By Richard Fallows
The view of the radio universe at the VHF frequencies of LOFAR is strongly affected by the Earth’s ionosphere. This dynamic region, which exists from about 60km altitude upwards, is where the neutral particles making up the lower atmosphere become ionised leading to radio waves from the rest of the cosmos being refracted and scattered as they pass through it.
Published by the editorial team, 11 June 2020
The effect for astronomers is that the images they are trying to take of distant radio sources can be heavily distorted, appearing to shift and shimmer and vary in their strength. It is exactly the same effect as us looking up at the stars in the night sky and seeing them twinkle in visible light due to the effects of the lower atmosphere, or trying to view a pebble at the bottom of a pool of disturbed water. However, in science, one person’s noise is another’s data: The ionospheric effects that most astronomers are trying to remove can also be used to gain information on the structure and dynamics of the ionosphere itself, and one team of researchers is doing exactly that.
The ionosphere is known to be highly active at polar latitudes, where spectacular displays of aurora can be seen, and at equatorial latitudes, but is much quieter at the mid-latitudes, which is one of the main reasons that LOFAR was built where it is. The effect of the ionosphere can be seen clearly in a movie showing how the intensity of a strong radio source, received by all LOFAR stations simultaneously, changes over the dense core of stations at LOFAR’s centre. Peaks and dips in the received intensity are shown in red and blue respectively, with the locations of the stations themselves marked as solid circles. The intensity pattern varies significantly with bands of intensity moving generally north-west to south-east over the core stations, but some patterns appearing to move in a different direction.
One of the main advantages of using LOFAR for studies such as these is its wide bandwidth. How much a radio wave is scattered depends, amongst other things, on its wavelength, with longer wavelengths (and so lower frequencies) generally scattered more than shorter wavelengths. This means that the intensity received at one wavelength will appear shifted in both time and frequency compared to another. When analysed over a wide band, these shifts can form an arc structure which gives information on the altitude of the scattering region and how fast it’s moving.
This observation showed two arc structures, indicating that the scattering causing such variations in the received intensity was caused by two different layers in the ionosphere, one with material at an altitude of several hundred kilometres, flowing north-west to south-east at a relatively slow speed, and the other with material very low down in the ionosphere, moving north-east to south-west at a much higher speed.
We think that this is the result of two, simultaneous disturbances in the ionosphere: One most likely comes from activity at high latitudes propagating southwards to affect the ionosphere above LOFAR, and the other is likely to be the result of activity lower down in the atmosphere bubbling up. It is, we think, the first time that these two effects have been directly observed simultaneously.
On 12 June 2020, LOFAR celebrates its tenth anniversary. The radio telescope is the world’s largest low frequency instrument and is one of the pathfinders of the Square Kilometre Array (SKA), which is currently being developed. Throughout its ten years of operation, LOFAR has made some amazing discoveries. It has been a key part of groundbreaking research, both in astronomy and engineering. Here we feature some – but definitely not all – of these past highlights, with surely more to come in the future.