November 1, 2003
A team from the Institut de Physique du Globe de Paris and the California Institute of Technology has successfully used the Global Positioning System (GPS) satellite constellation to map disturbances in the ionosphere following last November's magnitude 7.9 earthquake in Denali, Alaska.
Their paper has been published in the scientific journal Geophysical Research Letters. The research itself was carried out in support of ESA's Space Weather Applications Pilot Project, aimed at developing operational monitoring systems for space conditions that can influence life here on Earth.
The ionosphere is an atmospheric region filled with charged particles that blankets the Earth between altitudes of about 75 to 1000 km. It has a notable ability to interfere with radio waves propagating through it.
In the particular case of GPS
navigational signals, received on Earth from orbiting satellites,
fluctuations in the ionosphere known as 'ionospheric scintillations'
- have the potential to cause signal delays, navigation errors
or in extreme cases several hours of service lockouts at particular
The French and US team made use of dense networks of hundreds of fixed GPS receivers in place across California. These networks were originally established to measure small ground movements due to geological activity, but they can also be utilised to plot the ionosphere structure across three dimensions and in fine detail.
Then when the Denali earthquake occurred on 3 November 2002, the team had a chance to use this technique to investigate another distinctive property of the ionosphere, its ability to work like a natural amplifier of seismic waves moving across the Earth's surface.
There are several different
types of seismic waves moving the ground during an earthquake,
the largest scale and the one that does most of the movement
is known as a Rayleigh Wave. This type of wave rolls along the
ground up and down and side-to-side, in the same way as a wave
rolls along the ocean.
What the team were able to do following the Denali quake was detect a distinctive wavefront moving through the ionosphere. "Using the network allowed us to observe the propagation of the waves," explained co-author Vesna Ducic. "We could also separate the small total electron content signal from the very large total electron content variations related to the daily variation of the ionosphere."
The team observed a signal two to three times larger than the noise level, arriving about 660 to 670 seconds after the arrival of Rayleigh Waves on the ground. And because around six GPS satellites are visible to every ground receiver they were able to calculate the altitude of maximum perturbation around 290 to 300 km up.
The signals were weak and only sampled every 30 seconds, with a maximum resolution of 50 km and the overall noise rate high. But the ionospheric signal observed had a clear pattern consistent with models of seismic behaviour. The hope is that the technique can be improved in future, and used to detect earthquakes in areas without seismic detectors, such as the deep ocean or near islands.
"In the framework of Galileo we plan to develop this research," said Ducic. "Galileo will double the number of satellites and therefore will allow much more precise maps of the ionosphere. We can also foresee that Europe will develop a dense network of Galileo/GPS stations that will take part in the monitoring of these phenomena.
"ESA, together with the
French Ministry of Research and CNES have already decided to
fund a pre-operational project called SPECTRE - Service and Products
for Ionosphere Electronic Content and Tropospheric Refractive
index over Europe from GPS - devoted to the high-resolution mapping
of the ionosphere. We will be carrying out mapping above Europe
as well as California.
The Space Weather Applications Pilot Project is an ESA initiative which has already begun to develop a wide range of application-oriented services based around space weather monitoring.
The co-funded services under
development - of which this project is one - also include forecasting
disruption to power and communication systems, and the provision
of early warning to spacecraft operators of the hazards presented
by increased solar and space weather activities. The hope is
that an a seismic detection service based on ionospheric measurements
may in future supplement existing resources in Europe and elsewhere.
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