Abstract
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 (ΣP = 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 ΣP = 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.
Highlights
Field line resonance (FLR) is possible at any location in the Earth's magnetosphere where the ambient magneticCorrespondence to: A
The main conclusions derived from our previous studies (SL1, 2) are: (1) large-amplitude, narrow resonances are likely to be found at steep transverse gradients in the background AlfveÂn speed; (2) the coupling of energy from external sources to the resonance magnetic shell via surface waves is more eective when the azimuthal wave number is small; (3) in nonradiative FLRs wave energy is focused at low altitudes where the background AlfveÂn speed is the largest; (4) the characteristic transverse size of the resonance tends to be smaller for higher harmonics; and (5) the ®eld-aligned potential drop in nonradiative dispersive FLRs may be as large as several kilovolts and is likely to produce accelerated electrons leading to kilometer scale and smaller, discrete auroral arcs
The main qualitative conclusion to be derived from the results shown in Figs. 1 and 2 is that higherharmonic FLRs in the strongly inhomogeneous plasma develop quickly and with appreciable amplitudes even when the ionospheric conductivity is very low
Summary
Field line resonance (FLR) is possible at any location in the Earth's magnetosphere where the ambient magnetic. The main conclusions derived from our previous studies (SL1, 2) are: (1) large-amplitude, narrow resonances are likely to be found at steep transverse gradients in the background AlfveÂn speed; (2) the coupling of energy from external sources to the resonance magnetic shell via surface waves is more eective when the azimuthal wave number is small; (3) in nonradiative FLRs wave energy is focused at low altitudes where the background AlfveÂn speed is the largest; (4) the characteristic transverse size of the resonance tends to be smaller for higher harmonics; and (5) the ®eld-aligned potential drop in nonradiative dispersive FLRs may be as large as several kilovolts and is likely to produce accelerated electrons leading to kilometer scale and smaller, discrete auroral arcs All of these results were obtained for the approximations in which FLRs occur on the magnetic ®eld lines bounded by two perfectly conducting ionospheres.
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