Theoretical Physics  Space physics

The auroral generator region

Aurora Borealis, or more popularly the Northern Lights (in the southern hemisphere it is called Aurora Australis or the Southern Lights), is one of the most amazing and beautiful celestial phenomena we find in the nature. This spectacle of light on the dark northern (or southern) skies shows signs of mighty processes being in progress in the vault of the heavens. The aurora has been studied intensely by space scientists for several decades, but our understanding of the mechanisms behind the formation and evolution of auroras is still rather fragmented and uncertain. 

The discrete auroral arcs are associated with magnetic field aligned currents carried by electrons. At altitudes below a few Earth radii there are strong field aligned electric fields which accelerate the electrons down toward the Earth. These auroral electrons collide with particles in the upper atmosphere which start to emit light. This is the aurora. It is generally believed that the currents are driven by some type of generator in the outer magnetosphere where mechanical energy in the plasma motion is converted into electromagnetical energy (see the figure below). The generator resembles a bike's dynamo where mechanical energy from the rotating wheel is converted into electric energy which is used to light the headlight.

Neither the characteristic properties of this generator, nor its definite location has been investigated in any detail by using observational data. It is possible that the generator is rather extended or that it really consists of numerous smaller concentrated generators. However, there is no general consensus in the scientific community on the properties and the distribution of the auroral generator. The aim of this project is to increase our understanding of the auroral generator/generators by conduction observational investigations.

A schematic sketch of the auroral current circuit: In the magnetospheric generator (E·J<0), mechanical energy in the plasma motion is converted into electromagnetical energy. The generator power the auroral current circuit and supply energy to the acceleration of the electrons and the aurora. The acceleration region and the aurora can, hence, be viewed as loads (E·J>0) in this auroral current circuit.

In collaboration with, e.g., people from the Max-Planck-Institut für Extraterrstriche Physick, Germany, and members of the IRF (Swedish Institute of Space Physics) Uppsala group, we use data from both the four Cluster spacecraft and the FAST satellite to investigate plausible auroral generator regions in the magnetosphere. The Cluster Mission is in many respects suitable for studies of the generator region. For example, the full current density vector J can be derived from the simultaneous magnetic field measurements on the four satellites. The electric field can be obtained from three instruments on board the spacecraft. According to basic electrodynamics, the sign of the scalar product of the current density J and the electric field E can be used to distinguish between loads and generators.  By searching for negative power densities, E·J< 0, in the Cluster data we can investigate plausible generator regions in the Earth's magnetosphere.

We have used data from the Cluster satellites when they probe the plasma sheet and lobe region at high altitudes (about 20 Earth radii). From our Cluster data we have identified a few Concentrated Generator Regions (CGRs) in the plasma sheet close to the plasma sheet boundary. As the name suggests, these regions are rather small and concentrated. By comparing with conjugated FAST data (FAST crosses approximately the same magnetic field lines as Cluster but below the acceleration region at about 0.6 Earth radii) we see that the observed CGRs correlate well with auroral precipitation. Hence, we believe that our CGRs are related to the auroral activities at lower altitudes.

Department of Physics  Umeå University  Last changed on May 13, 2005. Responsible for this page: Maria Hamrin<