Activity on the Sun may be declining after last year’s peak, but, as the
four Cluster spacecraft can testify, our nearest star can still pack a
hefty punch.

During last week’s solar storm, the Cluster quartet was temporarily
disabled by a peppering of high-energy protons. Two days later, once
normal service had been restored, the Earth’s magnetosphere — the
invisible magnetic bubble that surrounds and protects our planet — was
dramatically squeezed by the impact of a huge cloud of matter ejected
from the Sun. As a result, the spacecraft made an unexpected journey
into interplanetary space, where they were exposed to the full blast of
the supersonic solar wind.

The onset of the storm was recorded by another ESA spacecraft, the Solar
and Heliospheric Observer (SOHO), which continually monitors events on
our turbulent Sun. Images from SOHO show that a huge cloud of gas (known
as a Coronal Mass Ejection or CME) erupted from the Sun at around 17:00
GMT (18:00 CET) on 4 November. A shower of high-energy protons was also
emitted by the Sun during this event. Travelling at almost the speed of
light, the protons swept past SOHO at 18:37 GMT (19:37 CET), causing a
‘snowstorm’ on the spacecraft’s imaging instruments.

Minutes later, these damaging particles caused a computer glitch on the
third Cluster spacecraft (Samba), which meant that the satellite could
not be commanded in the normal way. During the complex recovery
procedure, all of the instruments on the spacecraft had to be switched
off and reset using well-rehearsed back-up procedures.

By 6 November, all four Cluster spacecraft were up and running again and
SOHO data showed that the number of incoming protons was decreasing, but
another dramatic series of events was about to unfold.

At about 02:00 GMT (03:00 CET), the huge cloud of electrified solar gas
that had exploded from the Sun on 4 November swept past our planet. The
collision between the CME and the Earth’s magnetic field was detected
by various instruments on the Cluster quartet.

Sudden jumps in the strength of the magnetic field and the number of
magnetic waves were recorded by the FGM and STAFF instruments.

At the same time, the number of energetic protons recorded by the RAPID
instrument also increased once more. Unlike the first burst of high-
energy protons, which were produced by the initial explosion on the
Sun, these particles were created at the CME while it was travelling
towards the Earth.

“This CME was magnetised in the opposite direction to the Earth’s
magnetic field, so it produced a big shock when it struck the
magnetosphere,” said Cluster project scientist, Philippe Escoubet.

“The Cluster instruments showed a rapid rise in the strength of the
magnetic field, as well as marked increases in the density and velocity
of the particles — evidence that the sudden squeezing of the
magnetosphere had left the spacecraft ‘adrift’ in the solar wind,” he

“In fact, the spacecraft data show that the boundary region — the
magnetopause — crossed Cluster twice,” he explained. “This means that
the magnetic field was squeezed, expanded, squeezed again, and then
expanded once more over a period of less than four hours.”

“The collision had a very strong energising effect on the magnetosphere,
which resulted in a marvellous display of auroras to low latitudes in
Scotland and North America,” he said.

“This is the first time that Cluster has crossed the boundary of the
magnetosphere since June,” commented Philippe Escoubet. “We had not
expected the spacecraft to enter the solar wind for several weeks.”

“This event is very interesting for us because a similar solar storm
took place almost exactly one year ago, on 11 November 2000,” he said.
“On that occasion, too, a CME squashed the Earth’s magnetic bubble and
caused Cluster to be exposed to the charged particles in the solar wind.”

For further information please contact:

Dr. Philippe Escoubet Cluster project scientist

ESTEC, The Netherlands

Tel: +31 71 565 3454



* The Sun-Earth Connection
* The solar storm on 4 November captured by SOHO/LASCO
* More about Cluster


[Image 1:]
SOHO images showing the solar storm.

(Left) A Coronal Mass Ejection erupted from the Sun at 17:00 GMT (18:00
CET) on 4 November.
(Centre) Travelling at almost the speed of light, high energy protons
arrived at SOHO at 18:37 GMT (19:37 CET), causing a ‘snowstorm’
on the spacecraft’s imaging instruments.
(Right) By 6 November the number of incoming protons had significantly

This sequence of images was extracted from a time-lapse movie
[ (378KB)]
of observations from the SOHO/LASCO instrument.

[Image 2:]
Aurora in Scotland near Edinburg on 6 November 2001. Photo courtesy
Alexandre Vieira-Linhares, Scotland, United Kingdom.

[Image 3:]
Cluster data showing the effects of the Coronal Mass Ejection on 6
November. The data show that the magnetosphere was actually compressed
and then expanded on two occasions. Cluster first entered the solar
wind at around 02:00 GMT. The spacecraft then crossed back into the
magnetosphere before a second compression between 04:00 and 05:10 GMT
left them once again in the solar wind.

(Top) FGM data show how the magnetic field suddenly increased above 100 nT
at about 02:00 GMT and remained high for many hours.
(Centre) Data from the RAPID instrument show two sudden increases in high
energy protons (above 10 MeV) after the arrival of the CME.
(Bottom) STAFF data show large magnetic waves associated with the solar
wind when the spacecraft passed outside the magnetosphere.

(Data courtesy of A. Balogh, IC, London, P. Daly, MPAe, Germany, and N.
Cornilleau, CETP, France).

[Image 4:]
The location of the spacecraft after the magnetosphere was squeezed by
the Coronal Mass Ejection (CME). This resulted in the flotilla orbiting
outside the Earth’s magnetic shield.