SCIENCE

The Aurora from the Sun to Earth and Beyond

Dr Chris Davis, Rutherford - Appleton Laboratory, on 22 June 2001

This talk covered the structure of the Sun and its corona, and how it interacts with the Earth's magnetic field to produce what we know as the Northern Lights, or Aurora Borealis.

We are now beginning to understand the true nature of the Sun and how dynamic its behaviour is. We have known for some time that the number of sunspots follows an 11-year cycle, the spots appearing near the poles first, and over time, they drift in latitude towards the equator. A new cycle is seen to be starting when spots once more appear at the poles. The reason for this cycle is that the equator of the Sun super-rotates, that is it rotates faster than the layers at higher latitudes. There is a magnetic field interacting with the ionised gas at the surface of the Sun, and this follows the surface as it winds up with the fast equatorial rotation.

Eventually, the field is so contorted that it inverts, and the north and south poles change places, allowing the field to unwind before it winds itself up again. The period between field reversals is the 11 years we observe.

Sunspots are an indication of how much energy is stored in the Sun's contorted magnetic field. This energy is released in spectacular eruptions of massive amounts of matter being ejected into the Sun's corona, dragging the solar magnetic field into space and actively modifying it. These Coronal Mass Ejections (CMEs) can be clearly seen in film taken by the SOHO satellite sitting at the first Lagrangian point between the Earth and Sun, where it gets an untrammelled view of the Solar disk and corona.

These charged particles stream away from the Sun and when they reach the Earth, they interact with the Earth's field, which forms a kind of magnetic bubble, compressed on the sun-ward side, and extended on the night side. When the two magnetic fields oppose one another, there are two important reconnection points where the particles are slowed sufficiently to allow the two magnetic fields to merge allowing the energetic particles to cross from one to the other. One of these is on the day side, and one on the night side. As the particles are picked up by the Earth's field, they spiral down into the atmosphere, and when they hit the denser layers, they cause excitation of the atmospheric gases, which then emit the light seen as the aurora. The effects can be more pronounced on the night side, as the day side reconnection can cause energy to build up in the tail of the Earth's magnetic field. All this energy is then released in one burst, known as a sub-storm, when the charged particles are thrust into the Earth's atmosphere.

If the reconnection on the day side matches that on the night side, the system is said to be in a steady state. The reconnection points occur at fixed locations relative to the Earth, and the aurora would appear at the same place all the time. However, the CMEs can have a dramatic effect on the interaction between the fields; often reconnection on the day side exceeds that on the night side for several minutes at a time. During this phase, more field lines are open to the energetic solar particles in these places.The stronger the solar storm therefore, the closer to the equator can the auroral effects be seen.

The power density of the phenomenon is about 0.05 W/m² (by comparison, normal sunlight is about 80 W/m²). If the Sun has a southward field (i.e. opposite to the Earth), the total is equivalent to about 6000 power stations, of which about one third is deposited into the atmosphere. The particles begin to affect the gases of the atmosphere at a height of 250 km, and the most energetic reach down to 100 km. They ionise the oxygen and nitrogen, and the colours we see are the emissions from these energetic ions releasing the energy from their electrons as they fall to a lower state. The typical green colour is generated when oxygen atoms emit light after absorbing energy from the more energetic incoming particles. These tend to penetrate further in to the atmosphere and so to excite the lower strata of oxygen (at an altitude of around 100 km). The often-seen red aurora is generated higher up (around 250 km) as oxygen atoms emit light after absorbing energy from less energetic particles. If extremely energetic particles are present, these cause nitrogen, low in the atmosphere (again around 100 km) to emit a mauve light.

The SOHO space craft gives us about one hour's notice of a serious solar storm heading earthward. Such a storm can have significant effects to our other systems. They can interrupt radio transmissions of all sorts, and hence satellite based systems can fail, like the GPS appliances used in drilling location, surveying and navigation. Power distribution networks can be damaged as the fluctuating magnetic fields can induce electrical effects in the long cables.

The talk was accompanied by a full set of pictures showing everything from the spectral emissions of the aurora, diagrams of the interplanetary magnetic fields, the radars used to study the lights in Scandinavia, and much more, ending with a spectacular photograph from the Hubble telescope showing both auroral rings on Saturn. A copy of these was donated to the BRLSI and can be seen on application.

Andy Pepperdine