H.K. Moffatt Dynamo Instability and Feedback in a Stochastically Driven System
Transcript of H.K. Moffatt Dynamo Instability and Feedback in a Stochastically Driven System

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DYNAMO INSTABILITY AND FEEDBACK IN A STOCHASTICALLY
By H.K. MoffattDept. of Applied Mathematics and Theoretical
University of Cambridge, England
ABSTRACTIn 5 1 , a method of treatment of the equation
DRIVEN SYSTEM
Phys
where aijk(j,t) is a random tensor field of known statistical propertiesis reviewed, with particular reference (i) to the magnetohydrodynamicturbulent dynamo problem, and (ii) to the problem of diffusion of apassive scalar field by turbulent motion. The result of Steenbeck,Krause and Radler (1.966), that in the former context dynamo action canoccur (i.e. the ensemble average of can grow without limit) providedthe statistics of the turbulent field l a ck reflexional symmetry, isdiscussed within the framework of the above general equation.In f f 2 and 3 , the feedback mechanism in the magnetohydrodynamiccontext is considered. It is supposed that a velocity field lackingreflexional symmetry is generated in an electrically conducting fluidby a random body force of known statistical properties. Conditionsare then conducive to the growth of large scale magnetic field perturbations. The growth is limited by the fact that the growing Lorentzforce progressively modifies the statistical structure of the velocityfield, until ultimately a statistical equilibrium is achieved. It isshown that in this equilibrium the magnetic energy density may exceedthe kinetic energy density by a factor O(L/e> >>1, where L is the scaleof the magnetic field, and the scale of the turbulence.
1. INSTABILITIES OF THE LINEAR DIFFUSION EQUATIONWITH A RANDOM CONVECTNE TERMLet aijk(x,t) be a random tensor field, statistically homogeneous
in 3 and stationary in t, with

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(1.3)
In this context, it has been established by Steenbeck, Krause andRadler (1966) (hereafter SKR) that a sufficient condition for unlimitedgrowth of ( 1 04)ze =  ~ . v e X V V , ( 1 0 5 )
then, with y = 5 = 08 , (1.1) becomes the gradient of the equationb t
describing the convection and diffusion of a passive scalar contaminant.In this context, the effect of the turbulence on a localised @fieldis equivalent to that of an eddy diffusivity; the expression for thisdiffusivity was obtained by Taylor (1921) for A = 0, and the firstcorrection (for small )r ) by Saffman (1962).covered by the method described below, and in addition a simple expression for the limiting form of the eddy diffusivity for largemay be obtained.
These results may be re
The method of treatment of (1.1) is as follows:Let z(x,t)

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where Eij = (aijkyL) . Subtracting this from (1.1 ) ,Equation ( 1 . 7 ) , wit h the initial condition yi = 0, establishesrelation between yi and Yk, say
( 1 *7)a linear
where Kof integration is over all 5' and over 0 6 16 .
is a functional of aijk and of the parameter A . The rangeidWe can now construct the tensor E needed in ( 1 . 6 ) , Viz.,ij
(1 .10 )V dependss ationari tyiJk o n l y on 5 and f because of the assumed homogene ity and
of aijk; it also depends implicitly on A , We maycertainly suppose that
(1 .11)l i j k ( ~8 r)+ o as IxI+.o a h t f as r  *  ao0 ,but the scalesexample b e defined i n terms of its moments) may depend onas on the statistical properties of aijk.
, to characteristic of this tensor (whic h could f oras well
Let us no w consider solutions q(x,t) o f (1.6) having scales L, Tsatisfying
L > > l , T >> t o .Then in (1.9) we m ay expand Yk(z*,tl) in Taylor series about (r,t),and integrate ter m by term, obtaining

