BAA / RMetS Joint Meeting, 2004 November 27

The Aurorae

Mr Pinnock opened with a video of the northern lights, filmed from Alaska, which served both to introduce the phenomenon as well as to illustrate two features which were often not well appreciated: the fine structuring of the light, and its dynamic nature – the rippling structures changing from one second to the next. The speaker explained that whilst the altitude of aurora varied between a hundred and a few hundred kilometres, the image in question, a rare red aurora, was known from its colour to be at around 100-110km. It was also explained that whilst the physical mechanism producing the light was essentially understood, the structuring was not yet well understood.

Historically, aurorae had fascinated humanity for thousands of years, and the speaker recalled an excellent recent article by Stevenson et al.¹, which reported the discovery of a late Babylonian astronomical table, interpreted to contain the earliest known record of auroral observation, dating from 567 BC. The phenomenon was now understood to be triggered by the collision of charged interplanetary solar wind particles, primarily electrons and protons, with the Earth's atmosphere. Upon reaching the outer layers of atmosphere, such particles would typically be absorbed, resulting in the excitation of the absorbing particle. The energy would subsequently be re-emitted in the form of light of a colour characteristic to the material of the absorber – the same radiation mechanism used in television tubes. Mr Pinnock explained that there was considerable scientific interest in the effect, as it was a probe of many other processes which were much more difficult to measure. In addition, it was also known that enhanced solar activity posed a range of hazards to technology, especially satellites in orbit, but in extreme cases also on the ground, and so understanding the underlying processes was seen as a high priority.

It was now well-established that aurorae were primarily seen in two oval circumpolar belts, at latitudes 65-75° N/S, called the auroral ovals. This had first been realised in the analysis of data collected in the International Geophysical Year of 1957-8, but deeper investigation essentially had to await satellite technology, for example NASA's Dynamics Explorer probes, launched in 1981. Though many of the brightest aurorae were associated with the enhanced solar wind following coronal mass ejection (CME) events on the Sun, the speaker wished to make clear that there was always some background activity – the so-called quiet-time auroral oval. However no activity was visible at latitudes as low as the UK's 52° without the impact of solar features such as CMEs with the Earth's magnetic field.

The speaker showed images of a CME taken from the SOHO solar observatory – huge eruptions of solar material, often taking no more than an hour to develop. As the erupted particles travelled out through the solar system, they suddenly impacted with a shock front upon reaching the Earth's magnetic field, compressing it on the day side. Most of the charged particles in the solar wind were deflected by the Earth's magnetic field, but some penetrated the outer limits of the geomagnetic field and were then guided by it towards the polar regions, where they triggered aurorae. However, wishing to clear up one common misconception, the speaker emphasised that during storms as many as 90% of the plasma particles in the Earth's magnetosphere actually originated from the ionosphere, not the solar wind. Whilst the solar wind triggered the phenomenon, for the most part the visible glowing was caused by excited atmospheric material. Mr Pinnock explained that after the impact of CMEs, the auroral oval was observed to enlarge, both towards the poles and the equator. A series of "substorms", each lasting perhaps an hour, might extend the oval a few degrees in latitude, and thus several in succession were required to generate any visible effect in Scotland, and even more for London.

Returning to the subject of the technological hazards of solar wind phenomena, the speaker recalled that in recent years a handful of satellites had been lost to the damaging effects of solar storms. The collision of high-energy particles with the semiconductors used in the electronics of satellites tended to destroy small regions of it, and so the collision of such a particle with a satellite's on-board computer could easily result in the loss of the spacecraft. Such particles did not reach the ground, but there were still other hazards. The speaker recalled in particular the geomagnetic storm of 1989 March, which had blacked out much of Toronto. In this case, surging ionospheric currents had induced current surges in a power line, causing the dramatic explosion of a transformer. The hazards of such currents had to be considered in the design of any large metal structures at auroral latitudes, the speaker added, pipelines being another example.

Reviewing future plans, the speaker discussed proposals to study artificially-induced aurora, using huge antenna arrays to deliver 2MW of radio power into the ionosphere, exciting it at an altitude of 230km. Mr Pinnock also remarked that aurorae had been observed on other planets: the Galileo probe had detected polar brightenings on Jupiter caused by a similar phenomenon, however the formation mechanism had been shown to be rather different from its terrestrial equivalent, and so in the process, we had come to understand a new way in which aurora could arise.

To close, the speaker remarked that there was sadly little UK amateurs could usefully do in the observation of aurorae – the UK was not far enough north – but those who were interested were encouraged to sign up for the Lancaster University e-mail alert system², whereby they would receive regular updates on current space-weather activity.

Following the applause for Mr Pinnock's talk, the meeting broke for lunch. After the break, Prof Chris Collier, President of RMetS, took the chair, and welcomed the first speaker of the afternoon session, Dr Damian Wilson of the Meteorological Office.