Jovian Decametric Radio Emission
How scientists learn about Jupiter by observing its radio
Dr. Leonard N. Garcia
Jupiter's Rotation Period
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
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
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
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.
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]
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
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,
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? |
other moons as well. Do any of these other moons influence Jupiter radio emissions? |
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
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.
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
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
spacecraft are confirming some explanations of Jupiter radio emission but are also
discovering new radio phenomena that raise many more questions.
Belcher, J.W., The Jupiter-Io
Connection: An Alfven Engine in Space, Science, vol. 238, pp 170-176, 1987.
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.