20 August 1994

Science: Jupiter’s surprise radio broadcast

When fragments of Comet Shoemaker-Levy 9 slammed into Jupiter, the planet
brightened dramatically at radio wavelengths. But the increase in brightness
has puzzled astronomers because it happened at high frequencies, where
the intensity of radio emission was expected to drop. In contrast, at the
lower frequencies, where radio amateurs had hoped to hear the comet crash
with simple antennas, little or nothing happened (New Scientist, Science,
9 July).

During the impacts, which occurred in July, Michael Klein of the Jet
Propulsion Laboratory in Pasadena observed Jupiter with a 34-metre dish
at Goldstone, California. He found that the radio emission from Jupiter
rose by between 20 to 30 per cent at around a frequency of 2.3 gigahertz.
This was ‘synchrotron radiation’ from electrons spiralling round field
lines at close to the speed of light in Jupiter’s intense magnetic field.
‘Never, in 23 years of Jupiter observations, have we seen such a rapid and
intense increase in radio emission,’ says Kline.

Others astronomers, using radio telescopes in Germany, Australia, and
the US, witnessed similar increases at other high frequencies. The radio
emission from Jupiter reached a peak on 23 July, just after the last fragment
hit, and has declined since. The magnitude of the increase varied with
frequency, which is what is expected for synchrotron radiation.

Klein says that the increase surprised Jupiter watchers, who had expected
dust shaken free from the disintegrating comet to absorb the electrons that
produce synchrotron radiation. Instead, extra electrons were supplied by
a source which, as yet, is a mystery.


However, the impacts had little effect at the lower frequencies of 16.7
to 32 megahertz in the amateur band, says Tom Carr of the University of
Florida. Bursts at those frequencies are normally produced when Jupiter’s
moon Io passes through certain zones of the planet’s magnetic field, and
they can be predicted in advance.

Predicted bursts did occur during the week of impacts, but Carr detected
only two small bursts close to impact times, and he is not convinced they
were related to the impacts. Although the lack of activity on the short-wave
band may have disappointed some would-be observers, Carr says it may help
astronomers understand processes on Jupiter. In normal times, the giant
planet is the second brightest source of radio waves in the sky after the