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Polar Science Mission

The mission

The Earth's magnetosphere acts as a shield against the continuous bombardment of plasma, known as the solar wind, emanating from the Sun's atmosphere. The dipole nature of the Earth's magnetic field results in the formation of two cusp regions within the magnetosphere where the magnetic field is weak. It is in these cusp regions that the solar wind plasma may gain entry to the magnetosphere. This transfer of mass and energy drives dynamic processes within the magnetosphere. A visible manifestation of one such process is the aurora.

The Polar spacecraft was launched into a polar orbit on the 24th February 1996. The elliptical nature of the orbit along with the relatively short orbital period of 18 hours allows for frequent observations of the cusp region at various altitudes. The extensive suite of scientific instrumentation carried aboard Polar is used to study the detailed nature of the entry of solar wind plasma into the magnetosphere and the magnetosphere?s reaction to this plasma entry. The Polar spacecraft is one of the International Solar Terrestrial Program (ISTP) fleet of spacecraft currently studying the environment around the Earth and the interactions between this region and the out flowing solar wind


A number of research topics relating to the cusp region are being perused utilizing Polar data sets along with data from other spacecraft in the ISTP fleet, these are:

  • The position of the cusp. The precise location of the cusp region is not only dependent on the nature of the Earth?s magnetic field but also on the solar wind plasma and the interplanetary magnetic field (IMF) embedded within the plasma.
  • Non-adiabatic motion of solar wind plasma in the cusp region. Adiabatic motion of the plasma within the cusp is controlled by the orientation and strength of the Earth?s magnetic field, non-adiabatic motion occurs when the motion of the plasma becomes unstable so generating perturbations, waves, in the Earth?s magnetic and electric fields.
  • Charge neutrality at the equatorward edge of the cusp. The solar wind plasma contains both negatively charged electrons and positively charged ions. The less massive, faster moving electrons could more rapidly gain access to the Earth?s magnetosphere. Were this to happen the electrons would effectively become separated from the ions in the solar wind plasma as they enter the magnetosphere. The resultant electric field generated from this charge separation acts to inhibit the motion of the less massive electrons and so neutralize the resulting electric charge.
  • Anti-parallel versus component merging of the IMF and the Earth?s magnetic field. The flow of the solar wind plasma convects the IMF through interplanetary space. On arrival at the Earth the two magnetic fields can merge into one field. Two possible criteria exist for merging: anti-parallel merging requires the two magnetic fields to be oppositely orientated, whilst component merging requires only a cartesian component of the two fields to be oppositely orientated.
  • Conic ionospheric plasma distributions in the cusp region. The ingress of solar wind plasma into the Earth?s magnetosphere represents a transport of energy and momentum. Part of this energy is available to drive processes with the magnetosphere. One such process is the acceleration and heating of ionospheric plasma populations along the Earth?s magnetic field. An effect of this energization is the formation of conic distributions within the cusp region.
  • The origin of the plasma in the low latitude boundary layer (LLBL). Equatorward of the cusp region is a region inaccessible to solar wind plasma. The layer bounding these two regions on the low latitude side of the cusp consists of plasma whose character is consistent with neither that of the two adjacent regions. This plasma may originate from diffusion of plasma out of the two bounding regions and/or consist of the part of the population of the solar wind plasma that can access this boundary layer.
  • The effect of strong radial IMF on anti-parallel magnetic merging. The variation in the IMF means the position on the outer boundary of the magnetosphere, known as the magnetopause, where the IMF and the Earth?s magnetic field are oppositely directed also varies. Intervals where the IMF is directed either outward towards the Earth or inward towards the Sun have a noticeable effect of the merging site.

8th August 2000
Ian Krauklis

Mullard Space Science Laboratory - Holmbury St. Mary - Dorking - Surrey - RH5 6NT - Telephone: +44 (0)1483 204100 - Copyright © 1999-2005 UCL

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