This volume is based on lectures and discussions presented at a NATO Advanced Study Institute on ELF and VLF Radio Wave Propagation, which was held in Norway April 1974. The study of propagation of electromagnetic waves with frequen cies below 100 kHz has long traditions in ionospheric physics. To-day, this frequency range is still of great importance, both to the physicist, who uses the waves as diagnostic tools to study the earth's environment and to the engineer who exploits the characteris tics of these waves to improve communications, navigation and timing systems. In recent years the active interest in the field has led to very rapid progress in the development of propagation theory as well as in the application of this theory to the solution of practical problems. The intention of the Organizing Committee for this Conference was to bring together theoreticians and experi mentalists working on the various aspects of wave propagation, in order to stimulate a fruitful discussion and exchange of ideas.

Inhalt
I: Properties of the Lower Ionosphere.- The Photochemistry of the Mesosphere.- Ionizing Radiation Affecting the Lower Ionosphere.- Lower Ionosphere Ion Chemistry.- Coupling between the Neutral and the Ionized Atmosphere.- II: VLF Propagation Theory.- FORTRAN Program for Obtaining Wavefields of Penetrating, Nonpenetrating, and Whistler Modes of Radio Waves in the Ionosphere.- Intermode Coupling Processes in the Ionosphere in the Frequency Range 0.1 Hz to 300 kHz.- The Coalescence of Coupling Points and 'Cross- Over' for Ion Cyclotron Whistlers.- Theory of Energy Flux and Polarisation Changes of a Radio Wave with two Magnetoionic Components Undergoing Self Demodulation in the Ionosphere.- A Numerical Method for the Integration of the Equations Governing the Propagation of E.M. Waves in a Plane Stratified Magnetoionic Medium.- The Effect of Ground Conductivity on VLF Propagation.- Theory of the Terrestrial Propagation of VLF Electromagnetic Waves.- III: VLF Waves as a Diagnostic Tool Ionospheric Variations and their Influence on the Propagation.- The Use of VLF Propagation Results in Ionospheric Modelling.- The Application of Inversion Techniques to Determine the Accuracy and Resolution of Electron Density Profiles Derived from VLF Propagation Data.- Irregular Variations in the High Latitude Ionosphere and their Effects on Propagation.- VLF Phase Perturbations Associated with Geomagnetic Activity.- Determination of D-Region Electron Loss Rates and Electron Density Profiles from VLF Measurements during Solar Flare X-Ray Events.- The Polarization of VLF Radiowaves Reflected from the Ionosphere.- The Measurement of Electron Density below to km Report on a Discussion Session.- IV: Propagation of ELF Waves in the Earth-Ionosphere Duct Theory and Experiments.- ELFPropagation Theory.- Multimode Propagation in the Earth- Ionosphere Waveguide.- ELF Noise Measurements.- The Measurement of the E.L.F. Horizontal Electric Field Component of the Background Noise in the Sea.- Study of Typical Night Time Atmospheric Waveforms Observed at Poitiers (France).- Sanguine.- ELF Propagation Irregularities on Northern and Mid-Latitude Paths.- A Possible Explanation of the Nighttime Variations of ELF Signal Strengths in Connecticut.- Preliminary Discussion of ELF and VLF Propagation Data.- The Ionospheric Reflection-Level and Leakage-Rate for the Earth-Ionosphere Transmission-Line at ELF.- Some Results of Observations of IPDP- Type Pulsations at the Soviet Observatories.- V: Magnetospheric Propagation.- Ray Tracing.- Multipath Doppler Shifts in Man-Made VLF Signals.- Whistlers and Discrete ELF/VLF Emissions.- Controlled VLF Experiments.- Propagation Effects Observed in ELF/VLF Signals in the Ionosphere.- Transmission of ELF Pulses to the GEOS Satellite.- Some Spectral Peculiarities of ELF-VLF- Emissions at Subauroral Ground Stations.- VI: Application of ELF-VLF Waves.- ELF-VLF Applications in Navigation and Communications.- Timing Systems.- The Use of Propagation Data in Earth Resource Studies.- Transportable VLF Transmitter.- On-Line Scanning of VLF Transmitter.- Participants.