This section is devoted to the derivation of a priori estimates of

. We begin with the observation that the total mass is conserved. Moreover, if we multiply (1.3), (1.4), and (1.5) by

, and

, respectively, and sum up the resulting equations, we have by using (1.2) that

Integrating (1.2) and (2.1) over
, we arrive at our first lemma.

Lemma 2.1.

For any

, one has

where

is the nonnegative function defined by

The next lemma gives us an upper bound of the density
, which is crucial for the proof of Theorem 1.1.

Lemma 2.2.

For any
,
holds.

Proof.

Notice that (1.3) can be rewritten as

from which and (2.4), we find that

satisfies

In view of Lemma 2.1 and (2.6), we have by using Cauchy-Schwarz's inequality that

Letting

denote the material derivative and choosing

, we obtain after a straightforward calculation that

which, together with (2.8), yields Lemma 2.2 immediately.

To be continued, we need the following lemma because of the effect of magnetic
.

Lemma 2.3.

The magnetic field

satisfies the following estimates:

Proof.

Multiplying (1.5) by

and integrating over

, we have

where we have used Cauchy-Schwarz's inequality, Lemma 2.1, and the following inequalities:

Since
because of Lemma 2.1, we thus obtain the first inequality indicated in this lemma from (2.11) by applying Gronwall's lemma and then Sobolev's inequality.

To prove the second part, we multiply (1.5) by

and integrate the resulting equation over

to deduce that

where we have used Cauchy-Schwarz's inequality, Sobolev's inequality (2.12), Lemma 2.1, and the first part of the lemma. This completes the proof of Lemma 2.3.

Lemma 2.4.

The following estimates hold for the velocity

:

Proof.

Multiplying (1.3) by

and then integrating over

, by Young's inequality we obtain

It follows from (1.2) and (1.3) that

Thus, inserting (2.16) into (2.15), and integrating over

, we see that

where the terms on the right-hand side can be bounded by using Lemmas 2.1–2.3 as follows:

Therefore, taking

appropriately small, we conclude from (2.17)–(2.20) that

where, combined with the fact that

due to Lemma 2.1, we obtain the first part of Lemma 2.4 by applying Gronwall's lemma and then Sobolev's inequality. Similarly, multiplying ((1.4) by

and integrating the resulting equation over

, we get that

where we have also used Cauchy-Schwarz's inequality. Integration of (2.22) over

gives

where Lemmas 2.1–2.3 and the first conclusion of this lemma have been used. Therefore, from the above inequality we obtain the second part, and so Lemma 2.4 is proved.

Notice that (1.3), ((1.4) can be written as follows:

where

and

. Thus, by Lemmas 2.1–2.4, we see that

which immediately implies

Hence, we have the following lemma.

Lemma 2.5.

There exists a positive constant

, such that

where
and
.

To prove the uniqueness of strong solutions, we still need the following estimates.

Lemma 2.6.

The pressure

satisfies

. Furthermore, if the compatibility conditions (1.9), (1.10) hold, then

Proof.

It follows from the continuity equation (

1.2) that

satisfies

which, differentiated with respect to

, leads to

Multiplying the above equation by

and integrating over

, we deduce that

where we have used the inequality

which follows from the definition of

. Therefore, applying the previous Lemmas 2.1–2.5 and Gronwall's lemma, one has

which proves the first part of the lemma.

We are now in a position to prove the second part. We first derive the estimate for the longitudinal velocity

. To this end, we firstly rewrite (1.3) as

Differentiation of (2.34) with respect to

gives

which, multiplied by

and integrated by parts over

, yields

On the other hand, by virtue of (1.2) we have

from which and (2.36) we see that

Using the previous lemmas and Young's inequality, we can estimate each term on the right-hand side of (2.38) as follows with a small positive constant

:

Putting the above estimates into (2.38) and taking

sufficiently small, we arrive at

so that, using the relation between

and

again, one infers from (2.40) that

where the first term on the right-hand side of (2.41) is integrable on

due to the previous lemmas. Thus, integrating (2.41) over

, we deduce from (1.3) that

Letting

and using the compatibility condition (1.9), we easily obtain from (2.42) that

which, together with

and Gronwall's lemma, immediately yields

In a same manner as that in the derivation of (2.44), we can show the analogous estimate for the transverse velocity
by using the previous lemmas, (2.44), and the compatibility condition (1.10) as well. Thus, we complete the proof of Lemma 2.6.

Remark 2.7.

From the a priori estimates established above, one sees that the compatibility conditions are used to obtain the second part of Lemma 2.6 only. However, this is crucial in the proof of the uniqueness of strong solutions.