6 - Structure and perturbations

Summary

The researcher looked round him once more:

and now the Facts accumulated in such bewildering profusion, that the Theory was lost among them.

Lewis Carroll, Sylvie and Bruno

In order to concentrate on the basic physics of the solar wind acceleration, we considered in Chapter 5 a spherically symmetric and stationary problem, with a radial magnetic field. Unfortunately, the solar wind is more complicated. We now introduce some of these complications, trying however to keep a bias towards basics. More details may be found in the books, with some updates in and. Most of this chapter considers the wind having already been accelerated to a large velocity, which we note vw.

Basic large-scale magnetic field

Parker's spiral

Figure 6.1 (left) reminds us of the geometry considered in the previous chapter: a radially expanding solar wind with a radial magnetic field. This is an application of the frozen-in magnetic field concept (see Section 2.3). Since the magnetic Reynolds number is extremely large, any magnetic flux tube in the steady plasma flow will hold the same fluid parcels later on, so that the magnetic field lines are dragged by the flow and tend to be aligned with the radial flow lines. Conservation of the magnetic flux then yields a radial magnetic field varying as B ∝ r^-2.