Cluster Reveals How The Sun Shakes Magnetic Field Of Earth*

by Staff Writers
Paris, France (ESA) Jul 29, 2007

Space is a hostile region for both astronauts and satellites. One
constituent of this hazardous environment around the Earth are very
energetic electrons, which are able to perturb or permanently damage
satellites. Ultra Low Frequency (ULF) waves, which travel along the
Earth's magnetic field lines, are a prime candidate for generating these
killer electrons, but the source of these waves remains unclear.

A recent study reveals how a rare conjunction of ground based
instrumentation and a dozen satellites at a range of altitudes provide a
means to trace the energy source of these waves from the solar wind into
the Earth's magnetosphere down to the ground. Part of this satellite
constellation, the four spacecraft of the ESA Cluster mission, was
located at the border of the magnetosphere and played a major role in
discriminating between the various theoretical ULF wave generation
scenarios.

In 1859, the greatest geomagnetic storm ever recorded adversely affected
a significant portion of the world's 200 000 km of telegraph lines for
several hours, at that time the only means of effective long distance
communication. Meanwhile near London (UK), some of the first
ground-based magnetometers were monitoring the behaviour of the Earth's
magnetic field.

Surprisingly, quasi-sinusoidal oscillations of the magnetic field lines
with periods of a few minutes were recorded continuously for several
hours, as if a celestial musician had plucked the magnetic field lines
or strings of the Earth's magnetic guitar (see also "Earth's Magnetic
Field" linked from the right-hand side navigation).

These oscillations are now believed to play a significant role in the
transport of mass, energy, and momentum within the Earth's
magnetosphere. Known as ultra low frequency (ULF) waves, they can
effectively accelerate electrons to very high velocities. At
geosynchronous altitude ( about 35 600 km), these highly energetic, so
called 'killer', electrons, can inflict critical damage to
telecommunications satellites and represent a hazard to astronauts.
Understanding how these waves are generated is a strong focus of recent
space research.

Several ways of exciting these waves have been proposed. Most of them
involve the solar wind as the external driver. The solar wind is a
continuous stream of solar particles impacting and shaping the Earth's
magnetic environment. However, understanding the global nature of these
geomagnetic pulsations and the tracing of the energy transfer from the
solar wind to the ground is a difficult task. It requires a fortuitous
alignment of several satellites, together with ground-based instruments,
to observe the oscillations simultaneously.

A space armada and ground based instruments to track ULF waves

On 25 November 2001, such a serendipitous alignment occurred during the
recovery phase of a large geomagnetic storm. A low frequency type of ULF
wave, called Pc5, was continuously recorded for many hours by the
CARISMA magnetometer chain in Northern Canada.

During a 3-hour subset of this interval, these Pc5 waves were picked up
by more than a dozen scientific satellites, all collocated in the dusk
sector at different altitudes. They included: the four ESA Cluster
satellites outbound from the magnetosphere across its boundary, the
magnetopause, at altitudes higher than 110 000 km, the NASA Polar
spacecraft traversing the magnetosphere at about 58 000 km and four NOAA
geostationary satellites at about 35 600 km; GOES-8, -9, -10 and -12.

The Polar satellite links Cluster data to Ground-Based measurements
A detailed analysis of the data, published in December 2005 in the
Journal of Geophysical Research, revealed the following scenario. On 25
November 2001, the solar wind velocity was about 750 kms-1, nearly twice
its average speed.

The impact of this fast flow of solar particles on the Earth's
magnetosphere induced undulations of the magnetopause such as those
depicted in Animation 1. As a consequence, the Cluster satellites
outbound from the magnetosphere crossed the magnetopause not once but
periodically, roughly every 11 minutes or at 1.5 mHz, a typical period
for Pc5 waves.

The authors of this study surmise that these undulations induced
compressional waves propagating inward from the magnetopause, which
coupled to terrestrial magnetic field lines close to the location of
Polar and several geosynchronous satellites. All these spacecraft, over
a period of hours, picked up clear signatures of this magnetic Field
Line Resonance (FLR) with the same period as the magnetopause flapping.

Meanwhile, ionospheric signatures of this FLR were measured by SuperDARN
HF radars over a large portion of the northern hemisphere. Magnetically
conjugated magnetometers of the CARISMA network were recording the
ground footprint of this 1.5-1.6 mHz FLR. Taking into account all these
data, the authors of this study found that these Pc5 waves result from
the excitation of a magnetospheric waveguide either by the
Kelvin-Helmoltz instability or via overreflection acting on the duskside
magnetopause.

"This excellent conjunction of ground- and space-based instrumentation
provides a means to trace energy extracted from the solar wind into the
Earth's magnetosphere before ultimately being deposited into the upper
atmosphere. During this event, the Polar data provided a direct link
between Cluster measurements of magnetopause undulations and FLR
signatures detected by SuperDARN radars and the CARISMA magnetometer
network.

This is the first study to have such an advantageous instrumentation
collecting data over the same time interval," comments lead author Dr.
Jonathan Rae, scientist at the University of Alberta, Edmonton, Canada
and lead author of this study.

"ULF waves affect the entire magnetosphere. This study underlines once
again the scientific need of a constant monitoring by ground-based
geophysical instruments to put data measured in space in a global
context. In situ, Cluster is also revolutionizing our understanding of
ULF waves by using the unique capability of its four spacecraft flying
in formation, the subject of a PhD very recently defended at Royal
Institute of Technology (KTH), Stockholm, Sweden, by Tommy Eriksson,"
says Philippe Escoubet, ESA Cluster and Double Star project scientist.

Additional Recent Results on ULF Waves

Finally a few words on a very recent result on ULF waves, found thanks
to Cluster data and published 29 June 2007 in Geophysical Research
Letters. Dr. Qiugang Zong and co-authors report the first direct
observations of ULF-energetic particle resonance in the outer Van Allen
radiation belt.

In particular, the modes of oscillations of the Pc5 waves observed are
successfully related to theoretical models and the energy at which the
electrons are accelerated by theses waves is quantified. "The results
reported in this paper cast new lights to understand very energetic
particle acceleration in the Van Allen radiation Belts - the number one
space threat," says Dr. Zong, scientist at the University of
Massachusetts-Lowell (USA) and at Peking University (China).

Rae, I. J., E. F. Donovan, I. R. Mann, F. R. Fenrich, C. E. J. Watt, D.
K. Milling, M. Lester, B. Lavraud, J. A. Wild, H. J. Singer, H. Reme,
and A. Balogh (2005), Evolution and characteristics of global Pc5 ULF
waves during a high solar wind speed interval, J. Geophys. Res., 110,
A12211, doi:10.1029/2005JA011007 Zong, Q.-G., X.-Z. Zhou, X. Li, P.
Song, S. Y. Fu, D. N. Baker, Z. Y. Pu, T. A. Fritz, P. Daly, A. Balogh,
and H. Reme (2007), Ultralow frequency modulation of energetic particles
in the dayside magnetosphere, Geophys. Res. Lett., 34, L12105,
doi:10.1029/2007GL029915