If our eyes were radio rather than optical wide-band detectors it is
well known that for us the brightest object in the sky would still be
the Sun; that planets, stars and the Milky Way would still shine feebly
(and that we would still occasionally be blinded by man-made sources).
What is less well known is that quite a different earthbound overcast
would hover about us, with its climatic zones, its seasonal changes, its
unpredictable storms and scintillating transparence. To be sure, we can
get a sort of glimpse of this peculiar type of weather when we tune our
receiver to radio broad- casting from some remote spot, or photograph
the Earth from space at certain specific wavelengths. Nevertheless no
one has ever looked at the ionized shroud of the Earth without the help
of sophisticated apparatus, and this is one of the reasons why in this
domain the phenomena are not easily abstracted from the use of specific
techniques. For generations, the study of the ionosphere has been deeply
interwoven with the practice of radio communication and detection. Today
however, ionospheric physics is best thought of as a branch of space
physics; that part of physics which deals with processes at work in the
solar system and methods developed for its exploration.