Left: The development of the theta aurora on 15 September 2005, as seen by NASA’s IMAGE far-ultraviolet wideband imaging camera. Each frame is projected onto a grid of magnetic latitude against magnetic local time, with local noon at the top and dawn to the right. 

The footprint of the Cluster satellites is indicated by a red dot. Times are indicated in the clock.

Initially the aurora conformed to a standard ‘oval’ configuration, but then, at about 16:40 GMT, a small feature emerged from the nightside and grew into a transpolar arc, connecting the two sides of the oval and intersecting with the footprint of the Cluster satellites at around 17:15 GMT. The arc eventually moves past the satellites and dissipates, corresponding with the interplanetary magnetic field turning southward from an initially northward direction.

Right: Cluster measurements during the development of the theta aurora. The uncharacteristically energetic plasma corresponds to the times where the ‘arc’ of the theta aurora is seen to intersect the spacecraft footprint in the IMAGE data.