By Storm Dunlop
We are all used to blaming things (rightly or wrongly) on the weather, but now it seems that this tendency has been extended to space weather. Space weather, for those who are uncertain, describes the effects that flares and other events on the Sun produce on Earth.
Consult many of the sites on the World Wide Web that are devoted to events on a particular day in history, and you will be told that on 16 August 1989, a geomagnetic storm caused the Toronto Stock Exchange to crash. The trouble is that this is an urban myth. The Toronto Stock Exchange did crash that day, but because of hardware and software failures, not because of a geomagnetic storm.
Why did they blame the Sun? Probably because 1989 did see a catastrophic event in Canada caused by a geomagnetic storm, which has even been described as ‘The Cosmic Wake-up Call’. Less kindly perhaps, it might be called ‘The Day Quebec Hydro’s Network Collapsed’, when 6 million people were suddenly plunged into darkness, left without electricity, stranded in lifts, woke to unheated homes, and went without a hot breakfast.
But what is a geomagnetic storm? Let’s take that disastrous event in March 1989 as an example.
On 6 March 1989, the Sun’s rotation carried into view a gigantic sunspot group, about 70,000 km across. (That’s big, very big. Sunspot groups don’t come any larger.) Between 6 and 19 March it was phenomenally active, with at least 195 explosive solar flares, with 11 of the most extreme ‘X-class’ flares. One that occurred on 6 March emitted a surge of charged particles and X-rays that overwhelmed the detectors on the Geostationary Orbiting Environmental Satellite 7 (GOES-7). But that was nothing compared with what happened on 10 March. There was a rare ‘white-light flare’ – only a few of which have ever been seen – of extreme intensity, and a coronal mass ejection (CME), directed straight at the Earth.
It takes time for the plasma ejected in a CME to reach the Earth, and the flood of charged particles began to arrive on 12 March. By the middle of 13 March, the magnetopause, the boundary that separates the Earth’s magnetosphere (where the Earth’s magnetic field is dominant) from the interplanetary region governed by the Sun’s magnetic field and the solar wind, had been compressed from its normal distance of about 55,000 km on the sunward side, to about half that amount. Geostationary satellites, orbiting at 35,500 km from the Earth, and normally protected from the solar wind, were exposed to the full blast of particles. These can severely damage detectors and computers on board satellites and sometime even render them completely useless.
Many Earth-observation satellites, including the meteorological, polar-orbiting satellites are much closer to Earth, in what are known as low Earth orbits (LEO), which are generally taken to be at altitudes of 160 to 2000 kilometres. The International Space Station (ISS) is in an orbit that carries it between 320 and 400 km above the surface. Satellites in LEO may still be damaged by the intense flux of charged particles occurring during solar storms, and there are often communications problems. (Some GPS signals were affected by the March 1989 storm.) But there is yet another effect, because, although low in density, there is some residual atmosphere at these altitudes.
The charged particles collide with atoms and molecules in the atmosphere, giving rise to aurorae and, at the same time, cause heating of the upper atmosphere. This expands outwards, raising the density at satellite altitudes and thus slowing down LEO satellites, and causing their orbit to decay more rapidly. Some may even re-enter and burn up. In the March 1989 storm, the density increased to five to nine times its normal level. One satellite started tumbling uncontrollably.
Under relatively quiet conditions, aurorae are most frequently seen in two zones, known as the auroral ovals, roughly centred on the magnetic poles, but reaching lower latitudes on the midnight meridian. These auroral ovals may be regarded are roughly fixed in space as the Earth rotates beneath them. There is always a flow of charged particles, known as the auroral electrojet, in the ionosphere following the route of the auroral ovals.
When a major geomagnetic storm occurs, the particle influx into the ionosphere not only causes extreme changes in radio communications, but the electrojets become exceptionally strong and the whole system expands towards the equator.
During the extreme geomagnetic storm of 13 March 1989, intense red aurorae were seen over the whole of the southern United States and as far south as Mexico, Cuba, and the Cayman Islands. (Red aurorae, rather than the more common green form, often accompany major geomagnetic storms.) The auroral oval around the south magnetic pole also expanded, and aurorae were seen in Australia and New Zealand, and even as far north as South Africa, where aurorae are extremely rare.
But that is not all. When the currents flowing in the electrojets strengthen, they create correspondingly strong electrical currents at the Earth’s surface. All electrical equipment, such as the major transformers used in high-voltage transmission lines, are ‘earthed’, that is, bonded electrically to the underlying ground. The immense induced currents created by a geomagnetic storm may find it easier to flow through man-made electrical connections and transmission lines than through the Earth’s surface. This is especially the case where, as in Sweden and (in particular) in Canada, the underlying rocks are granite or similar rocks, which have poor electrical conductivity. On 13 March, the immense currents surged through Hydro Quebec’s transmission lines, burning out some transformers and causing other fault-sensing equipment to disconnect whole sections of the transmission grid. The ‘knock-on’ effect caused damage to electrical equimpment as far south as New Mexico and Arizona in the United States. Quebec Hydro’s whole system failed, leading to a blackout over a large part of Canada and some of the northeastern United States. Blackouts also occurred in Sweden, and the grid in the United Kingdom was affected, but there no major interruptions took place.
Such major geomagnetic storms are infrequent, but even though lessons were learned from the storm of 1989, our dependence on satellites, long-distance communications, and electrical power has only increased, so these solar storms remain a major hazard today.
Image credits: Green aurora photograph by Denis Buczynski. Red aurora photograph by Alex Cherney. Do not reproduce without permission.