Coronal activity from dynamos in astrophysical rotators

Citation
Eg. Blackman et Gb. Field, Coronal activity from dynamos in astrophysical rotators, M NOT R AST, 318(3), 2000, pp. 724-732
Citations number
56
Categorie Soggetti
Space Sciences
Journal title
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
ISSN journal
00358711 → ACNP
Volume
318
Issue
3
Year of publication
2000
Pages
724 - 732
Database
ISI
SICI code
0035-8711(20001101)318:3<724:CAFDIA>2.0.ZU;2-G
Abstract
We show that a steady mean-field dynamo in astrophysical rotators leads to an outflow of relative magnetic helicity and thus magnetic energy available for particle and wind acceleration in a corona. The connection between ene rgy and magnetic helicity arises because mean-field generation is linked to an inverse cascade of magnetic helicity. To maintain a steady state in lar ge magnetic Reynolds number rotators, there must then be an escape of relat ive magnetic helicity associated with the mean field, accompanied by an equ al and opposite contribution from the fluctuating field. From the helicity flow, a lower limit on the magnetic energy deposited in the corona can be e stimated. Steady coronal activity including the dissipation of magnetic ene rgy, and formation of multi-scale helical structures therefore necessarily accompanies an internal dynamo. This highlights the importance of boundary conditions which allow this to occur for non-linear astrophysical dynamo si mulations. Our theoretical estimate of the power delivered by a mean-field dynamo is consistent with that inferred from observations to be delivered t o the solar corona, the Galactic corona, and Seyfert 1 AGN coronae.