Nonducted mode of VLF propagation between conjugate hemispheres; Observations on OGO's 2 and 4 of the ‘walking-trace’ whistler and of Doppler shifts in fixed frequency transmissions

Abstract

Evidence for nonducted VLF propagation between conjugate hemispheres has been found in records from the broadband VLF receivers aboard the polar satellites OGO 2 (419–1521 km) and OGO 4 (412–908 km). The nonducted signals described here are received in the ionosphere between 47° and 56° invariant latitude. They have never been observed on the ground and include natural whistlers and fixed‐frequency signals (10.2–12.5 kHz) from the U.S. Navy Omega transmitters. In a time‐frequency spectrogram, these nonducted whistlers appear as rising tones with a lower cutoff frequency in the approximate range of 5 to 8 kHz. They have been named ‘walking‐trace’ (WT) whistlers, since a rapid increase in travel time as a function of satellite latitude causes successive examples of the rising trace to ‘walk through’ other whistlers having equal dispersions and produced by the same sequence of lightning sources. A train of WT whistlers exhibits a nearly constant lower cutoff frequency that is equal to the maximum value of the lower hybrid resonance (LHR) frequency above the satellite and an upper cutoff frequency that decreases with increasing satellite latitude. Reflected waves following a WT whistler can also be received if the LHR frequency below the satellite reaches values greater than above it. Observed spectral shapes of such whistlers resemble a fishhook. Fixed‐frequency Omega signals observed by OGO 4 in the hemisphere conjugate to the transmitter frequently have characteristics smiliar to those of the WT whistlers. The Omega signals exhibit two features that are not apparent in the natural whistlers: an enhancement of signal strength and a Doppler shift that increases with latitude and may reach hundreds of hertz. The main characteristics of the above phenomena are explained by tracing nonducted rays between conjugate hemispheres in a model magnetosphere. An equatorial electron concentration profile is derived from the WT whistlers.

Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 1. Observations and interpretation

“…The diagnostic potential of the LHR in terms of ion composition or effective mass m eff has been demonstrated in passive studies of wave activity observed aboard low altitude polar orbiting satellites [e.g., Barrington et al , 1965; Brice and Smith , 1965] and now once again becomes evident as we investigate active probing at altitudes below ∼5000 km with IMAGE. At much higher altitudes, the phenomenon of MR reflection has in the past received considerable attention, but mostly as an aid in modeling the propagation of various VLF waves such as non ducted lightning whistlers [e.g., Edgar , 1976; Walter and Angerami , 1969; Bortnik et al , 2003] and emissions such as chorus [e.g., Bortnik et al , 2008] and hiss [e.g., Sonwalkar and Inan , 1989; Abel and Thorne , 1998]. Now, with the prospect of remotely sensing the distribution of f lh along B 0 , there is the prospect of using radio sounding at frequencies lower than the 6 kHz limit of the RPI programs.…”

“…These sounding operations have allowed us to observe the results of WM sounding in an altitude range not previously explored and at frequencies lower than those previously employed. In making preliminary assessments of the new data, we are able to draw upon existing knowledge of several important whistler mode propagation phenomena, notably (1) non‐ducted propagation, wherein wave‐normal directions are not constrained to remain within a small cone of angles around the direction of the geomagnetic field but are strongly affected by large‐scale spatial variations of the plasma parameters [e.g., Walter and Angerami , 1969; James , 1972; Cerisier , 1973; Sonwalkar et al , 1984], (2) the phenomenon of “magnetospheric reflection” (MR), in which the raypath of a wave propagating at large wave‐normal angle is reversed near a point where the refractive index surface undergoes an important change in topology and size (e.g., Kimura [1966], Smith and Angerami [1968], who introduced the term “magnetospheric reflection,” Edgar [1976], and Shklyar et al [2010]), (3) spreading of wave‐normals (and hence time delays) due to encounters with geomagnetic field aligned density irregularities (FAIs) of transverse scale sizes varying from 10 m to 100 km [e.g., James , 1972; Sonwalkar et al , 1984, 2004a], and (4) partial or complete reflection of downcoming waves at the sharp lower ionospheric boundary [e.g., Helliwell , 1965; Brittain et al , 1983; Sonwalkar et al , 2004a].…”

Modeling of Doppler‐shifted terrestrial VLF transmitter signals observed by DEMETER

“…One of the first reports concerned observations from the OGO-4 spacecraft of VLF transmitter signals propagating from the northern hemisphere through the plasmasphere to the spacecraft in the southern hemisphere [Walter and Angerami, 1969]. The Doppler shifts observed on OGO-4 were a few hundred Hz.…”

Experimental study of the relationship between the propagation distance and the polarization characteristics of whistlers

“…tennas have been measured and the following results found: ( 1 ) the polarization of whistlers which penetrate the ionosphere close to the observing station is righthanded over a wide frequency range; and (2) some whistlers are found to have propagated over a distance of %1500 km. These whistlers exhibit a right-handed polarization at some frequencies and a left-handed polarization at other frequencies.…”