New Scientist: Since the early days of radio, we have known that signals that cannot be picked up by day may be heard clearly at night from hundreds of kilometres away. by David Hambling
'This is down to changes in the ionosphere, a layer of charged particles in the atmosphere that starts around 60 kilometres up (for more on this mysterious layer see “No-fly zone: Exploring the uncharted layers of our atmosphere“). The curvature of Earth stops most ground-based radio signals travelling more than 70 kilometres without a boost. But by bouncing between the ionosphere and the ground they can zigzag for much greater distances. At night the ionosphere is denser and more reflective.
'It’s not the first time we’ve tried to improve radio communication by tinkering with the ionosphere. HAARP, the High Frequency Active Auroral Research Program in Alaska, stimulates the ionosphere with radiation from ground-based antennas to produce radio-reflecting plasma.
'Now the USAF wants to do this more efficiently, with tiny satellites – such as CubeSats – carrying large volumes of ionised gas directly into the ionosphere.'