By Jonathan Amos

Science reporter, BBC News, San Francisco

C/NOFS was launched this year The upper reaches of Earth's atmosphere are much lower than expected, a US Air Force satellite has found. Currently, the ionosphere - a layer of charged particles that envelops the planet - is at an altitude of about 420km, some 200km lower than expected. The behaviour of the ionosphere is important because disturbances in its structure can upset satellite communications and radar. The C/NOFS satellite's data will be used to try to forecast such problems. "C/NOFS is focussing on one small aspect of 'space weather' which is the impact of the upper atmosphere - a region called the ionosphere - on radiowaves that try to get from satellites down to the ground," said Don Hunton, the technical manager on the satellite programme at the US Air Force Research Laboratory. "Sometimes the ionosphere interrupts satellite communications and sometimes it doesn't, and we're trying to understand how to forecast, or predict ahead of time, when those problems are going to arise," he told BBC News. Study platform The ionosphere is a weak plasma - a layer where ultraviolet light from the Sun has so excited the upper atmosphere that its constituent atoms and molecules have been broken apart into free electrons and positive ions. And just as light waves will be bent as they pass through water, so radiowaves will refract when they move through the charged particles that make up the ionosphere. Please turn on JavaScript. Media requires JavaScript to play. The C/NOFS (Communication/Navigation Outage Forecasting System) was launched back in April and put into an elliptical equatorial orbit at altitudes between 400 and 860km to investigate this enigmatic layer. Its sensors record the density and temperature of the plasma, as well as the strength and direction of electric and magnetic fields within it. One of C/NOFS' first discoveries has been simply to identify where precisely in the sky the ionosphere is right now; and it is a lot lower than expected. During the night it has been detected at about 420km, rising to 800km during the day. Scientists here at the American Geophysical Union meeting said more typical values would be 640km during night-time and about 960km during the day. Future forecasts To some extent, this should not be too surprising. The ionosphere reacts to the Sun's 11-year cycle of activity and our star is currently in a very quiet phase. "We are in the depths of a very low solar minimum right now and as a result the ionosphere is lower and less dense than, we believe, at any other time in the history of the space age when measurements have been made," said Dr Hunton. But despite this quiescence, C/NOFS has still managed to witness a fair amount of activity in the ionosphere. It can see structures, or bubbles, in the layer. It is these features that will distort, weaken, or even block radiowaves trying to pass through to the ground. C/NOFS' mission will be to try to understand how these structures start and evolve so that some warning can be given of their impact on satellite transmissions. Jonathan.Amos-INTERNET@bbc.co.uk



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