Auroral arcs

Polar wind






Auroral arcs

Aurora - also known as the polar lights - have been observed since ancient times. They have represented - and they still do - a puzzle to geophysicists. 

They are caused by the precipitation of charged particles in the ionosphere. They are therefore the manifestation of magnetospheric current systems.

Changes in the auroral patterns reflect changes in the structure of the magnetosphere under the action of the changing interplanetary conditions.


Even when viewed from space, aurora often appear as "curtains'' that are aligned with the geomagnetic field lines.

Studies at the Belgian Institute for Space Aeronomy (BIRA-IASB) have looked at the magnetospheric origin of the current system driving discrete auroral arcs.  The basic idea is that a discrete arc can be regarded as the projection of a magnetospheric interface somewhere in the magnetotail. This could help to explain the observed scale size (thickness) of discrete arcs.


The accompanying figure illustrates how the magnetospheric plasma interface (in this case a tangential discontinuity) generating an EMF at the surface of a plasma sheet density irregularity (cloud or plasma sheet-lobe boundary) would be projected down into the terrestrial ionosphere.

Electric potential differences across this interface are produced by thermo-electric charge separation at the interface between the two plasma regions. These potential differences map down into the ionosphere and drive Pedersen currents, provided the local ionospheric conductivity is large enough, i.e., that the system becomes loaded. Because of micro-instabilities in the interface region, electrons are scattered in the loss cone; they enhance the ionization at low altitude and increase the Pedersen conductivity. Large Pedersen currents flow in the ionosphere.

The diminution of the resistivity reduces electric potential gradients in the ionosphere and leads to the formation of field-aligned double layers such that the circulation of the electric field along the whole circuit is equal to zero (assuming a time-independent magnetic field distribution).

The field-aligned potential drop accelerates the scattered and precipitating electrons to several keV. An auroral arc is formed by the bombardment of these electrons in the atmosphere. This produces an additional enhancement of the ionospheric conductivity, smaller ionospheric potential differences but larger field-aligned ones. Provided the plasma cloud in the magnetosphere is a large reservoir of electrons or if the cloud is propagating in the +x direction, the source of precipitated electrons is constantly fed and the auroral arc lasts as long as a large EMF is maintained in the magnetotail.


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