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Plasma Regions

Plasma Regions

Solar Wind & Bow Shock

Artist's impression showing the main regions/boundaries of the Earth's magnetosphere.

The bow shock is the region where the solar wind is decelerated from supersonic to subsonic speed before being deflected around the Earth. The thickness of the bow shock and the length scales of various processes at play in this region have been revealed in unprecedented detail by the Cluster multi-point measurements.

Further upstream of the bow shock, i.e. towards the Sun, Cluster is providing new measurements which contribute to a better understanding of various phenomena including: plasma turbulence and its role in the heating of the solar wind, backstreaming ions, pickup ions, and plasma bubbles (see Featured highlight below).

Cluster carries five instruments to measure the characteristics of electromagnetic waves in various frequency domains - STAFF, EFW, WHISPER, WBD and DWP - thus it is also ideally suited to provide new insight into the transmission of electromagnetic waves through the bow shock and the magnetosheath.

Featured highlight in this region:
Cluster and Double Star discover density holes in the solar wind
Cluster and Double Star discover density holes in the solar wind

The Magnetopause

A three-dimensional cut-away view of Earth's magnetic bubble called the magnetosphere

The magnetopause is the border of the magnetosphere, a thin plasma layer that separates the magnetic field of the solar wind from the Earth's magnetic field. At this location the plasma pressure of the solar wind is in equilibrium with the magnetic pressure inside the magnetosphere. Due to a continuous variation of the solar wind pressure, this boundary is constantly in motion. Observations with the Cluster flotilla of spacecraft have helped to characterise this motion.

Although the magnetopause is usually considered as an impenetrable boundary, some plasma from the solar wind can enter the magnetosphere.

Various processes have been proposed to account for this penetration of plasma:

  1. Pressure pulses - when the solar wind dynamic pressure suddenly increases, leading to an indentation of the magnetopause
  2. Reconnection between the interplanetary magnetic field and the Earth's magnetic field
  3. Kelvin-Helmholtz vortices (see Featureed highlight below)
  4. Impulsive penetration where plasma filaments, which have a higher momentum than the surrounding solar wind plasma, impact and possibly penetrate the magnetopause.

Cluster measures the three-dimensional size and motion of the structures observed at the magnetopause and is also able to determine the local geometry of the magnetopause, which makes it possible to distinguish between these mechanisms.

Featured highlight in this region:
Cluster finds magnetic reconnection within giant swirls of plasma
Cluster finds magnetic reconnection within giant swirls of plasma

Polar Cusps

An illustration depiciting the spacecraft formation (in close-up) and the loction of the disturbance in the northern magnetic cusp.

At almost any location near the surface of the magnetopause, the Earth's magnetic field provides a sort of natural barrier to the solar wind particles. However, there are two regions, one in each hemisphere, where solar wind particles have a direct access to the Earth's ionosphere. Located above each pole, these regions are known as the polar cusps and are an excellent site for monitoring the coupling between the solar wind and the magnetosphere.

Cluster obtains high resolution measurements in these regions allowing full three-dimensional ion (CIS) and electron (PEACE) distribution functions to be measured. The exterior cusp is also seen as a very turbulent region with vortices in the plasma flow. Cluster measures the flow with high time-resolution and at four locations in space to determine the characteristics of these vortices (see Featured highlight below). The Cluster orbits are devised to ensure that the cusps are crossed as many times as possible in the required spacecraft configuration.

Featured highlight in this region:
From 'macro' to 'micro': turbulence seen by Cluster
From macro to micro: turbulence seen by Cluster

The Magnetotail

Illustration of Earth's magnetosphere (in blue) immersed in the solar wind.

The magnetotail is characterised by magnetic field lines stretched by the solar wind flow in the anti-sunward direction. The outer region consists of two magnetotail lobes and the inner region of the plasma sheet boundary layer and the central plasma sheet. The latter region is also known as the neutral sheet. The lobes are large reservoirs of magnetic energy which contain less than one particle per cubic centimetre.

Cluster is the first space mission to study in 3D the various phenomena occuring in this region. In particular, the Cluster data have enabled a leap forward in our understanding of magnetic reconnection, mapping for the first time, with multiple spacecraft, the core region of this universal phenomenon (see Featured highlight).

Featured highlight in this region:
Magnetic heart of a 3D reconnection event revealed by Cluster
Magnetic heart of a 3D reconnection event revealed by Cluster

Plasma Sheet Boundary Layer

Animation of generation process of ion beams

The Plasma Sheet Boundary Layer (PSBL) is often the most active plasma region of the magnetotail. Ion beams are observed coming from and flowing towards the Earth. Multi-point measurements are essential to derive the exact origin of these beams and to learn more about the generating mechanism (see Featured highlight below).

Electric currents are also observed in this region. They are aligned with the Earth's magnetic field, flowing towards and away from the Earth. The Fluxgate Magnetometer (FGM), one of 11 instruments on board Cluster, enables the current flowing in the vicinity of the four spacecraft to be calculated – for the first time ever - without having to make assumptions about the shape and orientation of the current sheet.

Featured highlight in this region:
Cluster explains nightside ion beams
Cluster explains nightside ion beams

The auroral zone

Auroral oval (in false colour) as seen from space, overlaid on top of a visible image of Earth. The red indicates the brightest aurora and blue the dimmest. The brightest aurora is found at midnight.

The auroral zone is a ring of light emission created by the precipitation of particles in the atmosphere and centred around the magnetic pole. The cusp and boundary layers on the dayside, and the plasma sheet and plasma sheet boundary layer on the nightside are the sources of these precipitations. Transient plasma flows and particle precipitation into the low-altitude cusp are of great interest, as they appear to be produced by phenomena occurring on the magnetopause, such as flux transfer events or solar wind dynamic pressure variations. The spatial scales and time variations of these transients are very important in distinguishing between these two mechanisms. The four Cluster spacecraft cross the low-altitude polar regions at an altitude of a few 100 km at intervals ranging from a few minutes to up to 40 minutes, allowing the time variations of the transient events to be studied.

The ionosphere is now believed to be an important source of plasma for the magnetosphere and, in particular, for the magnetotail. The polar wind and the nightside auroral zone contribute to the filling of the magnetosphere with plasma. Cluster has enabled measurements of these quantities using its comprehensive plasma physics package.

Featured highlight in this region:
Cluster quartet probes the secrets of the black aurora
Cluster quartet probes the secrets of the black aurora

Last Update: 1 September 2019
17-Sep-2019 20:12 UT

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