Abstract
Abstract. Solutions for ultra-low frequency (ULF) wave fields in the frequency range 1–100mHz that interact with the Earth's ionosphere in the presence of oblique background magnetic fields are described. Analytic expressions for the electric and magnetic wave fields in the magnetosphere, ionosphere and atmosphere are derived within the context of an inductive ionosphere. The inductive shielding effect (ISE) arises from the generation of an "inductive" rotational current by the induced part of the divergent electric field in the ionosphere which reduces the wave amplitude detected on the ground. The inductive response of the ionosphere is described by Faraday's law and the ISE depends on the horizontal scale size of the ULF disturbance, its frequency and the ionosphere conductivities. The ISE for ULF waves in a vertical background magnetic field is limited in application to high latitudes. In this paper we examine the ISE within the context of oblique background magnetic fields, extending studies of an inductive ionosphere and the associated shielding of ULF waves to lower latitudes. It is found that the dip angle of the background magnetic field has a significant effect on signals detected at the ground. For incident shear Alfvén mode waves and oblique background magnetic fields, the horizontal component of the field-aligned current contributes to the signal detected at the ground. At low latitudes, the ISE is larger at smaller conductivity values compared with high latitudes. Key words. Ionosphere (ionosphere-magnetosphere interactions; electric fields and currents; wave propagation)
Highlights
Ultra-low frequency (ULF) waves in the 1–100 mHz band are generated by processes involving the interaction of the solar wind with the Earth’s magnetosphere
While the altitude of the ionosphere is much smaller than typical ULF wavelengths, the anisotropic conductivity of the ionosphere and currents generated by the waves give complicated ground level wave field solutions (Hughes, 1974; Hughes and Southwood, 1976; Ellis and Southwood, 1983; Yoshikawa and Itonaga, 1996; 2000)
We focus on the inductive shielding effect (ISE), extending the work of Yoshikawa and Itonaga (1996, 2000) for middle to low latitudes
Summary
Ultra-low frequency (ULF) waves in the 1–100 mHz band are generated by processes involving the interaction of the solar wind with the Earth’s magnetosphere. There are a number of mechanisms that can “shield” ULF waves within the context of the magnetosphere/ionosphere/atmosphere/ground system These are (i) the atmospheric shielding effect (ASE) (Hughes, 1974; Nishida, 1964, 1978), (ii) the inductive shielding effect (ISE) (Yoshikawa and Itonaga, 1996), and (iii) a 90◦ rotation of the wave fields (NDR) (Hughes, 1974, 1983; Hughes and Southwood, 1976). The restriction that the wave in the atmosphere is evanescent in the vertical direction is retained These general solutions for ULF wave interaction with the ionosphere show that the magnetic field dip angle has a significant effect on the inductive shielding effect for a highly conducting ionosphere. The consequences for the amplitude of ULF waves observed by ground magnetometers at middle and low latitudes are discussed
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