2, and S. Haaland3,4 1Norwegian Defence Research Establishment, P.O. Box 25, 2027 Kjeller, Norway 2University of Oslo, Department of Physics, Norway 3University of Bergen, Department of Physics, Norway 4Max-Planck Institute for Solar System Research, Lindau, Germany Received: 7 May 2008 – Revised: 9 July 2008 – Accepted: 22 August 2008 – Published: 22 September 2008 Abstract. The Cluster-mission has enabled a study of the wealth of measurements on various aspects of the interac- near-Earth magnetospheric lobes throughout the waning part tion between the solar wind and the magnetosphere. Conse- of solar cycle 23. During the first seven years of the mis- quently, there are now available adequate data sets to carry sion the satellites crossed this region of space regularly from out specialized studies on particular topics within this frame- about July to October. We have obtained new and more ac- work. In that respect, it should also be noted that this time curate plasma densities in this region based on spacecraft po- period coincides with the waning phase of solar cycle 23 pro- tential measurements from the EFW-instrument. The plasma viding measurements over a range of magnetospheric condi- density measurements are found by converting the potential tions. measurements using a functional relationship between these In the present paper we will discuss plasma density mea- two parameters. Our observations have shown that through- surements obtained during traversals of the magnetospheric out this period a full two thirds of the measurements were lobes. These are the regions of space tailward of the polar cap contained in the range 0.007–0.092 cm−3 irrespective of so- delimited by the mantle and the plasmasheet. It is usually as- lar wind conditions or geomagnetic activity. In fact, the sumed that open field lines prevail in the lobes. The Cluster most probable density encountered was 0.047 cm−3, stay- orbit transverse these regions during several hours each orbit ing roughly constant throughout the entire observation pe- from July to October. riod. The plasma population in this region seems to reflect an The lobes are also noted for their notoriously tenu- equilibrium situation in which the density is independent of ous plasma, making accurate density measurements diffi- the solar wind condition or geomagnetic activity. However, cult. This is due to the high charging levels of the space- −3 the high density tail of the population (ne>0.2 cm ) seemed craft attained here and the cold plasma populations encoun- to decrease with the waning solar cycle. This points to a tered. However, using the technique described in Peder- source region influenced by the diminishing solar UV/EUV- sen et al. (2008) such problems can be overcome. This intensity. Noting that the quiet time polar wind has just method relies on measurements of the spacecraft potential, such a development and that it is magnetically coupled to and, through a proper calibration procedure, relates these val- the lobes, it seems likely to assume that this is a prominent ues to the ambient plasma density. source for the lobe plasma. Even though the Cluster satellites are equipped with ion Keywords. Magnetospheric physics (Polar cap phenom- emitters (see Riedler et al., 1997) it is usually not possible ena; Solar wind-magnetosphere interactions; Instruments to bring the spacecraft potential to low enough values to en- and techniques) able reliable measurements of the plasma density with tra- ditional methods in the lobes. In addition, the ion emitters are usually only operated every second orbit and even less frequently as the mission progressed. Hence, the spacecraft 1 Introduction potential method yields the most accurate and readily avail- able plasma density measurements throughout this scantily The four Cluster spacecraft (see Escoubet et al., 1997a) have mapped region. now been in orbit for more than seven years, providing a The high spacecraft potentials encountered in the lobe Correspondence to: K. R. Svenes regions was established early on by e.g. Lindqvist (1983), (ksv@ffi.no) who used ISEE-1 data to show that positive potentials in the Published by Copernicus Publications on behalf of the European Geosciences Union. 