Memoirs on Differential Equations and Mathematical Physics Volume 44, 2008, 23–44

Jamel Benameur and Ridha Selmi

ANISOTROPIC ROTATING MHD SYSTEM IN CRITICAL ANISOTROPIC SPACES

Abstract. The three-dimensional mixed (parabolic-hyperbolic) nonlin-
ear magnetohydrodynamic system is investigated in the whole space R^{3}.
Uniqueness is proved in the anisotropic Sobolev spaceH^{0,}^{1}^{2}. Existence and
uniqueness are proved in the anisotropic mixed Besov–Sobolev spaceB^{0,}^{1}^{2}.
Asymptotic behavior is investigated as the Rossby number goes to zero.

Energy methods, Freidrichs scheme, compactness arguments, anisotropic Littlewood–Paley theory, dispersive methods and Strichartz inequality are used.

2000 Mathematics Subject Classification. 35A05, 35A07, 35B40.

Key words and phrases. Mixed (parabolic-hyperbolic)M HDsystem, existence, uniqueness, critical spaces, Strichartz inequality, asymptotic be- havior.

R^{3}

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H^{0,}^{1}^{2}

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B^{0,}^{1}^{2} ^{} ^{} ^{} ^{} ^{!} ^{} ^{} ^{"} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{} ^{$} ^{} ^{} ^{} ^{%} ^{!} ^{} ^{}

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Anisotropic Rotating MHD System in Critical Anisotropic Spaces

1. Introduction

This paper deals with an incompressible mixed magnetohydrodynamic system with anisotropic diffusion with the small in the limit Rossby number.

Namely, we consider the following system denoted by (M HD^{ε}_{ν}_{h}):

∂tu−νh∆hu+u· ∇u−b· ∇b+1 ε∂3b+1

εu×e3=−∇p in R^{+}×R^{3}

∂tb−νh∆hb+u· ∇b−b· ∇u+1

ε∂3u= 0 in R^{+}×R^{3}
divu= 0 in R^{+}×R^{3}

divb= 0 in R^{+}×R^{3}
(u, b)|t=0= (u0, b0) in R^{3},

where the velocity fieldu, the induced magnetic perturbation b and pare unknown functions of time t and the space variable x = (x1, x2, x3) = (xh, x3),e3is the third vector of the Cartesian coordinate system andνh is a positive constant which represents both the cinematic viscosity and the magnetic diffusivity. ∆hdenotes the horizontal Laplace operator defined by

∆h=∂_{1}^{2}+∂_{2}^{2}andεis a small positive parameter destined to go to zero. It
is clear that the system is hyperbolic with respect to the directionx3 called
the vertical direction. About the physical motivations, we refer the reader
to [3] and references therein.

If we denote byU = (u, b), thenU is a solution of the following abstract system:

(S^{ε})

∂tU+a2,h(D)U +Q(U, U) +L^{ε}(U) = (−∇p,0) in R^{+}×R^{3}
divu= 0 in R^{+}×R^{3}

divb= 0 in R^{+}×R^{3}
U|t=0=U0 in R^{3},

where the quadratic term, the linear perturbation and the viscous term are respectively defined by

Q(U, U) =

u· ∇u−b· ∇b u· ∇b−b· ∇u

,
L^{ε}(U) = 1

εL(U) = 1 ε

∂3b+u×e3

∂3u

and

a2,h(D)U =−νh∆hU.

In the isotropic case, that is, when the global Laplace operator is taken
instead of the horizontal one, some isotropic magnetohydrodynamic systems
were studied by several authors ([1], [2], [11]). However, according to our
knowledge, the first paper dealing with the anisotropic case is due to the
authors in [3]. It deals with existence, uniqueness in H^{0,s} for s > ^{1}_{2} and
asymptotic behavior of the solution as ε → 0 for the same (M HD_{ν}^{ε}_{h}).

Nevertheless, in [10], the author studied the case of anisotropic pure fluid

J. Benameur and R. Selmi

and proved the uniqueness in the critical Sobolev space and both existence and uniqueness in the anisotropic Besov–Sobolev space.

