Highly Nonlinear Whistler Modes and EMHD Reconnection

Details Highly Nonlinear Whistler Modes and EMHD Reconnection Highly Nonlinear Whistler Modes and EMHD Reconnection

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm41a1426s&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SM41A-1426

Physics

2723 Magnetic Reconnection (7526, 7835), 7526 Magnetic Reconnection (2723, 7835), 7852 Solitons And Solitary Waves (4455)

Scientific paper

Nonlinear low-frequency whistler modes whose wave magnetic field exceeds the ambient field are excited in a large laboratory plasma . The waves are excited with magnetic loop antennas whose axis is parallel or antiparallel to the uniform dc background magnetic field. In one field topology, called a whistler spheromak, the wave creates magnetic null points which propagate with the wave packet along the ambient field. Since the whistler wave dispersion depends on the total magnetic field, the wave properties become amplitude-dependent. The spheromak decreases the average magnetic field, hence propagates slower with increasing amplitude. In the opposite polarity, called a whistler mirror, the propagation speed increases with amplitude since the wave field adds to the ambient field. With increasing amplitude, whistler spheromaks steepen and form whistler shocks in which the product of amplitude A and axial width w satisfies A· w2 =const. The spheromak width can be much smaller than a half wavelength of a linear whistler. The interaction of nonlinear whistler waves has been studied in collisions of counter-propagating modes. Counter-helicity spheromaks merge into a stationary field-reversed configuration, a whistler FRC. Its free magnetic energy is dissipated into electron heat and light emission. Dissipation occurs within a magnetic O-type null rather than the classical X-type null. Thus, it is a case of magnetic annihilation rather than reconnection. Work supported by USAF Materiel Command and NSF PHY.

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