VLF Measurements Research Articles

Noise in the geomagnetic tail

Present observations have revealed a variety of magnetic wave phenomena in the tail, from ULF to ELF frequencies. However, only VLF measurements of electric fields have been made. These measurements reveal that the tail is electrically quiet at VLF frequencies, except in the near Earth plasma sheet during substorm expansion phases. The magnetic waves observed include: waves with periods of about 2 min which cause the plasma sheet boundary position and the neutral sheet location to oscillate; waves from 10 −1 to 1 Hz which occur throughout the plasma sheet during plasma sheet expansions; and ELF waves which occur sporadically in the plasma sheet.

Observations of SAR arcs from OV1-10

Three midlatitude red arcs were measured by the night airglow photometer on board OV1-10 (1966–111B) during the winter of 1966–1967. One arc was observed as a conjugate phenomenon. Another arc was observed for 2 days at a constant local time and was found to decrease in intensity, increase in width, and move to higher L values with universal time. Both arcs were found to be associated with the plasmapause, the location of which was determined from the VLF measurements made by OV3–3 (1966–70A). These observations are shown to be consistent with the theory that SAR arcs are generated at the plasmapause as a consequence of the turbulent dissipation of ring current energy.

Rapid access of solar electrons to the polar caps

Abstract : Simultaneous measurement of solar electrons and protons in interplanetary space and in the magnetotail were made during the onset of the 2 November 1969 solar particle event. These particle measurements, when compared with continuous trans-polar VLF measurements on three propagation paths, indicate the solar electrons have access to the magnetotail and north polar cap with a time delay t such that O approximately equal or < t less than 1 min. (Author)

VLF measurements of the Poynting Flux along the geomagnetic field with the Injun 5 satellite

Poynting flux direction for proton whistlers determined from Injun 5 observations, obtaining data on source region and propagation in ionosphere

The position and height of auroral absorption deduced from VLF phase measurements

The main findings from VLF measurements during auroral absorption are as follows: The geographical extent of auroral absorption seems to be at least a factor of two larger than found by HF riometers. The VLF reflection layer is markedly depressed during auroral disturbances. At night large and rapid fluctuations in the reflection layer may occur simultaneously over distances of more than 500–1000 km. During day-time the depressed VLF reflection layer seems to be relatively homogeneous over the same distances.

Movement of off-path ionospheric irregularities deduced from short-path VLF measurements

A short radio path is used to measure the spatial and temporal characteristics of fluctuations of VLF signals reflected from the ionosphere. The experiment uses multiple frequency transmissions from a horizontal VLF antenna received at a triangle of spaced receivers. Power spectrum and cross spectrum analyses of the phase and amplitude records are used to demonstrate that much of the fluctuation in the records results from Doppler beating of the principal signal and signal components reflected from off-path irregularities. A new technique for determining the position and movement of irregularities in the ionosphere is described for use when Doppler lines are an important part of the records. Some typical records are shown and some implications concerning previous published analyses of fading records are discussed. The risk of interpreting periodic features in radio records to result necessarily from a wave structure in the ionosphere is made evident.

Interpretation of pre-sunrise electron densities and negative ions in the D-region

Pre-sunrise rocket measurements of the electron density in the D-region were carried out at Wallops Island, Virginai, on 15 July 1964, 14 June 1965, and 14 June 1966, when the solar zenith angle was near 95°; these rocket measurements were complemented by ground-based, steep-incidence VLF amplitude measurements of the NSS skywave. On 15 July 1964 and 14 June 1965, the rocket measurements revealed the presence of electrons in the D-region when the solar zenith angle was near 95°; and the VLF recordings showed absorption commencing at χ = 94°, approximately. However, during the pre-sunrise period on 14 June 1966, the VLF absorption commenced when the solar zenith angle was near 98°; and a rocket experiment near χ = 94° revealed an electron density profile almost identical to the one obtained on 14 June 1965, the exception being a ledge of ionization between 85 and 90 km. Based on the rocket electron density and VLF measurements, and assumptions of the height distribution of ozone, it is suggested that the VLF amplitude decreases near χ = 94° and χ = 98° are caused by the photodetachment of electrons from high-electron-affinity negative ions by solar u.v. radiation. Also, it is believed that an occasional VLF amplitude decrease commencing near χ = 99° is caused by the photodetachment of electrons from a low-electron-affinity negative ion by solar visible radiation.

On the use of VLF measurements for obtaining information on the lower ionosphere (especially during solar flares)

Three types of VLF propagation measurements which can be used to determine some characteristics of the lower ionosphere are described. These methods are: 1) determination of the field strength (and ideally the phase) of a continuous wave transmitter as a function of distance, 2) determination of the relative phase and amplitude of a broadband signal, such as an atmospheric, and 3) observing simultaneously the phase and amplitude of a continuous wave signal at a single point remote from the transmitter. The latter method is discussed in detail. Although it cannot alone yield much information about the normal steady-state ionosphere, it can be used to investigate changes in the electron density distribution. In this paper phase and amplitude variations observed during solar flares are used to deduce the corresponding D-region changes, both for a sharply bounded and for an exponentially varying model of the ionosphere. It is further suggested that the exponential model can be used to explain VLF observations made during Polar Cap Absorption events, as well as the somewhat anomalous diurnal variation of VLF signals observed on equatorial paths.