TURBULENT MAGNETIC-FIELD IN THE DISTANT MAGNETOTAIL - BOTTOM-UP PROCESS OF PLASMOID FORMATION

The GEOTAIL magnetic field observations in the earth's magnetotail are examined in terms of a turbulent magnetic reconnection process. It is found that the observed Fourier power spectral density of the turbulent magnetic fields can be approximated by a power-law spectrum which changes its slope around a turnover frequency with 0.04 Ha, in spite of the differences among a wide variety of magnetic perturbations observed in the distant magnetotail plasma sheet. The turbulent magnetic field data are classified into two cases (Nishida et al. in this issue): one is accompanied with a plasmoid bipolar signature on B-z magnetic field, and the other without the bipolar signature. The higher frequency regions above the turnover frequency always show similar spectral slope in the cases with and without a bipolar signature in B-z magnetic field component, although the spectral feature in the lower frequency regions exhibits differences in appearance. When there is no bipolar signature on B-z magnetic field, the lower frequency range is almost a flat spectrum. On the other hand, the slope of the lower frequency region becomes steep when a large scale bipolar signature is observed. The observations may indicate that 1) a small-scale tearing mode vortex with the scale of plasma sheet thickness is excited in the magnetotail, and 2) the large-scale plasmoid is coalescently formed from the small-scale tearing mode vortices.