The propagation of a radio signal and the propagation in the time domain is reviewed for linear amplitude systems. The particular case of the propagation of a ground wave pulse is considered in detail. A stretching in the form or shape of the pulse is noted as a result of the filtering action of the propagation medium. Theoretical transfer characteristics for the media of propagation of LF signals are introduced and methods of computation are considered. The particular case of a signal transmitted between two points on the earth's surface is considered from the viewpoint of propagation in the time domain. The field of LF waves propagated around the earth is, in large measure, influenced by the reflection and transmission processes at the ionosphere. Such processes are evaluated theoretically with the aid of Maxwell's equations together with an equation which describes the electron motion in the presence of a static magnetic field, a superposed electrodynamic field together with mechanical collisions between electrons and ions, such as the Langevin equation of motion of the electron. The use of full mathematical rigor in the application of these equations is feasible and indeed desirable at LF. Thus, the application of these equations to an electron-ion model plasma with arbitrary orientation of the superposed magnetic induction results in anisotropic transmission and reflection properties. The full rigor can be applied to model plasmas in which the electron density and collision frequency vary with altitude.

Propagation of the Low-Frequency Radio Signal

Numerous papers have been dealing with extremely low frequency (ELF) phenomena recently and a comprehensive bibliography with over one thousand entries and abstracts has been prepared by Brock-Nannestad (1962). A chapter of a recent book by Wait (1962a) discusses ELF waves and it also contains a number of references to published papers. It will not be attempted to relist here all the references of the above publications. Instead, after a brief historical review of the work on ELF propagation and a summary of the fundamental concepts, much of the discussion will be devoted to recent analytical and experimental results.

Terrestrial Extremely-Low-Frequency Propagation

\n Major solar eruptions occasionally cause magnetic superstorms on the Earth. Despite their serious consequences, the low frequency of their occurrence provides us with only limited cases through modern instrumental observations, and the intensities of historical storms before the coverage of the Dst index have been only sporadically estimated. Herein, we examine a solar-terrestrial storm that occurred in 1946 March and quantitatively evaluate its parameters. During the ascending phase of Solar Cycle 18, two moderate sunspot groups caused a major flare. The H α flaring area was recorded to be ≥600–1200 millionths of solar hemisphere, suggesting that this was an M- or X-class flare in soft X-ray intensity. Upon this eruption, a rapid interplanetary coronal mass ejection (ICME) with an average speed of ≈1590 km s−1 was launched. Based on measurements in four known mid-latitude and relatively complete magnetograms, the arrival of this extreme ICME caused a magnetic superstorm, which caused an initial phase with the H-component amplitude of ≥80 nT, followed by a main phase whose intensity was reconstructed as ≤−512 nT using most negative Dst* estimates. Meanwhile, the equatorial boundary of the auroral oval extended down to ≤41${^{\\circ}_{.}}$8 in invariant latitude and formed a corona aurora in Watheroo, Australia. Interestingly, during this magnetic superstorm, larger magnetic disturbances were recorded at dusk and near the dip equator on the dayside. Its cause may be associated with a strong westward equatorial electrojet and field-aligned current, in addition to the contribution from the storm-time ring current.

Intensity and time series of extreme solar-terrestrial storm in 1946 March

The effect of collisions on the propagation of radio waves in the ionosphere

This paper describes the use of the generation of extremely low frequency (ELF) from heating of the D region of the ionosphere at high latitudes to estimate the degree of electron density ionization of the D region. In particular, the heating facility near Fairbanks, Alaska, known as the high-power auroral simulation (HIPAS) facility, was used in this work. The theory of electron density synthesis is described, results of actual data presented, and electron densities determined. A new future ionospheric measurement technique is discussed.

J.J. Gibbons

Papers

Polarization computations by means of the multislab approximation

Calculation of group indices and group heights at low frequencies

Theoretical computation of ionospheric absorption at low frequencies