Take a bunch of fast-moving electrons, place them in orbit and
then hit them with the shock waves from a solar storm. What do you
get? Killer electrons. That's the shocking recipe revealed by ESA's
Cluster mission.
Killer electrons are highly energetic particles trapped in
Earth's outer radiation belt, which extends from 12 000 km to 64
000 km above the planet's surface. During solar storms their number
grows at least ten times and they can be dislodged, posing a threat
to satellites. As the name suggests, killer electrons are energetic
enough to penetrate satellite shielding and cause microscopic
lightning strikes. If these electrical discharges take place in
vital components, the satellite can be damaged or even rendered
inoperable.
On 7 November 2004, the Sun blasted a solar storm in Earth's
direction. It was composed of an interplanetary shock wave followed
by a large magnetic cloud. When the shock wave first swept over the
ESA-NASA solar watchdog satellite SOHO, the speed of the solar wind
(the constant flow of solar particles) suddenly increased from 500
km/s to 700 km/s.
Shortly afterwards, the shock wave hit Earth's protective
magnetic bubble, known as the magnetosphere. The impact induced a
wave front propagating inside the magnetosphere at more than 1200
km/s at geostationary orbit (36 000 km altitude) around Earth. The
quantity of energetic electrons in the outer radiation belt started
to increase too, according to Cluster's RAPID instruments (Research
with Adaptive Particle Imaging Detectors). Cluster's four
satellites sweep around an elliptical orbit, coming as close as 19
000 km and going out as far as 119 000 km.
Understanding the origin of the killer electrons has been a
focus for space weather researchers. Thanks to previous data
collected by Cluster and other space missions, scientists proposed
two methods by which electrons can be accelerated to such harmful
energy levels. One relies on very low frequency (VLF) waves of
3 kHz, the other on ultra low frequency (ULF) waves of
0.001 Hz. This latest work disentangles the problem.
Which waves are responsible? Both of them. "Both VLF and ULF
waves accelerate electrons in Earth's radiation belts, but with
different timescales. The ULF waves are much faster than the VLF,
due to their much larger amplitudes," says Qiugang Zong from Peking
University (China) and University of Massachusetts Lowell (USA),
lead author of the paper describing this result.
The data show that a two-step process causes the substantial
rise of killer electrons. The initial acceleration is due to the
strong shock-related magnetic field compression. Immediately after
the impact of the interplanetary shock, Earth's magnetic field
lines began wobbling at ultra low frequencies. In turn, these ULF
waves were found to effectively accelerate the seed electrons
provided by the first step, to become killer electrons.
Although the analysis has been a long one, the results have been
worth the wait. Now astronomers know how killer electrons are
accelerated. "Data from the four Cluster satellites allowed the
identification of ULF waves able to accelerate electrons," says
Malcolm Dunlop, Rutherford Appleton Laboratory, Didcot (UK) and
co-author of this study.
Thanks to this analysis of Cluster data, if the killer electrons
happen to be ejected towards Earth, we now know that they can
strike the atmosphere within just 15 minutes. "These new findings
help us to improve the models predicting the radiation environment
in which satellites and astronauts operate. With solar activity now
ramping up, we expect more of these shocks to impact our
magnetosphere over the months and years to come," says Philippe
Escoubet, ESA's Cluster mission manager.
SOURCE
