In January 1994, three geostationary satellites suffered total failures of their momentum wheel control circuitry. Services were affected for hours, and one satellite never fully recovered. At the time it was well known that satellites could suffer damage when high energy particles were emitted from the sun during an explosive Solar Particle Event (SPE). Protons and electrons travelling at relativistic speeds (a significant fraction of the speed of light such as 0.3c) could impact with orbiting satellites causing damage in a number of different ways. In this case, however, no SPE had been observed.

It is now believed that the events above were associated with long lasting streams of high energy electrons which are now referred to as high speed solar wind streams. These occur most frequently around the time of solar minimum and are closely associated with coronal holes. A coronal hole is like a window in the magnetic field structure of the sun's corona, that allows solar particles to flow more freley outwards from the sun towards the Earth.

What appears to be important in predicting whether an electron flux is dangerous to satellies is not so much the electron intensity but the total integrated flux. The USAF uses empirically derived thresholds to issue warnings to satellite operators. Damaging conditions are believed to exist when the daily electron flux (number of high energy electrons > 2 MeV per square centimetre per steradian per day) meets either of the following conditions:

* greater than 3 x 10⁸ per day for 3 consecutive days; or
* greater than 10⁹ for a single day.

Such conditions often occur about 2 days after the onset of a large geomagnetic storm.

Damage to satellite systems is caused through a phenomenon called Deep Dielectric Charging. The high energy electrons penetrate the spacecraft outer surface and bury themselves in dielectric materials such as circuit boards and the insulation in coaxial cables. The buildup gives rise to intense electric fields which when they exceed the breakdown potential of the material produce sudden discharges, like a mini-lightning stroke. It is this discharge that damages the system, burning components and destroying semiconductors. Designers can take measures to minimise the effect of such deep dielectric charging. This generally involves increased complexity and weight for the satellite however.

Material Prepared by John Kennewell and Andrew McDonald. © Copyright IPS - Radio and Space Services.
Comments or suggestions can be directed to education@ips.gov.au