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In the days before space-based
radio astronomy, observations on many frequencies required waiting until nightfall
because the Earth's ionospheric activity interfered with signals in many bands of
interest. Two such bands are 18 MHz and 20 MHz (16 and 15 meters, respectively),
on which information on lightning-type discharges from Jupiter are received. Near-real-time
maps of ionospheric absorption in the D layer (caused by solar x-ray activity) are
available on the Solar Terrestrial
Dispatch website for 5 through 30 MHz, which is where long-range high frequency
(HF) communications occur. The F2 layer is where signals are usually reflected,
but absorption in the lower D layer can be severe enough to limit reception. This
"Radio Signals from Jupiter Studied by N.B.S." story from the August 1957 issue
of Radio & TV News magazine presents the state of the science at the time.
Signals from Jupiter Studied by N.B.S.
The reception cones of Jupiter's radio emissions as limited by
Jupiter's ionosphere.
These antennas direct radio signals originating 500 million miles
away from earth to recording equipment located in the trailer.
Distant planet emits pulse-type radiation that appears to indicate the presence
of a surrounding ionosphere.
For about two years astrophysicist Roger Gallet at the Boulder Laboratories of
the National Bureau of Standards has been studying radio signals of tremendous power
from Jupiter. Gallet's work rules out thunderstorms as the possible source since
lightning discharges, unlike the signals being received from the planet, broadcast
on all frequencies at the same time and have other different characteristics.
The actual signals consist of 2-second pulses having 100 thousand times more
energy than that contained in a strong local lightning discharge, and 30-millisecond
pulses of infrequent repetition. Concerning the origin of the signals, it is suggested
that they may have a shock-wave origin possibly from geyser-like phenomena or volcanic
activity, although different from any such activity we know on earth, because the
material constituting Jupiter is different from Earth.
Perhaps the most important evidence on Jupiter that has been collected is that
which seems to prove that the huge planet has a strongly ionized upper atmosphere
- an ionosphere - similar to our own. And just like our ionosphere its electronic
density varies in relation to the amount of ultraviolet radiation given off by the
sun. Emissions, recorded at a specific frequency, come through a cone of transmission
radiating from the source. This seems to indicate that the radio waves within the
cone are penetrating Jupiter's ionosphere, but the oblique waves outside the cone
are being reflected back to Jupiter by its ionosphere.
It has also been found that the cone is larger for 20 than for 18 megacycles.
These are the two frequencies on which the observations are conducted.
Interestingly enough, this radio astronomy work must be done only at night when
our ionosphere is less ionized and the Jupiter waves can come through.
Here is screen shot of near-real-time maximum usable frequency (MUF) for regions on the earth.
Posted June 26, 2023
(updated from original post
on 1/27/2014)
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