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where
The ran ge of integration in these moments is over all , and overt < P < 0; for t >> to, the time range is effectively  W < t & 0, andthe tensors defined b y (1.13) are then simply constant tensors, determined solely by the statietical properties of aijk and by .
The expansion (1.12) may b e rearranged, using (1.6) repeatedly,so that only spacederivatives appear; in this way we obtain
(1.14)(I) h 6 ) 0 ) fo)where, i n fact, Aijkt = Aijk4 + Bijm A,lk, and higher coef fi cien ts may
be similarly obtained. Equatio n (1.6) then becomes an equation withconstant coefficients
(clThe term involving Aijke may be recognized as describing an 'eddydiffusion' effect whi ch certainly appears i n bot h the particular cas esgiven b y (1.2) and (1.4). The term involving A(') is less familiar.It appears i n the magnetohydrodynamic case (1.2), and i s described i nthat context (for notational reasons) as the effect' by Ste enb eckt a1 (1966)*; (would 'SKReffectt or thelicity effect'  see Moffatt1970a  not be more apt?).described by (1.4), the reas on being essentially that the kernel f unc tio nV i n (1.10) is in this case expressible as a gradient and s o the
ijk
It does not appear in the second s itu ati on
i jk*Footnote: E.N. Pa rk er anticipated the SKReffect through inspiredphysical reasoning (Parker 1955); he used the symbol P where SKR use# :

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expression for A given by ( 1 . 1 3 ) vanishes.ijkThe case when a. t) is statistically isotropicjkx*
Suppose now that ai k(z , is statistically isotropic, i. thatits statistical properties are invariant under rotations of the frameof reference. Then, in Particular, the tensors Aijk,,, are isotropic,i.e.
(4
(1 .16 )
If the statistical properties of aijk(r,t) are invariant under re= olexions of the frame of reference as well as rotations, then Aijk4 
when m is even, and in particular 1 = 0. Only if the random fieldlacks reflexional symmetry can we have O( # 0. Note that pz is a pseudoscalar*, while p, , & and ,$$ are pure scalars.
When ac # 0, he first term on the right of (1 .15 ) clearly dominates the development of _Y(_x,t) provided the scale L is sufficientlylarge, and we then have simply
2 SOCVAYIa tAny mode for which
then grows exponentially according to
+ negligible terms.
if K is sufficiently small. For example, the Fourier components
(1 .18 )
(1 .20 )
(1 .21 )= f K f o , and one of these Fourier components will growsatisfy v . o
exponentially if it is represented in the Fourier transform of the initial field y (2). 0

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In the reflexionally symmetric case, o( = 0, and ( 1 . 15 ) becomes
In the first example, with E = , V,,(v,,~) = 0 so that0v.z = V ' z , and we have an eddy diffusivity .I / (1 2 4 )
One would of course expect these diffusivities to turn out to be positive in real physical situations, and they do turn out to be positivein every limiting case amenable to analysis. However no general proofthat they must invariably be positive appears to be available, and thereremains the intriguing possibility that in certain circumstances the(net) diffhsivity may be negative with interesting inverse diffusionconsequencies.
t) is statistically anisotropic but reflexionallyJhe case when aisymmetricneed not vanish. We might for(0 )*ijkn these circumstances again,
example have a situation where
where , and 2 are any three vectors. Putting(1.25)
in ( 1 . 15 ) , and retaining only the O(K) term on the righthand sidethen have
.26)we

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The determinant al condition for a nontrivial solut ion gives a cubicequation for m with roots
Hence we have an instability when K is in a direction satisfying
(1.29)[This situa tion cannot arise i n the magnetohydrod ynamic conte xt, i nwhich, becau se of the particular structure (1.2) of aijk, Aijk is antisymmetric i n i and j; the particular for m (1.25) would t he n require
are imaginary and1,2933 = 0 and I ; the three roots mLthere can be no question of inst abi lit y2
2. THE ESTABLISHMENT OF A NONLINEAR EQUILIBRIUMIt has been shown in f l that wh en th e statistical properties of
the a field lack reflexional symmetry, equation (1.1 ) leads to exponential grow th of Fourier components of the xfield of sufficientlysmall wavenumber. In a real physical situation, the gro wth mustultimately be l imited by some backreaction which modifies the sta 'tistical properties of the ai jkfield.context, this back r eaction is provided by the Loren tz fo rc e; the effecthas be en analysed when the &field consists of a random fie ld ofdecaying inertial waves i n a rotating fluid (Moffatt 1970b).be en furthe r discussed i n general terms by Roberts (1971).here8jtamine the magnetohydrodynamic situation , on the assump tio n thata body force z(,~, t) f know n statistical properties is applied to thefluid,ln order to gener ate the veloc ity field from whi ch the magneticfield may be nourished and sustained.equations governing the velocity field ;(x,t) and the Al fv dn vel ocity
i j k
In the magnetohydrody namic
It hasWe shall
We assume that ( f ) = 0. Thefield xr(lf,t) =(,up)f z(t,t) are

