SOHO sun watcher improves space weather forecasting
ESA SCIENCE REPORT
Posted: August 4, 2000
"For thousands of years my ancestors in Norway marvelled at the space weather seen in the Northern Lights," Dr Brekke says. "But auroras never hurt a sailor or a farmer. It's only with our modern electrical, electronic and space technologies that the Sun's effects become damaging, and personally hazardous for astronauts. The more we do in space, the more serious and potentially costly the problems of space weather will become."
A big solar explosion last month demonstrated the leading role of the SOHO spacecraft in the early-warning system for space weather. On July 14, SOHO's ultraviolet telescope EIT saw the bright flash of a solar flare near the centre of the Sun's disk, at 10:12 Universal Time (GMT). The flare's intensity peaked at 10:24, and half an hour later SOHO's LASCO instrument detected a mass of gas racing out from the Sun in a coronal mass ejection, or CME. The gas appeared as an expanding halo around the Sun because the CME was heading towards the Earth.
Next, a burst of energetic particles from the solar explosion hit SOHO. It was the most intense storm of energetic particles seen in the present solar cycle. In the imaging instruments it looked like a snowstorm that continued for some hours and spoiled the images. But by then the world's space-weather reporting centres were already aware of the eruption.
Travelling more slowly than the energetic particles, the interplanetary shock wave driven by the gas of the CME arrived at SOHO a day later, at 14:19 UT on 15 July. The solar-wind instrument CELIAS on SOHO registered a jump in the wind speed from 500 to 800 kilometres per second, increasing to over 900 km/s an hour later. As the spacecraft is stationed 1.5 million kilometres out, on the sunward side of the Earth, the CME slammed into the Earth's magnetic field half an hour later than at SOHO, provoking auroral displays that peaked in the early hours of 16 July. Satellite operators and electric-power engineers experienced many disturbances in their systems but they reported no major failures, perhaps because they were forewarned. "SOHO has proved to be especially valuable for spotting the coronal mass ejections," Dr Brekke comments. "But dangerous energetic particles capable of killing an unprotected astronaut travel much faster and arrive half an hour after the sighting of a solar flare. To give more useful warnings, we'll have to find out how to predict the eruptions -- which means getting deeper into the solar physics that SOHO was designed to study."
SOHO was built in Europe for a project of international cooperation between ESA and NASA, and launched in 1995. It carries twelve sets of instruments provided by European and American scientific teams, for fundamental studies of the solar interior and atmosphere, energetic particle emissions, and the solar wind.
Two instruments on SOHO have proved to be especially valuable for continuous real-time monitoring of solar storms that affect space weather. One is the Extreme Ultraviolet Imaging Telescope (EIT, under French leadership) that provides images of the solar atmosphere at four wavelengths. It reveals flares and other stormy events in the atmosphere. The other is the Large Angle Spectrometric Coronagraph (LASCO, under US leadership) that takes images of the solar corona by blocking the light coming directly from the Sun itself with an occulter disk, creating an artificial eclipse within the instrument. It is the perfect tool for detecting the coronal mass ejections (CMEs) that are responsible for some of the most dramatic space weather effects on Earth.
A CME that erupts to one side of the Sun will miss the Earth. One coming towards us from the front side of the Sun is observed as a so-called halo event. As the gas spreads wider, LASCO sees it as a halo around the Sun. But the same appearance can occur with an eruption on the far side of the Sun, where the CME is going directly away from the Earth. SOHO scientists have learned how to avoid false alarms by using EIT observations. A CME on the near side provokes waves in the solar atmosphere, seen by EIT. The combination of positive indications from EIT and LASCO announces an earthward-heading CME.
When a CME hits the Earth's magnetic field, which shields us from direct effects of the solar wind, a magnetic storm results, measured by the Kp index. In 1995, before SOHO was operational, only 27% of major magnetic storms (Kp index of 6 or greater) were correctly forecast, and most forecasts were false alarms. The improvement offered by SOHO is apparent in a study of 25 front-side halo CMEs seen by LASCO and EIT during 1996 and 1997. Over 85% caused major magnetic storms and only 15 % of such storms were not predicted. Since the beginning of 2000, SOHO has detected more than 20 earthward-directed halo CMEs, most of them much weaker than the event of 14 July.
With a view to predicting CMEs before they occur, a programme of research into their origins uses another instrument on SOHO, the Coronal Diagnostic Spectrometer or CDS. Most of its observations concern eruptions occurring on the visible edge of the Sun. CMEs originating there do not head towards the Earth, but scientists can more easily distinguish the events in different layers of the Sun's atmosphere, and can use the novel CDS instrument to provide the equivalent of meteorological maps of the solar atmosphere.
A number of CME onsets have been identified with this technique. In a particularly clear event on 25 July 1999, CDS saw a magnetic loop rising through the atmosphere at 10 km/s for two hours before a CME, apparently carrying with it the gas that provided the mass of the mass ejection. Then a magnetic explosion occurred, releasing the CME and also causing a flare.
"At last we begin to see tell-tale events that precede eruptions on the Sun," says Dr Richard Harrison of the Rutherford Appleton Laboratory, UK, who is Principal Investigator for CDS. "Yet the link between CMEs, flares and dangerous outbursts of particles is still very vague. Geologists knew for a long time that earthquakes and volcanoes were related, but they had to wait for the plate-tectonics theory to understand the connection. I suspect that we need an equivalent theory of the Sun's magnetic weather if these awesome outbursts are to make sense."