Letter to the Editor

Galactic dynamo and helicity losses through fountain flow

¹ School of Mathematics and Statistics, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK e-mail: anvar.shukurov@ncl.ac.uk
² Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India
³ Department of Physics, Moscow University, 119992 Moscow, Russia
⁴ Nordita, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark

Received: 23 December 2005
Accepted: 19 January 2006

Abstract

Aims. Nonlinear behaviour of galactic dynamos is studied, allowing for magnetic helicity removal by the galactic fountain flow.Methods. A suitable advection speed is estimated, and a one-dimensional mean-field dynamo model with dynamic α-effect is explored. Results. It is shown that the galactic fountain flow is efficient in removing magnetic helicity from galactic discs. This alleviates the constraint on the galactic mean-field dynamo resulting from magnetic helicity conservation and thereby allows the mean magnetic field to saturate at a strength comparable to equipartition with the turbulent kinetic energy.