Sherwood 2015

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approvedabstract_v2.pdf2015-01-19 13:01:22Makoto Hirota

Theoretical description of explosive magnetic reconnection in collisionless two-fluid models

Author: Makoto Hirota
Requested Type: Consider for Invited
Submitted: 2015-01-19 12:59:29

Co-authors: Y.Hattori, P.J.Morrison

Contact Info:
Institute of Fluid Science, Tohoku University
2-1-1, Katahira, Aoba-ku
Sendai, Miyagi   980-857

Abstract Text:
A mechanism of explosive magnetic reconnection is investigated by analyzing nonlinear and non-quasi-steady evolution of collisionless tearing instabilities in two-fluid models. For simplicity, we revisit the effects of electron inertia and electron temperature that bring in two microscopic scales; the electron skin depth and the ion-sound gyroradius, respectively. It is known that an elongated current sheet with Y-shaped ends is formed in the presence of only electron inertia, whereas the X-shaped current sheet is formed when the effect of electron temperature is taken into account. Although several pioneering works [Ottaviani and Porcelli, PRL 71 3802 (1993), Bhattacharjee et al., PoP 12 042305 (2005)] have attempted to explain the explosive growth of the nonlinear tearing mode, we note that their predictions do not agree quantitatively with our high-resolution simulation results. In previous work [Hirota et al., NF 53 063024 (2013)], we have considered only the effect of electron inertia (namely, the Y-shape) and estimated the explosive growth rate by using a new variational method. This theory is also generalized to the case of the X-shape in the present work.

The concept of our variational method is inspired by the ideal MHD Lagrangian theory, in which the magnetic energy is considered to be part of the potential energy of the dynamical system. If an ideal fluid displacement keeps decreasing the magnetic energy, it is likely to grow by gaining the corresponding amount of the kinetic energy, and the most unstable displacement would decrease the magnetic energy most effectively. This argument assumes that the two-fluid effects are locally essential for changing the topology of magnetic field lines, but their impact on the global energy balance is negligible. By choosing a proper fluid displacement as a trial function, we can estimate the growth rate of the displacement, which is indeed explosive and agrees with the simulation results.

I will leave on 18th evening. So, I'd like to avoid afternoon session of 18th, if it exists.

March 16-18, 2015
The Courant Institute, New York University