On the generation and propagation of auroral electromagnetic ion cyclotron waves


  • E. J. Lund,

  • J. LaBelle


On a recent sounding rocket flight, electromagnetic ELF waves below the proton gyrofrequency were detected in an auroral arc. These waves would be resonant with ƒcH+ or ƒcO+ in the auroral acceleration region; the latter waves are associated with modulated parallel electron fluxes. Following previous papers, we use homogeneous linear theory to calculate temporal and convective growth rates for electron beam-driven electromagnetic ion cyclotron (EMIC) waves in various plasmas with H+, He+, and O+ ions, although the inhomogeneity in the source region makes this approximation questionable. We find that all three EMIC modes are unstable with growth rates inversely proportional to the mass of the ion associated with the mode; maximum convective growth occurs at frequencies of 0.8–1.0Ωi. However, the growth rate of the O+ EMIC mode is too low to account for the observations unless relatively large beam densities are used, implying that nonlinear or inhomogeneous plasma effects must play an important role in the instability. We also assess the propagation of these EMIC waves in a cold plasma. Our ray tracing calculations show that propagation effects can explain both the localization of the O+ waves and the absence of He+ waves at lower altitudes. We also show that the wider latitudinal spread and low power spectral density of the H+ waves can also be explained by propagation effects, but the H+ rays which can reach the ionosphere are not the rays for which the beam-driven instability produces the highest growth rates in a homogeneous plasma.