Due to differences in solar illumination, a geomagnetic field line may have one foot point in a dark ionosphere while the other ionosphere is in daylight. This may happen near the terminator under solstice conditions. In this situation, a resonant wave mode may appear, which has a node in the electric field in the sunlit (high conductance) ionosphere and an antinode in the dark (low conductance) ionosphere. Thus, the length of the field line is one quarter of the wavelength of the wave, in contrast with half-wave field line resonances in which both ionospheres are nodes in the electric field. These quarter waves have resonant frequencies that are roughly a factor of 2 lower than the half-wave frequency on the field line. We have simulated these resonances using a fully three-dimensional model of ULF waves in a dipolar magnetosphere. The ionospheric conductance is modeled as a function of the solar zenith angle, and so this model can describe the change in the wave resonance frequency as the ground magnetometer station varies in local time. The results show that the quarter-wave resonances can be excited by a shock-like impulse at the dayside magnetosphere and exhibit many of the properties of the observed waves. In particular, the simulations support the notion that a conductance ratio between day and night foot points of the field line must be greater than about 5 for the quarter waves to exist.
Bibliographical noteFunding Information:
Work at the University of Minnesota is supported by grant AGS‐1840891 from the National Science Foundation. We also acknowledge supercomputer support from the Minnesota Supercomputer Institute. R. L. L. would like to thank the University of Newcastle, where some of this work was carried out, for its support. Source code for the numerical simulations and data files associated with the results presented in this paper is available at the Data Repository for the University of Minnesota at https://doi.org/10.13020/d1g7-c676 website.
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- ULF waves
- magnetosphere-ionosphere coupling
- quarter wave modes