In
the mid-1960s the technique of interplanetary scintillation
was discovered. Interplanetary scintillation is the
apparent 'twinkling' of radio emissions from a compact
radio source.
The
visible light from a distant star will appear to twinkle
when viewed from earth, because the light has to pass
through the earth's atmosphere to reach us. The atmosphere
consists of many moving layers of air at different temperatures
and densities. Light refracts differently through each
layer of atmosphere, so the star appears to be wiggling
slightly when seen from the ground. This produces the
twinkling, or scintillation. Refraction of light through
the atmosphere means scientists build optical telescopes
on top of high mountains, so that light has passed through
as little of the atmosphere as possible before reaching
the telescope.
Fortunately
the atmosphere is transparent to radio waves, so radio
telescopes can be built in the flat fields around Cambridge!
The 'twinkling' of radio sources is not due to the Earth's
atmosphere, but due to moving clouds of charged particles
given off by the Sun. As radio waves pass through these
moving 'solar winds' they are diffracted, making the
radio source appear to twinkle when seen from Earth.
