Penetration of ELF currents and electromagnetic fields into the Earth's equatorial ionosphere

Abstract

The penetration of extremely low frequency (ELF) transient electromagnetic fields and associated currents in the Earth's equatorial E‐region plasma is studied theoretically and numerically. In the low‐frequency regime, the plasma dynamics of the E‐region is characterized by helicon waves since the ions are viscously coupled to neutrals while the electrons remain mobile. For typical equatorial E‐region parameters, the plasma is magnetically insulated from penetration of very long timescale magnetic fields by a thin diffusive sheath. Wave penetration driven by a vertically incident pulse localized in space and time leads to both vertical penetration and the triggering of ELF helicon/whistler waves that carry currents obliquely to the magnetic field lines. The study presented here may have relevance for ELF wave generation by lightning discharges and seismic activity and can lead to new concepts in ELF/ULF injection in the earth‐ionosphere waveguide.