Does Reconnection Onset Require Thin Current Sheets?

Details Does Reconnection Onset Require Thin Current Sheets? Does Reconnection Onset Require Thin Current Sheets?

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm31b1726b&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SM31B-1726

Physics

2723 Magnetic Reconnection (7526, 7835), 2744 Magnetotail, 2753 Numerical Modeling, 2772 Plasma Waves And Instabilities (2471)

Scientific paper

One mechanism for the onset of magnetic reconnection in a current sheet is accelerated growth of a tearing mode in the presence of a lower hybrid drift instability (LHDI). The LHDI causes current sheet thinning and anisotropic electron heating, both of which increase tearing mode growth far beyond normal nonlinear saturation levels. Implicit particle simulations in three dimensions yield noise initiated reconnection for thin current sheets with ρi/L ≍ 1.7-2.2, where ρi is the ion cyclotron radius and L is the current sheet thickness [1]. One asks, in more realistic current sheets with ρi/L ≤ 1 will there be spontaneous reconnection? The answer is interesting from two perspectives. Can the action of the LHDI explain onset in realistic current sheets? If so, why isn't reconnection always happening? Highly resolved explicit particle simulations of the LHDI in two dimensions with ρi/L =1, a more realistic value for the Earth's magnetotail, show smaller but still significant thinning and heating [2]. Recent implicit simulations in three dimensions show that reconnection onset is delayed in current sheets with ρi/L = 1, but that reconnection, once begun, proceeds rapidly. The delay is consistent with the expected decreased growth rates for both the LHDI and tearing modes. In simulations with even thicker current sheets, ρi/L ≤ 0.5, the time scales are longer and the signal to noise ratio less favorable. However, the results suggest either that onset does not occur at all or is long delayed, and possibly that reconnection onset occurs only in a sufficiently thin current sheet. 1. P. Ricci, J. U. Brackbill, W. Daughton, G. Lapenta, Phys. Plasmas 11, 4489 (2004). 2. W. Daughton, G. Lapenta, P. Ricci, Phys. Rev. Lett. 93(2004).

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