ESA studies the Sun-Earth climate link

Meteorologists can no longer view the Earth as an isolated system. Both long-term climate changes and day-to-day weather show links with the Sun`s activity. Scientists therefore study the nature of those links intensely. With data from ESA`s spaceprobes SOHO, Cluster, and Ulysses, we now have the information we need to solve the mystery of how the Sun`s activity affects the climate here on Earth. This study is the first step in setting up a new type of weather forecast – the space-weather bulletin.

For the Sun to affect the Earth`s weather, the Sun`s behaviour must vary in some way. At visible wavelengths, however, the Sun is remarkably constant. Satellite data show that there are dramatic changes going on beyond this narrow range. For example, the Sun emits a `wind` of charged particles and we know that this wind is variable. The ultraviolet radiation released by the Sun also varies. Studying the interaction between solar variability and the Earth environment is a science known as `space weather`.

This solar variability is caused by the ever-changing magnetic behaviour of the Sun. The Sun`s magnetic behaviour changes on an 11-year cycle, passing from `solar minimum` to `solar maximum`. At the peak of this cycle, one of which occurred last year, the solar wind is stormy because explosions on the Sun`s surface catapult particles outwards with an increased intensity. The energy released during such explosions can be up to one thousand million megatonnes (or 66 thousand million times the energy released by the Hiroshima atomic bomb). Such events are also the source of the variable ultraviolet emissions.
ESA`s solar fleet is observing these phenomena very carefully and from several points in space. The joint ESA/NASA spaceprobe, the Solar and Heliospheric Observatory (SOHO), is constantly watching the Sun, monitoring this activity. The solar wind gusts buffet the magnetic field of the Earth. ESA`s quartet of satellites, Cluster, monitors these effects close to Earth while Ulysses patrols the Sun in a tilted orbit, well away from the plane of the planets, to get a more `global` view of the solar wind.

These data, linked with meteorological and other data, are an invaluable source of information to study the Sun`s effects on the Earth. “All the data is being archived and made available to the science community,” says Alexi Glover, one of the space-weather team at ESA`s ESTEC organisation in the Netherlands. SOHO has been designed to work for 3 years. “If SOHO keeps going for another four or five years, we`ll have a whole solar cycle`s worth of consistent data. That would be invaluable.”

Scientists are looking at three main mechanisms that may explain this Sun-Earth link for our weather and climate. Firstly, the Sun`s varying ultraviolet emissions affect the production of ozone in the Earth`s atmosphere, changing our ozone layer, and affecting the large-scale circulation of air. Secondly, the solar wind`s gusts affect the electrical properties of the Earth`s upper atmosphere which somehow affects the lower layers of the atmosphere. Thirdly, during the solar minimum, the solar wind is weaker which enables galactic cosmic rays (GCRs) to enter the Earth`s atmosphere more easily. GCRs are particles that are heavier and more energetic than those carried by the solar wind and are accelerated much farther away in space. Scientists believe that the movement of GCRs, which is influenced by the solar wind, generates conditions that promote the formation of low-altitude clouds. The significance of each of these mechanisms is as yet unknown, and scientists also do not know if the mechanisms are interrelated. Every avenue is being explored.

ESA is currently looking at developing a network of space-weather services, which will issue space- weather information. Such information could complement the more conventional weather forecasting. It would include space-weather forecasting, a database of space-weather events, and the development of computer modelling of the physical processes involved.

“We have just issued an announcement of opportunity inviting the community to propose contributions to such a space-weather service.” says Glover. Someday perhaps ESA will be able to help scientists, industry, and commerce understand and exploit the Sun`s effect on our everyday lives.