Detection of Jupiter signals
From Figure 2.1 we can see that the signal levels on Earth from Jupiter are about 105 Jy which is comparable to the quiet Sun and low level solar storms. The same equipment used to detect solar emissions can be used to receive signals from Jupiter. Greater attention has to be paid the design and positioning of a suitable HF antenna that has some directionality (3) (4). A wire dipole 22 feet long set between 0.25 and 0.5 wavelengths above the ground will produce a beam at 21MHz such as is shown in Figure 4.1. If this antenna is connected to a communications receiver when Jupiter is positioned to be in the beam, a variety of noisy signals may be heard. These vary considerably and are related to the position of the moon Io. There are many web-sites dedicated to observations of the complex emissions from Jupiter (5) (6) (7) and one should look at these for details of emission classifications and technical details of equipment.
Figure 4.1 Dipole antenna pattern for detection of Jupiter noise
There are three major factors not related to observing conditions on Earth which have been identified to affect the probability of hearing Jupiter's decametric emissions at any given time (8)
An illustration of the Jovian radiation belts and the spiral motion of trapped electrons is given in Figure 4.2,
Figure 4.2 Jovian magnetosphere
4.2 Spectrum of Jupiter emissions
Signals from Jupiter were first detected in 1958 at a wavelength of ~3cm, but it was not until measurements were made in the HF band that it was realised how strong the emissions were. The spectrum is complex and is composed of two parts as shown in Figure 4.3.
Figure 4.3 Complex spectrum of Jupiter emissions
The thermal component A arises from the cold planet and its atmosphere at a temperature of 130° K. The intensity of the non-thermal component B is much greater at wavelengths longer than 10cm and is generated by the interaction of high energy electrons (~10MeV) with the planet's magnetic field that has a strength of about 1 gauss. A centimetric emission map of the active region is shown in Figure 4.4.
Figure 4.4 Radio emission map at centimetre wavelengths
Jupiter is a good 'target object' for the amateur radio astronomer with fairly basic equipment, however a large antenna is required and this can be difficult to erect in a limited space and in the midst of interfering electronic equipment in urban situations.
© Dr David Morgan 2011