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The Jovian Decametric Radio Emission

How scientists learn about Jupiter by observing its radio emissions. 

by Dr. Leonard N. Garcia

Jupiter's Rotation Period
After the accidental discovery of radio bursts from Jupiter, scientists sought to understand what caused this radio emission. They started with careful observations, recording the times they heard Jupiter and how intense Jupiter's decametric radio bursts were. (The word decametric means tens of meters since the wavelength of the radio bursts are in the tens of meters). After collecting these radio data they compared it with other information they had about Jupiter. They began to match the Jupiter radio bursts with the rotation of the planet. The only way observers know which part of Jupiter is facing them at a certain time is by knowing its rotation rate. At first, astronomers only knew Jupiter's rotation rate by watching the cloud patterns move across the planet; there are no surface features to track. From this information they found that Jupiter rotates once in about 10 hours, more than twice as fast as Earth.

The observers realized that whether we hear Jupiter or not depends a lot on what part of Jupiter is facing us at the time. The radio emission depends on the Jovian longitude. It seemed there were special longitudes where Jupiter was much more likely to be heard than others. These longitudes were like "landmarks" on a planet with no observable surface. These landmarks also mean that Jupiter isn't just sending out radio waves in every direction but rather it is beaming the radio waves into space.

Astronomers continued to make careful observations collecting years worth of data. With more data they were able to see subtle changes in the location of the radio sources on Jupiter. The emissions began to drift slowly. This meant that either the radio sources really were moving slowly or that scientists didn't have a very good estimate of how fast Jupiter was rotating. The way this drift was occurring led to the conclusion that their estimate of the rotation rate of Jupiter needed improvement. Many years of data were used to make an improved estimate of the rotation rate. The most accurate estimate we now have of the rotation rate of Jupiter is based on radio observations.

Jupiter's Magnetic Field
The next thing they realized was that the radio landmarks matched no cloud features. Not even the Great Red Spot seemed to follow along with the radio emission. The radio emissions seemed to follow a unique rotation period and it stayed very constant, neither slowing down nor speeding up.

Other characteristics of the radio emission taught us more about Jupiter. Radio waves like light waves can be polarized or unpolarized. We can picture light waves and radio waves as ripples flying through space. When we talk about light being polarized we mean that most of the light is traveling through space with one preferred way of rippling. These ripples can move up and down like ripples on a pond of water or they can move side to side like a snake slithering across the road. Light waves can even travel like a twisting corkscrew. When light is unpolarized all the waves appear to be traveling randomly as if they are traveling all of these ways at the same time.

Most of the radio waves from Jupiter are polarized. This discovery told us something about the cause of the radio waves, the conditions at the source of the waves and even about the conditions in space between Jupiter and Earth. Polarized radio waves implied that wherever these waves were coming from there was a magnetic field present. This was one of the first indications that Jupiter had a magnetic field.

Knowing that Jupiter has a magnetic field and knowing that the radio "landmarks" reappear at very constant intervals tells us that what we are seeing is likely rotating at the same rate as some inner part or core of Jupiter where the magnetic field is generated.


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.

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.

To learn more

Hubble Space Telescope Images - see Jupiter's Aurora and the trace of the current flowing between Jupiter and Io [via STScI]

The Goldstone-Apple Valley Radio Telescope (GAVRT)- see an observatory where decimetric radio observations of Jupiter are made

Polarization - learn more about this property of light [from The Physics Classroom]

Jupiter and Io - more information about Jupiter and Io from NASA

Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and Galileo - learn more about these spacecraft

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.

 

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