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			Geomagnetically Induced Currents
	IRF-Lund		GIC - A Space Weather Effect	Lund Space Weather Center

GIC - A CME hitting the Earth on the 25th of September, 1998

When a CME passes the Earth, the geomagnetic field will be disturbed. How disturbed it becomes depends, among other things, on the direction of the magnetic field in the frontmost part of the CME. If the magnetic field is directed southward (negative), that is opposite to the geomagnetic field, the disturbance will be large. If it is pointing northward, the disturbance will be smaller.
The aim is to be able to forecast GIC in power lines, so we would like to be able to find and model a relation between what happens in the solar wind and what GIC we get as a result. This is no easy task, but as you will see below, we can at least conclude that some form of relation exists.

This plot shows us measurements from the solar wind as measured by the spacecraft ACE. The solar wind passes ACE about one hour before it reaches the Earth.
We can see that around midnight on the 24th, the solar wind magnetic field turns strongly soutward (top panel).

At the same time, there is a sudden increase in both the density and the velocity of the solar wind (panels 2 & 3).

This behavior is typical for a rather fast CME. We can now expect a large disturbance to occur in the geomagnetic field about one hour after the southward turn of the solar wind magnetic field. The largest disturbances, caused by so called substorms, often tends to occur each time the solar wind magnetic field is strongly negative but turns a bit northward (i.e becomes less negative). Now lock below at what happend. Do not forget to take account of the time delay of about one hour when yo

Here we have a plot showing the geomagnetic field as measured at Lovö, not to far from Stockholm, Sweden. The data is provided by SGU, Sveriges Geologiska Undersökningar, in Uppsala.
We can see that there is a disturbance in the horisontal components of the geomagnetic field, both in the northward (top panel) and in the eastward component. The disturbance starts when the CME reaches the Earth and ends when the solar wind magnetic field becomes positive, just as expected.

Now we come to the effects on technological systems. The top panel in the plot on the right shows the geomagnetically induced current as measured in the earthing of a transformer neutral. The transformer is connected to the 400 kV grid in the south of Sweden.
The bottom panel shows the geomagnetically induced potential in a gas pipeline, situated some 300 kV from the transformer. We can clearly see that the patterns are much alike. It is a current, called an electrojet, in the ionosphere that causes the induced current and potential. Since the patterns look alike, we can conclude that the electrojet is wast enough to influence systems 300 km apart in much the same way.

In the above plots, we could see that the GIC seemed to follow the changes of the geomagnetic field, though the exact relation might not be obvious.
If we look at the change of the geomagnetic field, we see that the GIC very strongly depends on the change, that is we have approximately GIC = konst*dB/dt.

Observe that the geomagnetic field is measured about 200 km from the transfomer and about 500 km from the pipeline. We can again conclude that the electrojet influences an area many hundreds of klilometers across in much the same way.

In the case we look at here, it seems that GIC follows dX/dt (the change in the northward component of the geomagnetic field) a bit closer than it follows dY/dt. This might depend o the orientation of the power lines and the pipeline. The changes in the geomagnetic field, induced by the electrojet, in turn induces an electric field in the crust of the Earth. If the power lines lays along this electric field, we get strong GIC, otherwise we get weaker GIC (and GIP).

GIC - A CME hitting the Earth on the 25th of September, 1998
When a CME passes the Earth, the geomagnetic field will be disturbed. How disturbed it becomes depends, among other things, on the direction of the magnetic field in the frontmost part of the CME. If the magnetic field is directed southward (negative), that is opposite to the geomagnetic field, the disturbance will be large. If it is pointing northward, the disturbance will be smaller. The aim is to be able to forecast GIC in power lines, so we would like to be able to find and model a relation between what happens in the solar wind and what GIC we get as a result. This is no easy task, but as you will see below, we can at least conclude that some form of relation exists.

This plot shows us measurements from the solar wind as measured by the spacecraft ACE. The solar wind passes ACE about one hour before it reaches the Earth. We can see that around midnight on the 24th, the solar wind magnetic field turns strongly soutward (top panel). At the same time, there is a sudden increase in both the density and the velocity of the solar wind (panels 2 & 3). This behavior is typical for a rather fast CME. We can now expect a large disturbance to occur in the geomagnetic field about one hour after the southward turn of the solar wind magnetic field. The largest disturbances, caused by so called substorms, often tends to occur each time the solar wind magnetic field is strongly negative but turns a bit northward (i.e becomes less negative). Now lock below at what happend. Do not forget to take account of the time delay of about one hour when yo

Here we have a plot showing the geomagnetic field as measured at Lovö, not to far from Stockholm, Sweden. The data is provided by SGU, Sveriges Geologiska Undersökningar, in Uppsala. We can see that there is a disturbance in the horisontal components of the geomagnetic field, both in the northward (top panel) and in the eastward component. The disturbance starts when the CME reaches the Earth and ends when the solar wind magnetic field becomes positive, just as expected.
Now we come to the effects on technological systems. The top panel in the plot on the right shows the geomagnetically induced current as measured in the earthing of a transformer neutral. The transformer is connected to the 400 kV grid in the south of Sweden. The bottom panel shows the geomagnetically induced potential in a gas pipeline, situated some 300 kV from the transformer. We can clearly see that the patterns are much alike. It is a current, called an electrojet, in the ionosphere that causes the induced current and potential. Since the patterns look alike, we can conclude that the electrojet is wast enough to influence systems 300 km apart in much the same way.
In the above plots, we could see that the GIC seemed to follow the changes of the geomagnetic field, though the exact relation might not be obvious. If we look at the change of the geomagnetic field, we see that the GIC very strongly depends on the change, that is we have approximately GIC = konst*dB/dt. Observe that the geomagnetic field is measured about 200 km from the transfomer and about 500 km from the pipeline. We can again conclude that the electrojet influences an area many hundreds of klilometers across in much the same way. In the case we look at here, it seems that GIC follows dX/dt (the change in the northward component of the geomagnetic field) a bit closer than it follows dY/dt. This might depend o the orientation of the power lines and the pipeline. The changes in the geomagnetic field, induced by the electrojet, in turn induces an electric field in the crust of the Earth. If the power lines lays along this electric field, we get strong GIC, otherwise we get weaker GIC (and GIP).