UCL Astrophysics Group


Magnetosphere-Ionosphere Coupling, Aurorae

The bright displays near the magnetic poles of planets such as the Earth, Jupiter and Saturn are known as aurorae. These emissions are caused when charged particles from the solar wind, or from the planet's outer magnetosphere, 'spiral' down along magnetic field lines and collide with neutral molecules. The resulting is the production of energetic photons when the molecules relax back to their lower energy states.

At the gas giants, the upper atmosphere is principally composed of hydrogen. The auroral emissions are thus dominantly UV photons from quadrupole transitions of H2 (neutral hydrogen) and infrared photons from ro-vibrational transitions of the H3+ molecular ion. The Planetary and Plasma Physics group has a strong heritage of infrared auroral observations of the gas giants Jupiter and Saturn, using ground-based telescopes. More recently, we have started to investigate the application of sophisticated image processing techniques to UV auroral images of these planets acquired by the Hubble Space Telescope (HST).

Our group at UCL have also been involved in developing global circulation models for the gas giant atmospheres. A very challenging problem with such models is the representation of magnetosphere-ionosphere coupling, the process which transfers angular momentum from the planet's atmosphere to its 'disc-like' magnetosphere. This transfer is associated with millions of Amps of electric current flowing between these two regions - upward-flowing current at the planet's surface corresponds to energetic electrons flowing downward into the atmosphere and axciting auroral emissions. Thus the detailed modelling of aurorae also makes use of these displays of light as a diagnostic of the physics operating globally throughout the planet's magnetosphere. Recently we have been investigating, using GCM for Jupiter, how the winds and temperatures in the planet's upper atmosphere respond to changes in the upstream solar wind which, in turn, cause strong reconfigurations in the structure of the magnetosphere.