Reduction of rocket-borne atmospheric density measurements using Monte Carlo simulation.

摘要

The HEX II atmospheric rocket experiment, conducted near 9:30 U.T. February 14, 2007 from the Poker Flat Rocket Range in Alaska was designed to lead to a better understanding of density and velocity gradients in the lower thermosphere. In a supporting role to this mission, cold cathode ionization gauges recorded composite density along three rocket payload trajectory paths from 90 km to 200 km. Rocket-borne instruments experience near sonic to hypersonic velocities and encounter a range of viscous, slipflow, and collisionless flow conditions at these heights. These variable conditions necessitate the use of tailored analytic expressions appropriate to the flow conditions, requiring increasingly unacceptable levels of approximation beyond their applicable range. In contrast, Direct Monte Carlo-based simulations (DSMC) reproduce the full behaviour of a gas through direct representation of gas molecules and their kinetics by a matrix of state variables, allowing accurate prediction of evolving macroscopic gas characteristics under a wide variety of mean free path conditions.;The HEX II ion density measurements are corrected for atmospheric ram and wake effects using G.A. Bird's DSMC DS3VD software and a simulated payload in a model atmosphere. Density measurements were likewise corrected through the use of analytic expressions appropriate to either viscous or collisionless flow. The ambient atmospheric densities reduced by the DSMC simulation factors are maximum +4.5%/- 5.7% uncertain, instrument error excluded.;However, DSMC reduced ambient densities near 115 km were outside the bounds of estimated model atmosphere variation(+200%/-50%). Other sources of error, such as instrument and calibration uncertainty, must be not only identified but also accounted for to improve these results. DSMC ram reduction factors were typically only +/- 2% different from collisionless analytic factors above 120 km, wake regions excluded. Therefore, less costly analytic ram factors may be used in place of DSMC simulations for a carefully designed instrument chamber within the collisionless region, typically above 120 km.