Io Plasma Torus: Nonequilibrium Effects

Details Io Plasma Torus: Nonequilibrium Effects Io Plasma Torus: Nonequilibrium Effects

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.2315w&link_type=abstract

American Astronomical Society, DPS meeting #28, #23.15; Bulletin of the American Astronomical Society, Vol. 28, p.1155

Statistics

Computation

Scientific paper

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 ~ 5000s from one extreme of electron density to another. However, the commonly observed [Sii]lambda lambda 6716,6731 doublet has much smaller Einstein A coefficients [(5780s)(-1) and (1890s)(-1) , 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 lambda lambda 6716,6731 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-ther\-mal ``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.

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