Lecturer: Andre Galli (UB) Date: 2006-10-12 10:00 Place: Aniara
Energetic Hydrogen and Oxygen Atoms Observed on the Nightside of Mars
Andre Galli
University of Berne
After almost 3 years in orbit the ASPERA-3 experiment on Mars Express has given us plenty of ion, electron and neutral particle data that need to be compared to theoretical models. Ion and neutral particle data, and recent UV limb emission measurements are consistent with each other; they indicate that the martian exosphere and ionosphere are much thinner than theoretically predicted for solar minimum conditions.
The talk will be restricted to measurements done with the neutral particle detector (NPD). The focus lies on measurements of energetic hydrogen and oxygen atoms (ENAs) observed on the nightside of Mars. We have derived energy spectra and total fluxes, and have compiled maps of hydrogen ENA outflow. The hydrogen ENA intensities reach 10$^{5}$ cm$^{-2}$ sr$^{-1}$ s$^{-1}$, but no oxygen ENA signals above the detection threshold of 10$^{4}$ cm$^{-2}$ sr$^{-1}$ s$^{-1}$ are observed. These intensities are considerably lower than most theoretical predictions. We explain the discrepancy as due to an overestimation of the charge-exchange processes in the models for which too high an exospheric density was assumed. Recent UV limb emission measurements, also done with NPD, point to a hydrogen exobase density of 10$^{10}$ m$^{-3}$ and a very hot hydrogen component, whereas the models were based on a hydrogen exobase density of 10$^{12}$ m$^{-3}$ and a temperature of 200 K predicted by Krasnopolsky and Gladstone [1996]. Finally, we estimate the global atmospheric loss rate of hydrogen and oxygen due to the production of ENAs.
s lies on measurements of energetic hydrogen and oxygen atoms (ENAs) observed on the nightside of Mars. We have derived energy spectra and total fluxes, and have compiled maps of hydrogen ENA outflow. The hydrogen ENA intensities reach 10$^{5}$ cm$^{-2}$ sr$^{-1}$ s$^{-1}$, but no oxygen ENA signals above the detection threshold of 10$^{4}$ cm$^{-2}$ sr$^{-1}$ s$^{-1}$ are observed. These intensities are considerably lower than most theoretical predictions. We explain the discrepancy as due to an overestimation of the charge-exchange processes in the models for which too high an exospheric density was assumed. Recent UV limb emission measurements, also done with NPD, point to a hydrogen exobase density of 10$^{10}$ m$^{-3}$ and a very hot hydrogen component, whereas the models were based on a hydrogen exobase density of 10$^{12}$ m$^{-3}$ and a temperature of 200 K predicted by Krasnopolsky and Gladstone [1996]. Finally, we estimate the global atmospheric loss rate of hydrogen and oxygen due to the production of ENAs.