In this paper, we extend those results to the rotating (M HD^{ε}_{ν}_{h}) system,
which presents the difficulty to be coupled in a nonlinear way. In addition,
as the considered system is a perturbed one, it is quite natural to ask about
the asymptotic behavior of the solution as the Rossby number εtends to
zero. We note that this perturbation presents the difficulty of being singu-
lar. Precisely, we establish uniqueness results in H^{0,}^{1}^{2}(R^{3}) and B^{0,}^{1}^{2}(R^{3}),
uniform local existence for arbitrary initial data and global existence for
small initial data inB^{0,}^{1}^{2}(R^{3}). Moreover, we establish a convergence result
asε→0.

Let us first say that H^{0,}^{1}^{2}(R^{3}) is the space of regularity L^{2}(R^{2}) in xh

andH^{1}^{2}(R) inx3, andB^{0,}^{1}^{2}(R^{3}) is alsoL^{2}(R^{2}) inxh butB_{2,1}^{1}^{2} (R) inx3. As
in [7], forH^{1}^{2}(R^{3}) in the case of Navier–Stokes equation, we say here that
H^{0,}^{1}^{2} andB^{0,}^{1}^{2} are critical spaces for the system (S^{ε}). This means that they
are invariant by the following scaling of (S^{ε}): ifU(t, x) is a solution of (S^{ε})
with the data U0(x), thenUλ(t, x) =λU(λ^{2}t, λx) is also a solution of (S^{ε})
with the dataλU0(λx).

The main idea is to use the structure of the convection operator together
with the incompressibility condition to compensate the lack of information
due to the incomplete diffusion operator that describes the anisotropy ef-
fect. This very fine analysis is performed with the help of Littlewood–Paley
decomposition in order to deal with scale invariant spaces such as H^{0,}^{1}^{2}
andB^{0,}^{1}^{2}.

The uniqueness result in the anisotropic homogenous Sobolev spaceH^{0,}^{1}^{2}
is dealt with by the following theorem:

Theorem 1. The system (M HD^{ε}_{ν}_{h}) has at most one solution U^{ε} such
thatU^{ε}∈L^{∞}_{T} (H^{0,}^{1}^{2}(R^{3}))and∇hU^{ε}∈L^{2}_{T}(H^{0,}^{1}^{2}(R^{3})).

The proof of this theorem is partially based on a technical lemma inspired
from [10] and adapted here for the case of (M HD^{ε}_{ν}_{h}). By this lemma, we
establish a doubly logarithmic estimate for the H^{0,−}^{1}^{2} norm of W^{ε}, the
difference of two solutions, and we use Osgood lemma to finish the proof.

ThoughW^{ε}belongs toH^{0,}^{1}^{2}, it will be estimated inH^{0,−}^{1}^{2}. This is due to
the fact that the equation satisfied byW^{ε}is hyperbolic in the variablex3.
As in [10], we are not able to establish existence inH^{0,}^{1}^{2} but only unique-
ness. This is due to the noninclusion of H^{0,}^{1}^{2} in L^{∞}_{v} (L^{2}_{h}). Such inclusion
holds forB^{0,}^{1}^{2}(R^{3}) and plays an essential role in proving the existence result.

In order to state such result, we introduce Besov type spaces that take into
account Lebesgue regularity in time on the dyadic blocs. These spaces are
denoted forp≥1 byLf^{p}_{T}(B^{0,}^{1}^{2}) and defined as in [6] by

kuk_{L}_{f}p

T(B^{0,}^{1}^{2})=X

q∈Z

2^{q}^{2}k∆^{v}_{q}ukL^{p}([0,T];L^{2}(R3)).

Anisotropic Rotating MHD System in Critical Anisotropic Spaces

In the case of critical anisotropic Besov–Sobolev spaceB^{0,}^{1}^{2}, existence and
uniqueness results are given by the following theorem:

Theorem 2. LetU0 = (u0, b0) ∈ B^{0,}^{1}^{2}(R^{3}) be a divergence free vector
fields. There exists a positive time T such that for allε >0there exists a
unique solution U^{ε} of (M HD^{ε}_{ν}_{h}), where U^{ε}∈gL^{∞}_{T} (B^{0,}^{1}^{2}(R^{3}))with ∇hU^{ε}∈
Lf^{2}_{T}(B^{0,}^{1}^{2}(R^{3}))and satisfies the following energy estimate:

kU^{ε}k_{L}_{g}^{∞}

T(B^{0,}^{1}^{2}(R3))+√

ν_{h}k∇^{h}U^{ε}k_{L}_{f}2

T(B^{0,}^{1}^{2}(R3)) ≤√

2kU0k_{B}^{0,}^{1}_{2}. (1)
Moreover, if the maximal time of existenceT^{∗} is finite, then

kU^{ε}k_{L}_{g}^{∞}

T∗(B^{0,}^{1}^{2}(R^{3})) = +∞, (2)

and if there exists a constant c such that kU0k_{B}^{0,}^{1}_{2}_{(}R3) ≤ cνh, then the
solution is global.

