Subionospheric VLF propagation


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Subionospheric VLF propagation Prepared by Morris Cohen Benjamin Cotts Forrest Foust Stanford University Stanford CA IHY Workshop on Advancing VLF through the Global AWESOME Network 1 Radio waves on the ionosphere Magnetosphere Ionosphere Microwave MF HF Waves LF Waves Atmosphere N Lehtinen Earth 2 Ideal parallel plate waveguide Isotropic Ionosphere Perfect Reflections reflection coefficient is 1 Ionosphere Reflection Height Transmitter Receiver Earth Perfect Reflections Flat Earth 3 Basic waveguide analysis n k j a 2 2 Ionosphere Propagation 80 km Transmitter Receiver Earth Fields within waveguide Modal components of propagating waves Magnetic Field B Transverse Transverse Transverse Electromagnetic Magnetic TM Electric Field E Electric TE TEM Vertical direction Azimuthal direction 4 Radial direction propagation Above 1 8 kHz Below 1 8 kHz All Frequencies Typical Spectrogram TE and TM Weak TEM TEM Wave 5 Typical conductivities Nighttime Ionosphere Daytime Ionosphere 10 7 to 10 5 S m 80 90 km 70 75 km Salt water 4 S m Fresh water 10 2 S m Wet soil 10 3 to 10 2 S m Dry soil 10 4 to 10 26S m Basic plasma conductivity e Electron response ee e Applied electric field e eeee ee eeee Applied electric field forced rearranging of electrons Polarization opposes field shields it from propagating further Characteristic plasma response time 1 p p2 Ne e ee ee Polarization field Debye Shielding 7 Ionospheric Conductivity z Electrons in motion forced to orbit magnetic field Applied electric field can generate currents in other directions Anisotropic conductivity Gyrofrequency is a function of magnetic field and e mass y x r r r F qe v B ce qe B 0 me 8 Mode conversion Incident Wave Pure TM wave Incident fields are rotated by electron response TE and TM waves can be converted into each other Reflected Wave Mixed TM and TE wave 9 Anisotropic Conductivity Incident Wave Direction of wave incidence matters Different reflection coefficients Reflected Wave Incident Wave Reflected Wave 10

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