Monte Carlo simulation ofDregion sampling

Abstract

Monte Carlo calculations were made of the aerodynamic properties of rocket payload designs that might be suitable for measuring the composition of the D region of the ionosphere. A conical design with the sampling orifice at its tip was used for most of the calculations. Variations were made in the altitude parameters, the cone angle, and the angle of attack. The molecules in the computations were classified as undisturbed free-stream molecules or as molecules that collided with the surface of the payload or with another molecule that had previously collided with the payload. This classification enabled determination of the fraction of the gas entering the sampling orifice that might have been changed from its ambient state by the influence of the payload. The results indicate that a cone half-angle of _<25 ø would result in >_90% of the sample being undisturbed free-stream molecules representative of the composition of the ambient atmosphere. For the blunt configuration used for D region measurements so far, only about 8% of the sample would be certain to be representative of the composition of the ambient atmosphere. Much information has been obtained about the properties of the earth's ionosphere, and the processes that occur there are quite well understood, especially in the higher regions. However, the lowest ionospheric region, the D layer (about 60-to 90-kin altitude), has not been thoroughly investigated [Thomas, 1971]. Obtaining in situ composition data there is difficult owing to the .relatively large air density in the D region, which complicates both the instrumeptation and the interpretation of the data. The instrumentation must include a vacuum pump in the rocket, if mass spectrometers proven at higher altitudes are to be used, and a payload design with suitable aerodynamic flow properties. The interpretation of the data must include possible changes in the relative measurement sensitivity with altitude, such as might result from a change in shock wave thickness, as well as the relation of the measurements to the actual ambient conditions [Hoult, 1964; Golubev et al., !970]. At altitudes above 120 km these factors can be calculated by the free-molecule theory; below 70 km, continuum theories can be used [Sonin, Copyright ¸ 1972 by the American Geophysical Union. 1967; Hoult, 1965]. However, in the D region, transition flow conditions generally prevail, the velocity distribution of the molecules at a point in the flow differing significantly from an equilibrium Maxwell-Boltzmann distribution. There is reason to believe that continuum equations can not accurately describe such flow conditions [Vogenitz et al., 1968; Vogenitz and Takata, 1972]. A very successful method of describing flows in the transition regime is the Monte Carlo direct simulation tech•nique, a probabilistic numerical experiment in which the motion of a representative set of molecules flowing past a body is followed by digital computation and collisions in the'gas are computed by statistical sampling. A considerable virtue of this te...