We use Friedrichs’s scheme to prove global in time existence result in
L^{2}(R^{3}). To establish global in time existence result in B^{0,}^{1}^{2}(R^{3}), we use
again Friedrichs’s scheme and Littlewood–Paley theory. A suitable re-
arrangement of the nonlinear term allows to apply a technical lemma due
to [10]. Then, absorption techniques yield an estimate of the approximate
solution. Using standard compactness argument, we finish the proof. To
prove local in time existence result in B^{0,}^{1}^{2}(R^{3}), we decompose the initial
data into low and high frequency parts. The low frequency part will be the
initial data of a linear problem and the high one will be the initial data of
the remainder which is nonlinear. For the former, classical arguments give
explicitly the result. For the latter, Littelwood–Paley theory, and especially
Bony decomposition, plays a crucial role for estimation of the nonlinear
part. The target is to establish an estimate where the norm of the solution
will be bounded by an expression that depends on the life span T of the
solution and the high frequency part of the initial data. Thus, since we are
looking for a local in time result, we can choose, in the appropriate order,
the cut-off integerN as big as needed andT as small as needed. This is the
idea behind the frequency decomposition of the initial data. We note that
the uniform life span of the solution will not depend, as usual, on the norm
of the initial data but only on its frequency repartition.

Concerning the asymptotic behavior of the solution as the Rossby number εtends to zero, we prove the following convergence result:

Theorem 3. LetU0= (u0, b0)∈ B^{0,}^{1}^{2}(R^{3})∩L^{2}(R^{3})be a divergence-free
vector field and (U^{ε})the family of solutions given by Theorem 2. If we set
for χ∈ D(R)U_{R}^{ε} =χ(^{|∇}_{R}^{h}^{|})U^{ε}andUe_{R}^{ε} =U^{ε}−U_{R}^{ε}, then

1. ∀α∈]0,^{1}_{4}[, ε >0andR >0there existsNα(ε, R)∈Nsuch that
Nα(ε, R)^{−−→}_{ε→0} +∞

J. Benameur and R. Selmi

and

sup

|p|≤Nα(ε,R)k∆^{v}_{q}U_{R}^{ε}kL^{4}_{T}(L^{∞})=o(ε^{α}), ε→0.

2. ∀η >0,

lim sup

ε→0

|∇h|^{1−η}Ue_{R}^{ε}

L^{2}_{T}(B^{0,}^{1}^{2})−−−−−→

R→+∞ 0.

The proof uses a Strichartz inequality and Fourier analysis. In fact,
dispersive effects are of great importance in the study of nonlinear partial
differential equations, since they yield decay estimates on waves inR^{3}.

The structure of this paper is as follows. The next section is devoted to introduction of anisotropic Lebesgue spaces and anisotropic Littlewood–

Paley theory. In the third section, we prove Theorem 1. The fourth section deals with the proof of Theorem 2. In the last section, we prove Theorem 3.

2. Notation and Technical Lemmas

2.1. Anisotropic Lebesgue spaces. Let us define anisotropic Lebesgue spaces and recall some of their properties which are useful in the sequel.

Definition 1. We defineL^{p}_{h}(L^{r}_{v}) to be the spaceL^{p}(R_{x}_{1}×R_{x}_{2};L^{r}(R_{x}_{3}))
endowed with the norm

kfkL^{p}_{h}(L^{r}_{v})=kf(xh,·)kL^{r}(R_{x}

3)

L^{p}(R_{x1}×R_{x2}).

Similarly, L^{r}_{v}(L^{p}_{h}) is the space L^{r}(R_{x}_{3};L^{p}(R_{x}_{1} ×R_{x}_{2})) endowed with the
norm

kfkL^{r}_{v}(L^{p}_{h})=kf(·, x3)kL^{p}(R_{x}

1×R_{x}

2)

L^{r}(R_{x}

3).

