Heating of the Solar Wind

Abstract

Examination of observations by Explorer 34 of the bulk speed, temperature, and speed gradients in the solar wind indicate the properties of the solar wind to be highly variable. It is shown that the temperature and speed when averaged over periods of 3 hours are connected by the relation sqrt(T)=(0.036+/- 0.003)V-(5.54+/1.50). Other measurements are used to indicate that this relation may be valid at all epochs of the solar cycle. Although one-fluid models are not quantitatively consistent wiht this relation, the two-fluid theory of Sturrock and Hartle gives a consistent temperature corresponding to the very quiet time (low speed) which it represents. Observations at the bulk-speed gradients show heating of the plasma to a temperature above that predicted by the above equation at positive gradients and an abscence of additional heating at negative gradients. Most such hot spots are associated with large positive bulk-speed gradients. The dependence of the heating effect upon the sign of the speed gradient indicates that the heating takes place as a result the collision of plasma streams rather than as a result of Kelvin Helmholtz instability. Neither this heating effect nor that due to shocks is a dominant process in raising the proton temperature on a large scale. An attractive means for reproducint the relation shown in the above equation may be by heating close to the Sun (R < 50 Rsun) as a result of damping of hydrodynamic waves by Barnes's mechanism. Support for this hypothesis is found from measurements of the ratio of helium temperatures to hydrogen temperatures, which indicate a value close to 4, consistent with hydromagnetic heating for the wind as a whole.