## The Jovian Decametric Radio Emission

#### by Dr. Leonard N. Garcia

 The Decametric Radio Source When charged particles like electrons and protons move through a magnetic field their paths are changed. The particles are accelerated and start to move in spirals around magnetic field lines towards either the south or the north pole. Charged particles that are accelerated emit radiation that depends on the energy of the charged particles. For charged particles moving in Jupiter's magnetic field the energy is such that radio waves are generated there. The frequency of these radio waves increase the stronger the magnetic field is. This radio emission is called cyclotron emission after a type of particle accelerator. Electrons spiraling in Jupiter's magnetic field are thought to be the cause of the radio noise we hear.  The decametric radio waves have frequencies between 10 and 40 MHz. These types of radio waves from Jupiter are never heard above 40 MHz. This seems to be the maximum frequency. >From our knowledge of the cause of the radio waves and knowing that the frequency depends on the strength of the magnetic field we can estimate the maximum strength of Jupiter's magnetic field.

 Left: The probability of detecting radio "landmarks" or sources A, B, and C are plotted against Jupiter's Central Meridian Longitude (CML). The A source has the highest probability of being detected.[Garcia, 1996] Right: Probability plotted against Io phase and CML shows Io-related and non-Io-related sources. The vertical stripes show non-Io-A and non-Io-C.[Garcia, 1996]

 How dense is the Io Torus around Jupiter? How is it distributed around the planet and how does it change with time and with Io's volcanic activity? What about the Sun? How much of an effect does it have any on the emission at Jupiter? Jupiter has other moons as well. Do any of these other moons influence Jupiter radio emissions? Jupiter beams its radio emissions in certain directions. How wide are these beams? How are they shaped? Are they always the same shape? Where precisely is the radio emission coming from? Are there separate sites for Io-related and non-Io-related emission?

Other sources astronomers use for information about Jupiter:

We have learned more about Jupiter and its magnetic field by sending spacecraft there. The Pioneer 10 & 11 spacecraft, and the Voyager 1 and 2 spacecraft flew by Jupiter in the 1970s and 1980s and, during the short period of time they were there, allowed scientists to develop more detailed models of Jupiter's magnetic field. The Galileo spacecraft has orbited Jupiter for several years and is providing a wealth of new data about Jupiter and its moons. Astronomers will be studying the data from Galileo for many years to come.

Jupiter does emit radio waves of a different sort at frequencies above 100 MHz. These are the decimetric radio waves and are believed to be emitted by extremely energetic electrons moving at close to the speed of light close to the planet near its equator. (Decimetric means tenth of a meter since the wavelength of this type of radio emission is several tenths of a meter). Jupiter's rotation period was confirmed and other properties of the magnetic field including its axial tilt were determined using decimetric radio observations.

Recently the Hubble Space telescope has been used to observe Jupiter's aurora in the ultraviolet and has found evidence of the powerful currents that are flowing between Jupiter and Io.

These spacecraft are confirming some explanations of Jupiter radio emission but are also discovering new radio phenomena that raise many more questions.