Geomagnetic disturbances, (eg. storms) are believed to
occur when the Earth encounters a disturbance in the
solar wind. This may be in the form of a plasma cloud,
stream or region of greater than normal density and/or
velocity. Sources of these disturbances in the solar
wind may be explosive events on the sun that produce
coronal mass ejections, or the less energetic high speed
streams that emanate from coronal holes.
Predicting geomagnetic disturbances is even now far from
an exact science. Forecast activity does not always
eventuate, and occasionally unforeseen activity occurs.
Prior to the use of satellite sensors, geomagnetic
prediction methods tended to rely on a 27-28 day
repetition interval for recurrent geomagnetic activity,
particularly around solar minimum, related
to the presence of coronal holes. Ground based sensors,
such as those which monitor
radio spectral emissions, together with climatology, were used to
forecast the likely effects of the energetic solar events.
Satellites have now added two more direct methods by
which we may forecast geomagnetic activity. These
satellites are usually placed at the Lagrange point of
gravitational neutrality between the Sun and the Earth
(located about 1.5 million km due sunward from the Earth).
Examples of such spacecraft are SOHO, ACE and WIND.
SOHO carries on board a white light coronagraph (LASCO)
that can see coronal mass ejections (CME's) as they leave
the sun out to many solar radii. If the CME is heading
toward the Earth it will appear as a halo around the Sun.
(Unfortunately CME's travelling away from the Earth in
the direct opposite direction will also present a halo
aspect). Thus halo CME's are of particular interest in
geomagnetic forecasting, and should be able to give many
ten's of hours warning of an approaching storm.
Satellites like ACE and WIND carry in-situ plasma
detectors, that directly detect plasma disturbances as
they travel past the satellite toward the Earth. The
data here is less ambiguous than is the coronagraph data,
but the warning time is only about one hour - this is the
time taken by the solar wind disturbance to move from the
satellite to the Earth, 1.5 million km downwind.
Although it was expected that SOHO would dramatically
improve the forecasting of geomagnetic storms, this has
not proved to be the case. Not only are there many
events seen apparently travelling toward the Earth which
'come to naught', but there are also significant
geomagnetic disturbances which occur without prior
warning by way of an observed halo CME.
We are thus still seeking sensors that will complement
all of the ground and space based sensors that we use at
present, to better improve our forecast success.
One suggestion is the use of a "solar sail", a sail to
provide a force due to the solar wind, to orbit a sensor
along the Sun-Earth line (as for SOHO),
but at a distance much closer to the Sun. In-situ plasma
sensors would then be able to provide a warning time much
in excess of the current one hour. Such an orbit is
not normally feasible (it is termed a non-Keplerian
orbit), because it is not gravitationally stable.
However, by applying a force to the satellite using a
very large and light "sail" and the solar wind,
such an orbit is possible, and in fact is under active consideration.
Another technique is the use of a large VHF radio-
telescope to measure the scintillation of far distant
(generally quasar type) radio sources. In the absence of
any disturbances in the interplanetary medium, the
signals received from these point radio sources is
constant in amplitude. However, if the radio signal
passes through a plasma cloud (eg CME) on its way to the
Earth it is absorbed and refracted in passing through the
plasma. The amplitude and phase of the radio signal as
received on the Earth varies from one moment to the next
- the signal shows scintillation. The signal from the radio-star
(quasar) "twinkles" just as the light from visible stars twinkles as it passes
through the atmosphere.
If we measure the signals from several hundred radio
stars spread across the sky we can plot a map of the sky.
Those areas of our map that show high scintillation
activity are those parts of the sky in which there is a
coronal mass ejection or other disturbance travelling
along. Continued observation will tell us how the CME is
moving, whether it will impact the earth or not and
how fast it is travelling.
Interplanetary scintillation sensors may thus
enable better geomagnetic forecasting.
The frequencies used by radio-telescopes generally
range from about 80 to 300 MHz. Lower frequencies are
subject to more scintillation, and are thus able to
better detect smaller plasma densities. Higher
frequencies can see closer in to the sun, and are thus
theoretically able to provide greater warning time. At
these frequencies, the radio telescopes need to be very
large to provide a beam narrow enough to resolve closely
separated radio stars. For instance, at 100 MHz (a
wavelength of 3 metres), a linear dimension of 200
metres is required to form a beam of 1 degree in that
dimension. The antenna generally employed at these
frequencies is an array of many hundreds of dipoles that
are electronically combined and steered to scan the sky.
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