In the frame of anisotropic Lebesgue spaces, the H¨older inequality reads
kf gkL^{r}_{v}(L^{p}_{h})≤ kfk_{L}r0

v(L^{p}_{h}^{0})kgk_{L}r00
v (L^{p}_{h}^{00}),
where ^{1}_{r} =_{r}^{1}^{0} +_{r}^{1}^{00} and ^{1}_{p} = _{p}^{1}^{0} +_{p}^{1}^{00}.

Young’s convolution inequality takes the following form:

kf ? gkL^{r}_{v}(L^{p}_{h})≤ kfk_{L}r0

v(L^{p}_{h}^{0})kgk_{L}r00
v (L^{p}_{h}^{00}),
where 1 +^{1}_{r} =_{r}^{1}0 +_{r}^{1}00 and 1 + ^{1}_{p} =_{p}^{1}0 +_{p}^{1}00.

The following lemma will be useful in the sequel

Lemma 1. Let1≤p≤q andf :X1×X2→Rbe a function belonging
to L^{p}(X1;L^{q}(X2)), where (X1;dµ1) and (X2;dµ2) are measurable spaces.

Then f ∈L^{q}(X2;L^{p}(X1))and

kfkL^{q}(X2;L^{p}(X1)) ≤ kfkL^{p}(X1;L^{q}(X2)).

Anisotropic Rotating MHD System in Critical Anisotropic Spaces

2.2. Anisotropic Littlewood–Paley theory. The basic idea of Little- wood–Paley theory consists in a localization procedure in the frequency space. The powerful point of this theory is that the derivatives and, more generally, the Fourier multipliers act on distributions whose Fourier trans- form is supported in a ball or a ring in a very special way that we will re- turn on. Such theory in its anisotropic form allows to introduce anisotropic Sobolev and Besov spaces. To do so, we use an anisotropic dyadic decom- position of the frequency space. We begin by defining for any function a the following operators of localization:

∆^{h}_{j}a=F^{−1}(ϕ(2^{−j}|ξh|)F(a)) for j ∈Z,

∆^{v}_{q}a=F^{−1}(ϑ(2^{−q}|ξ3|)F(a)) for q∈N,

∆^{v}_{−1}a=F^{−1}(ϑ(|ξ3|)F(a))
and

∆^{v}_{q}a= 0 for q≤ −2.

The functionsϕand ϑrepresent a dyadic partition of unity inR; they are
regular non-negative functions and satisfy supp(ϑ) ⊂ B(0,^{4}_{3}), supp(ϕ)⊂
C(0,^{3}_{4},^{8}_{3}). Moreover, for allt∈R,

ϑ(t) +X

q≥0

ϕ(2^{−q}t) = 1.

Furthermore, we define the operatorsS_{q}^{v} andS_{j}^{h} by
S_{q}^{v}u= X

q^{0}≤q−1

∆^{v}_{q}^{0}u
and

S_{j}^{h}u= X

j^{0}≤j−1

∆^{h}_{j}^{0}u.

In this way, we are considering a homogeneous decomposition in the hor- izontal variable and an inhomogeneous one in the vertical one. We define respectively the corresponding Sobolev space and the mixed Besov–Sobolev space by the following definitions:

Definition 2. Letsands^{0} be two real numbers such thats <1,ube a
tempered distribution and

kukH^{s,s}^{0} = X

j,q

2^{2(js+qs}^{0}^{)}k∆^{h}_{j}∆^{v}_{q}uk^{2}L^{2}

^{1}_{2}
.

The spaceH^{s,s}^{0}(R^{3}) is the closure of D(R^{3}) in the above semi-norm.

Definition 3. The anisotropic Besov spaceB^{0,}^{1}^{2} is the closure ofD(R^{3})
in the following norm

kuk_{B}^{0,}^{1}_{2} =X

q∈Z

Z

ξ_{h}

Z

2^{q}^{−1}≤|ξ3|≤2^{q}

|ξ3| |Fu(ξ)|^{2}dξ
^{1}_{2}

.

J. Benameur and R. Selmi

The above norm is equivalent to the one defined by
kuk_{B}^{0,}^{1}_{2} =X

q∈Z

2^{q/2}k∆^{v}_{q}ukL^{2}(R^{3}).

