Now consider the usual solar limit with so that
(2.29) simplifies to (2.31), namely

The

** Alfvén's Theorem**. `` In a perfectly conducting fluid (
), magnetic field lines move with the fluid: the field lines
are ` frozen' into the plasma. "

This theorem states that motions along the field lines do not change the field but motions transverse to the field carry the field with them.

** Proof**. The proof will be developed in several steps. Firstly, we need to
make use of
Gauss's divergence theorem, which is

where is a

where is a closed curve around the

(i) From
, for all time, we may integrate
over the volume of the plasma to deduce

(ii) Next we consider the time behaviour of the * magnetic flux*,
, through a closed curve C, around an * open* surface .

(iii) Consider how the curve moves with the fluid motion to the curve in the time interval .

The motion of the surface enclosed by curve to the surface enclosed by curve generates a volume enclosed by the surface as depicted in Figure (2.3). The volume is enclosed by the closed surface that is made up of the top surface, enclosed by , the bottom surface enclosed by and the sides.
(iv) Consider the total flux through the closed surface in (iii).
At time , when the magnetic field is
, we have from (2.37)

(v) Consider the contribution to the total flux from the curved side.

A small element of length on the curve traces out the shaded region in Figure (2.5). Then is given by the outward normal, , times the area of the shaded region. This area is approximately the area of the parallelogram with sides and . Hence, on the sideThus, from (2.39) we have

Hence,

so that the flux through the curve , at time , is equal to the flux through the curve minus the contribution from the sides.

(vi) How does change in time? This is simply the difference
between the value of at time and at time
. Thus, the change in flux is

If is small then we can approximate the integrand in the surface integral, by . Hence,

Here we have used the vector identity

(vii) The final step in the proof is to use the induction equation,
for , i.e. (2.31). In this limit we have,

Thus, (2.43) becomes,

As we have

Thus, we reach the conclusion that does not change in time and so

where is any closed contour moving with the fluid. The magnetic lines of force are

The same fluid occupies the interior of the flux tube at time as did at time the earlier time . If the area of the flux tube is small, then the field strength will be approximately constant across the area of the tube and we obtain the important result that

so that

Therefore, if the area, , is reduced by the fluid motion then the field strength, , becomes stronger.