A Revised Sensitivity Model for Cassini INMS: Results at Titan

Abstract

Cassini Ion Neutral Mass Spectrometer (INMS) measurements from roughly a hundred Titan encounters over the Cassini mission yield neutral and ion densities systematically lower, by factors approximately 2 to 3, than estimates from several other spacecraft systems, including the Attitude and Articulation Control System, and Navigation system. In this paper we present a new INMS instrument sensitivity model, obtained by re-analyzing (1) the capture and transmission of neutral gas through the instrument, and (2) the detector gain reduction during pre-launch testing. By correcting for an under-estimation of gas leakage out of the instrument into space by the original calibration model, and adjusting for the gain change, the new model brings INMS densities into much closer agreement with the other Cassini systems. Accordingly, the INMS ion densities are revised upward by a constant detector sensitivity correction factor of 1.55 ± 21 %, while the neutral sensitivities have a complex instrument pointing direction dependence, due (mostly) to the effect of the INMS vent and antechamber-to-closed source tube. In the special case of on-ram pointing the neutral densities are revised upward by a constant factor of 2.2 ± 23 %. The corrected neutral and ion sensitivities given here are applicable to all previously published INMS results at Titan, Enceladus and elsewhere in the Saturn system. The new model gives reliable densities at high ram angles, in some cases above 90 degrees, thereby expanding the list of Titan flybys from which INMS densities may be extracted. We apply the model to obtain accurate densities from several off-ram Titan flybys which gave unusual neutral density vs. altitude profiles, or unreasonably high densities, with the original calibration.

Cassini INMS measurements of Enceladus plume density

“…Early comparisons between INMS measurements of the density of Titan's atmosphere to navigation, showed INMS to be a factor of approximately three less sensitive than the value measured during ground calibration. As reported in Teolis et al (2015), that discrepancy is a factor of 2.2 ± 23%, caused by venting within the INMS, and included in the calibration factors in Section 3.…”

“…Based on ground (Waite et al, 2004) and flight calibration at Titan (Teolis et al, 2015), each INMS count during E7, E14, E17, and E18, corresponds to 2100 molecules/cm 3 for the 28-u mass channel and 1500 molecules/cm 3 for the 44 u channel. The next section discusses calibration uncertainty, which is 25%.…”

Solar cycle variations in ion composition in the dayside ionosphere of Titan

“…The N 2 + empirical production rate is given by (with a small correction factor very close to unity) [ Richard et al ., ]: where P stands for the production rate, is the reaction rate between methane and CH 3 + , and quantities in the brackets are the INMS measured densities. The empirical production rates of Richard et al [] and Sagnieres et al [] are very similar, differing slightly due to different INMS calibration factors, which until recently were in a state of flux [ Teolis et al ., ].…”

“…Figure shows the density profiles of N 2 and CH 4 , from 33 flybys, listed in Table . Data points with ram angle greater than 30° are filtered out as high ram angles may cause inaccurate measurements by the instrument [ Teolis et al ., ]. Data points are color coded by the F 10.7 index, and the colorbar to the right shows the corresponding values.…”

Suprathermal Electrons in Titan’s Sunlit Ionosphere: Model–observation Comparisons

“…While using as default the correction factor 2.9, we present in Section 3 also P e,EI,M /P e,EI,O ratios obtained from the use of a calibration factor of 2.1 (slightly reduced from the value of 2.2 suggested by Teolis et al 2015). A decrease of the utilized number density profiles by a multiplicative factor of 2.1/2.9 changes CAPS/ELS-derived electron production rates, P e,EI,O , by the same multiplicative factor.…”