When is the magnetosphere like an elephant?

The magnetosphere is so vast, it cannot be entirely measured in exact detail. Observations are gathered using a suite of different instruments located onboard spacecraft, or arranged in networks across the surface of the Earth. Rather like in the classic story of the blindfolded men and the elephant, a large-scale picture of the magnetosphere must be built up from simultaneously sensing the properties of many different aspects of the beast.

At present, two magnetospheric spacecraft missions are active: Cluster, launched in 2000 comprising four satellites which fly together in formation, and five Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites, launched in 2007, with each occupying a different orbit.

Ground-based remote-sensing equipment includes ionospheric radars, which measure motions of the ionised upper atmosphere, and auroral cameras, which measure the light emitted by the atmosphere when it is struck by energetic particles from the magnetosphere above. Radars on the ground include the international Super Dual Auroral Radar Network (SuperDARN), a collaboration between 11 nations, which observe across both the Arctic and Antarctic polar regions. Auroral cameras can be located in arrays on the ground, such as those deployed in Canada in support of the THEMIS mission. If they are flown on a spacecraft such as the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite, they can gain a global panorama of the aurora from their viewpoint in space.

The “winds” measured by SuperDARN occur at an altitude of around 300 km. They are not driven by pressure gradients, but by the interaction of constantly changing electric and magnetic fields with the charged particles in the ionosphere. In turn, the electric fields are generated by the interaction of the solar wind with the magnetosphere, which sets the whole plasma within near-Earth space into circulation. The ionospheric winds are a projection of the churning of the magnetosphere above, and it is that churning that ultimately leads to the formation of the aurora.

In an analogy with weather monitoring, the radars provide the global “wind” pattern, the auroral cameras detect the presence of “clouds” and “rainfall,” while the various spacecraft are rather like weather balloons which can study the details of “raindrop” formation directly inside a cloud.

Many other instrument networks exist. For instance, magnetometers sense the electrical currents generated in the atmosphere by the ionospheric winds, while ionosondes, riometers and radars such as those run by the European Incoherent Scatter (EISCAT) radar consortium, measure changes in the ionosphere caused by the auroral “rain”. It is a formidable task to assimilate all the data from so many different instruments and locations. UK scientists are at the forefront of this exciting and constantly developing field.