![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh-7HQdKrmB9mVWCfPB_WykiTAZuNJov6ZKL9jZFSO5s9EhtNvOU-mqKOb78nvga6QLto5SbLekW45CmYRRGWX_Vau7bcWvEOR2BIf73CSKjnoDhFul1K0rcgnpJEI1LuQCVf3pSx9cjJM/s320/Lyman-alpha_Feldman2000.png)
While I have not read the Walker et al. paper covering the details of this new model since it hasn't been accepted by Icarus and posted online, according to this paper, it is a 3-D global rarefied gas dynamics model that uses both volcanic plumes and sublimation of surface sulfur dioxide (SO2) frost as sources for the gas in Io's atmosphere. The model takes into account changes to the column density of the atmosphere as a result to time-of-day changes to the surface temperature, distribution of SO2 surface frost, distribution of volcanic plumes (though they use persistent volcanic thermal hotspots as plume sites rather than confirmed plume locations or the locations of large surface changes), plasma heating from Jupiter's magnetosphere, and heat loss to space. For this paper, the authors used a backward monte carlo method to simulate how their model atmosphere would appear in different types of observations.
In Gratiy et al. 2009, the authors compared their model results to three observations: disk-integrated, high-spectral resolution measurements in the mid-infrared near 19-µm published by Spencer et al. 2005; disk-resolved, far-ultraviolet observations in the hydrogen Lyman-α band published in Feldman et al. 2000; and disk-integrated, millimeter wavelength observations published in Io After Galileo in the Io's Atmosphere chapter by Lellouch et al.
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The next dataset the authors compared their model to was the Lyman-α data disk-resolved observations published in Feldman et al. 2000 and Feaga et al. 2009 (the latter paper was discussed here last year). This data was acquired using the Space Telescope Imaging Spectrograph (STIS) on Hubble between 1997 and 2001.At these far ultraviolet wavelengths, areas where Io's atmosphere are denser absorb sunlight, appear dark in Lyman-α images. Sunlight is better able to reach the surface and reflect back into space in order to be seen by Hubble. Thus, areas where Io's atmosphere is thinner appear brighter in Lyman-α images. An example of one of these images is shown at top. Gratiy et al. could not reproduce this data with their model, suggesting that they do not properly simulate the latitudinal variation in the column density of Io's atmosphere (thicker at the equator, thinner at the mid-latitudes and poles). In particular, they had difficulty reconciling the observed sharp increase in Lyman-α brightness, and therefore the sharp decrease in atmospheric column density, 45° North or South of the equator, and with the utter lack of atmosphere beyond 60° North or South latitude. This cutoff, the authors suggest, is more consistent with the distribution of surface changes and volcanic hotspots, as opposed to surface frost. However, there don't seem to be variations in the Lyman-α images resulting from known volcanic plumes. The lack of an east-west asymmetry on either side of the central meridian in Io's equatorial brightness in the far-ultraviolet data that would be expected from the Walker et al. model suggests that the surface thermal inertia is much lower than they used for that model.
In the final comparison, Gratiy et al. compared their model to disk-integrated millimeter-wave SO2 emission line profiles obtained at IRAM 30-meter telescope in Spain, published in Io After Galileo in the Io's Atmosphere chapter by Lellouch et al. and disk-resolved data in Moullet et al. 2008. The authors determined that strong atmospheric winds explain the wider SO2 emission lines in the IRAM data compared to what would be expected from thermal Doppler effects alone.
To be honest, this was a difficult paper for me to get through, hence why it took a month and a half for me to get this summary up. So, I apologize for not explaining the paper's conclusions as well as I could have. Basically, the authors hope that by comparing their rarefied gas dynamics model of Io's atmosphere with real observations they can make some improvements to that model that also provide new information about Io, such as the presence and strength of atmospheric winds, the surface thermal inertia, and the relative contribution of frost sublimation and volcanic plumes to Io's atmosphere.
Link: Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model [dx.doi.org]
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