Like many great discoveries, the story of discovery of radio emission from Jupiter and it’s moon Io is full of surprises. This first detection of radio emission from any planetary body was made by B. F. Burke and his graduate student K.L. Franklin in 1955. They had been operating a newly built radio antenna known as the ‘Mill’s Cross’ to conduct a sky survey. The aerial was tuned to 22.2 MHz and it received radio waves
within a beam only 2.5 degrees wide. They set the telescope to the declination of Crab nebula in constellation Taurus and allowed the sky to drift through the beam as the earth rotated.
After Crab, and another radio source IC 433 had transited, a strong and curious bit of radio noise appeared on their recorder graph. They first attributed this noise to the farming activity going on around the observatory. However, they were surprised to note that the noise occurred almost at the same sidereal time each day and it also slowly drifted westwards from its sidereal position day by day. As any good astronomer
would know, this could only mean that not only the radio source is located far away among the stars, but it is shifting relative to the fixed stars!
Another colleague H. Tatel, who had tried unsuccessfully to detect hydrogen gas on Jupiter a few days back, somewhat facetiously suggested that perhaps this curious radio noise could be coming from Jupiter. At this suggestion, they all went out and had a good laugh when they saw the brilliant Jupiter high up in the sky. The very notion of such intense radio waves coming from a planet was simply preposterous!
But nature thought otherwise. Out of curiosity, when Franklin looked up the position of Jupiter in an almanac, he was startled to find it located about right! This set all their alarm bells ringing and soon with more systematic observations it became clear that Jupiter indeed was the culprit! The radio noise was recorded only when Jupiter was within the narrow confines of their radio beam and the noise source exhibited the same change of direction as Jupiter did during its retrograde loop of 1955. This discovery took the entire scientific world by surprise and soon all the major radio telescopes were trained on to Jupiter to listen and understand it’s mysterious radio
noise. Thus, nature revealed one of its secrets in most unexpected manner and the truth proved contrary to all conventional wisdom.
What causes the radio noise on Jupiter?
Since that momentous day in 1955, when the radio noise of Jupiter was first recorded by Burke arid Franklin, many ground and space telescopes (such as Hubble) and several interplanetary spacecrafts, such as VOYAGER, ULYSSIS and GALILEO have studied the phenomenon from close range.
Although there are more unsolved questions than answers, the broad facts, if put simply, are as follows:
(1) The most intense part of radio emission is received at DECAMETRIC wavelengths, with waves longer than Io metres or frequencies shorter than 30 MHz. Mainly in the
frequency range of 18-22 MHz, intense and sporadic ‘radio bursts’ and ‘noise storms’ can be heard. These last from minutes to hours and are quite unpredictable. The central
role of Jovian moon Io was discovered by Bigg in 1964 when he noticed that the radio noise was very intense whenever the Earth, Jupiter and Io were in a special configuration. This happens only when Jupiter’s meridional sector 200-270 degrees faces earth and the phase angle of Io is in range 205 – 260 degrees. The rotation period of radio source was found to be 9h 55m 29s, about 5 minutes longer than Jovian equatorial rotational period of 9h 50m 30s. This puts the noise source close to the polar regions of Jupiter (It does not rotate like a solid ball but spins faster at the equator than the poles).
(2) The location of radio emission is in the intense magnetosphere (a vast region filled with charges, radiation and magnetic field) of Jupiter. The Jovian magnetosphere is
about 20,000 times more energetic than earth’s and any astronaut foolish enough to venture here would die instantly of radiation poisoning. The charged particles are confined to a doughnut shaped cloud in the equatorial plane where the Galilean moon Io orbits (about 422,000 km from Jupiter). The strong magnetic field of Jupiter, shaped somewhat like a bar magnet, soon guides these particles to it’s polar regions, where they gyrate about the field lines and produce the radio noise. Figure 1 shows a radio image of Jupiter (central oval), the radiation belt along the Io’s orbit (red and blue zones to each side) and the particles being guided to polar regions (blue crescents).
ATCA 13cm (Images courtesy of CSIRO)
(3) The Hubble Space Telescope pictures taken in ultraviolet light have recently shown the spectacular images of intense auroral activity. These aurorae are produced when some of these particles plunge into Jupiter’s upper atmosphere. Figure 2 shows the images of these auroral ovals over the north and south Jovian poles. The power dissipated in these aurorae is about 100 million mega watts, compared to ‘mere’ 10,000
mega watts for earth’s aurorae.
