Electrostatic wave variety and the origins of BEN

Spiky pulses in broadband electrostatic “noise” (BEN) in the plasma sheet boundary layer (PSBL) observed by Geotail led to a model for BEN as Bernstein–Greene–Kruskal (BGK) modes, which has been explored by a number of simulations. A recent modified model proposed by Grabbe (Phys. Rev. Lett. 84 (2000a) 3614; Rec. Res. Dev. Plasma 1 (2000b) 89) for BEN well into the magnetotail, in which these trapped-particle modes only occur in the top of the spectrum, whereas the bulk of the spectrum below that which propagates obliquely to the magnetic field consists of standard modes driven by unstable electron/ion beams, predicts that such solitary waves are only present in the source region, whereas they are not in BEN that has propagated well outside the source region. Motivated by that prediction and other features of the model, observations are examined from POLAR of BEN obtained poleward of and within the near-Earth extension of the PSBL. The wave data examined for BEN poleward of the near-Earth PSBL (in the plasma mantle) exhibit frequent turbulent waveforms, but little evidence of solitary waves. However, the wave data near the PSBL source region shows a persistent level of turbulence, within which solitary-like waves are embedded, several of which saturate the receiver. The non-solitary portion of the observed wave data, which appears throughout these regions, fits well the theory of standard electron/ion beam instabilities. However, the higher-frequency portion above the plasma frequency, which is confined to the PSBL vicinity, appears nonlinear in character, with a stronger magnetic field apparently playing a vital role in that nonlinearity.