Cluster - EDI - Scientific Objectives

The ability of the EDI instrument to make accurate and highly sensitive measurements of the electric field and of the perpendicular gradient of the magnetic field makes possible a variety of studies that comprise the essence of the Cluster mission. Cluster has been designed primarily to study small-scale structures in three dimensions in the Earth's plasma environment. The processes leading to the formation of such structures are believed to be fundamental to the key processes of interaction between the solar wind and the magnetospheric plasmas. With Cluster it is possible to obtain differential quantities by measurements of particle and field properties at the four spacecraft locations. These differences can be used to form quantities such as the gradient, curl, and divergence of the fields, and of the plasma moments such as velocity and pressure.

The electric field plays a very important role in the physics of collisionless shocks. In a laminar shock, the electrons are magnetized and follow equipotentials, while the ions are unmagnetized and are decoupled from the electrons because of the inertia. Charge separation occurs, which causes an electric field along the shock normal, which in turn slows down the ion population. How this electric field is distributed in the shock layer is almost completely unknown.

The magnetopause is an example of a current sheet formed when two magnetized plasmas interact with each other. In the simplest physical picture, in which the magnetic fields are frozen into the plasma, the two interacting plasmas remain separate. Therefore the major interest is in those processes that violate the frozen flux condition and then lead to transfer of mass, momentum and energy across the current sheet.

Violation of the frozen flux theorem implies that the measured electric field differs from the convection electric field. Processes that lead to such deviations are expected to operate only on small spatial scales. In magnetic reconnection, for example, there is the diffusion region around the X- line which separates the regions of different magnetic field topology. But reconnection also implies the presence of non-zero electric fields tangential to the magnetopause over much larger scales. Because it is necessarily rather small (of the order of 1 mV/m), it has not been measured in the past except in a few cases. Thus the systematic measurement of the tangential field and its spatial scale remains one of the outstanding tasks of the Cluster mission.

As a consequence of the transfer processes, a boundary layer of solar wind plasma exists inside the magnetopause. The significance of the various portions of the boundary layer for the transport of magnetic flux, and thus for the cross-magnetotail potential, can be assessed from measurement of the electric potential across the layer. Previous estimates relied on single-spacecraft measurements which become highly suspect in the presence of boundary motions or non-stationary conditions. The availability of measurements on the four Cluster spacecraft will go a long way towards improving the accuracy of the potential measurement.

For the polar-cusp region, a major objective will be the study of plasma turbulence, because eddy diffusion or turbulent convection has been invoked as the dominant plasma transport mechanism in that region. Correlations between the four spacecraft will help to confirm or deny this type of transport.

Another important topic where electric field measurements can contribute to our understanding is that of current sheets. Current sheets in the magnetosphere, such as in the magnetotail, are critical regions in that they are the most important sites of particle energization. In these current sheets, the magnetic field is small, the gyroradius can be large compared to the scale size, and consequently the electric field can play a dominant role in the particle motion. Attention is being focussed on these regions as sites for magnetic field reconnection, where magnetic field energy can be transformed into particle kinetic energy.

Prime objectives in the inner magnetosphere include studies of the electric fields associated with convection, ULF waves, and particle injections.

The concept of plasma convection in the magnetosphere has unified a number of high-latitude geophysical phenomena. However, there has been a paucity of direct measurements of the convection electric field in the inner (4--12 R_E) equatorial magnetosphere. Plasma injections are the sudden appearances of energetic plasma at all energies and directions in the equatorial magnetosphere during magnetospheric substorms. There are probably strong electric fields associated with these plasma injections which may be transient and/or localized at the injection boundary. EDI will measure these fields, including inductive fields, with a time resolution of up to several tens of Hz at all magnetospheric activity levels.

last update 08/12/2004
by Rosmarie Mayr-Ihbe,
Contact Person: Götz Paschmann,


Scientific Objectives
General Bow Shock Magnetopause, Boundary Layer, Polar Cusp Magnetotail Inner Magnetosphere

General The ability of the EDI instrument to make accurate and highly sensitive measurements of the electric field and of the perpendicular gradient of the magnetic field makes possible a variety of studies that comprise the essence of the Cluster mission. Cluster has been designed primarily to study small-scale structures in three dimensions in the Earth's plasma environment. The processes leading to the formation of such structures are believed to be fundamental to the key processes of interaction between the solar wind and the magnetospheric plasmas. With Cluster it is possible to obtain differential quantities by measurements of particle and field properties at the four spacecraft locations. These differences can be used to form quantities such as the gradient, curl, and divergence of the fields, and of the plasma moments such as velocity and pressure.