The interest of this decomposition resides in the fact that any vertical de-
rivative of a function localized in vertical frequencies of size 2^{q} acts as a
multiplication by 2^{q}.

The following lemma is an anisotropic Bernstein type inequality (see [10]).

Lemma 2. Letube a function such that supp(F^{v}u)⊂R^{2}

h×2^{q}C, where
C is a dyadic ring. Letp≥1andr≥r^{0} ≥1be real numbers. The following
holds:

2^{qk}C^{−k}kukL^{p}_{h}(L^{r}_{v})≤ k∂_{x}^{k}_{3}ukL^{p}_{h}(L^{r}_{v})≤2^{qk}C^{k}kukL^{p}_{h}(L^{r}_{v}), (3)
2^{qk}C^{−k}kukL^{r}_{v}(L^{p}_{h})≤ k∂_{x}^{k}_{3}ukL^{r}_{v}(L^{p}_{h})≤2^{qk}C^{k}kukL^{r}_{v}(L^{p}_{h}), (4)
kukL^{p}_{h}(L^{r}_{v})≤C2^{q(}^{r}^{1}^{0}^{−}^{1}^{r}^{)}kukL^{p}_{h}(L^{r}_{v}^{0}) (5)
and

kukL^{r}_{v}(L^{p}_{h})≤C2^{q(}^{r}^{1}^{0}^{−}^{1}^{r}^{)}kukL^{r}_{v}^{0}(L^{p}_{h}). (6)
It is well known that the dyadic decomposition is useful to define the
product of two distributions. That is,

uv= X

q∈Z,q^{0}∈Z

∆^{v}_{q}u·∆^{v}_{q}^{0}v=Tvu+Tuv+R(u, v),
where

Tvu= X

q^{0}≤q−2

∆^{u}_{q}^{0}v·∆^{v}_{q}u=X

q

S_{q−1}^{v} v·∆^{v}_{q}u,
Tuv= X

q^{0}≤q−2

∆^{v}_{q}^{0}u·∆^{v}_{q}v=X

q

S_{q−1}^{v} u·∆^{v}_{q}v
and

R(u, v) =X

q

X

i∈{0,±1}

∆^{v}_{q}u·∆^{v}_{q+i}v.

The two first sums are said to be the paraproducts and the third sum is the remainder. This is known to be the Bony’s decomposition in the vertical variable (see [4], [5], [9]). In this framework, we have the following properties

∆^{v}_{q}(S_{q}^{v}^{0}_{−1}u·∆^{v}_{q}^{0}v) = 0 if |q−q^{0}| ≥5
and

∆^{v}_{q}(S_{q}^{v}^{0}_{+1}u·∆^{v}_{q}^{0}v) = 0 if q^{0} ≤q−4.

For the sake of simplification, we will denote by (aq), (bq) and (cq) generic positive sequences (depending possibly ont) such that P

q∈Z

√aq≤1, P

q∈Z

bq≤1 and P

q∈Z

c^{2}_{q} ≤1.

Anisotropic Rotating MHD System in Critical Anisotropic Spaces

Notice thatubelongs toH^{0,s}(R^{3}) if and only if

k∆^{v}_{q}ukL^{2} ≤C2^{−qs}cqkukH^{0,s}, (7)
and thatubelongs toB^{0,}^{1}^{2}(R^{3}) if and only if

k∆^{v}_{q}ukL^{2}≤C2^{−q/2}bqkuk_{B}0,1

2. (8)

In the sequel, it will be useful to introduce mixed Besov–Sobolev type spaces that take into account Lebesgue regularity in time on the dyadic blocs.

Those spaces will be denoted, forp≥1, byLf^{p}_{T}(B^{0,}^{1}^{2}) and defined as in [6]

by

kuk_{L}_{f}^{p}

T(B^{0,}^{1}^{2})=X

q∈Z

2^{q/2}k∆^{v}_{q}ukL^{p}([0,T];L^{2}(R3))<+∞.
Remark that we have

kuk_{L}p

T(B^{0,}^{1}^{2})≤ kuk_{L}_{f}p
T(B^{0,}^{1}^{2}),
where

kuk^{p}

L^{p}_{T}(B^{0,}^{1}^{2})=
Zt
0

ku(t)k^{p}

B^{0,}^{1}^{2}dt.

