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D. S. Balsara, J. Kim (NCSA/UIUC), M.-M. Mac Low (Amer. Mus. Natural Hist.)
The growth of magnetic fields in astrophysical bodies has recently been the subject of intense debate (e.g. Kulsrud and Anderson 1992). Mean field dynamo theory has been called into doubt and several authors have proposed alternatives. Kulsrud et al (1997) suggested that fast stretching and turbulent processes might, in themselves be adequate for field growth. Other authors such as Vainshtein and Zeldovich (1972) and Childress and Gilbert (1995) have advocated stretch, twist, and fold mechanisms for field growth.
Some numerical studies of turbulence at modest Mach numbers and with a mean impressed helicity have been carried out (Balsara 2000), suggesting the plausibility of fast dynamos. However, the question remains whether flows without a mean helicity can sustain magnetic field growth. Some clues that this might be possible were seen in the SNR simulations of Balsara, Benjamin and Cox (2001) where it was shown that the interaction of strong magnetosonic shocks with turbulence amplifies the turbulence and thereby causes strong helicity fluctuations. In the present work we show that a strongly supersonic medium lacking an initial net helicity that is rendered turbulent by energy input from supernovae can sustain growth in magnetic energy. We supply energy via supernovae at rates that are somewhat higher than the present-day galactic supernova rate but consistent with supernova rates anticipated in the early galactic ISM (and also prevalent in many present-day starburst galaxies). The results are interesting because there is no apparent source of mean helicity in the simulations. The field, however, grows rapidly over several orders of magnitude in a few million years. We analyze the nature of the field and its growth rates on various length scales.
M-MML was partially supported by an NSF CAREER grant AST99-85392.