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.
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
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 January 27, 2014