3. Uniqueness

Following the ideas in [10], we establish Lemma 3 to prove uniqueness
inH^{0,}^{1}^{2}.

Lemma 3. LetU = (u, b) andV = (v, c)be two divergence free vector
fields, which belong to L^{∞}_{T} (H^{0,}^{1}^{2}), such that∇^{h}U and∇^{h}V in L^{2}_{T}(H^{0,}^{1}^{2}).

LetW = (w, β)∈L^{∞}_{T} (H^{0,}^{1}^{2})with∇^{h}W ∈L^{2}_{T}(H^{0,}^{1}^{2})be a solution of

∂tW+νh∆hW+Q(W, W + 2U) +1

εL(W) = (−∇p,0) divw=divβ= 0

W

t=0 = (0,0).

For all0< t < T, ifkWk_{H}^{0,}^{−}^{1}_{2} ≤^{1}e, then
d

dtkWk^{2}_{H}0,−1

2≤Cf(t)kWk^{2}_{H}0,−1

2 1−lnkWk^{2}_{H}0,−1
2

ln 1−lnkWk^{2}_{H}0,−1
2

, wheref is a time-locally integrable function defined by

f(t) = 1 + 2kUk^{2}

H^{0,}^{1}^{2} + 2kVk^{2}

H^{0,}^{1}^{2}

1 + 2k∇hUk^{2}

H^{0,}^{1}^{2} + 2k∇hVk^{2}

H^{0,}^{1}^{2}

. Proof. We begin by noting thatQ(W, W + 2U) is explicitly given by

Q(W, W + 2U) =

u· ∇w+w· ∇v−b· ∇β−β· ∇c u· ∇β+w· ∇c−β· ∇u−c· ∇w

.

J. Benameur and R. Selmi

If we take the scalar product inH^{0,−}^{1}^{2}, we obtain
1

2 d

dtkWk^{2}_{H}0,−1

2+νhk∇hWk^{2}_{H}0,−1

2≤ X

q≥−1

2^{−q}(∆^{v}_{q}Q(W, W + 2U)|∆^{v}_{q}W)L^{2},
where

∆^{v}_{q}Q(W, W + 2U)|∆^{v}_{q}W

L^{2} =

= ∆^{v}_{q}(u· ∇w)|∆^{u}_{q}w

L^{2}+ ∆^{v}_{q}(w· ∇v)|∆^{v}_{q}w

L^{2}−

− ∆^{v}_{q}(b· ∇β)|∆^{v}_{q}w

L^{2}− ∆^{v}_{q}(β· ∇c)|∆^{v}_{q}w

L^{2}+
+ ∆^{v}_{q}(u· ∇β)|∆^{v}_{q}β

L^{2}+ ∆^{v}_{q}(w· ∇c)|∆^{v}_{q}β

L^{2}−

− ∆^{v}_{q}(β· ∇u)|∆^{v}_{q}β

L^{2}− ∆^{v}_{q}(c· ∇w)|∆^{v}_{q}β

L^{2}.
We mention that the above nonlinearities are of two types: those where two
variables are the same, for example (∆^{v}_{q}(u· ∇w)|∆^{u}_{q}w)L^{2}, and those where
the three variables are different like (∆^{v}_{q}(β· ∇c)|∆^{v}_{q}w)L^{2}. The former are
estimated in [10], for the latter it suffices to note, after applying Cauchy–

Schwarz or H¨older inequality, thatkwk,kβk ≤ kWkandk∇^{h}wk,k∇^{h}βk ≤
k∇^{h}Wk. The same holds forU andV compared to their components (u, v)

and (b, c).

We return to the proof of the uniqueness result and suppose thatU^{ε}and
V^{ε} are two solutions of (S^{ε}) with the same initial data, such that U^{ε} and
V^{ε} belong to L^{∞}_{loc}(H^{0,}^{1}^{2}) with ∇^{h}U^{ε} and ∇^{h}V^{ε} belonging to L^{2}_{loc}(H^{0,}^{1}^{2}).

We will prove thatW^{ε}=U^{ε}−V^{ε}is such thatW^{ε}= 0 inL^{∞}_{T} (H^{0,−}^{1}^{2}) with

∇hW^{ε}= 0 inL^{2}_{T}(H^{0,}^{1}^{2}).

