title: Circulation-strain sum rule in stochastic magnetohydrodynamics.
abstract: We study probability density functions (PDFs) of the circulation of velocity and magnetic fields in magnetohydrodynamics, computed for a circular contour within inertial range scales. The analysis is based on the instanton method as adapted to the Martin-Siggia-Rose field theory formalism. While in the viscous limit the expected Gaussian behavior of fluctuations is indeed verified, the case of vanishing viscosity is not suitable of a direct saddle-point treatment. To study the latter limit, we take into account fluctuations around quasistatic background fields, which allows us to derive a sum rule relating PDFs of the circulation observables and the rate of the strain tensor. A simple inspection of the sum rule definition leads straightforwardly to the algebraic decay rho(Gamma)-1/Gamma(2) at the circulation PDF tails.
authors.affiliation: Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ 21945-970, Brazil.
authors.forename: L
authors.initials: L
authors.lastname: Moriconi
authors.forename: F A S
authors.initials: FA
authors.lastname: Nobre