Influence of the finite ionospheric conductivity on dispersive, nonradiative field line resonances

摘要

The influence of the finite ionosphericconductivity on the structure of dispersive, nonradiative field line resonances(FLRs) is investigated for the first four odd harmonics. The results are basedon a linear, magnetically incompressible, reduced, two-fluid MHD model. Themodel includes effects of finite electron inertia (at low altitude) and finiteelectron pressure (at high altitude). The ionosphere is treated as ahigh-integrated conducting substrate. The results show that even very lowionospheric conductivity (Σ_P = 2 mho) is notsufficient to prevent the formation of a large-amplitude, small-scale,nonradiative FLR for the third and higher harmonics when the backgroundtransverse plasma inhomogeneity is strong enough. At the same time, thefundamental FLR is strongly affected by a state of low conductivity, and whenΣ_P = 2 mho, this resonance forms onlysmall-amplitude, relatively broad electromagnetic disturbance. The difference inconductivities of northern and southern ionospheres does not produce significantasymmetry in the distribution of electric and magnetic fields along the resonantfield line. The transverse gradient of the background Alfvén speed plays animportant role in structure of the FLR when the ionospheric conductivity isfinite. In cases where the transverse inhomogeneity of the plasma is not strongenough, the low ionospheric conductivity can prevent even higher-harmonic FLRsfrom contracting to small scales where dispersive effects are important. Theapplication of these results to the formation and temporal evolution ofsmall-scale, active auroral arc forms is discussed.