W^{ε}satisfies the following equation

∂tW^{ε}+νh∆hW^{ε}+Q^{ε}(W^{ε}, W^{ε}+ 2U^{ε}) +1

εL^{ε}(W^{ε}) = − ∇p^{ε},0
.
Lemma 3 implies that for all 0< t < T ifkW^{ε}k_{H}0,−1

2 ≤^{1}_{e}, then
d

dtkW^{ε}k^{2}_{H}0,−1
2 ≤

≤Cf(t)kW^{ε}k^{2}_{H}0,−1

2 1−lnkW^{ε}k^{2}_{H}0,−1
2

ln 1−lnkW^{ε}k^{2}_{H}0,−1
2

, wheref is a locally time-integrable function defined by

f(t) = 1 + 2kU^{ε}k^{2}

H^{0,}^{1}^{2} + 2kV^{ε}k^{2}

H^{0,}^{1}^{2}

1 + 2k∇hU^{ε}k^{2}

H^{0,}^{1}^{2} + 2k∇hV^{ε}k^{2}

H^{0,}^{1}^{2}

.

By the Osgood lemma, one infers the uniqueness inH^{0,}^{1}^{2}.

To investigate uniqueness inB^{0,}^{1}^{2}, note thatW^{ε}will be estimated in the
norm

kS_{0}^{v}W^{ε}(t)k^{2}L^{∞}_{v} L^{2}_{h}+X

q≥0

2^{−q}k∆^{v}_{q}W^{ε}(t)k^{2}L^{2}+

Anisotropic Rotating MHD System in Critical Anisotropic Spaces

+νhsup_{x}_{3}
Zt
0

S_{0}^{v}∇hW^{ε}(τ,·, x3)^{2}

L^{2}_{h}dτ+

+νh

X

q≥0

2^{−q}
Zt
0

∇h∆^{v}_{q}W^{ε}(τ)^{2}

L^{2}dτ.

The exact estimations are easy to obtain, since we use functions localized in low vertical frequencies.

4. Proof of Existence Results
4.1. Proof of the existence result in B^{0,}^{1}^{2}.

4.1.1. Global existence for small initial data. For a strictly positive integer n, we define Friedrichs’s operators by

Jn(u) =F^{−1}(1B(0,n)Fu(ξ)),
J_{n}^{v}(u) =F^{−1}(1_{{ξ,|ξ}_{3}_{|≤n}}Fu(ξ))
and

Jen(u) = (Jn−J_{1/n}^{v} )(u).

Let us consider the following approximate magnetohydrodynamic system
denoted (M HD_{ν}^{n}_{h})

∂tu−νh∆hJenu+Jen(Jenu· ∇Jenu)−Jen(Jenb· ∇Jenb) +1

ε(Jenu×e3)+

+1

ε∂3Jenb=∇X

i,j

∆^{−1}∂i∂jJen(JenuiJenuj+JenbiJenbj)+

+1 ε∇X

i

∆^{−1}∂i(∂3Jenb−Jenu×e3)i,

∂tb−νh∆hJenb+Jen(Jenu· ∇Jenb)−Jen(Jenb· ∇Jenu) +1

ε∂3Jenu= 0, divu= 0,

divb= 0, U

t=0=JenU^{0}.

The above system is an ODE that can be rewritten in the following abstract form:

∂tU =Fn(U),

whereU = (u, b) and the expression ofFnis given by the system (M HD^{n}_{ν}_{h}).

Note that sinceU^{0} ∈ B^{0,}^{1}^{2}, U(0) = JenU^{0} belongs toL^{2}. Moreover,Fn is
a continuous function fromL^{2} into L^{2} and the Cauchy–Lipschitz theorem
implies that (M HD_{ν}^{n}_{h}) has a unique local solutionU_{n}^{ε} inC^{1}([0, Tn(ε)[, L^{2}).

