Monday 31 March 2014

Variations in scintillation

Ionospheric scintillation is the variation in the amplitude and phase of radio signals as they pass through the ionosphere. What is happening is small-scale variations in the electron density result in differences in the ionosphere's refractive index at radio-frequencies. It is sort of the radio equivalent of the "twinkle of stars". The localised variations in refractive index cause momentary focusing, defocusing or shifting of the incoming radio waves. For broad, diffuse structures in the sky (like the synchrotron emission from the Galaxy) all of these effects average out for different directions. But for a point-source (such as bright "radio stars") the singular line -of-sight can result in dramatic differences.

One of KAIRA's regular experiments has a number of "beams" on the sky. These are sort of like pixels in an image and they monitor the variation in power levels coming from that direction. Most of these have been set up for specific geodetic directions. But we also have some that track astronomical sources.

The following plot shows the radio power coming from the bright radio object Cassiopeia A (or "Cas A" for short).

As can be seen, there are some huge variations. There is a natural variation as Cas A rises higher and lower in the sky (it never sets completely as it is circumpolar at these latitudes). However, this is only a variation between approx. 96.5 and 97.5 on the arbitrary scale used above. The point where the strong scintillation occurs shows huge variations.

Compare the difference between, say, samples around 25000 and around 35000. Not only can the effect be very great, but it can change very quickly too.

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