Using Iridium data to map the Birkeland currents
Iridium Communications Inc. is a satellite communications company that operates a global network of over 80 low earth orbit communications satellites. The Iridium Home Page provides information on their products and services (http://iridium.com).
The satellites are arranged into 6 polar orbit planes with about eleven satellites in each plane plus some spares. The Iridium satellites are in low-Earth orbit (LEO), at an altitude around 800 km.
Each satellite must be oriented in space so that the solar panels provide power and the communications antennae point toward the specified cell on Earth. The orientation of a satellite in space involves the Attitude Control System (ACS). A magnetometer is also part of the ACS.
Magnetometers also respond to electric currents external to the Earth, those that are generated in near-Earth space. A well-known current system in near-Earth space is the Birkeland current system, named after the famous Norwegian scientist who studied them using ground based instrumentation around 1908.
Each Iridium satellite is around 13 m in length, 4 m wide and weighs 700 kg on Earth. They complete one revolution around the Earth in about 100 minutes. This means that an Iridium satellite is in view for a person observing from the Earth surface for about 9 minutes. A person making a voice/data connection through Iridium gets routed from satellite to satellite as the antennae come into and disappear from view. Communication across the constellation is required to route the data to ground stations (gateways).