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Abstract. We have produced a computer assisted tomography program that optimizes a three-dimensional model to fit observational data. We have used this program with interplanetary scintillation data from Nagoya, Japan, and Cambridge, England. The program iterates to a least squares solution fit of observed data using solar rotation and solar wind motion to provide perspective views of each point in space accessible to the observations. We plot the optimized model as Carrington maps in velocity V and density N e for the two data sets with resolutions of 10 ø in hellographic longitude and latitude. We map the model to 1 AU and compare this to in situ observations from the IMP spacecraft. Tomography methods can be used to improve upon the inherent averages made in the point P approximation used to display heliospheric data. IPS observations covering a large range of solar elongations and obtained over an extended period of time provide a global view of the inner heliosphere. The multiple perspectives required for tomographic analysis are provided by both solar rotation and outward solar wind motion. Effects due to the evolution of solar structures as they rotate during the period of observation can be minimized by selecting observations from a quiet part of the solar cycle. From this comparison we find12,049

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Abstract. Mercury's sodium atmosphere is known to be highly variable both temporally and spatially. During a week-long period from November 13 to 20, 1997, the total sodium content of the Hermean atmosphere increased by a factor of 3, and the distribution varied daily. We demonstrate a mechanism whereby these rapid variations could be due to solar wind-magnetosphere interactions. We assume that photon-stimulated desorption and meteoritic vaporization are the active source processes on the first (quietest) day of our observations. Increased ion sputtering results whenever the magnetosphere opens in response to a southward interplanetary magnetic field (IMF) or unusually large solar wind dynamic pressure. The solar wind dynamic pressure at Mercury as inferred by heliospheric radial tomography increased by a factor of 20 during this week, while the solar EUV flux measured by the Solar EUV Monitor (SEM) instrument on board the Solar and Heliospheric Observatory (SOHO) increased by 20%. While impact vaporization provides roughly 25% of the source, it is uniformly distributed and varies very little during the week. The variations seen in our data are not related to Caloris basin, which remained in the field of view during the entire week of observations. We conclude that increased ion sputtering resulting from ions entering the cusp regions is the probable mechanism leading to large rapid increases in the sodium content of the exosphere. While both the magnitude and distribution of the observed sodium can be reproduced by our model, in situ measurements of the solar wind density and velocity, the magnitude and direction of the interplanetary magnetic field, and Mercury's magnetic moments are required to confirm the results.