Geomagnetic Effects on Communication Cables

Introduction

The telegraph was the first man-made system to involve the use of long electrical conductors and consequently was the first such system to be affected by magnetic disturbances. In the history of these effects there are many accounts of the telegraph system being unusable when aurora were observed overhead. There are even accounts of telegraph engineers using the "celestial power" associated with the aurora to send messages.

In the 20th century the technology for cable communications changed but all cables have been affected by the induced voltages produced by geomagnetic disturbances.

Early Effects

In the early days of the telegraph, a variety of methods were used for recording the signal transmitted over the wires. Bain's "chemical telegraph" used specially prepared paper: current from a stylus caused a chemical reaction leaving a coloured mark on the paper. During the magnetic storm of February 19, 1852, the current increased so much that a "flame of fire" followed the pen and set fire to the paper.

The early phone system had ground connections only through lightning protection devices at the ends of the lines. The breakdown voltage of these devices was higher than would be produced during most geomagnetic disturbances. However, during the magnetic disturbance on March 24, 1940, phone communications in the US were disrupted and voltages in excess of 500 V were thought to have occurred. In Sweden, several magnetic storms produced voltages large enough to start arcing in the carbon protectors. Once started, the arc continued even while the voltage began to decrease and the carbon protectors were heated to such an extent that they caught fire.

How Magnetic Storms Affect Cable Systems

The diagram shows the power feed for a submarine cable.  Power supplies at either end of the cable drive an electric current along the cable that powers the repeaters.

In the twentieth century open wire systems were replaced by coaxial cables. The introduction of coaxial cables increased the bandwidth of communication systems but required repeater amplifiers to compensate for the cable losses. These repeaters are connected in series with the centre conductor of the cable and are powered by a direct current supplied from the terminal stations at the ends of the cable. The varying magnetic field that occurs during a geomagnetic disturbance induces a voltage directly into the centre conductor of a coaxial cable. This voltage will add to or subtract from the voltage coming from the cable power supply.

Effects on Continental Cable Systems

On 4 August, 1972, an outage of the L4 coaxial cable system in the mid-western US occurred during a major geomagnetic disturbance. An examination of this disturbance showed that, at the time of the outage, the Earth's magnetic field had been severely compressed by the impact of high-speed particles from the Sun.

The resulting magnetic disturbance had a peak rate of change of 2200 nT/min, observed at the Geological Survey of Canada's Meanook Magnetic Observatory, near Edmonton, and a rate of change of the magnetic field at the cable location estimated at 700 nT/min. The induced electric field at the cable was calculated to have been 7.4 V/km, exceeding the 6.5 V/km threshold at which the line would experience a high current shutdown.

Effects on Submarine Cable Systems

During the magnetic storm of February 10, 1958, transatlantic communication from Clarenville, Newfoundland, to Oban, Scotland proceeded as alternately loud squawks and faint whispers as the naturally induced voltage acted with or against the cable supply voltage.

Effects on Fibre Optic Cables

New submarine cables are using optical fibres to carry the signals, but there is still a conductor through the cable to carry the power to the repeaters. At the time of the March 1989 storm, a new transatlantic telecommunications fibre-optic cable was in use. It did not experience a disruption, but large induced voltages were observed on the power supply cables. Future cables, because of improvements in the fibre optics, may use fewer repeaters and require a lower driving voltage. However, downsizing the power feed equipment without taking account of the induced voltages may leave future systems more vulnerable to geomagnetic effects.