FRACTAL PROPERTIES OF THE STRETCH-TWIST-FOLD MAGNETIC DYNAMO

This paper presents direct numerical simulations of the stretch-twist- fold (STF) dynamo. For more than two decades, this dynamo has been vie wed as the prototype of the fast dynamo process; and because of its ap parently conceptual simplicity, it was generally not thought to be nec essary to investigate its quantitative properties in detail via numeri cal simulations. Furthermore, it has been generally assumed that the S TF dynamo is not characterized by small-scale fluctuations, as is usua lly the case for many other dynamo processes. Numerical simulations sh ow, however, that the STF dynamo process is accompanied by the generat ion of small-scale fluctuations in the magnetic field. Therefore, it c annot be taken as an a priori given that the STF dynamo is a large-sca le dynamo; however, our results suggest that the STF dynamo does gener ate large-scale magnetic fields. In any eventuality, the magnetic fiel ds generated by the STF process do not behave as was previously expect ed: As we show, these fields become chaotic, first, in the sense that magnetic field lines acquire multifractal properties; and, second, bec ause the field itself becomes chaotic [i.e., the (intermittency) fract al dimensions are no longer trivial]. [S1063-651X(96)08905-2]