(4) Unlike the earth’s radiation belts and magnetosphere, where most particles are captured from the sun, the Jovian aurora and it’s radio emission is powered by charged particles ejected by its curious moon Io in the form of several volcano that erupt often and spew material hundreds of kilometers into space. A GALILEO spacecraft picture of one such volcanic eruption is shown in Figure 3 (blue plume, Image courtesy NASA/JPL ). Most ejected material falls back and covers entire surface by yellow, brown sulfur compounds. However, the volcanic gases make the atmosphere of Io a good electrical conductor. Io acts as an electrical generator as it moves through the Jupiter’s magnetic field, developing about 400,000 Volts and generating a current of about 3 million Amperes that flows between Io and the polar regions of Jupiter. This current is the main source of the decametric radio noise and the aurorae.
A simple antenna system to receive the radio waves from Jupiter and Io
A simple radio antenna setup is described here that you can build in your backyard or rooftop. With this antenna you can receive the radio noise described above and conduct some scientific experiments to better understand the fascinating radio phenomenon of Jupiter and Io. The simplest antenna that you can build is a dipole cut to the frequency of interest. Choose a good open space far from busy traffic, tall buildings, transformers and overhead power lines in order to minimise the unwanted radio noise. Now choose your frequency of reception between 18 and 23 MHz. Say, this frequency is fMHz. The required length L of the half-wave dipole would be
L (metres) = 142.5/f (Mhz).
As an example, say we choose 21 MHz for frequency. Therefore we are going to need a
dipole of length L = (142.5/21) = 6.79 metres. Add about 40 cm length. The total length would be 7 m and 20 cm. Use #12 gauge (dia ~ 2 mm) stranded copper wire. The extra length is necessary to tie the wire to the insulators, but finished length should be L as calculated above. It does not matter if the wire is insulated or not and almost any copper wire would work just as well. Although, the antenna dimensions are optimized for frequency f, it would work well for range of frequencies near this value.
Now we require some pieces of insulators to support and stretch the wire horizontally. You can buy these from local electronics shop or else take a plastic ruler and saw off few 2.5 inch long sections and drill holes on their both ends to construct your own insulators. Cut the wire exactly into two half sections and tie them to the insulators as shown in the Figures here. Tie some nylon twine to the end insulators and using them fix the wire horizontally between two vertical poles or walls, as shown in the figure. To start with, an approximate north-south orientation of the dipole axis should be tried.
It is necessary that the height of the antenna above the ground be kept at H = 75/f metres. Remember again that f is frequency in MHz and, therefore, for 21 MHz operation, the height must be 75/21 = 3.57 metres above ground. At this height, the reception pattern of the dipole would be quite broad with maximum reception in vertical direction (perpendicular to dipole), zero reception along the axis of dipole, and intermediate for other directions. Now, buy sufficient long length of RG-59/U (75 Ohms, 0.24 inch outer Dia.) type co-axial cable to connect antenna to the radio receiver. This type of cable is commonly used in cable television networks and it is freely available. Do not use more than 50 feet length to avoid signal losses. You can also try using 300 Ohms twin parallel-wire type TV antenna cable but the losses would be more. Connect 
coaxial cable to dipole following the figure and clamp it on the central insulator as shown. Once antenna end is ready, connect the other end of the co-axial cable to a short-wave radio receiver. Many good receivers have external antenna socket and the cable can be easily connected to it using a suitable jack. If no antenna socket exists, connect the cable’s copper shield to the chassis ground and the inner conductor to the antenna end of the tuning gang capacitor. Tune the receiver at a spot close to 18-23 MHz frequency and by slowly tuning within this range try to pick up the strong radio noise of Jupiter, with occasional modulation or intense rapid bursts of radio noise. Of course, you must ensure that Jupiter is out there, sufficiently high up to be received by the dipole! Also, make sure that the radio waves are indeed from Jupiter by noting if the noise appears/disappears as Jupiter rises/sets at your horizon. In case, if you happen to receive weak or no signal, keep trying for several days till, as we mentioned before,
Earth, Jupiter and Io come in their most favourable configuration to generate a strong noise (This may happen about once in two days; more exact daily predictions can be obtained from the authors). The non-Io dependent part of the radio emission should be heard more often. Once you have familiarised yourself with the character of the noise, try replacing the loudspeaker by a multi meter set to the most sensitive range of AC voltage reading. This way one can take a quantitative measure of radio wave intensity for scientific experiments.
What scientific experiments you want to conduct with this antenna depends on your ingenuity and interest. We can suggest a few here:
(1) Localization of radio source on Jupiter from rotational period.
(2) Probability curve of wave intensity as function of Earth, Jupiter and Io position.
(3) Do other moons of Jupiter have any effect on the radio noise?
(4) Does the activity on Sun (sunspots, flares, etc.) affect the radio noise from Jupiter’s magnetosphere?
(5) How the radio noise intensity varies with time and the frequency of reception?
Some more sensitive antenna systems with capability to detect the polarization of Jupiter’s radio waves would be presented in the forthcoming issues of ‘Khagol’.
DIYA 