4.3 Rotation of Jupiter's magnetosphere and Io's plasma torusercury and Venus
Jupiter has a magnetic field about 20,000 times stronger than that of
Earth, and the field is running by the Jupiter's rapid rotation. So, the
charged particles of the magnetosphere move at high speed. Since a magnetic
coupling force acts among the same charged particles, and the life of the
plasma in the magnetosphere of Jupiter is long.
There is Io-plasma torus in which the plasma is distributed in the doughnut
shape is located at about six times the distance of the Jupiter’s radius.
The plasma torus rotates around Jupiter in a cycle of about 9.93 hours
in the anti-clockwise direction due to the rotation of Jupiter. On the
other hand, Io of Jupiter's satellite moves slowly in the anti-clockwise
direction. The period of rotation of Io is 42.46 hours.
Since the orbital radius of Io is 421,700km, the orbiting velocity (Vorbiting of Io) is 17.3
km/sec at the orbital orbit of Io, while the orbiting velocity of the plasma
torus on the orbit of Io is Vplasma torus at Io = 74.1 km/sec. Therefore, the relative velocity of the plasma torus from
Io is given by the difference. i.e. V plasma torus at Io - Vorbiting of Io =56.8 km/sec. Since the
kinetic energy of H+ of this speed is given at 1/2 (mv2) = 17 eV, it can be ionized by collision of proton. As shown in Fig.12,
it was clarified that extreme ultraviolet rays on the plasma torus of Io
became stronger downstream of Io by using the Planetary Observation Space
Telescope Hisaki.
Fig.12 Local electron heating in the Io plasma torus associated with Io
from HISAKI satellite observation.
(Reproduced from http://www.sci.tohoku.ac.jp/news/20160512-7885.html )
[Journal of Geophysical Research Space Physics：https://doi.org/10.1002/2015JA021420]
The rotation of the Io plasma torus that is caused by the rotation of
the magnetosphere of Jupiter brings a very high-speed tailwind to the Io.
New ions are released by sputtering on the surface of Io and cause a strong
light region in the plasma torus. The excited ion captures free electron
and it becomes atom. The atom is not accelerated by the rotation of the
magnetosphere. But collisions between atoms and ions are repeated in the
plasma torus. Those particles are accelerated by the tailwind of the plasma
torus and are flowed forward of the orbit. Then, the emission region in
the Io plasma torus is flowing along the counterclockwise direction in
Io's orbit. Since the life of free electrons is short in plasma, it is proton (H+) that maintain Io’s plasma torus.
The magnetic field is generated perpendicular to the direction of rotation by the rotating motion of the proton around Jupiter,
and the plasma region is formed in a doughnut shape because the plasma is confined by its magnetic field.
index -4.3-