Dd. Barbosa, NEUTRAL CLOUD THEORY OF THE JOVIAN NEBULA - ANOMALOUS IONIZATION EFFECT OF SUPERTHERMAL ELECTRONS, The Astrophysical journal, 430(1), 1994, pp. 376-386
The standard model of the Jovian nebula postulates that its particle s
ource is the extended cloud of neutral sulfur and oxygen atoms that es
cape from the satellite Io and become ionized through electron impact
from the corotating plasma. Its energy source is the gyroenergy acquir
ed by newly formed pickup ions as they are swept up to corotation velo
city by the planetary magnetic field. Elastic collisions between plasm
a and electrons cool the ions and heat the electrons, while inelastic
collisions cool the electrons and excite the ions to radiate intense l
ine emission, which is the primary energy-loss mechanism for the plasm
a. This neutral cloud theory of the Io plasma torus, as it has come to
be known, has been the subject of recent criticism which asserts that
the theory cannot account for the observed charge state of the plasma
which features O+ and S2+ as the dominant ions. It is shown in this w
ork that the inclusion of a small population of superthermal electrons
is required to achieve the correct ion partitioning among various cha
rge states. The superthermal electrons are an essential element of the
theory: they are a consequence of the highly nonthermal velocity dist
ribution of pickup ions generating plasma waves which dissipate by acc
elerating hot electrons to kiloelectron volt energies. Coupled ionizat
ion and energy balance equations are solved for a homogeneous plasma m
odel featuring the addition of a superthermal-electron population. The
results of the analysis show excellent agreement with the observation
s. It is also argued that the anomalous ionization effect of the super
thermal electrons is responsible for the overall spatial bifurcation o
f the nebula into a hot multiply charged plasma region outside of 5.7
Jovian radii and a cool singly ionized plasma inside this distance.