E. C. Whipple

Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions

Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma. Citation: Pedersen, A., et al. (2008), Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions,

The Electron Drift Instrument for Cluster

The Electron Drift Instrument (EDI) measures the drift of a weak beam of test electrons that, when emitted in certain directions, return to the spacecraft after one or more gyrations. This drift is related to the electric field and the gradient in the magnetic field, and these quantities can, by use of different electron energies, be determined separately. As a by-product, the magnetic field strength is also measured. The present paper describes the scientific objectives, the experimental method, and the technical realization of the various elements of the instrument.

Active Spacecraft Potential Control

Charging of the outer surface or of the entire structure of a spacecraft in orbit can have a severe impact on the scientific output of the instruments. Typical floating potentials for magnetospheric satellites (from +1 to several tens of volts in sunlight) make it practically impossible to measure the cold (several eV) component of the ambient plasma. Effects of spacecraft charging are reduced by an entirely conductive surface of the spacecraft and by active charge neutralisation, which in the case of Cluster only deals with a positive potential. The Cluster spacecraft are instrumented with ion emitters of the liquid-metal ion-source type, which will produce indium ions at 5 to 8 keV energy. The operating principle is field evaporation of indium in the apex field of a needle. The advantages are low power consumption, compactness and high mass efficiency. The ion current will be adjusted in a feedback loop with instruments measuring the spacecraft potential (EFW and PEACE). A stand-alone mode is also foreseen as a back-up. The design and principles of the operation of the active spacecraft potential control instrument (ASPOC) are presented in detail. Flight experience with a similar instrument on the Geotail spacecraft is outlined.

Extension of the Harris magnetic field model to obtain exact, two‐dimensional, self‐consistent X point structures

[1]xa0Harris [1962] developed a one-dimensional current sheet model in a magnetic field that was self-consistent with a collisionless Maxwellian plasma. This model has served as a starting point for many current sheet studies. Kan [1973] showed how this approach could be extended to two dimensions, but there has been little further application of this extension to magnetospheric problems. We have followed Kans technique and give exact analytic expressions for plasma and magnetic field structures in the vicinity of a two-dimensional magnetic X point. The plasma is Maxwellian with arbitrary electron to ion mass ratio, but in the present stage of work the flow is constrained to be perpendicular to the plane containing the X point. The model is flexible in that it contains parameters that should allow it to be adapted to particular observational geometries and to be extended further using perturbation techniques to higher dimensions.

Multifluid model of a one‐dimensional steady state magnetotail current sheet

[1]xa0The central current sheet of the magnetotail has been modeled frequently as a nearly one-dimensional configuration that might have a significant role in magnetospheric physics as a site of particle acceleration. Yet comparisons of data with tail models have mainly used the Harris solution in spite of its deficiencies: No normal magnetic field, uniform cross-tail drift, and vanishing asymptotic plasma density. We present a time-independent one-dimensional multifluid model that separates ions into groups coming from sources above and below the current sheet. Electrons are a separate group. A disadvantage is that equations of state are required for closure. The isotropic pressure equation of state is supported by observation and gives reasonable results. This model is more realistic; it encodes more information than a single fluid or an ion and electron fluid model. This fluid model can be easily used to compare with magnetotail data. Field and plasma moment data provide boundary conditions for a model calculation. We examine symmetric models that predict specific relations between various quantities, such as magnetic field or density. Solutions are obtained similar to Harris model, but with non-uniform out-of-plane ion flow and non-vanishing asymptotic density. Sheet thickness has weak dependence on the normal magnetic field and modest dependence on electron-to-ion pressure ratio. The ratio of sheet center to asymptotic density decreases with asymptotic ion pressure. This one-dimensional configuration of the current sheet, in Earths reference frame, provides an example of conversion of electromagnetic energy into fluid energy.

Correction to “Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions”

[1] Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma.