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PREDICTING when solar storms will arrive has always been hit-and-miss, but a
study of how they interact with the normal solar wind will now let researchers
predict an accurate 12-hour storm window.

The Sun blows out its regular solar wind at a speed of 250 to 800 kilometres
per second, but every so often it belches out a bigger mass of material known as
a coronal mass ejection (CME)—often the start of a solar storm. The
initial speed of a CME can vary from 20 to 2000 km/s. Because of this widely
varying speed, and the complicated interactions between CMEs and the solar wind,
a storm can take anywhere from 36 hours to four days to reach Earth.

To get a more accurate time of arrival for the potential disruption rushing
to us from space, Natchimuthuk Gopalswamy of the Catholic University of America
in Washington DC has been studying the speeds of CMEs. It is difficult to judge
the speed of an approaching CME using the data from a satellite near the
Sun-Earth line, such as NASA’s SOHO solar observatory. “It’s like having a
baseball thrown straight at your face,” explains Ernest Hildner, director of the
Space Environment Center in Boulder, Colorado.

So Gopalswamy analysed past records of 17 CMEs collected by satellites that
happened to be at an angle to the line between Earth and the Sun. By comparing
this data with measurements from SOHO and Earth-based observatories, Gopalswamy
was able to analyse the friction set up between the solar wind and CMEs. This
moderates the solar storm to a breezy 300 to 600 km/s by the time it reaches
Earth.

Understanding the interactions lets Gopalswami calculate a 12-hour window
when a CME will hit the Earth. “We have essentially established a benchmark,” he
says. Hildner says they have predicted solar storms to within 12 hours before,
“but with lots of mistakes” in terms of false alarms.