The geomagnetic field extending several Earth radii into space obstructs the solar wind, the continuous stream of charged particles from the Sun. It redirects this stream to an arc around this obstacle, thus modifying the magnetic field and generating electric currents that may penetrate into the Earth atmosphere. Already Gauss and Weber observed such fluctuations of the magnetic field on ground. Here at the institute Bartels 1949 defined a 3 hour average as unit for the magnitude of the magnetic effects of the solar particle stream, the so called Kp value (www.gfz-potsdam.de/kp-index). Kp-values have a significance in solar-terrestrial relationship comparable to the Richter scale for the magnitude of earthquakes. Magnetic storms, as they are also called, often lead to polar light, seen as aurora borealis in arctic and aurora australis in antarctic latitudes or sometimes even in Göttingen when their Kp value exceeds 6. Data from pace instruments which continuously observe the Sun and our magnetosphere are used to forecast space weather and to predict the probability of sightings  (www.polarlicht-vorhersage.de).

Sightings at the sky above Göttingen

Oktober 10, 2024

During the night hours lucky people could observe bright polar light at moments of clear sky among the clouds above Göttingen. This event was classified as G4-storm with a Kp-value of 9.

© Wedeken, Foto 10. Oktober 2024, 22:59 h MESZ, Rohnsterrassen, Göttingen

May 10, 2024

The strongest geomagnetic storm since October 2003, a G5 with a Kp-value of 9.0 excited polar light that could be observed all over Germany. This foto showing the clear sky above Göttingen was taken by Dr. Klaus Reinsch, Institute for Astrophysics and Geophysics of the university in Göttingen.

© Reinsch, Foto 10. Mai 2024, 21:53 h MESZ, Leinebrücke, Schiefer Weg, Göttingen

What exactly is polar light?

Gauss and co-workers detected short-term variations of the terrestrial magnetic field, but they had no idea that such fluctuations are related to the polar light.

For more than a century scientists assumed that the geomagnet is similar to a rod magnet with field lines oriented in half circles leading from the north to the south pole in perfect rotational symmetry. Since we have access to space, we learned that things are different. Charged particles from the Sun can not move across the field lines and are re-routed to an arc around the Earth. This permanently flowing solar wind depresses the geomagnet on its dayside and stretches the field lines to a long tail at the night side. Far out, field lines from the north pole run almost anti-parallel to those leading to the south pole.

Sometimes the Sun is much more active. Then violent eruptions can eject solar material. Such a cloud of charged particles can reach a speed of up to 1500 km/s and can escape solar gravity. If it is Earth-directed, it will arrive in 1-2 days and will be guided all around the magnetic bubble. Field lines of different polarity can get in contact to each other due to the disturbance and reconnect. Such a magnetic shortcut generates a high-voltage, a process that we know from an electric fence. This high voltage accelerates ambient electrons along the field lines until they reach the Earth atmosphere and excite the gases near the footpoint. Heavy eruptions may affect footpoints at lower latitudes, sometimes as low as in Göttingen.

The release of energy far away at the nightside of the Earth is not well reported in public outreach news. Similarly, a valid explanation of the observed colours is missing.

The basic process of light emission in gases is the energy input of an atom by a colliding electron and the energy release by emission of a photon within nanoseconds. However, resonance effects limit the number of possible energy levels similar to a guitar that can play only a small number of tunes. Every gas has its specific colours. Since the resonances are known, physicists have calculated all the possible energy levels and summarized them. A comprehensive data base is provided by the National Institute of Standards and Technology, NIST.

Both colours, the red at 630/636 nm and the green at 597 nm are associated to oxygen A lookup in the data base reveals at first glance, that the related energy levels do not release their energy within nanoseconds as Neon does, but stay energized over seconds and minutes. This is a difference of nine orders of magnitude! Such almost stable energy levels accumulate to huge numbers. We name them metastable and their afterglow phosphorescence.

There is, however, a competing process for such long-living levels. They can release their energy without radiation by collision with a neighbouring atom, if there is any. This introduces a pressure dependant side-track. Collision will dominate the de-excitation, if the ambient pressure is too high. In consequence the red colour is less dynamic and can only occur at higher altitudes, since the related energy level has a significant longer lifetime than that of green light.

The observed colours are pressure dependant and the atmospheric layering follows the lifetime of the related energy levels.