Charge-transfer Interactions between Solar Wind Protons and Neutral Particles in the Vicinity of the Sun

Abstract

BECAUSE of its large mean free paths interstellar matter near the solar system has a good chance of reaching within a few astronomical units of the Sun before undergoing collisions with solar protons flying radially outwards from the Sun. The trajectories of interstellar particles crossing the solar system are governed by the gravitational field of the Sun. The particles will be accelerated towards the centre of gravitation, but are also focused at the same time. Because of this effect, the particle density n of interstellar matter within the solar system will be larger then the density n₀ at infinity. The function n = n(r,θ) is dependent on the radial distance r from the Sun and the angle θ between the space-vector r and the velocity V₀ with which interstellar matter is approaching the solar system. This function n(r,θ) is given by¹ where p₀ is the impact parameter of the individual trajectory leading to the space-point (r,θ). The space dependent particle velocity v(r) has the form M being the solar mass and γ the gravitational constant. The function H = H (r,θ) and the constant β are given by Because every space-point is hit by two particle trajectories, one corresponding to the angle θ, the other to the angle 360−θ, the total density is given in the form Near the Sun, the ionizing effect of extreme solar ultraviolet radiation has to be taken into account. I shall assume that all the interstellar matter crossing the surface of a sphere of radius R_c = r_E/3 (ref. 1) centred on the Sun, where r_E is the Earth's radius, will become ionized. I shall therefore neglect the contributions to the density of trajectories which cross this sphere R_c before reaching an outer space-point. Thus the density profile along the orbit of the Earth shows a semiannual variation, having minima at June 22 and December 22, if we identify v₀ and v_s as the velocity of the Sun with respect to the local standard of rest (v_s = 20 km s⁻¹.)