In astronomy, interplanetary scintillation refers to random fluctuations in the intensity of radio waves of celestial origin, on the timescale of a few seconds.
It is analogous to the twinkling one sees looking at stars in the sky at night, but in the radio part of the electromagnetic spectrum rather than the visible one.
Interplanetary scintillation is the result of radio waves traveling through fluctuations in the density of the electron and protons that make up the solar wind.
Scintillation, meaning rapid modification, in radio waves due to the small scale structures in the ionosphere, known as ionospheric scintillation,[1] was observed as early as 1951 by Antony Hewish, and he then reported irregularities in radiation received during an observation of a bright radio source in Taurus in 1954.
The array consisted of 2,048 dipoles over almost five acres of land, and was built to constantly survey the sky at a time resolution of about 0.1 seconds.
[7] Soon after observations were under way, Hewish's student Jocelyn Bell turned this assumption on its head, when she noticed a signal which was soon recognized as emanating from a new class of object, the pulsar.
The root mean square (RMS) intensity fluctuations are often expressed relative to the mean intensity from the source, in a term called the scintillation index, which is written as This can be related to the phase deviation caused by turbulence in the solar wind by considering the incident electromagnetic plane wave, and yields The next step, relating the phase change to the density structure of the solar wind, can be made more simple by assuming that the density of the plasma is highest towards the sun, which allows the "thin screen approximation."
[16] Thus interplanetary scintillation measurements can be used to determine the size of compact radio sources, such as active galactic nuclei.