UCLA BPPL - Nonlinear Vortices with Magnetic Null Points
When the magnetic field of a whistler vortex exceeds the background magnetic
field, magnetic null points can be formed where EMHD breaks down locally.
The J × B force is the primary reason for
the nonlinearity (J 
B > B0).
The penetration and propagation depends both on the
direction and strength of the vortex fields. When the vortex field adds to
the ambient field, the vortex can propagate in the whistler mode.
 A quasi steady-state current flows
through a loop antenna and creates a
dipolar field opposing a uniform
background magnetic field. Two 3-D
radial null points are formed on axis.
(b) The antenna current is switched off
and the field-reversed configuration is
maintained by electron currents in the
plasma. (c) The vortex stretches but does
not split and propagate like a small
amplitude vortex. The null points prevent
propagation in the whistler mode. Note
that the vortex does not significantly
tilt during its free relaxation.',
800,
639) "Click image to see larger version (87 kB)")
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Evolution of a nonlinear EMHD vortex. (a) A quasi steady-state current flows through a loop antenna and creates a dipolar field opposing a uniform background magnetic field. Two 3-D radial null points are formed on axis. (b) The antenna current is switched off and the field-reversed configuration is maintained by electron currents in the plasma. (c) The vortex stretches but does not split and propagate like a small amplitude vortex. The null points prevent propagation in the whistler mode. Note that the vortex does not significantly tilt during its free relaxation. |
In the opposite case, when a magnetic null point is formed, whistler propagation is not possible. Many interesting phenomena arise in the regime of nonlinear EMHD, some of which have been studied in our machine.
When an oscillating magnetic field of a loop antenna excites vortices of opposite polarity the nonlinear plasma response produces locally dc and harmonic magnetic fields.
 Loop field adds to
background field and produces a strong
mirror field. (b) Loop field opposes
the background field and produces a
magnetic null point with fan and spine.',
800,
577) "Click image to see larger version (59 kB)")
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Comparison of field lines for opposite directions of the dipolar loop field. (a) Loop field adds to background field and produces a strong mirror field. (b) Loop field opposes the background field and produces a magnetic null point with fan and spine. |
 Antenna current pulses of alternating
polarities are applied to a loop antenna
producing mirror and null point
configurations. (b) Toroidal magnetic
field component differs greatly for
both polarities. (c) Fourier analysis
of the current waveform (yellow) and
the toroidal field component (white and
magenta). The
<b>J</b> × <b>B</b>
nonlinearity produces a dc field
[see dashed line in (b)] and new
harmonics (white bars) absent in
the applied spectrum.',
900,
382) "Click image to see larger version (31 kB)")
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Nonlinear magnetic field generation. (a) Antenna current pulses of alternating polarities are applied to a loop antenna producing mirror and null point configurations. (b) Toroidal magnetic field component differs greatly for both polarities. (c) Fourier analysis of the current waveform (yellow) and the toroidal field component (white and magenta). The J × B nonlinearity produces a dc field [see dashed line in (b)] and new harmonics (white bars) absent in the applied spectrum. |
When two nonlinear vortices propagate against each other they merge into a single vortex. When a large amplitude vortex relaxes it can become unstable to tilting. The relaxation of a nonlinear vortex with magnetic null points involves magnetic reconnection and annihilation.
 Just after
switch-off of the two loop antenna
currents two distinct vortices with
separate 3-D null points exist. (b)
As the vortices expand two adjacent
null points touch and merge into a
single degenerate null point. (c)
Subsequently the degenerate null
point opens into a null line joining
the O-type nulls of the toroidal
current layers. A field-reversed
configuration is formed. It decays
slowly without significant tilting.',
800,
567) "Click image to see larger version (224 kB)")
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Nonlinear interaction between two strong EMHD vortices. (a) Just after switch-off of the two loop antenna currents two distinct vortices with separate 3-D null points exist. (b) As the vortices expand two adjacent null points touch and merge into a single degenerate null point. (c) Subsequently the degenerate null point opens into a null line joining the O-type nulls of the toroidal current layers. A field-reversed configuration is formed. It decays slowly without significant tilting. |
References
- Generation of DC magnetic fields by rectifying nonlinear whistlers (260 kB), R. L. Stenzel and J. M. Urrutia, Phys. Rev. Lett. 81, 2064-2067 (1998). [Link to original publication]
- Measurements of helicity and reconnection in electron MHD plasmas R. L. Stenzel, J. M. Urrutia, and M.C. Griskey, "Magnetic Helicity in Space and Laboratory Plasmas," Geophys. Monograph 111, edited by M. Brown, R. Canfield, and A. Pevtsov (Am. Geophys. Union, Washington, DC, 1999), pp. 179-186.
- 3D EMHD reconnection in a laboratory plasma, R. L. Stenzel, J. M. Urrutia, M. C. Griskey and K. D. Strohmaier, Earth Planets & Space, 53, 553-560 (2001).
- A new laboratory experiment on magnetic reconnection (479 kB), R. L. Stenzel, J. M. Urrutia, M. C. Griskey, and K. D. Strohmaier, Phys. Plasmas 9, 1925-1930 (2002). [Link to original publication.]