Modulated electron‐acoustic waves in auroral density cavities: FAST observations

Persistent subplasma-frequency kinetic electrostatic electron nonlinear waves

“…The lower part of this "thumb" curve is what has been called the EAW branch. 9,10 While the EAW curve does give some guidance as to the lowest frequency that can be easily excited as a KEEN mode, it is clear that KEEN modes for the wavenumber shown can be driven up very far away ͑in -k space͒ from that locus. The frequency axis is normalized to the EAW frequency, showing that KEEN waves can be readily produced over a 2:1 range of frequencies ͑80% of EAW frequency to 160% of EAW frequency͒ for this typical drive wavenumber.…”

“…Similarly, for a strongly two-temperature electron distribution there is also a weakly damped electron sound mode, the EAW, whose small signal behavior can be obtained from perturbation theory and which is comparable to a KEEN wave, with hot electrons partly shielding the cold electron plasma density variation. 4,5,9,10 From the linear theory for this strongly twotemperature case, the ratios of the vacuum contribution to the fast electron contribution to the cold electron contribution are 1: +1/ ͑k 2 Dhot 2 ͒ :− pcold 2 / 2 , with the first term having the much smaller magnitude. Returning to KEEN waves, whether driven up ponderomotively or created via BGK, the two dynamic species are the trapped electrons and the free electrons and they seem to be behaving in a similar way.…”

“…The theory just cited [4][5][6][7] and work by many others [9][10][11][12][13][14][15][16] ͑previous to the work by Strozzi et al 8 ͒ addressed the problem of the possible persistence of such EAW-style modes but only when imposed as initial states ͑with necessarily complicated structure in the electron velocity distribution function near the phase velocity͒. No obvious means exist to impose such complicated initial states other than by a simple act of will by theorists in a computer model.…”

“…The acronym EAW has been previously used to denote waves in the linear wave frequency gap between the electron and ion plasma frequencies, with phase velocities not far from the electron thermal velocity, in contexts ranging from ionosphere observations, possibly in plasmas with appropriate amounts of clearly defined populations of cold and hot electrons, 4,5 or in plasmas with Landau damping set equal to zero ͑perhaps because of some special adjustment to the electron distribution function at some designated phase velocity͒, 3,6,7 or as a postulated initial plasma distribution function. 6,7,[9][10][11][12][13][14][15][16] Rather than postulating such distributions, when we drove up the distribution function from the initial Maxwellian by using a limited-time PF driver with a chosen pair of frequency and wavenumbers, we found ͑as described below͒ that the trapped electrons obtained kinetically were not simply a means of negating Landau damping but an essential oscillation component quite distinct from and equal in effect to the rest of the ͑untrapped͒ electrons. It also appeared that it would be quite difficult to produce such waves at very low amplitudes, so they should not be confused with the result of any linear perturbation.…”

Electron acoustic solitary waves with non-thermal distribution of electrons

“…Such large value of electric fields suggest that the nonlinear effects have a bearing in the generation of BEN. Electrostatic solitary structures have been observed in the auroral acceleration region (Temerin et al, 1982;Bostrom et al, 1988;Dubouloz et al, 1991a, b;Mozer et al, 1997;Ergun et al, 1998;Bounds et al, 1999), in Earth's high altitude polar magnetosphere (Franz et al, 1998), in Polar cap boundary layer (Tsurutani et al, 1998), in the plasma sheet boundary layer Omura et al, 1994), on cusp field lines (Cattell et al, 1999) and in AKR source region (Pottelette et al, 1999).…”