ESA's Cluster and NASA's Image satellites have worked together to explain a particular type of very high-latitude aurora, which is the most visible manifestation of the Sun's effect on Earth.
Although separated by some 150 million kilometers, the Sun and Earth are connected by the solar wind. This stream of plasma, electrically charged atomic particles, is launched by the Sun and travels across the solar system, carrying its own magnetic field with it.
Depending on how this 'interplanetary magnetic field' is aligned with Earth's magnetic field when it arrives, there can be various results. At the point where the two fields meet, Earth's magnetic field points north. If the interplanetary field is pointing south, then 'magnetic reconnection' can occur, where magnetic field lines pointing in opposite directions spontaneously break and reconnect with other nearby field lines which opens the door to solar wind plasma entering the magnetosphere or Earth's magnetic 'bubble.'
The ultimate result can be colourful displays in the night sky known as the Northern or Southern Lights, produced when the particles are channelled along Earth's magnetic field lines and strike atoms high in the atmosphere. The interaction with oxygen atoms results in a green or more rarely red glow in the night sky, while nitrogen atoms yield blue and purple colours.
But when the interplanetary magnetic field points northward, auroras can occur at even higher latitudes and one type is known as a 'theta aurora' because seen from above it looks like the Greek letter theta an oval with a line crossing through the centre.
Lead author Robert Fear from University of Southampton said that the possibilities have been debated since the first satellite observations of the phenomenon were made in the 1980s, but one idea is that the process of magnetic reconnection on the night side of Earth causes a build-up of 'trapped' hot plasma in the higher latitude lobes.
Fear added that they found that the energetic plasma signatures occur on high-latitude magnetic field lines that have been 'closed' by the process of magnetic reconnection, which then causes the plasma to become relatively hot.
Fear continued that because the field lines are closed, the observations are incompatible with direct entry from the solar wind and by testing this and other predictions about the behaviour of the theta aurora, our observations provide strong evidence that the plasma trapping mechanism is responsible for the theta aurora.
The paper will be reported in Science.
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