|Ilse van Bemmel, Cyril Tasse
| For radio astronomers the top layer of the Earth's atmosphere, the ionosphere, can be a real nuisance. The layer contains free electrons, which bend the light wave. Because the electron density varies in time and direction, the amount of bending also varies with time and direction. This causes radio sources to "dance", much like stars do in visible light. The bending becomes stronger for longer wavelengths, which makes LOFAR very sensitive to ionospheric effects.
The effect is nicely demonstrated by cutting a 6 hour LOFAR observation into time-slices of 2 minutes. The observations are of the 3C196 field, at 150 MHz. The 2-minute images are stitched together to form a movie. The central source, 3C196, is not moving, because the calibration removed any ionospheric effects in this particular direction. However, for any other direction in the field the ionosphere will be different, which causes all other sources to dance around their actual positions. Here is the link to the movie: http://www.astron.nl/dailyimage/pictures/20110204/3C196_3.avi
A zoom on the sources is seen on the left, ordered by brightness. The top left source is 3C196.