Summary of Project Outcomes

The theme of this ISSI Team has been to study the process of low frequency Magnetohydrodynamic waves coupling in a three-dimensional magnetosphere. These waves of interest are the global scale waves with wavelengths comparable to the size of the magnetosphere. We focus on the resonant coupling of two particular wave modes: the “fast” mode (similar to a sound wave but driven by magnetic pressure) and the “Alfven” wave (similar to wave on a string, where field lines play the role of a string). Typically, disturbances from the solar wind can be communicated throughout the magnetosphere by the fast mode. On particular field lines the fast wave can resonantly couple to Alfven waves, and these can couple efficiently to the ionosphere. The resonant wave coupling process is well-studied in 2D equilibria but has only recently been studied in 3D equilibria by our Team members.

We have developed new simulation codes to study the wave coupling process, and this has guided us to develop a theoretical description with novel features present that are absent in 2D models. We have published theoretical analyses identifying the locations where efficient coupling occurs, and also shown this can be cast as a minimization principle.

The above figure shows (first panel) the afternoon equatorial plane with coloured ridges indicating where the resonant Alfven waves are. (There is a plasma density enhancement – a  plume – centred on (x,y)=(6,6), making the medium 3D.) The remaining panels show hodograms of the velocity (or electric field) components in the plane at three locations, labelled 1,2 and 3. In 2D theory, hodogram ellipses would all be aligned with the vertical axis. Importantly, at location 2, the ellipse is inclined, and corresponds to the presence of an azimuthal electric field component.

The Teams theoretical proof of the existence of resonant Alfven waves (driven by fast modes) having an azimuthal electric field is something new that only occurs in 3D. The Team wanted to corroborate the theoretical ideas with the first ever observations of such a wave.

The figure above shows how hodograms (this time for the magnetic field – see the bottom row) vary as a satellite crosses a plume boundary: from (c) to (d) the ellipse rotates towards the radial direction and then back again (d) to (e) – exactly as we predicted.

That the Alfven waves can have an azimuthal electric field is unexpected and means they can interact strongly with radiation belt particles. Our Teams activities are motivating a re-evaluation of particle interactions with these waves, which is fundamental to understanding Space Weather.