Jupiter and Io
We have learned many things about Jupiter by listening to its radio waves. But is the orientation of Jupiter the only thing that influences this radio emission? We know that Jupiter has many moons; could they affect the radio waves? It turns out that Io, one of the larger moons of Jupiter, has a very big effect on whether we hear any Jupiter radio emission or not. Io is a large moon, about the size of our own Moon, but it is still tiny compared to the enormous planet Jupiter. Io is very unique since it is the most volcanically active body in the solar system. Io is continually flexed by the gravitational pulls of Jupiter and the other satellites. This flexing causes Io to be molten and volacanos on its surface are almost continually erupting. Tons of material, mostly sulfur compounds, are ejected each second. Some fraction of this material escapes Io and travels into space. Once in space the molecules soon lose their electrons, becoming ionized, and are trapped within Jupiter's magnetic field. These ions form a vast donut-shaped ring around Jupiter called the Io Torus.
	Left: An image taken by one of the Voyager spacecraft which shows Io (just above Jupiter's Great Red Spot) and Europa in the foreground with Jupiter in the background.[NASA's JPL] Right: Another Voyager image showing Io and a volcanic eruption. This volcano is called "Loki", after one of the gods of Norse mythology.[NASA's JPL]
Scientists have found that Io enhances Jupiter's emission of decameter radio waves. As Io orbits Jupiter there are only certain positions in its orbit where our chances of hearing radio emissions become much greater. Jupiter's magnetic field moves rapidly past Io as it orbits. When conductors, such as metals, move through a magnetic field a current is produced in the conductor. This is how generators at power plants on Earth create electricity. We already know that there are electrons trapped in Jupiter's magnetic field, and Io with its thin conducting atmosphere, moves through this field and a powerful current is generated between Jupiter and Io. This current may be "energizing" the decametric radio emission. This picture is actually a bit more complicated. It appears that Io and Jupiter don't form a simple electrical circuit. It seems that Io somehow "disturbs" the magnetic field of Jupiter as the field sweeps by the moon. This disturbance remains for some time after Io passes by. It is the disturbance which carries the current. The orbital position of Io can be defined by something called the Io phase. The Io phase is 0 degrees when Io is directly behind Jupiter as seen from Earth. The Io phase increases as Io orbits until it becomes 180 degrees when Io crosses in front of Jupiter as seen from Earth. The "landmarks" or sources referred to at the beginning have both Io-related and non-Io-related components. The non-Io-related sources have a chance of being observed regardless of where Io is in its orbit. The Io-related sources all have higher probabilities of being heard than their corresponding non-Io-related sources. These sources have been labeled A, B and C roughly in order of the likelihood of observing them; the Io-related sources are Io-A, Io-B and Io-C. These sources are often shown on CML versus Io-phase plots. CML stands for Central Meridian Longitude and is defined by the longitude of Jupiter facing the Earth at a certain time. When we plot how often we detect Jupiter's radio emission on this CML versus Io-phase plot we start to see how our data group into narrow bands and distinct regions. This illustrates the fact that Jupiter's orientation and Io's orbital position play a large role in detecting decametric radio emissions.

Jupiter and Io

We have learned many things about Jupiter by listening to its radio waves. But is the orientation of Jupiter the only thing that influences this radio emission? We know that Jupiter has many moons; could they affect the radio waves? It turns out that Io, one of the larger moons of Jupiter, has a very big effect on whether we hear any Jupiter radio emission or not.

Io is a large moon, about the size of our own Moon, but it is still tiny compared to the enormous planet Jupiter. Io is very unique since it is the most volcanically active body in the solar system. Io is continually flexed by the gravitational pulls of Jupiter and the other satellites. This flexing causes Io to be molten and volacanos on its surface are almost continually erupting. Tons of material, mostly sulfur compounds, are ejected each second. Some fraction of this material escapes Io and travels into space. Once in space the molecules soon lose their electrons, becoming ionized, and are trapped within Jupiter's magnetic field. These ions form a vast donut-shaped ring around Jupiter called the Io Torus.

Left: An image taken by one of the Voyager spacecraft which shows Io (just above Jupiter's Great Red Spot) and Europa in the foreground with Jupiter in the background.[NASA's JPL]
Right: Another Voyager image showing Io and a volcanic eruption. This volcano is called "Loki", after one of the gods of Norse mythology.[NASA's JPL]

Scientists have found that Io enhances Jupiter's emission of decameter radio waves. As Io orbits Jupiter there are only certain positions in its orbit where our chances of hearing radio emissions become much greater. Jupiter's magnetic field moves rapidly past Io as it orbits. When conductors, such as metals, move through a magnetic field a current is produced in the conductor. This is how generators at power plants on Earth create electricity. We already know that there are electrons trapped in Jupiter's magnetic field, and Io with its thin conducting atmosphere, moves through this field and a powerful current is generated between Jupiter and Io. This current may be "energizing" the decametric radio emission.

This picture is actually a bit more complicated. It appears that Io and Jupiter don't form a simple electrical circuit. It seems that Io somehow "disturbs" the magnetic field of Jupiter as the field sweeps by the moon. This disturbance remains for some time after Io passes by. It is the disturbance which carries the current.

The orbital position of Io can be defined by something called the Io phase. The Io phase is 0 degrees when Io is directly behind Jupiter as seen from Earth. The Io phase increases as Io orbits until it becomes 180 degrees when Io crosses in front of Jupiter as seen from Earth. The "landmarks" or sources referred to at the beginning have both Io-related and non-Io-related components. The non-Io-related sources have a chance of being observed regardless of where Io is in its orbit. The Io-related sources all have higher probabilities of being heard than their corresponding non-Io-related sources. These sources have been labeled A, B and C roughly in order of the likelihood of observing them; the Io-related sources are Io-A, Io-B and Io-C. These sources are often shown on CML versus Io-phase plots. CML stands for Central Meridian Longitude and is defined by the longitude of Jupiter facing the Earth at a certain time. When we plot how often we detect Jupiter's radio emission on this CML versus Io-phase plot we start to see how our data group into narrow bands and distinct regions. This illustrates the fact that Jupiter's orientation and Io's orbital position play a large role in detecting decametric radio emissions.

	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]

Questions scientists are still asking:
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.

Questions scientists are still asking:

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.

Bibliography

Belcher, J.W., The Jupiter-Io Connection: An Alfven Engine in Space, Science, vol. 238, pp 170-176, 1987.

Carr, T.D., M.D. Desch, and J. K. Alexander, Phenomenology of magnetospheric radio emissions, in Physics of the Jovian Magnetosphere, edited by A.J. Dessler, Chapter 7, pp. 226-284, Cambridge University Press, New York, 1983.