The estimations of magnetic field fluctuations atmospheres in turbulent

Authors
Citation
Na. Silant'Ev, The estimations of magnetic field fluctuations atmospheres in turbulent, ASTRON ASTR, 370(2), 2001, pp. 533-540
Citations number
19
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY AND ASTROPHYSICS
ISSN journal
0004-6361 → ACNP
Volume
370
Issue
2
Year of publication
2001
Pages
533 - 540
Database
ISI
SICI code
0004-6361(200105)370:2<533:TEOMFF>2.0.ZU;2-M
Abstract
We qualitatively analyze the equations of the magnetohydrodynamics for vari ous quadratic fluctuations of the magnetic field and the vector potential. For the stationary inhomogeneous state, rye have established the exact inte gral relations between the magnetic field fluctuations b(r, t) and the mean magnetic field B-0(r, t). We estimate alpha approximate to (eta + beta )B- 0 (del x B-0)/B-0(2) where eta and beta are the ohmic (molecular) and turbu lent diffusivities, respectively. The alpha - coefficient describes the enh ancement of the mean magnetic field. We found that the exact Seehafer formu la B-0.<u x b > = -eta <b.(del x b)> is also valid for the locally stationa ry and homogeneous evolution of magnetic fluctuations (u is the Euler turbu lent velocity). It is shown that tile usual a-coefficient; more correctly t he term alphaB(0).(del x B-0), presents with opposite signs in equations fo r B-0(2) and <b(2)>, i.e. the rate of increase of the total magnetic energy does not depend directly on this coefficient. T We show that at the moment tb of the maximum of magnetic fluctuations, <b(2)> approximate to B-0(2). The same also holds for the locally homogeneous and stationary regime if th e spectrum of tile turbulence has a slow decrease in the inertial region of wave numbers. The detailed analysis of two-dimensional turbulence has show n that the Zeldovich estimate <b(2)> approximate to beta/etaB(0)(2) is vali d only at the maximum of the vector potential fluctuations. This estimate i s corrected such that at tb it also gives <b(2)> approximate to B-0(2). We also give the approximate expression for tile alpha -coefficient which take s into account the back reaction of the mean magnetic field onto the turbul ence itself.