Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsm51a1280c&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #SM51A-1280
Physics
2744 Magnetotail, 2764 Plasma Sheet, 2772 Plasma Waves And Instabilities (2471)
Scientific paper
In the near-Earth plasma sheet, earthward convection can be inhibited by plasma compression (the Erickson-Wolf effect), resulting in the formation of a weakly magnetized region from which the normal magnetic field component B_n increases in both the tailward (x) and earthward (-x) directions. Starting from a model plasma sheet equilibrium with B_n at the midplane (z = 0) proportional to (1+x/L) mid-way along the tail, 3D fully electromagnetic PIC simulations demonstrate that a strong ballooning/interchange type instability develops in the region of tailward increasing B_n. Unlike the classic MHD ballooning mode, however, the kinetic instability has a frequency which is below the ion bounce, diamagnetic, and magnetic drift frequencies, and a cross-tail (y) wavelength which is comparable to the ion gyroradius ρin in the normal field (k_y ρin ≍ 2π). The spatial extent of the mode in the x direction is initially of the order of the B_n gradient length L; at late times, the ballooning fingers of magnetic flux extend throughout and earthward of the B_n gradient region. The polarization of the mode is dominated by strong perturbations in the parallel magnetic field and approximately anti-correlated plasma density, the cross-tail Ey field, and the electrostatic potential. The ion distribution develops a weak v_y perturbation, while the x- and field-aligned flow velocities remain very small, a consequence of the ion's gyromotion essentially averaging the perturbed electric fields to zero. Thus the ions do not move earthward with the inward moving fingers of magnetic flux. The perturbed magnetic fields are generated by strong transverse electron Hall currents which are coupled to a complex system of field-aligned currents. The earthward speed of the flux enhancements is typically about 400 km/s. A characteristic signature of the mode is a local increase in B_z by about a factor of two without any significant ion flow.
Coroniti Ferdinan V.
Pritchett Philip L.
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