Upper limits for the Martian exospheric number density during the Planet B/Nozomi mission

Dissociative recombination (DR) of ionospheric O-2(+) ions is an important source of suprathermal atomic oxygen in the exosphere as previous studies a bout the Martian upper atmosphere have shown. Because of the weaker gravita tional attraction a hot oxygen corona on Mars should be denser than that ob served on Venus. Since the most important mechanism for the production of t he hot oxygen atoms in the Martian exosphere is DR, we investigated the var iability of this production mechanism depending on solar activity. The Japa nese Nozomi spacecraft will have the possibility to detect with the neutral mass spectrometer (NMS) for the first time in-situ the theoretically predi cted hot oxygen corona on Mars, if the corona number density above the cold background atmosphere is of the order of 10,000 cm(-3). Due to a problem i n the propulsion system Nozomi failed its planned arrival rendezvous with M ars in October 1999 and will, therefore, arrive at the red planet not befor e January 2004. Solar activity will reach its maximum in 2001, so the relat ed production rate of hot oxygen atoms will be in the medium range during t he new arrival date of Nozomi. We used the ionospheric profiles from the Vi king mission for low solar activity conditions (F-10.7 approximate to 70) a nd the Mariner 9 mission with a solar activity of about 120 for medium sola r wind activity. The latter is comparable to the level we expect for the Ma rs arrival of Nozomi. The resulting influence of the hot oxygen corona numb er density distribution was calculated with a Monte Carlo technique. This t echnique is used to compute a hot particle density distribution function. W e studied the atomic diffusion process in the Martian atmosphere by simulat ing the collision probability, particle direction and energy loss after col lisions by generating random numbers. Compared to previous studies we have improved the Monte Carlo model by using more and smaller altitude steps and more detailed treatment of particles with a temporary downward motion. Thi s has resulted in an increased amount of collisions and a shift to lower en ergies in the energy spectrum. Our results show that the hot oxygen compone nt should begin to dominate above the cold background atmosphere at an alti tude of about 500 km above the Martian surface. The NMS instrument on board of Nozomi should detect the hot oxygen component after its arrival at Mars in January 2004, at an altitude of about 600 km above the Martian surface. Since the solar activity will decrease during the mission the measurements during the first orbits will be the most significant ones. The first in-si tu measurements of the hot oxygen number density would be very important fo r adjusting atmospheric escape models by separating ballistic, satellite an d escape trajectories of the hot oxygen atoms, which are significant for st udies of the evolution and solar wind interaction of the Martian atmosphere . (C) 2000 Elsevier Science Ltd. All rights reserved.