On the other hand, the fact thatJen is a projector implies thatJenU_{n}^{ε}is also
a solution of (M HD^{n}_{ν}_{h}). By uniqueness, it follows that

JenU_{n}^{ε}=U_{n}^{ε}

J. Benameur and R. Selmi

andU_{n}^{ε}is a solution of the following system also denoted (M HD_{ν}^{n}_{h}):

(M HD^{n}_{ν}_{h})

∂tu^{ε}_{n}−νh∆hu^{ε}_{n}+Jen(u^{ε}_{n}· ∇u^{ε}_{n})−Jen(b^{ε}_{n}· ∇b^{ε}_{n}) +1
ε∂3b^{ε}_{n}+
+1

ε(u^{ε}_{n}×e3) =∇X

i,j

Jen∆^{−1}∂i∂j(u^{ε}_{i,n}u^{ε}_{j,n}+b^{ε}_{i,n}b^{ε}_{j,n})+

+1 ε∇X

i

∆^{−1}∂i(∂3b^{ε}_{n}−u^{ε}_{n}×e3)i,

∂tb^{ε}_{n}−νh∆hb^{ε}_{n}+Jen(u^{ε}_{n}· ∇b^{ε}_{n})−Jen(b^{ε}_{n}· ∇u^{ε}_{n}) +1

ε∂3u^{ε}_{n}= 0,
divu^{ε}_{n}= 0,

divb^{ε}_{n}= 0,
U_{n}^{ε}

t=0=JenU^{0}.

TheL^{2} energy estimate implies that
1

2 d

dtkU_{n}^{ε}(t)k^{2}L^{2}+νhk∇hU_{n}^{ε}(t)k^{2}L^{2} = 0.

So, one deduces the global existence in L^{2}.

To prove the existence result inB^{0,}^{1}^{2}, we introduce the following lemma
due to [10].

Lemma 4. Let u and v be two vector fields defined on R^{3} such that
u(t) is divergence-free for all t ∈ [0, T]. There exists a real sequence (aq)
satisfyingaq =aq(u, v, T)>0and P

q∈Z

√aq <1such that

ZT 0

|(∆^{v}_{q}(u· ∇v)|∆^{v}_{q}v)L^{2}|dt≤

≤Caq2^{−q}

k∇^{h}uk_{L}_{f}2

T(B^{0,}^{1}^{2})kvk_{L}_{g}^{∞}

T(B^{0,}^{1}^{2})k∇^{h}vk_{L}_{f}2
T(B^{0,}^{1}^{2})+
+kuk_{g}^{1}^{2}

L^{∞}_{T}(B^{0,}^{1}^{2})k∇huk^{1}^{2}_{f}

L^{2}_{T}(B^{0,}^{1}^{2})kvk_{g}^{1}^{2}

L^{∞}_{T}(B^{0,}^{1}^{2})k∇hvk^{3}^{2}_{f}

L^{2}_{T}(B^{0,}^{1}^{2})

.

We apply the operator ∆^{v}_{q} and use theL^{2} energy estimate to obtain
1

2 d dt

∆^{v}_{q}U_{n}^{ε}(t)^{2}

L^{2}+νh

∇h∆^{v}_{q}U_{n}^{ε}(t)^{2}

L^{2}≤

≤ ∆^{v}_{q}(u^{ε}_{n}· ∇u^{ε}_{n})|∆^{v}_{q}u^{ε}_{n}

L^{2}|+ ∆^{v}_{q}(u^{ε}_{n}· ∇b^{ε}_{n})|∆^{v}_{q}b^{ε}_{n}

L^{2}

+
+ ∆^{v}_{q}(b^{ε}_{n}· ∇b^{ε}_{n})|∆^{v}_{q}u^{ε}_{n}

L^{2}+ ∆^{v}_{q}(b^{ε}_{n}· ∇u^{ε}_{n})|∆^{v}_{q}b^{ε}_{n}

L^{2}

. (9) We note the following rearrangement:

∆^{v}_{q}(b^{ε}_{n}· ∇b^{ε}_{n})|∆^{v}_{q}u^{ε}_{n}

L^{2}+ ∆^{v}_{q}(b^{ε}_{n}· ∇u^{ε}_{n})|∆^{v}_{q}b^{ε}_{n}

L^{2} =

=

∆^{v}_{q}(b^{ε}_{n}· ∇(u^{ε}_{n}+b^{ε}_{n})

|∆^{v}_{q}(u^{ε}_{n}+b^{ε}_{n})

L^{2}−

− ∆^{v}_{q}(b^{ε}_{n}· ∇u^{ε}_{n})|∆^{v}_{q}u^{ε}_{n}

L^{2}− ∆^{v}_{q}(b^{ε}_{n}· ∇b^{ε}_{n})|∆^{v}_{q}b^{ε}_{n}

L^{2}. (10)