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Jovian Decametric Emission - Origin and Mechanism
If charged particles (i.e. electrons or protons) move in a magnetic field and the velocity vector v is
perpendicular to the magnetic field B, the electron starts to move in a circle
(rotation or gyration around a magnetic field line) (right hand rule) (Fig.1).
If the velocity vector v is not entirely perpendicular to the magnetic field (i.e. a fraction of v
is in direction of B), it starts to move in a spiral (Fig.2).
Fig.1: Circling electron due Lorentz-Force |
Fig.2: Spiraling charged particle in a magnetic field |
In our case, electrons or protons (or other charged particles) are accelerated at Io due electric fields.
The particles travel along Jupiter's magnetic field lines towards Jupiter in spirals.
Spiraling electrons radiate right-circulary-polarized (RCP) electro-magnetic waves at or near the local gyrofrequency f.
f = 2.8 * Bg [MHz]
Bg ... local magnetic field in Gauss
Billions of electrons spiraling along Jupiter's magnetic field lines radiate electro-magnetic waves from 1 - 40MHz.
Since the emitted frequency depends on the local magnetic field strength, the emissions from other planets, such as Saturn,
Uranus and Neptune, are lower in frequency (<2MHz) due lower planetary magnetic fields and are not detectable on
the surface of Earth. The ionosphere of Earth is reflecting these emissions back to space.
In the following schematic (Fig.3) the mechanism is illustrated:
Simplifications:
* rotation axis = magnetic field axis
* emission shells are circular
Electrons are accelerated up to a few keV (or more) at Io due Alfvèn waves generated by Io's movment thru the Jovian
magnetic field. Those electrons travel in spirals to Jupiter on magnetic field lines (flux tubes).
As Jupiter's magnetic field increases, the frequency of the emitted electro-magnetic waves also increases. This is
shown with several discrete emission shells (several frequencies at a magnetic field strength from 1.1Gauss to 14.5Gauss
at distances of 2.5 to 1.1 jovian radii - source UFRO).
The radio waves are emitted in thin hollow cones, which axis are parallel to the magnetic field line - the emission can only be
detected at Earth, if the thin walls of the cone intersect the direction of Earth (see emission ray to Earth in the
schematic).
Actually the Io-related emission is emitted from a Previously Energized Flux Tube (PEFT), which is 20 to 0 degrees behind the
current Io Flux Tube (IFT). The electron acceleration due Alfvèn wave trains is not instantaneous, hence the angle between
the IFT and the PEFT.
Alfvèn wave: A low frequency hydromagnetic plasma wave generated by ions oscillating about their equilibrium positions.
Basically, these waves can be thought of as waves on a magnetic string. The magnetic field acts like a string, and the plasma
particles act like beads. Heavier beads (-> heavier plasma ions) means the waves are slower and the wavelength is longer.
These Alfvèn waves produce an electric field parallel to the magnetic field, which accelerates the electrons towards Jupiter.
The latest results (K. Imai et al. PRE VI 2005) show that there is only one radio active region (northern hemisphere) on Jupiter
for Io-A and Io-B and emission is only emitted when the PEFT passes through this (red) radio active region
(also see Jupiter Radio CG Gallery). Future research will most likely
show another source for Io-C and Io-D, maybe on the other hemisphere of Jupiter.
Fig.3: Schematic explaining the origin and mechanism of Jupiter's Decametric Emission
Based on the model by Imai Lab., Kochi National College of Technology
The effects of the particles hitting Jupiter's atmosphere via the flux tubes can be seen in this UV image, shot by the
Hubble Space Telescope. The effect of the Io Flux Tube (IFT) is very strong, but also flux tubes of other moons, such as
Ganymede and Europa, can be seen. Also clearly visible, the main oval and polar emissions within the oval.
Jupiter Aurora with footprints of the flux tubes of Io, Ganymede and Europa
NASA & J. Clarke (University of Michigan) * STScI-PRC00-38
References:
- K. Imai, F. Reyes, T.D. Carr, A. Lecacheux (PRE VI 2005): Recent progress in the measurement of jupiter's decametric radio source parameters by the modulation lane method
- Mechanism responsible for the emission: UFRO
- Imai Lab., Kochi National College of Technology
Last-Modified: Wed, 04 May 2005 10:38:25 GMT
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