In the last chapter we have seen that if a star has an open magnetic field in the equatorial region and is also rapidly rotating, a very strong stellar wind can be produced. In this chapter we consider the effects of the magnetic field in absence of rotation. If oscillations are induced in the field at the base of the wind, transverse ‘Alfvén’ waves will be generated. The dissipation of energy and momentum associated with the wave propagation can lead to the acceleration of the outer atmosphere in the form of an ‘Alfvén wave driven wind’. Open field regions can arise in a variety of configurations, depending on the circulation currents or dynamo properties of the interior of the star. Furthermore, the strength and geometry of the magnetic field can vary significantly from one location on the star to another, and the wind flow tubes will vary accordingly.
In the absence of a magnetic field, a star that has a spherically symmetric hot corona will produce a steady, radial, structureless wind, driven by the thermal gas pressure gradients in the corona (Parker, 1958), as discussed in Chapter 5. Within a few years after the solar wind was predicted by Parker, interplanetary space probes proved that indeed there is a wind from the sun that occurs at all times. However, the wind was found to be far from steady and structureless. To understand the spatial and temporal variability of the wind, Parker (1965) considered outflow in open magnetic field structures.