Planets
As the Moon all the planets emit thermal radiation corresponding to their surface temperature.
Because of the large distances involved the radioflux is rarher small:
A body with temperature T emits an intensity (or surface brightness)
I = 2760 T / λ2
If its diameter is D and we observe it from a distance d (both in km),
its angular diameter is (in arcmin) 3438 D/d and it subtends a solid angle
Ω = π(D/2)2/d2
so that the flux (in Jansky) is:
F = I Ω = 2760 π T [D/(2λd)]2
From this one can compute for the individual planets
Planet: |
Mercury |
Venus |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
Pluto |
|
Diameter |
4240 |
10170 |
6750 |
142800 |
120800 |
47200 |
44600 |
3000 |
km |
Distance |
0.61..1.39 |
0.28..1.72 |
0.52..2.52 |
4.2..6.2 |
8.5..10.5 |
19.2 |
30 |
39 |
AU |
Ω |
22..4.2 |
660..17 |
59..2.5 |
440..185 |
7....4.6 |
2.1 |
0.77 |
0.008 |
10-10sterad |
Flux (10GHz) |
4.7..0.9 |
152...4 |
5.4..0.2 |
20..9.4 |
2.9..1.9 |
0.065 |
0.018 |
0.0002 |
Jy |
(S+N)/N |
0.001..0.0002 |
0.04..0.001 |
0.001..0.0001 |
0.004..0.002 |
0.0007..0.0005 |
0.000015 |
0.000004 |
0.00000004 |
dB |
(Some of the surface temperateures are estimates only)
The S/N ratios are computed for our 7 m antenna on 10 GHz with a
system temperature of 150 K. They show that Venus and Jupiter are
the best candiates, whenever they are closest to the Earth.
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On 1 Jan. 2014 the Solar System offered a rather favourable situation:
Jupiter would be in opposition to the Sun on 5 Jan. and Venus would
pass through lower conjunction on 12 Jan.
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Venus
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The Record of 25 Jan. shows the observational method: The 10 GHz
antenna tracks alternatingly Venus for a while and a position nearby to
measure the background from sky and receiver. The background level
rises by about 0.2 dB within a quarter of an hour. The signal from Venus
is only 0.03 dB above the background.
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The slow change of the background level can be filtered out by FFT, and
one can determine the average signal from Venus ...
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The individual measurements show a good agreement with the predictions.
Unfortunately, no flux calibrations could be done, but as the 10 GHz
system has proven to be rather stable, one may assume that the system
temperature kept close to 150 K.
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The 10 GHz radio flux perdicted for the next few years: Venus gets
closest to us every 585 days, at about 0.3 AU. In the meantime it
may go as far away as 1.7 AU. Hence the signal level varies over
a large range, from 10 times that of Jupiter's to only one half.
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Jupiter
The outer layers of the giant gas planet Jupiter consist of hydrogen and
helium gas. Under the existing pressures and temperatures hydrogen acts
as a liquid and is electrically highly conductive. The planet's rapid
rotation (about 10 hrs period) then produces a strong magnetic field.
Quite similar to the Earth, a large magnetosphere is formed which can deflect
the charged particles of the solar wind. The volcanos of the moon Io eject gas,
which is ionized by the solar ultraviolet radiation and forms a doughnut-sheped
ring around Jupiter and supplies plasma which is trapped in the magnetic field
as radiation belts, like the Van Allen belts of the Earth. The movement of Io
in the magnetic field causes large currents to flow. These charged particles have
to follow the magnetic field lines towards Jupiter's magnetic poles where they
cause auroral emission in the upper atmosphere, like on Earth.
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A picture taken in ultraviolet light by the Hubble Space Telescope shows the polar region
of Jupiter with aurorae: they are produced by electrons which can only spiral along the
magnetic field lines and thus are funneled towards the magnetic poles. Here they collide with the
dilute gas of the upper atmosphere and excite the atoms to give off optical emission.
Apart from the permanent auroral ring which may vary in intensity, there are emission spots
linked to the Jovian moons. The brightest is associated with Io, whose sulphur volcanos
eject gas which is ionized by sunlight and thus constitute a source of charged particles.
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Aurorae are also accompanied with radio emission:
Early in 1955 B.Burke and K.Franklin picked up on 22 MHz sporadic radio
signals from Jupiter. After extendive observations it became clear that
burst-type emissions occurred when a certain Jovian longitude faces the
Earth. This indicates that the sources of these noise storms are fixed
with respect to Jupiter's rotating magnetic field. In 1964 E.K.Bigg found
that the location of the moon Io on its orbit also influences the
probability of detecting radio emissions. The details of how the radio
bursts are generated is not yet completely clear. The plot below summarizes
the statistics of occurance of radio bursts.
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Probability of Jovian decametric radio emission as a function
of the central meridian longitude (CML) and the orbital phase
of the moon Io (C.Higgins, 1966).
While it is impossible to predict when a noise storm actually
occurs, these data permit to predict when a storm would be
detectable by a terrestial observer.
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These radio bursts can be observed with rather simple equipment:
NASA's RadioJove Project
has produced a inexpensive receiver on 20.1 MHz, software to
predict observability of possible radio storms, software to
record and inspect observational data, and provides all the
instructions for a successful operation.
However, one very important requirement to pick up bursts from Jupiter
is to have a electronically quiet location: In a city or on a campus
this is rather difficult, but observations of solar radio bursts
are also possible with this equipment. Being located on a
computer-infested site, during
DF3GJ's operations
Jupiter was never detected, but nontheless it provided a lot of
interesting experiences and great fun!
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Since electrons can only move along or spiral around a magnetic field
line, the energetic electrons from the radiation belts are forced
on helical paths around the field lines, thereby radiating synchrotron
radiation, in a broad spectrum from 100 MHz to 10 GHz.
This radio image taken at 22 cm wavelength indicates the radiation belts
in the equatorial plane ...
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... while the image at the shorter wavelength of 13 cm (2.3 GHz) already
shows some thermal emission from Jupiter itself.
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The spectrum above about 10 GHz is dominated by thermal emission from the planet and its
atmosphere. Below this frequency there is a component of non-thermal emission from the
magnetosphere, viz. the layers high above the planet, in which strong magnetic fields
trap electrons and protons in radiation belts - as the Van Allen belts around the Earth.
The electrons can only move on helical orbits around the magnetic field, producing the
non-thermal synchrotron radiation.
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Our Observations
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The observation of 21 Jan. shows that it is much more difficult, but
still possible. The signal is only 0.015 dB above the background!
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FFT allows to get rid of the slow changes in the signal level ...
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The difficult measurements, the weather, and some problems with the receiver
allowed only a few observations. The measured flux initally is significantly
above the predicted level, but eventually appears to converge on it.
Unfortunately no flux calibrations could be done, but as the measurements of
Venus, done at the same time, agree well with predictions, and the 10 GHz
system is known to be rather stable, it seems well possible that the observed
discrepancies are genuine. Although one would not expect non-thermal emission
reaching as far high up as 10 GHz, it might be possible that we caught the
tail of a radio eruption. If this is a rare phenonmenon or if it does happen
occasionally, only future observations will reveal.
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Predicted 10 GHz radio flux for the next few years: Since the distance varies only
between 4.2 and 6.2 AU, the low level flux varies only by a factor of 2, due to the
yearly movement of the Earth.
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