A good weekend for aurora watchers?
17 February 2000
At 22:30 Central European Time on 17 February the ESA-NASA SOHO spacecraft spotted a large puff of gas from the Sun. Subsequent images from the LASCO coronagraph on SOHO confirmed that this coronal mass ejection is heading towards the Earth. The gas will reach the Earth's vicinity on Saturday or Sunday, 19 or 20 February. When it hits the Earth's magnetic shield, it is likely to cause a magnetic storm. It may provoke spectacular displays of auroras high in the air over sub-polar regions of the Earth, and perhaps even at lower latitudes in Scandinavia and North America.In a second event, at 13:00 on 18 February SOHO and other spacecraft registered a burst of energetic particles --- solar protons -- coming from an eruption on the Sun. They may be associated with a second large mass ejection seen by LASCO, which is not heading for the Earth.
Satellite controllers will be on the alert this weekend for harmful effects that the earthward-bound mass ejection may have on their spacecraft. Users of telecommunications and navigation satellites may be affected too, as may operators of ground systems that are vulnerable to the effects of solar storms, such as power transmission lines.
Most mass ejections from the Sun miss the Earth, or are minor events. When a large mass ejection is heading towards the Earth it appears as an expanding bright halo around the Sun, in SOHO/LASCO images. A mass ejection heading directly away from the Earth, on the far side of the Sun, has a similar appearance. But in this case the EIT telescope on SOHO saw shock waves in the solar atmosphere associated with the mass ejection, making it certainly an Earth-approaching event.
Half an hour or so before the mass ejection reaches the Earth, it will wash over SOHO itself, and the CELIAS solar-wind instrument on the spacecraft will gauge the density of the gas. Only then will scientists and engineers know how hard the Earths space environment will be hit.