2846 K. R. Svenes et al.: Magnetospheric lobe plasma densities region of 20–50 V were regularly obtained. Such observa- photo-electrons as well as active current collection or emis- tions have since been confirmed and extended by e.g. Laakso sion from the vehicle itself. (2002), who even observed spacecraft potentials in the re- However, a satellite in the tenuous lobe plasma region will gion 50–70 V on the Polar satellite. Similar measurements attain an equilibrium potential where collected ambient elec- have also been obtained by Cluster as reported by Pedersen trons and escaping photo-electrons balance each other. In et al. (2008). this plasma ion currents are negligible in comparison. With Previously, general surveys of magnetospheric densities, knowledge of the photo-electron escape current as a function based on the spacecraft potential method, have been carried of spacecraft potential it is then possible to estimate the elec- out by using data from both ISEE-1 (see Escoubet et al., tron density leading to the equilibrium current. Typical satel- 1997b) and Polar (see Laakso et al., 2002a, b). These have lite potentials in the lobes are in the range 30–50 V. Solar already established plasma densities of the order of 0.1 cm−3 radiation in the EUV range are required to generate photo- for the magnetospheric lobe regions. In the present paper we electrons with enough energy to escape a satellite at these will focus more on the lobes, studying them during different large positive potential values. solar wind conditions and magnetospheric activity. In such a tenuous plasma, the potential attained by the In the next two sections we will outline the spacecraft po- spacecraft will consequently be so high that most of the ion tential method itself as well as the selection criteria for our population will be excluded from the ion sensors yielding data base. Then there follows a discussion chapter and finally ion density measurements in this region of little value. The a summary of results. electron spectra on the other hand may be contaminated by photo-electrons. Even active sounder techniques are often in- accurate in this environment due to low plasma density and 2 Method high photo-electron flux. However, since the spacecraft po- tential will be a function of the properties of the surrounding The spacecraft potential method for measuring plasma den- plasma a proper calibration of these measurements will yield sity builds on a fit between the measured spacecraft poten- good estimates of the ambient plasma density with a high tial and the ambient plasma density obtained through cali- time resolution. bration. The method was initially established by Knott et The spacecraft potential is determined by measuring the al. (1983), and by Schmidt and Pedersen (1987), based on potential difference between the main body of the satellite observations from geostationary orbit. Later on the method and the much smaller electric field probes controlled to be was significantly extended by Pedersen (1995) and Escoubet near their local plasma potential by balancing their photo- et al. (1997b) using data from several more plasma regions. electron current with a current from a high impedance source Consequently, this method has now been established as ro- on the spacecraft itself. The potential distribution around bust enough to enable the kind of magnetospheric surveys as Cluster in a tenuous plasma has been modeled by Cully et mentioned previously (see Escoubet et al., 1997b; Laakso et al. (2007), who showed that plasma potential near the probes al., 2002a, b). located 44 m from the spacecraft is approximately 20% of the On Cluster the spacecraft potential measurements are ob- spaceraft potential relative to the ambient plasma potential. tained as part of the regular operation of the Electric Field By establishing a functional dependence between the and Wave (EFW) experiment. This consists of four spheri- spacecraft potential and the ambient plasma density through cal probes and preamplifiers located at the tips of radial wire a thorough calibration program, the spacecraft potential mea- booms mounted on each spacecraft. The booms are spinning surements may then be routinely converted to density mea- along with the spacecraft at 0.25 Hz, and the static electric surements. A detailed explanation of this calibration proce- field measurements are obtained by measuring the potential dure is given in Pedersen et al. (2008). The relationship thus difference between pair of probes using the 88 m baseline obtained is given by the equation: provided by this configuration. Time varying fields are mea- −Vsp/B −Vsp/D −3 sured according to the particular time resolution employed. Ne(EFW) = Ae + Ce [cm ] (1) A more detailed description of the EFW instrument and mea- surement methods are given in Gustafsson et al. (1997). where the coefficients A, B, C and D in principle are varying In general, any conductive surface immersed in a plasma over the solar cycle. Here, Vsp (given in units of Volt) is the will attain a potential relative to the surrounding plasma such potential difference between the spacecraft and the probes, that the sum of currents to it becomes zero. This equilibrium and as such represent the direct measurements. From the situation will be attained essentially instantaneously in a par- above equation it can be seen that B and D are given in units ticular environment, but the vehicle potential may change as of Volt while A and C are given in cm−3.