Model representations of the interaction between the solar wind and the supersonic interstellar medium flow. prediction and interpretation of experimental data

Abstract

The planning and conducting of physical experiments requires the development of theoretical models capable either of predicting possible experimental data or explaining those already obtained. The processes taking place in the physical world can be understood only in terms of the close interaction between theory and experiment. Developing any quantitative or qualitative model of a physical phenomenon requires a mathematical apparatus, on the basis of which such models can be constructed. The branch of theoretical science using the methods of magnetohydrodynamics and hydroaeromechanics for studying space physics problems is usually called cosmic gasdynamics; it is mostly used in developing models of physical phenomena occurring under space conditions.

In order to emphasize the importance of cosmic gasdynamics in the development of astrophysics and space research, we will present several examples of models constructed by aerodynamicists. These models not only played an important role in qualitative predictions but are still being developed due to the need for the quantitative interpretation of the experimental data.

The solar corona was long thought to be a formation in a state of gravitational equilibrium (Chapman model). However, it turned out that the pressure at infinity obtained on the basis of this equilibrium solution is considerably greater than the estimated pressure in the interstellar gas surrounding the solar corona. In [1] it was concluded that in this case the solar corona gas must expand and a solution describing this expansion was obtained by invoking the steady-state hydrodynamics equations in the spherically-symmetric approximation. The solution of these equations led to the theoretical prediction of the solar wind, a radial flow of fully ionized hydrogen plasma issuing from the solar corona at a low subsonic velocity but already hypersonic at the Earth’s orbit. Subsonic-to-supersonic transition is ensured by solar gravitation which in this case plays the role of a convergent-divergent nozzle. Within a year, the theoretical prediction of the solar wind [1] was confirmed by its experimental detection [2] onboard the Soviet spacecraft Luna-2. It turned out that at the Earth’s orbit the mean velocity of the solar wind V _E ≈ 450 km·s⁻¹, the mean proton temperature T _E ≈ 6 · 10⁴ K (the electron temperature is somewhat higher), and the mean concentration of protons (and electrons) n _E ≈ 10 cm⁻³.

The first hydrodynamic model of the supersonic solar-wind flow past the Earth’s magnetosphere [3] was only qualitative, since it considered a flow past a plane magnetic dipole in the approximation of a thin layer between the bow shock and an “obstacle” embedded in the flow. However, it was constructed before the actual discovery of the solar wind and provided further important impetus to the development of models of the supersonic solar wind flow past planets with a detached shock.

One more example is furnished by the gasdynamicmodel of the solar wind flow past cometary atmospheres, first suggested in

In this work, a model of the interaction between the supersonic solar wind and the supersonic flow of the local, i.e., surrounding the Sun, interstellar medium is considered; it was first suggested in [6] in a much simplified formulation. This model has been actively developed in connection with the flights of the spacecraft Voyager 1 and 2, Ulysses, Hubble Space Telescope, SOHO, and others, exploring the outer regions of the solar system.