The basic physics of a Buneman-like instability for electron streams with small extent perpendicular to the confining magnetic field is examined analytically and with a 2-1/2D particle-in-cell (PIC) simulation. The geometry in this scenario transforms energy associated with the parallel flow of electrons to large perpendicular electric fields in the lower-hybrid (LH) range of frequencies that cause ion acceleration and create magnetic-fieldaligned density striations.
A basic process capable of explaining observations of fast perpendicular ions in a wide range of plasma environments is described. Spatial symmetry breaking perpendicular to the confining magnetic field is shown to cause irreversible energy gain for ions gyrating through an electric field having a nonuniform amplitude. The efficiency depends on the ratio of the ion Larmor radius to the scale length of the amplitude gradient, and on the scaled frequency ν≡ω/Ωi. A Landau resonance is not required, and there is no lower threshold on the electric field, because the mechanism is active in the linear regime. Theory, numerics, and particle-in-cell simulations are used to illustrate the interaction for electrostatic fields in the lower-hybrid range of frequencies, but the process does not depend on a particular type of mode.
A basic process similar to transit‐time absorption is evaluated as a possible source of fast perpendicular ions under conditions typically encountered during sounding rocket flights in the auroral ionosphere. Electric fields localized within density striations aligned with the geomagnetic field have a nonuniform perpendicular amplitude that results in irreversible energy absorption. The efficiency of the process depends on the ratio of the ion Larmor radius to the perpendicular scale length of the amplitude gradient and on the scaled frequency ν ≅ ω/Ωi. This absorption mechanism does not depend on a Landau resonance and does not require the electric field to be larger than a threshold value, because it is active in the linear regime. Theory, numerics, and particle‐in‐cell simulations demonstrate the process for electrostatic fields with frequencies a few times the ion gyrofrequency, but the process does not depend explicitly on the fluctuations being lower hybrid waves. Calculations of the evolving ion distribution function due to coherent electric fields suggest that the effect of spatial localization should be considered in the analysis of fast ions of geophysical origin.
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