Magnetic footprints in the heavens

The aurora borealis, also known as the northern lights, is the visible result of interactions between the Earth and the Sun. Among other things, the aurora can tell us how the Sun distorts the Earth’s magnetic field.

“Ideally, we should have had two satellites with cameras that would be able to look down on both poles at the same time,” says associate professor Nikolai Østgaard of the Space Physics Section of the Department of Physics and Technology.

Displaced northern lights oval

The scientists do not have two satellites, but in 2001 and 2002 the orbits of the Polar and IMAGE satellites occasionally made it possible to observe both polar regions simultaneously.

Space research depends on satellites to be able to make proper measurements. And we still know so little about conditions in near-earth space that nearly all the knowledge we do have is basic knowledge.

“In our study we looked, among other things, at data from an aurora outbreak on September 13 2001 . On that occasion we found a displaced northern lights oval, when the auroras in the northern and southern hemispheres took roughly the same form, but were distorted vis-à-vis one another,” says Østgaard.

Charged particles produce the aurora

The aurora appears when charged particles are captured by the Earth’s magnetic field and bombard the upper atmosphere, where they are braked and their energy is transformed into light, which we can observe as the northern and southern lights in two huge ovals that surround the magnetic poles. Before they are braked by the atmosphere, the particles are “bound” to the lines of a magnetic field and they spin round these field lines, back and forth from north to south. If we think of the Earth as a magnetic dipole, we should expect that when we have the northern lights in the northern hemisphere there ought to be a simultaneous appearance of southern lights at the other end of the Earth, and the aurora borealis and the aurora australis (the southern lights) should appear to be identical and be located at the same relative positions over the two hemispheres. In practice it has proved to be quite difficult to investigate this. There are few observation stations in the southern hemisphere, and we are dependent on clear skies and darkness to be able to observe the aurora borealis from the ground. Furthermore, the field of view from an observation point on the ground is too limited to enable a single observer to take in the whole extent of the aurora.

The Sun’s magnetic field affects the field of the Earth

The data from September 2001 clearly showed that the Earth’s magnetic field is not symmetrical. In this case, the southern lights were displaced in the direction of the morning side relative to the northern lights, by as much as one and a half time zones. The explanation lies in the Sun’s magnetic field, which is projected into space by means of the solar wind, a current of charged particles that are continuously emitted by the Sun. The Earth’s magnetic field protects us from most of the solar wind, but within a limited area the two magnetic fields may become connected. It is in such situations that charged particles are brought into the Earth’s own magnetic field, where they generate the aurora borealis and aurora australis.

“The Sun’s magnetic field can thus force its way through, and also influence, the Earth’s magnetic field, distorting it in such a way that the end points of the field lines become asymmetrical. Our analyses show that the displacement of the northern lights can be predicted quite well by the angle of the magnetic field from the Sun,” says Østgaard.

The northern and southern lights are thus the footprints of the magnetic field lines that connect the northern and southern hemispheres. Their asymmetry is the visible result of the distortion of the magnetic field.

There exist rough models of how the magnetosphere behaves under given conditions. Østgaard and his colleagues have compared a number of observations with the models and found that while they support one another, the displacements are much greater than the models predict. There are thus several factors that drive the differences between the northern and southern lights, and the researchers now intend to find out more about many of these factors.

Space physics studies processes in near-earth space, particularly interactions between the Sun and the Earth. The Space Physics section of the Department of Physics and Technology is participating in a number of satellite experiments, and has also developed detectors that are used in satellites.

www.ift.uib.no/Romfysikk

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