Longwave Propagation General Remarks

It is worth noting that the investigation of longwave propagation (from ELF through LF) was greatly influenced by ground-based observation of whistlers, a mode of propagation that is strongly influenced by the earth's magnetic field. This mode allows ionospheric penetration but no major system application of this mode has been developed. However, a number of proposals have recently been made for which longwave transmitters would be orbited, providing longwave terrestrial coverage from space. These concepts would utilize the whistler mode for penetration but significant, and possibly unrealistic, transmitter powers would be required for a system to be useful. A rather thorough discussion on longwave propagation in the ionosphere is found in the aforementioned Handbook on The Ionosphere and its Effects on Radiowave Propagation (i.e., Chapter 4 of [ITU-R, 1998]). Additional background on the theory of longwave propagation is found in earlier works, including a document published by AGARD [1982], books by Wait [1970] and Galejs [1972], and the U.S. Air Force Handbook [Jursa, 1985],

For longwaves, the antenna system is a significant component of the overall system. Indeed, the sheer size requirement to efficiently launch longwaves may be a formidable restriction. As indicated by Kelly [1986], considerable ingenuity has been employed to develop relatively efficient antennas that are necessarily large in human terms but small in comparison with a wavelength dimension. Contrary to popular opinion, it is thought that the classic Marconi experiment in 1901, which demonstrated the feasibility of communication over global distances, was actually performed using signals in the MF band, and bordering the longwave part of the radio spectrum. Marconi's success subsequently led to the notion that signals were being refracted from upper atmospheric strata rather than being diffracted along the earth surface. It turns out that the greatest efficiency in coverage by this ionospheric "bounce" process is achieved at HF, since ionospheric properties permit the largest refraction height in this frequency band. At lower frequencies (viz., LF and below), the interaction is restricted to heights on the order of 100 km or less restricting the maximum range for a single hop, while at higher frequencies (viz., VHF and above), the ionosphere has a high probability of being transparent and signals are lost into outer space. Nevertheless, since it has been shown that the lower ionosphere is far less variable (and more predictable) than the upper ionosphere, the longwave communication channel is a highly reliable one at least in comparison with the so-called shortwave (or HF) channel. Furthermore, we find that the longwave signal traverses far less of the ionosphere than signals in other frequency regimes, with most of its lifetime spent in the "free space" between the earth and the lower ionosphere. This accounts for the relative stability of longwave signals, but there are other factors that tend to reduce some of this attractiveness. First and foremost, huge power-aperture products are essential to overcome the impact of environmental noise. Also, the longwave channel has a very limited bandwidth, which restricts the ultimate information rate possible to achieve. A major advantage provided by the longwave channel is its seawater and earth penetration capability, especially at ELF.

While the use of longwaves in the practical realm is under decline, there is still a vigorous scientific and popular interest. Amateur monitoring and student activities abound, and an educational corporation has been established to encourage the science of natural radio listening (e.g., "Project INSPIRE"). On the professional level, Stanford University has engaged in a vigorous VLF research program since the middle of the 20th century, and they are still involved in studies of the ionosphere and magnetosphere using natural and man-made VLF signals. They use VLF waves as diagnostic tools to investigate the physical processes in the Earth's low and high altitude plasma environment. Umran Inan heads the VLF group, and it has an illustrious senior staff including Bob Helliwell (who introduced the author to the notion of Whistlers in the 1960s) and Don Carpenter (who is associated with the discovery of "Carpenter's knee" or the plasmapause). The VLF group manages multiple ground-based stations in the continental United States, Canada, and Antarctica; and has observational programs on satellites. Research includes modeling of such phenomena as sprites, blue jets, and elves. Refer to Chapter 6 (on Resources) for information on activities affiliated with the Stanford VLF group, such as HAARP, POLAR, CLUSTER, and IMAGE.

Long wave bands have very attractive ground wave characteristics, which are virtually unaffected by the ionosphere and nuclear-induced Electromagnetic Pulse (EMP). The United States built the Ground Wave Emergency Network (GWEN), a high power VLF communication system operating at 150-175 kHz, and designed to provide survivable connectivity to bomber and tanker bases. The system has been placed in a sustainment mode (circa 2000), but is being considered for other non-military purposes, such as inland navigation.

4.3.2 Extremely Low Frequency

The ELF band (<3kHz) has been the subject of theoretical investigation for a number of years, and Bannister [1986] has reviewed the topic. ELF field strength predictions include methods developed by Pappert and Moller [1978] in which the ionosphere and the earth are both assumed to be homogeneous and sharply bounded. Although this model is a simple one, the bulk of the evidence, both experimental and theoretical, has shown its adequacy. The ionosphere acts almost like a perfect conductor at ELF, and as a result, the earth's magnetic field-induced anisotropy is vanishingly small. Thus, the refractive index is independent of propagation direction. Ionospheric effects are not dominating at ELF; nevertheless certain ionospherically-related phenomena have been observed. They include the following:

(a) Anomalous field strength fluctuations, which may arise as the result of magnetic storms [Bannister, 1980], enhanced particle precipitation, or movement of sporadic E patches [Pappert, 1980]. These events are typically nocturnal, have magnitudes in the range of 3-8 dB, and may be the result of interference between waves reflected from the normal E region and an enhanced sporadic E patch.

(b) Solar-induced effects, such as x-ray flare disturbances, and solar particle events (SPE), which may increase the signal attenuation rates by 1-2 dB per megameter of path length. Field [1982] has reported daytime attenuation rate enhancements of up to 4 dB per megameter.

(c) Non-radial propagation disturbances arising from inhomogeneities, such as the day-night terminator, the polar cap boundary, or significant sporadic E formations. The most significant effects are observed for the situation in which the great-circle path is approximately tangent to the boundary of one of these features.

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