On the 24th of October 2003, the SOHO spacecraft registered a huge Coronal Mass
Ejection (CME), emitted by the Sun. Several hours later this eruption reached
the Earth and was detected by a number of spacecraft including Cluster.

The ACE spacecraft, situated along the Sun/Earth direction, was situated about
1,500,000 km upstream from the Earth, monitoring the solar wind. At about 14:49
UT, ACE recorded a sharp increase on the proton velocity, which jumped from
about 450 km/s to more than 600 km/s . The proton density, which was about 3 to
4 particles/cm3 , increased to more than 20. The proton temperature in the solar
wind at this instant was also multiplied by a factor of 8.

The four Cluster spacecraft were in the southern magnetospheric lobe, inbound
towards their perigee. Note that the Sun, ACE, Cluster and the Earth were almost
aligned when the CME was ejected from the Sun. Cluster was situated close to the
inner magnetosphere (near to the ring current region) when it detected the
effects of the solar wind pressure on the magnetosphere: The sudden increase of
the solar wind pressure registered by ACE arrived at the Earth’s magnetosphere
about 40 minutes later. It provoked a huge compression of the dayside
magnetosphere. The Cluster spacecraft detected this compression by getting
suddenly out of the southern magnetospheric lobe into the Magnetosheath. They
thus detected the Magnetopause, moving earthward, at about 15:25 UT. They
remained into the Magnetosheath until about 17:00 UT, when they were only at a
6.8 RE (Earth radii) distance from the Earth. The transition between the lobes
and the Magnetosheath was characterised by an important ion density increase
(from close to 0 in the lobe to more than 160 particles/cm3 in the
Magnetosheath) as well as a very clear signature in the velocity components, as
measured by the CIS experiment onboard Cluster (P.I.: Henri RĂ‹me).

This is a very unusual position for the Magnetopause, which on the average is
standing ahead of the Earth at about 10 to 11 RE. Such compressions can have
dramatic space weather effects, particularly to geostationnary satellites which
are orbiting the Earth at a distance of about 6.6 RE. Further analysis of the
four spacecraft data will tell us at what speed the magnetopause moved which
will give information on the strength of the CME.


Iannis Dandouras, Iannis.Dandourascesr.fr
Claire Vallat, Claire.Vallatcesr.fr
Philippe Escoubet, philippe.escoubetesa.int

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[Image 1:
Solar wind density and velocity measured by ACE on 24 Oct. 2003 at L1. The CME
passage, defined by the sudden jump of solar wind density and velocity, was
observed at 14:49 UT. Later on the density reached a peak above 70 particles per
cc, which is about 15 times the usual value.

[Image 2:
The Cluster spacecraft above the Southern polar cap when they detected a CME
reaching the Earth on 24 October 2003.

[Image 3:
Data from the Cluster ion spectrometer instrument (CIS) on 24 October 2003. The
ion spectrogram (top panel), the ion density (middle panel) and the ion velocity
(lower panel) from spacecraft 1-Rumba are shown. The magnetopause was crossed
15:25 UT when we see the sudden fluxes of ions with a density reaching almost
200 particles per cc. After 17 UT the spacecraft re-enter the magnetosphere in
the cusp region.