Monte Carlo Modelling of Hot Particle Coronae

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1 ling of Hot Particle Coronae H. Lichtenegger Space Research Institute, Austrian Academy of Sciences, Graz, Austria N2 Workshop, Helsinki/FMI Oct

2 Goal 3D Monte Carlo model of hot particle coronae of terrestrial planets Investigation of the evolution of the hot particle corona (in view of the interaction of the early solar wind with planetary atmospheres/coronae) 2

3 Neutral Atmosphere CO 2,O, O 2, N 2 Ions + Electrons CO 2+, O +, O 2+, N 2+, Major Source of Suprathermal Atoms in Upper Atmosphere: Dissociative Recombination of Molecular Ions Assumption: ions are in ground vibrational level O 2+ + e O( 3 P) + O( 3 P) ev br = 0.22 O 2+ + e O( 3 P) + O( 1 D) ev br = 0.42 O 2+ + e O( 1 D) + O( 1 D) ev br = 0.31 O 2+ + e O( 1 D) + O( 1 S) ev br = 0.05 [Kella et al., 1997] Flux of hot oxygen at the exobase Oxygen exosphere above exobase 3

4 Production Rate Production Rate of Oxygen Rate coefficient [Mehr & Bondi, 1969; Alge et al., 1983] O 2 + [Kim et al., 1998] Electron Temperature CO 2 + [Kim et al., 1998] [Fox, 2004] 4

5 Production Rate Production Rate of Oxygen A: O 2+ + e O( 3 P) + O( 3 P) ev B: O 2+ + e O( 3 P) + O( 1 D) ev C: O 2+ + e O( 1 D) + O( 1 D) ev D: O 2+ + e O( 1 D) + O( 1 S) ev Different Channels Different Excitation States O( 1 D ) A C B O( 3 P ) D O( 1 S ) 5

6 Energy Distribution Velocity/Energy Distributions of Reaction Products A: O 2+ + e O( 3 P) + O( 3 P) ev B: O 2+ + e O( 3 P) + O( 1 D) ev C: O 2+ + e O( 1 D) + O( 1 D) ev D: O 2+ + e O( 1 D) + O( 1 S) ev Velocity vectors are assumed to be isotropically distributed D C B 160 km altitude A T i = 3000K km altitude C B A D 6

7 Collisions Collision Probability Statistically averaged Oxygen elastic cross-section O( 3 P)-O( 3 P) l( z) = 1 n( z) σ ( E) [Kharchenko et al., 2000] n(z) : density of background gas Elastic cross-section O( 3 P)-N CO 2 2 σ(z) : collision cross-section [Balakrishnan et al. 1998] [Kim et al., 1998] 7

8 Collisions Energy Loss via Collisions elastic inelastic quenching elastic inelastic quenching Simple collision model R E : random number representing the collision type and effectiveness of energy transfer between colliding particles. Angular dependence of the scattering cross sections is ignored 8

9 Energy Distributions Energy Distribution at Exobase Without secondary atoms 230 km altitude Secondary atoms included 230 km altitude 9

10 Exosphere Exosphere Density N&C88 L&B91 Ip88 Energy dependent cross sections Different energy distribution of DR products Different treatment of collisions Effect of secondary atoms Smaller stepsize for determination of C p [Krestyanikova & Shematovich, 2006] Effect of 3D modelling 10

11 Number of Collisions von Zahn et el Hedin et al Seiff, 1983 (Noon) Sounder Probe Day Probe North Probe This model: High activity: F10.7=200 Medium: F10.7=110 Low: F10.7=68 Altitude, km Temperature, K Fig. 2: Number of collisions depending on the simulation as a function of the start altitude of the traced particles. The insert shows the observed and modelled Venus thermosphere temperatures for low, moderate and high solar activity conditions. 11

12 Fig. 3: Calculated hot oxygen density distribution. The (+) shows data points inferred from PVO OUVS airglow observations (Nagy et al., 1981), while the (*) correspond to PVO OUVS observations shown by Nagy et al. (1988). One should note that the figure shows only the hot O atom density. By adding the cold background component the total (cold + hot) O number densities below 400 km would be higher. 12

13 Improvements Implementation of correct cross-section for O-CO 2 collisions Consideration of angular dependence of the scattering cross sections (determines the rate of momentum transfer between hot particles and cool background gas; elastic scattering of suprathermal O atoms with energies < 5 ev is characterized by a distribution in small scattering angles [Kharchenko et al., 2000]) Inclusion of quenching collisions (large amount of atomic O in metastable states) 13

14 3D-Exosphere at Venus Density [cm -3 ] Fig. 5: Day and night exosphere oxygen density distribution of the 3D Monte Carlo simulation for high solar activity from 210 km up to 2000 km. 14