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

Abstract. The influence of the finite ionospheric conductivity on the structure of dispersive, nonradiative field line resonances (FLRs) is investigated for the first four odd harmonics. The results are based on a linear, magnetically incompressible, reduced, two-fluid MHD model. The model includes effects of finite electron inertia (at low altitude) and finite electron pressure (at high altitude). The ionosphere is treated as a high-integrated conducting substrate. The results show that even very low ionospheric conductivity (Σ_P = 2 mho) is not sufficient to prevent the formation of a large-amplitude, small-scale, nonradiative FLR for the third and higher harmonics when the background transverse plasma inhomogeneity is strong enough. At the same time, the fundamental FLR is strongly affected by a state of low conductivity, and when Σ_P = 2 mho, this resonance forms only small-amplitude, relatively broad electromagnetic disturbance. The difference in conductivities of northern and southern ionospheres does not produce significant asymmetry in the distribution of electric and magnetic fields along the resonant field line. The transverse gradient of the background Alfvén speed plays an important role in structure of the FLR when the ionospheric conductivity is finite. In cases where the transverse inhomogeneity of the plasma is not strong enough, the low ionospheric conductivity can prevent even higher-harmonic FLRs from contracting to small scales where dispersive effects are important. The application of these results to the formation and temporal evolution of small-scale, active auroral arc forms is discussed.