Magnetic energy transient growth in the subcritical Kazantsev model

Abstract

We study average magnetic field growth in a mirror-symmetrical Kazantsev turbulent flow near the dissipative scales. Our main attention is directed to a subcritical regime, when an exponential decrease of magnetic energy is usually expected. We show that instead of damping, transient energy growth can be obtained, for example, in stochastic processes supported by the large-scale magnetic fields. We calculate the longitudinal correlation functions and demonstrate that they can tend to nonzero stationary solutions, whose localization width is inversely proportional to the square of the magnetic Reynolds numbers and with amplitude depending on the closeness of these numbers to the critical value. We present the local generation effect without any external support, predicted by Zeldovich in 1956. Numerically solving the initial-boundary Kazantsev problem on the nonuniform grids, we simulate this process by implicit schemes and discuss the possible consequences of subcritical growth for dynamo theory.

A short and sudden increase of the magnetic field strength and the accompanying spectral variability in the O9.7V star HD54879

“…Small-scale dynamo action which provides substantial magnetic flux and is modulated by a subcritical oscillating solar-type magnetic configuration also seems to be an option that deserves investigation (cf. Yushkov, Lukin & Sokoloff 2018).…”