Why geomagnetic storms affect telegraphs, telephones, power grids and piplines
The Space Environment Center website has information on solar conditions and the effects of the solar wind, under the headings of Geomagnetic Storms, Solar Radiation and Radio Blackouts. A surprising number of things are affected by solar weather. Solar Radation is just that, ionizing particles that can be a hazard to high flying. Radio Blackouts are apparently the result of an intensification of the lower ionospheric layers that strongly absorb radio waves on the sunlit side. Geomagnetic Storms are fluctuations in the geomagnetic field, which induce electrical currents affecting various activities.
Information from the site told me that geomagnetic fluctuations are reported by a number called Kp, which ranges from 0 to 9 depending on the strength of the fluctuations in the last three-hour period. Kp has to reach about 5 before minor disturbances are possible. The larger the value of Kp, the further south the aurora can be seen. For Kp = 3 or less, the aurora is restricted to Canada, but for Kp = 9, it can be observed further south than Colorado. Numbers G1-G5 are assigned to various intensities of geomagnetic disturbance from minor to extreme, with similar indexes S1-S5 and R1-R5 for the solar and radio disturbances. All these numbers and indices make it seem quite scientific and give people something to talk about, however small the information content. I was unable to find the current values of these indices on the website, but they must be there somewhere.
A steady magnetic field is almost without effect. It turns the compass needle, but that is about it. Most of the geomagnetic field, of about a gauss, is steady and changes very slowly with time. This part is due to currents deep within the earth, and can be described as due to a dipole not quite at the center of the earth and not quite aligned with the axis of rotation. The magnetic "pole" in Canada is a south pole--the flux enters here--and the north poles of compass needles point roughly towards it. The intensity of this field decreases as the cube of the distance from the center of the earth, and interacts with the interplanetary magnetic fields to guide the charged particles of the solar wind as they approach the earth, deflecting them so that they do not reach the surface.
There is a strong electric current in the upper atmosphere circling the magnetic pole called the auroral electrojet, with a strength of millions of amperes. The strength and position of this jet are affected by incoming solar radiation. The variation in these factors causes a change in the magnetic field produced by the electrojet, and this is at least one source of geomagnetic field variations. A change in magnetic field produces an electric field, which permeates all nonconducting materials and penetrates deeply into poorly conducting materials, such as the earth's crust. This means that different points are at different electric potentials. If these points are connected by a conducting path, an electric current will flow.
A telegraph line was a conductor grounded at its ends, a perfect device for detecting earth currents. There may be actual earth currents, if the earth has a finite conductivity, but the currents in the telegraph wire are really not earth currents, but a current induced in the line itself. Nevertheless, the phenomenon was called "earth currents" and proved bothersome. However, it was often possible to operate a telegraph line on these earth currents, without a battery, quite successfully. Sometimes the voltages were high enough to cause severe damage to the apparatus or danger to the operator. Such high voltages were bypassed by "protectors" than offered a path to ground. These protectors were sometime called "lightning arrestors" when the high voltages were thought due to lightning. Of course, a nearby lightning stroke, or one to a wire, does indeed cause a voltage spike, but the results are often disastrous in this case, and rather rarer than earth currents. The earth currents are greatest at the times of rapid geomagnetic fluctuations, of course. Telephone circuits are also susceptible to these effects, and must be provided with protectors. With a two-wire circuit, the earth currents are common mode, it should be noticed. The currents are called GIC by the acronym-lovers, geomagnetically induced currents. It really should be geomagnetically induced electric fields.
There is a strong vertical electrical field near the surface of the earth, of some 100V/m, but the very low conductivity of the atmosphere means that the currents are very small. Near thunderstorms, this field can become quite large, eventually causing an electrical discharge as the air breaks down electrically. Electric current is a response to an electric field by a conducting medium.
One can also picture the changing magnetic field directly inducing a current in a wire. This actually requires a magnetic field linking a loop, a fully metallic loop or one with an earth return. In the previous picture, the wire and the earth were merely parts of one conductor in the electric field produced by the magnetic disturbance. It might be interesting to investigate some actual cases to find out the relative importance of the two points of view. There is little doubt that both occur.
These days, geomagnetic storms appear to be a major hazard to power distribution systems. On 13 March 1989, at the peak of the 11-year solar activity (sunspot) cycle 23 that began in 1986, the entire Hydro Québec power grid went down during a severe geomagnetic storm. The hazards of high winds, ice loading and lightning to transmission lines was well-known, but the geomagnetic hazard seemed to have been overlooked. It appears that high currents between grounding points caused protective devices to operate, and transformers to fail. The large direct currents through transformers would saturate the cores, and the reduced impedance would allow excessive currents that would burn out the transformers (I deduce that this is the explanation).
A power system that goes totally down is subject to extreme stress on restarting. All thermostatically controlled load is switched on at once, and the load on the system at restoration can be 600% of the normal load.
The author of the paper on the effects of geomagnetic storms on power systems in the website said that the effects are particulary strong in regions underlain with igneous rocks, which have a low electrical conductivity. He says that the "high resistivity causes more of the current to flow through the wires," an explanation that I doubt, even if the effect is real. Canada is largely on a shield of old igneous rock. I think the current in each conductor would depend on its resistance, the potential between grounding points being the same. This may be the old concept of "earth currents" again.
Pipelines are low-conductivity paths buried in the ground, and indeed earth currents of hundreds of amperes have been observed in them. Should these currents cause a problem, it would seem easy to prevent it by insulated joints. The pipes are continuously grounded, unless the protective coating of the pipe is insulating. Pipelines are affected by such a number of interesting phenomena from widely different fields (see, for example Hydrates) that this might make a good topic for a paper.
Composed by J. B. Calvert
Created 25 November 2000