Doubly charged positive ions (dications) are an important component of planetary ionospheres because of the large energy required for their formation. Observations of these ions are exceptionally difficult due to their low abundances; until now, only atomic dications have been detected. The Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made on board the recent Mars Atmosphere and Volatile Evolution mission provide the first opportunity for decisive detection of molecular dications, CO2++ in this case, in a planetary upper atmosphere. The NGIMS data reveal a dayside averaged CO2++ distribution declining steadily from 5.6 cm−3 at 160 km to below 1 cm−3 above 200 km. The dominant CO2++ production mechanisms are double photoionization of CO2 below 190 km and single photoionization of CO2+ at higher altitudes; CO2++ destruction is dominated by natural dissociation, but reactions with atmospheric CO2 and O become important below 160 km. Simplified photochemical model calculations are carried out and reasonably reproduce the data at low altitudes within a factor of 2 but underestimate the data at high altitudes by a factor of 4. Finally, we report a much stronger solar control of the CO2++ density than of the CO2+ density .
Solar Wind (SW) electron precipitation is able to deposit a substantial amount of energy in the nightside Martian upper atmosphere, potentially exerting an influence on its thermal structure. This study serves as the first investigation of such an issue, with the aid of the simultaneous measurements of both neutral density and energetic electron intensity made on board the recent Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. We report that, from a statistical point of view, the existing measurements do not support a scenario of noticeable neutral heating via SW electron precipitation. However, during 3%−4% of the MAVEN orbits for which data are available, strong correlation between nightside temperature and electron intensity is observed, manifested as collocated enhancements in both parameters, as compared to the surrounding regions. In addition, our analysis also indicates that neutral heating via SW electron precipitation tends to be more effective at altitudes below 160 km for integrated electron intensities above 0.01 ergs·cm−2·s−1 over the energy range of 3−450 eV. The results reported here highlight the necessity of incorporating SW electron precipitation as a heat source in the nightside Martian upper atmosphere under extreme circumstances such as during interplanetary coronal mass ejections.