Jovian Decametric Emission - Origin and Mechanism

locsaucyElectronics - Devices

Oct 18, 2013 (3 years and 9 months ago)

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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 dep
ends 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 r
eflecting 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 du
e 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 in
creases. This is shown with several discrete emission shells (several frequencies at a
magnetic field strength from 1.1

Gauss to 14.5

Gauss at distances of 2.5 to 1.1 jovian radii
-

source UFRO).


The radio waves are emitted in thin hollow cones, which axi
s 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 Previous
ly 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 p
lasma
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 t
here 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. 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 Co
llege 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, s
uch 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 tu
bes 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 mod
ulation lane
method



Mechanism responsible for the emission: UFRO



Imai Lab., Kochi National College of Technology