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and
As befo re, let V,(x_,t) = and suppose that the scale L of 1 slarge compared wit h the sc ale e of any relevant statistical propertyof the sfield generated.
( Is )
The equation satisfied b y 1 is
where & = 2 Y , and the mai n objective is to obtain an expression fo r(~ ~h _) in terms of 1 so that (2.4) may be integrated (see the expressions (2.6) and (2.13) below).
Assuming that ('2) = 0 (and this is something that will req uir efurther consideration i n retrospect), the equations for U, and & are
where N.L. indicates t erms quadratic in the fluctuating quantitiesand B.and the field V, (which grows exponentially on the linear analysis of $ 1 )sufficiently strong, fo r these nonlinear terms to be negligible. W emay also neglect the terms _h.w and 2.Vx on account of the slow variation of the field p. We are then left with two coupled linear e quationsdescribing forced Alf vLn waves i n an effectively uniform field 1.
We shall suppose that the forcing field; is sufficie ntly we ak
We can now Four ier transform these equations and construct th e( k _ , O ) of z;onvolution (uAh_) i n terms of the spectrum tensor F
omitting the details , whic h are straightforward, we obtainid

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where(2.7)
The quantity in square brackets under the integral sign is real in viewof the Hermitian symmetry condition Fjk(z,d) = Fspectrum tensor of a real vector field. It is no surprise that theexpression is non zero only ifThe expression (2.6) is evidently nonlinear in 1 due to the dependenceof on in the denominator. As 1 grows in strength, :>Iultimately decreases, and the mechanism responsible for the growth ofV is correspondingly controlled.
Evaluation of the integral (2.6) for given F
* (k,f,a) or thekj Fjk(4,u lacks reflexional symmetry.
c ( 5 , a ) is in generaljka very difficult (if not impossible) matter.this lecture to consider a choice of F j k ( , k , U ) which is very special,but which nevertheless serves to illustrate the type of effect thatmay be expected in general. Suppose that, locally,
It may be sufficient for
and suppose that
f ( 5 , t )"This is a forcing wave at the *resonance1 frequency U = koV,,withhelical structure, Note that
(2.10)and correspondingly,
where 5 = (ko, 0, 0). We put angular brackets round fo to indicatethat this amplitude may be a random variable with an ensemble average.Suppose further that the dissipative effects are weak, viz. that

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Xk. * v,so that, to lowest order, 6 = i( g + A ) k : ; then, from (2.6))
(2.12)
(2.13)
The implications fo r dynamo theoryLet us now suppose that varies slowly, and that the statistical
properties of the Efield are such that the formula (2.13) holds everywhere. This is admittedly unrealistic, i n that it requires a forc inghelicity wave whose wavevector k is everywhere locally aligned wit hthe mean field L; but it is the only assumption that leads to a reasonably simple treatment of the feedback phenomenon. Equation (2.4) th enbecomes
4
3t = AVA($=%) + A V 2 V 9(2.14)
where A = > ( A +~)~k;(f:)form
This equation admits solutions of the
provided Vo(t) satisfies(2.15)
(2.16)For consistency, we require K e k o .utions of this type is that they satisfy the forcefree condition( 1 . 19 ) ) and are therefore preferentially amplified on the basis oflinear theory. According to the nonlinear theory of this section, thestructure of suc h modes remains unaltered whe n t he backreaction is included, but the gro wt h rate i s modified.
The solution of (2.16) ie given by
The particular interest of s o l 

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1here C is a constant determined b y the value of V at some time t = tbeyond which the approximations (2 .12 ) are satisfied.order ( A K 2 )  ' = L 2 / x , the magnetic energy density M(t) = +VE (thereis in this situation negligible energy in the fluctuating magneticfield) asymptotic lly attains the value
0In a time of
( 2 . 18 )The kinetic energy density is given by
Hence
( 2 . 2 0 )The magnetic energy in the ultimate steady state is therefore necessarily an order of magnitude greater than the kinetic energy of thebackground velocity field,
3 . DISCUSSIONThere are three questions that arise in the course of the above
analysis that require special comment: (i) Can we be sure that a meanvelocity fielduniform Reynolds stress distribution implied by the nonuniform largescale magnetic field? (ii) Is the particular assumption ( 2 .11 ) concerning the spectrum tensor of the forcing field too special for theresult to have any significance?sider a forcing field whose statistical properties lack reflexionalsymmetry? Let us take these questions in turn.
= does not develop as a result of the non
(iii) Is it at all realistic to con