Bow Shock The electric field plays a very important role in the physics of collisionless shocks. In a laminar shock, the electrons are magnetized and follow equipotentials, while the ions are unmagnetized and are decoupled from the electrons because of the inertia. Charge separation occurs, which causes an electric field along the shock normal, which in turn slows down the ion population. How this electric field is distributed in the shock layer is almost completely unknown.


Magnetopause, Boundary Layer and Polar Cusp The magnetopause is an example of a current sheet formed when two magnetized plasmas interact with each other. In the simplest physical picture, in which the magnetic fields are frozen into the plasma, the two interacting plasmas remain separate. Therefore the major interest is in those processes that violate the frozen flux condition and then lead to transfer of mass, momentum and energy across the current sheet. Violation of the frozen flux theorem implies that the measured electric field differs from the convection electric field. Processes that lead to such deviations are expected to operate only on small spatial scales. In magnetic reconnection, for example, there is the diffusion region around the X- line which separates the regions of different magnetic field topology. But reconnection also implies the presence of non-zero electric fields tangential to the magnetopause over much larger scales. Because it is necessarily rather small (of the order of 1 mV/m), it has not been measured in the past except in a few cases. Thus the systematic measurement of the tangential field and its spatial scale remains one of the outstanding tasks of the Cluster mission. As a consequence of the transfer processes, a boundary layer of solar wind plasma exists inside the magnetopause. The significance of the various portions of the boundary layer for the transport of magnetic flux, and thus for the cross-magnetotail potential, can be assessed from measurement of the electric potential across the layer. Previous estimates relied on single-spacecraft measurements which become highly suspect in the presence of boundary motions or non-stationary conditions. The availability of measurements on the four Cluster spacecraft will go a long way towards improving the accuracy of the potential measurement. For the polar-cusp region, a major objective will be the study of plasma turbulence, because eddy diffusion or turbulent convection has been invoked as the dominant plasma transport mechanism in that region. Correlations between the four spacecraft will help to confirm or deny this type of transport.


Magnetotail The electron drift instrument will provide reliable surveys of the convection electric field in the tail, not only in the equatorial plane but also along the north-south direction where strong gradients seem to exist near the plasma sheet boundary layer. These surveys should lead to a better understanding of the entry of solar wind/lobe plasma into the central plasma sheet and the circulation of this plasma to the frontside magnetosphere. Another important topic where electric field measurements can contribute to our understanding is that of current sheets. Current sheets in the magnetosphere, such as in the magnetotail, are critical regions in that they are the most important sites of particle energization. In these current sheets, the magnetic field is small, the gyroradius can be large compared to the scale size, and consequently the electric field can play a dominant role in the particle motion. Attention is being focussed on these regions as sites for magnetic field reconnection, where magnetic field energy can be transformed into particle kinetic energy.


Inner Magnetosphere Prime objectives in the inner magnetosphere include studies of the electric fields associated with convection, ULF waves, and particle injections. The concept of plasma convection in the magnetosphere has unified a number of high-latitude geophysical phenomena. However, there has been a paucity of direct measurements of the convection electric field in the inner (4--12 R_E) equatorial magnetosphere. Plasma injections are the sudden appearances of energetic plasma at all energies and directions in the equatorial magnetosphere during magnetospheric substorms. There are probably strong electric fields associated with these plasma injections which may be transient and/or localized at the injection boundary. EDI will measure these fields, including inductive fields, with a time resolution of up to several tens of Hz at all magnetospheric activity levels.


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© Max-Planck-Institut für extraterrestrische Physik last update 08/12/2004 by Rosmarie Mayr-Ihbe, Contact Person: Götz Paschmann,

Scientific Objectives

General

Bow Shock

Magnetopause, Boundary Layer and Polar Cusp

Magnetotail

Inner Magnetosphere