Northern Lights: insight into the origin puzzle

Shape of the arcs regardless of latitude

Aurora. © Jan Curtis, Fairbanks, Alaska
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Luminous arches, shimmering veils - the grandiose sky fireworks of the Northern Lights fascinate not only viewers but also science. Researchers have now gained new insights into the differences between two types of arcs of the aurora, using ESA's Cluster Satellites. They report about it in the journal "Journal of Geophysical Research".

The basic process of aurora formation is similar to what happens in the tube of an old television: accelerated electrons hit the screen and make it glow. The same applies to the northern lights: Here too it is electrons that fall into an "acceleration region" at a height of about 5, 000 to 8, 000 kilometers and are hurled from there into the ionosphere. Here, in the upper atmosphere, they collide with the charged atoms and molecules of the ionosphere and transfer part of their energy to them. As a result, the particles begin to shine - a polar light is created. So far the general principle. However, scientists have been researching for years what exactly happens, and what mechanisms are behind them - and still not all the secrets are revealed.

U- or S-shape - depending on the latitude?

Electrons that "walk" down the magnetic field lines: artistic representation. © ESA

It is now known that the almost static electric fields, which are aligned parallel to the magnetic field of the earth, play a decisive role in the acceleration of the electrons. These are thrown in high latitudes in two directions - both down and up, out of the atmosphere. The resulting structures belong to two different types: on the one hand symmetrical, U-shaped auroras, on the other hand S-shaped, asymmetric ones. As early as 2004, Göran Marklund, a professor at the Royal Institute of Technology in Stockholm, noted that these variations occur at the boundaries between magnetic field regions with different properties.

The U-type arises where the so-called central plasma field, which lies in the magnetic tail of the earth over the equatorial regions, and the adjacent plasma boundary layer abut each other. In contrast, the S-type was more likely to be found in the higher latitudes, at the boundary between the boundary layer and the polar cap. From these observations Marklund developed a model that assumed that the conditions in the lower latitudes generally promoted the emergence of symmetrical aurora forms, while the magnetic field properties of the higher latitudes were more likely to give rise to the S-shape,

Satellites contradict theoretical model

But the new measurements of the cluster satellites do not confirm this idea. As expected, one of the probes registered a transverse, symmetrical structure as it crossed the boundary between the central plasma field and the boundary layer, but only 16 minutes later a second cluster probe in the same area encountered asymmetric, Sif rmige structure, which was actually considered to be untypical for this region. At the same time, however, it appeared that in the short period between the first and second measurements, the plasma field had actually changed markedly: the currents and particle movements decreased and, in the second time, actually resembled the asymmetric one Conditions as they prevail over the poles. display

The researchers conclude that it is less responsible for the position of the auroras but rather for the very changeable conditions prevailing for their formation for the specific type of arc. This realization represents an important step in the understanding of the Northern Lights, yet many questions remain unanswered. For example, how the acceleration process is initiated and maintained. Further measurements of the cluster satellites in the next few years should provide a solution here.

(NASA / ESA, 12.02.2007 - NPO)