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Figure 6. Simulated spectra for a closed source (top) and open source (bottom) INMS measurement. These spectra were created using the total cross sections from Kanik et al. [1993], Krishnakumar and Srivastava [1988; 1990], Harrison et al. [1966], and Tawara and Kato [1987]; branching ratios from the NIST library; absolute calibration from the INMS flight model; and neutral densities at 950 km from the Toublanc et al. [1995] model neutral atmosphere. The spectra show the spectral position and concentration of the various cracking products resulting from the electron impact ionization of the sampled neutral species that takes place in the ion source. Masking from dissociative ionization of the primary species renders some constituents, such as C2H4, HCN, and CH3, hard to detect. In such cases, however, it is possible that the neutral densities can be deduced from ion densities measured with CAPS and INMS, allowing important chemical pathways to be distinguished by which, for example, HCN and polyacetylenes are formed in the upper atmosphere. Closed source measurements will be used to determine the density of N2 as a function of altitude, from which the scale height and thus the thermal structure of the upper atmosphere can be derived.