Generation of Broadband Electrostatic Waves in Earth's Magnetotail

The theory that broad-band electrostatic waves (BEN) in Earth's magnetotail are trapped-electron (“BGK”) modes is reexamined. Electron/ion beams analyzed for a realistic magnetized-plasma source model with $\kappa$ distributions are found to drive an unstable spectrum of broad angular range over several orders of magnitude in $f$, up to $(0.1–0.2)f_{\mathrm{pe}}$. Analysis indicates that trapping essential for the BGK paradigm is good only at the highest $f$, whereas most of the spectrum has minimal trapping and can be driven by electron/ion beam instabilities. A new model is proposed in which trapped-electron modes exist only at the highest $f$ band, whereas electron/ion beam instabilities drive the bulk of the broad-band spectrum below that. BEN wave data from ISEE-1 and ISEE-3 show large angles of propagation with respect to the magnetic field for $f<\mathrm{}{f}_{\mathrm{ce}}$ as predicted by the new model but not the BGK model. However $f>\mathrm{}{f}_{\mathrm{ce}}$ is observed only in a narrow angular range around the magnetic field and may be BGK modes. This predicts that the BEN solitary waves in the source region are not in BEN well into the lobe.