edu.ucar.opensky::articles:18176 UCAR/NCAR - Library OpenSky Support PO Box 3000 Boulder CO 80307-3000 opensky@ucar.edu http://opensky.ucar.edu/ homepage pointOfContact 2025-12-26T03:58:48.129193 ISO 19115-2 Geographic Information - Metadata - Part 2: Extensions for Imagery and Gridded Data ISO 19115-2:2009(E) https://n2t.org/ark:/85065/d7rb764b Not much helicity is needed to drive large-scale dynamos 2012-06-01T00:00:00Z publication https://n2t.org/ark:/85065/d7rb764b https://doi.org/10.1103/PhysRevE.85.066406 Pietarila Graham, J. author Blackman, E. author Mininni, Pablo D. author Pouquet, Annick author UCAR/NCAR - Library publisher Understanding the in situ amplification of large-scale magnetic fields in turbulent astrophysical rotators has been a core subject of dynamo theory. When turbulent velocities are helical, large-scale dynamos that substantially amplify fields on scales that exceed the turbulent forcing scale arise, but the minimum sufficient fractional kinetic helicity fh,C has not been previously well quantified. Using direct numerical simulations for a simple helical dynamo, we show that fh,C decreases as the ratio of forcing to large-scale wave numbers kF/kmin increases. From the condition that a large-scale helical dynamo must overcome the back reaction from any nonhelical field on the large scales, we develop a theory that can explain the simulations. For kF/kmin8 we find fh,C3%, implying that very small helicity fractions strongly influence magnetic spectra for even moderate-scale separation. UCAR/NCAR - Library OpenSky Support PO Box 3000 Boulder CO 80307-3000 opensky@ucar.edu http://opensky.ucar.edu/ homepage pointOfContact PDF Text theme Resource Type 2016-01-01T00:00:00Z publication Copyright 2012 American Physical Society. None eng geoscientificInformation