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(i) Generation of a mean velocity fieldThe mean momentum equation, from ( 2 . 1 ) is
?+ + u..v7cu, =  v % , + v . v ~  v . < ~ ~  h h _ >9 v 2 u ( 3 . 1 )c> tso that a mean velocity field will develop if either i . v i or IT)is rotational; if these terms are irrotational, they can be accommodatedby a mean (effective) pressure field X,(z,t) = ( x ) .
For the particular field ( 2 . 1 5 ) considered above E. vx is certainlyirrotational; indeed iA(v,,l) 0 since the growing field is forcefree,so that ,V.v_V= $ v V 2 .tensor axisymmetric about the direction of 1, nd depending only on zand t, since 1 depends only on z and t.02 is a possible principal axis of P for all L ; hence
Furthermore pij +.U  h h is a symmetric ij)Clearly p13= p,, = 0, since
ij
and so this term is also irrotational, Hence no mean flow will develop,but a mean pressure field periodic in % will be established.
For a more general field than ( 2 . 1 5 ) consisting of a superpositionof such Fourier components, but having different wavevectors 5 , itseems certain that a mean velocity field ,~,(x,t) develop, but whatits structure will be and how it will modify equation ( 2 . 1 6 ) remains anopen question.(ii) The effect of a forcing field with a continuous spectrum
The assumption (2 .11 ) is indeed too restrictive, and not only because the result ( 2 . 1 3 ) would then appear to have only local and notglobal significance. There is some justification for choosing theresonant frequency 0 = koV, since the response to a random 5field containing a continuous spectrum of wavenumbers $ and frequencies rJ will(for weak dissipative effects) peak around the resonant frequencies ofthe undamped system. However, in singling out a particular resonantfrequency, we overestimate the response, which for a continuous spectrum,

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is controlled by a narrow band of frequenoies around the resonant frequency. Moreover, we should take account ofe he resonant frequencies given by the Alfvgn dispersion relationeffects can be taken into account in an asymptotic evaluation of theintegral (2.6) for particular choices of continuous spectrum tensors;but it is better first to take some account of the third question (iii)raised above.
kl = f $.I. These
(iii) What physical mechanism can generate a lack of reflexionalsymmetry?It was recognized by Steenbeck et a1 (1966) that, as mentioned in
the introductory paragraphs, a lack of reflexional symmetry is likelyto arise only in a rotating fluid in which a definite direction relativeto the rotation vector 4 can be distinguished, i.e. only if the 'external conditions' themselves lack reflexional symmetry. A randomforce field f is unlikely in itself to lack reflexional symmetry. Ifhowever a reflexionally symmetric Lfield acts upon a rotating fluid,the resulting :field will lack reflexional symmetry if the statisticalproperties of the ffield lack symmetry about planes perpendicular to
, e.g. if (with the convention d > ) only waves for which k,&>Oare present.
An analysis similar to that described in f2, but with the additionof a Coriolis force 2 4 A g in equation (2 .1 ) has been carried out(Moffatt 1971), and full account is taken of the considerations underheading (ii) above. In this calculation, it is supposed that the spectrum of f is isotropic over the halfspace &.$>O and reflexionallysymmetric, and characterised by wavenumbers O ( 4  ') and frequenciesO( (.3, ). Under the assumptions
3 h. , CJ, e n ,a result analogous to (2.20) is obtained,s.,

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where C is a dimensionless constant of order unity.much greater than Eof a mean magnetic field is sufficiently large compared with the lengthscale of the background forcing field.
Again Mult can beif the length scale L available for the growthult
REFERENCES
Moffatt, H.K. 1970a,b. J. Fluid Mech. 41, 435 and 44 , 705.Moffatt, H.K. 1971. (submitted for publication).Parker, E .N . 1955. Astrophys. J. 2,93.Roberts, P.H. 1971. Lectures in applied mathematics (ed. W.H. Reid).
Providence: American Mathematical Society.Saffman, P.G. 1960. J. Fluid Mech, 8, 73.Steenbeck, M., Krause, F. and Radler, KOH., 1966. 2. Naturforsch.a,69.Taylor, G.I. 1921 Proc. Lond. Math. Soc. 2 0 , 196.Weiss, N.O. 1971. Q uart. J. Roy. Astr. Soc. (to appear).