Radio Emission

Radio emissions from Jupiter provided the first clues that the planet had a strong magnetic field and a large magnetosphere [251]. Jupiter is a very strong radio source, especially at a wavelength of 30-0.6 MHz. Radiation in the radio part of the spectrum was first detected from Jupiter in the 1950s. Soon it was discovered that the radio bursts seemed to happen when certain longitudes were near Jupiter's central meridian. However, these radio bursts did not appear to be associated with any physical feature seen on Jupiter. Finally, in 1964 E. K. Biggs realized that they corresponded to particular orbital positions of the moon Io. Once again we see the strong influence of Io on Jupiter's electromagnetic environment [252].

These 30-0.6 MHz emissions are known as decametric emissions, and are only one of four classes of radio emission. We know that these decametric radio waves are "synchrotron radiation that spiral around the magnetic field lines" [253]. Synchrotron radio emission is produced when electrons are trapped in a magnetic field. The other three emissions are decimetric, millimetric, and kilometric emissions.

Decimetric emission is detected at wavelengths between 10 cm and several meters. These come from the inner magnetosphere at 1.3-3 Rj, and appear strongest at 1.5 Rj. This is the highest energy part of the radiation belts [254].

Millimetric emission is part of the thermal spectrum of Jupiter's atmosphere, detected at wavelengths up to 10 cm [255].

Kilometric emission can only be detected from space, and this emission from Jupiter was not discovered until Voyager reached the Jovian system [256].

There is evidence that Jupiter's radio emissions and aurora are controlled by the solar wind. Radio emission in wavelengths of 0.3-3 MHz is often referred to as hectometric radiation. Research by Gurnett et al., [257] indicates that the emissions are triggered by interplanetary shock propagating outward from the Sun. The same electrons that produce the hectometric emission also produce aurora. Thus, aurorae are also controlled by the interplanetary shock in the solar wind. It is believed that Jupiter's radio emissions are generated along high-latitude magnetic field lines by the same electrons that produce Jupiter's aurora, and both the radio emission in the hectometric frequency range and the aurora vary considerably. Evidence already existed that the solar wind plays a role in controlling the intensity of hectometric radiation. The Cassini and Galileo spacecraft confirmed this effect on j- ®

Jupiter's hectometric radiation and aurora. Simultaneous, coordinated observa- ein tions of Jupiter's hectometric radio emission and extreme ultraviolet auroral nmnd emissions were carried out by the Cassini and Galileo spacecraft on 30 December, O 3 ¡5

2000, during the Cassini fly-by of Jupiter. This observation revealed that both of irroite these emissions were triggered by interplanetary shocks propagating outward ¡5

from the Sun [258]. We also learned that Jupiter's synchrotron emission can vary u Ifl n on relatively short time scales, even as short as a matter of days [259].

When an interplanetary shock interacts with Earth's magnetosphere, we know that the magnetosphere becomes strongly compressed. This results in a large-scale reconfiguration of the magnetic field and associated acceleration and energization of plasma in the magnetosphere. The stresses associated with the compression lead to large field-aligned currents and electric fields, particularly along the high-latitude magnetic field lines where the electrons that carry the field-aligned currents strike the atmosphere and produce the aurora. It is inferred that similar processes are responsible for the Jovian hectometric radiation and aurora [260]. The observations made by the Cassini and Galileo spacecraft present strong evidence that Jovian hectometric radiation and Jovian auroral extreme ultraviolet emissions are triggered by interplanetary shocks [261]. Thus, even radio emission from Jupiter is affected by interplanetary shock waves in the solar wind.

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