Encyclopedia of the Solar System 2nd ed

532 Encyclopedia of the Solar System

FIGURE 10 The ionization of an extended atmosphere of neutral

atoms (yellow) around Jupiter’s moon Io is a strong source of

plasma, which extends around Jupiter in a plasma torus. Electrical

currents generated in the interaction of Io with the surrounding

plasma couple the moon to Jupiter’s atmosphere where they

stimulate auroral emissions. The main ring of auroral emissions is

associated with currents generated as the plasma from the Io torus

spreads out into the vast, rotating magnetosphere of Jupiter. Credit:

John Spencer.

energy of the planet, in the cases of Jupiter and Saturn, or
by acceleration in the distorted and dynamic magnetic field
in the magnetotails of Earth, Uranus, and Neptune. In a
nonuniform magnetic field and particularly in a rapidly ro-
tating magnetosphere, the ions and electrons drift at differ-
ent speeds around the planet, producing an azimuthal elec-
tric current. If the energy density of the energetic particle
populations is comparable to the magnetic field energy den-
sity, the azimuthal current produces magnetic perturbations
that significantly modify the planetary magnetic field. Table
4 shows that this occurs at Jupiter and Saturn, where the
high particle pressures inflate and stretch out the magnetic
field and generate a strong azimuthal current in the magne-
todisc. Even though Uranus and Neptune have significant

radiation belts, the energy density of particles remains small

compared with the magnetic field and the azimuthal current

is very weak. In Earth’s magnetosphere, the azimuthal cur-

rent, referred to as thering current,is extremely variable,

as discussed in Section 5. Relating the magnetic field pro-

duced by the azimuthal current to the kinetic energy of the

trapped particle population (scaled to the dipole magnetic

energy external to the planet), we find that even though

the total energy content of magnetospheres varies by many

orders of magnitude and the sources are very different, the

net particle energy builds up to only 1/1000 of the mag-

netic field energy in each magnetosphere. Earth, Jupiter,

and Saturn all have energetic particle populations close to

this limit. The energy in the radiation belts of Uranus and

TABLE 4 Energetic Particle Characteristics in Planetary Magnetospheres

Earth Jupiter Saturn Uranus Neptune

Phase space densitya 20,000 200,000 60,000 800 800

Plasma betab < 1 > 1 > 1 ∼0.1 ∼0.2

Ring current,B(nT)c 10–200 200 10 < 1 <0.1

Auroral power (W) 1010 1014 1011 1011 < 108

aThe phase space density of energetic particles (in this case 100 MeV/Gauss ions) is measured in units of (cm (^2) ssr
MeV)−^1 and is listed near its maximum value.
bThe ratio of the thermal energy density to magnetic energy density of a plasma,β=nkT/(B (^2) μ 0 ). These values
are typical for the body of the magnetosphere. Higher values are often found in the tail plasma sheet and, in the case
of the Earth, at times of enhanced ring current.
cThe magnetic field produced at the surface of the planet due to the ring current of energetic particles in the
planet’s magnetosphere.