R. C. Woodward (U Wisc), W. H. Smyth (AER)

The complex structure of the Io plasma torus makes the task of
modeling it difficult and computationally expensive; several
equilibrium assumptions and/or simplifications are therefore commonly
used to make the problem more tractable. Because many of these
assumptions were untested, we made them optional in our semi-empirical
model of the torus [*B.A.A.S.* **26**, 1139
(1994)]. Here we explore the effects of three of these assumptions on
the modeled latitudinal emission brightnesses of the torus.
**First:** Perhaps the least-often questioned assumption
is that the distribution of atomic states of an ion species is in
local equilibrium with the surrounding electrons, based on the fact
that the inverse Einstein A coefficients of most observed lines are
short compared to the ion's travel time along a field line of ~ 5000 s
from one extreme of electron density to another. However, the commonly
observed [S II] 6716,6731Å doublet has much smaller Einstein A
coefficients [1/(5780 s) and 1/(1890 s), respectively], suggesting
that their upper states may not fully equilibrate. We have therefore
performed a full time-evolved nonequilibrium calculation of emissions
from the S+ ion. While the differences from the corresponding
equilibrium calculations are not huge, neither are they negligible: we
show that the nonequilibrium 6716,6731Å calculated emissions are
latitudinally more extended and, consequently, that parallel ion
temperatures estimated from equilibrium models are significantly too
high. **Second:** Although there is strong evidence of a
second electron population in the torus, hotter but less dense than
the first, this population is frequently neglected in emission
brightness calculations for computational ease.
**Third:** The various plasma species, while not
necessarily in thermal equilibrium with one another, are generally
each regarded as being in a purely thermal (*i.e.* Maxwellian)
velocity distribution. Recent data from Ulysses, however,
do not support this--the quasi-thermal "kappa" distribution
appears to be a better description of the data--but thermal
distributions are still commonly used. We present comparisons of
modeled emission brightnesses calculated with and without each of
these simplifications.