Theoretical studies of the solar wind phenomenon

This paper is a review of current theoretical topics concerning the solar wind. Broadly speaking the questions outstanding at the present time concern the loss of angular momentum to the sun, the origin of the fluctuations observed in the wind at the orbit of earth, conditions in the wind in regions yet unvisited by spacecraft (inside the orbit of Venus, beyond the orbit of Mars, and out of the plane of the ecliptic), conditions at the terminus of the wind, etc. The question of angular momentum loss is important in understanding the evolution of the sun to its present form with a slowly rotating surface. Evidence from both comet and spacecraft observations of the wind indicate that the rate at which angular momentum is being carried away by the solar wind is very large, of the order of 10³¹ dyne/cm in the gas flow and half as much by the interplanetary magnetic field. But theory cannot account for more than about 10³⁰ dynes/cm in the gas without special assumptions.The fluctuations presently observed in the wind at the orbit of earth have scales ranging upward from 10² km. Their presence is puzzling because fluctuations with scales less than about 10⁶ km are not expected to survive from the sun. Presumably, therefore, the fluctuations are generated by the velocity differences of more than 100 km/sec in the wind from different regions in the solar corona and by instabilities produced by the anisotropy of the electrons of the wind plasma.Conditions in the wind at places far removed from the orbit of earth can be inferred from the behavior of cosmic rays. The evidence is that the wind becomes relatively placid beyond about 5 AU, extending from there out to 30–300 AU without much small-scale turbulence. There are also some suggestions that the wind may perhaps be less turbulent toward the sun from 1 AU, and that the wind may be faster and more turbulent at higher solar latitudes. But the ambiguity of the situation does not permit a firm conclusion on this yet.