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Aurora

Trying to understand the mystery

We know that auroras are produced by magnetospheric electrons that follow the magnetic field lines and that are accelerated by electric fields up to energies of several thousands of volts.

The acceleration mechanism itself, however, remains somewhat of a mystery. When those accelerated electrons hit atoms and molecules in the tenuous upper atmosphere at 90-120 km altitude, they excite or ionise them. As a consequence, those atoms and molecules emit light of specific colours.

Because of the shape of the geomagnetic field, auroras occur predominantly in ring-shaped regions that encircle the geomagnetic north and south poles. Depending on the electron energies and on the composition of the upper atmosphere, the auroras display different colors, mostly green for auroras at lower altitudes and red for auroras at higher altitudes.

Auroras are very dynamic: Their shape and colours can change considerably in a few minutes’ time. By studying the evolution of auroras with time, scientists hope to improve their insight in the complex coupling between the magnetosphere and the ionosphere.

 

Models

Researchers at the BIRA-IASB try to understand the fundamental processes involved in the formation of auroras. For example, we try to find out what source of energy lies at the basis of auroral acceleration. In order to do that, we develop theoretical and numerical models of the magnetosphere – ionosphere coupling.

AuroraThanks to such models, and thanks also to observations in space, we can partially predict the position, size, and luminosity of auroral arcs. Similarly, we can predict the strength of the electric currents that are associated with auroras.

In this way, we are able to evaluate the total amount of energy that is transferred from the magnetosphere to the ionosphere and the upper atmosphere. (Image credits Cyril Simon)

 

Observations

At the BIRA-IASB, we use optical and radar observations from the ground to verify our models, but also measurements performed in space.

  • The radar observations are carried out with the EISCAT radar (European Incoherent Scatter Radar) in Tromsø (in the north of Norway). The optical observations are performed by the stations of the ALIS-network (Auroral Large Imaging System) in northern Scandinavia.

    With simultaneous images of aurora from different stations, and thanks to tomography, we can make a 3D-reconstruction of the auroral emissions in various wavelengths (red, green, blue). The optical and radar measurements are complementary. We obtained simultaneous observations with ALIS and EISCAT during two campaigns in 2008 and 2009.

  • The space observations provide a completely different perspective on auroras, because they are collected by spacecraft that cross the magnetic field lines above aurora, either below or above the electron acceleration region (such as the four Cluster spacecraft).

 

 

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