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Interplanetary scintillation
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.
==Early study==
Scintillation in radio waves due to the ionosphere 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.〔Hewish (1955), p. 238.〕 Hewish considered various possibilities, and suggested that irregularities in the solar corona would cause scattering by refraction and could produce the irregularities he observed.〔Hewish (1955), pp. 242–244.〕 A decade later, while making astrometric observations of several bright sources of celestial radio waves using a radio interferometer, Hewish and two collaborators reported "unusual fluctuations of intensity" in a few of the sources.〔Hewish (1964), p. 1214.〕 The data strongly supported the notion that the fluctuations resulted from irregularities in the density of the plasma associated with the solar wind, which the authors called interplanetary scintillation,〔Hewish (1964), p. 1215.〕 and is recognized as the "discovery of the interplanetary scintillation phenomenon."〔Alurkar (1997), p. 38.〕
In order to study interplanetary scintillation, Hewish built the Interplanetary Scintillation Array at the Mullard Radio Astronomy Observatory. 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. This high time resolution set it apart from many other radio telescopes of the time, as astronomers did not expect emission from an object to feature such rapid variation.〔Manchester (1977), pp. 1–2.〕 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. Thus "it was an investigation of interplanetary scintillation that led to the discovery of pulsars, even though the discovery was a by-product rather than the purpose of the investigation."〔Lyne (1990). p. 4.〕
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