Transauroral Path Research Articles

Observations of the reduction in the available HF band on four high latitude paths during periods of geomagnetic disturbance

Abstract Four high frequency propagation paths were provided by a transmitter located within the polar cap and four receivers located variously within the polar cap and at sub-auroral latitudes. Of these paths, one was contained entirely within the polar cap at all times, two were trans-auroral at all times, and one varied from trans-auroral during the day to polar cap during the night. Fourteen frequencies within the HF band were transmitted each hour for the duration of two 24 day experimental campaigns during the summer of 1988 and the winter of 1989. The received signals were analysed to determine signal recognition and signal strength. The mean quiet-time behaviour of the propagation was determined and compared with four periods during which the ionosphere was perturbed by (relatively mild) geomagnetic storm conditions. The daily occurrence of correctly recognised signals decreased during the disturbed periods, both on polar cap and trans-auroral paths. Several geophysical causes for this decrease are identified, including a decrease in F-region electron density, decreasing the MOF, as a consequence of negative storm effects and an extended mid latitude trough. On polar cap paths, an increase in the LOF caused by mild PCA events also decreased the available HF band. Storm-time auroral-E makes propagation possible on frequencies higher than expected during the night on trans-auroral paths, mitigating to some extent the degradation in propagation. The storm-time decrease in the available HF band is quantified, falling to 0.3–0.5 of undisturbed levels even during relatively small geomagnetic storms. During the two experimental campaigns, 50% of days are identified as being storm-disturbed.

Observations of the directional characteristics of ionospherically propagated HF radio channel sounding signals over two high latitude paths

The author presents measurements of the directional characteristics of the Doppler and time delay components of a pulsed channel sounding signal received over two high latitude paths. A variation in bearing with Doppler frequency was frequently evident, an effect attributed to Doppler shifts imposed on the signal when scattered from ionospheric irregularities drifting across the reflection points. For a transauroral path, for which the ionospheric reflection points were subauroral, standard deviations of up to ~ 2.5° were observed in the azimuthal power distribution of the received energy. Much greater disturbances with azimuthal standard deviations of up to ~35° were recorded on signals received over a path contained entirely within the polar cap path.

Enhanced MUF propagation of HF radio waves in the auroral zone

Abstract Four high frequency propagation paths were monitored from a transmitter located within the polar cap by four receivers located variously within the polar cap and at sub-auroral latitudes. Of these paths, one was contained entirely within the polar cap at all times, two were trans-auroral at all times, and one varied from trans-auroral during the day to polar cap during the night. Fourteen frequencies within the HF band were transmitted each hour for the duration of two 24 day experimental campaigns during the summer of 1988 and the winter of 1989. From an analysis of the received signals the confidence of signal recognition and signal strength were determined. During geomagnetically undisturbed periods the propagation behaviour resembled that of mid-latitude paths. During geomagnetically disturbed times, however, night-time propagation occurred on frequencies up to and sometimes over 10 MHz above the undisturbed night-time MUF, for periods of 2 to 6 h. These features appeared on the trans-auroral paths only and were attributed to E region (and occasionally F region) enhancement by auroral precipitation. APEs (auroral E propagation events) occurred on over 50% of nights. The occurrence of APEs also coincided with ionospheric storm periods when the HF band available for propagation was otherwise significantly narrowed due to a depletion of the F region electron density.

Delay, Doppler, and amplitude characteristics of HF signals received over a 1300‐km transauroral sky wave channel

Channel probe observations of propagation conditions along a 1294‐km transauroral path between Sondrestrom, Greenland, and Keflavik, Iceland, were made during the period from March 13 to April 2, 1992. The midpoint of this path was located at a corrected geomagnetic latitude of 72°. The objective of these measurements was to supplement the existing data base describing propagation conditions on the HF transauroral channel with data pertaining to a period around the time of solar maximum. Received signals for this path fell into three distinct groups depending on their amplitude and delay and Doppler spread characteristics. These are (1) strong, specularly reflected ionospheric returns characteristic of a quiescent daytime ionospheric channel during magnetically quiet conditions; (2) strong specular multipath signals reflected from horizontal gradients of electron density and regularly encountered at night; and (3) weak scatter returns that are also a persistent nighttime phenomenon. The scatter returns are usually observed at delays exceeding those anticipated for the one‐hop return and, very often, at frequencies that are well above the MUF for the great circle propagation path. The multipath and scatter returns exhibit large delay and Doppler spreads indicative of spatially extensive distributions of drifting and randomly moving irregularities. Two measurement events are discussed which illustrate these conclusions: a noontime measurement with Kp = 3, and a midnight measurement with Kp = 2. The noontime measurement exhibited a scatter return from an isolated irregularity region in addition to the usual ionospheric reflected signals. A simple irregularity drift model produced delay and Doppler shift curves that were consistent with those observed for the scatter component of the received signal and supported a hypothesis of an irregularity region drift speed of 1200 m s−1 parallel to the great circle propagation path.

Wideband probing of the transauroral HF channel: Solar minimum

The Naval Research Laboratory HF Channel Prober is a high‐resolution, computer‐controlled, coded‐pulse sounder designed to study the time variable behavior of the wideband HF radio channel. Multispectral time histories of the channel pulse response and the channel scattering function are tools used in the study. Pulse response delay resolution down to 1 μs and unambiguous Doppler up to ±30 Hz are possible, depending on the choice of experiment format and operating parameters. Measurements were made quarterly between October 1985 and July 1986 on a 2300‐km transauroral path between a transmitter site at Frobisher Bay, Canadian Northwest Territories, and a receiver site near Rome, New York. Data are presented in the form of ionograms, pulse response time histories (power and coherent video), and channel scattering functions. Data for mid‐latitude and transauroral paths are compared, and their similarities and differences are discussed. The mid‐latitude channel is multimodal but is usually characterized by a single specular return per mode. The character of transauroral channel is quite variable, depending strongly on the degree of magnetic disturbance, the location of the midpath reflection point relative to the position of the auroral oval, and the mode. During quiet magnetic conditions, and with the path midpoint well below the auroral oval, channel behavior is shown to resemble a mid‐latitude channel at certain times and a specular multipath channel at other times (i.e., one where each mode comprises several distinct multipath components). Doppler spread in these cases tends to be small. At night, when the midpath point lies within the auroral oval, the return signal is diffuse, and the Doppler spread appears to be larger by an order of magnitude.

Quantitative study of substorm‐associated VLF phase anomalies and precipitating energetic electrons on November 13, 1979

The phase anomalies associated with substorms are observed on VLF signals propagating on transauroral paths (transmitters at OMEGA‐ALDRA (13.6 kHz), GBR (16.0 kHz), and OMEGA‐NORTH DAKOTA (13.6 kHz)) which were continually received at Inubo, Japan, during the events on November 13, 1979. Detailed comparisons are made between these phase anomalies and geomagnetic bays, and quantitative relations are obtained with precipitating energetic electrons (E > 30, E > 100, and E > 300 keV) detected on board the TIROS‐N and NOAA 6 satellites. It is concluded that two types of VLF phase anomalies exist which, in turn, are associated with two phases in the history of energetic electron precipitation into the atmosphere. The first type of phase anomaly is associated with direct injection of energetic electrons into the outer magnetosphere and atmosphere which, in turn, is completely correlated in time with development of the auroral electrojet current system. The second type arises from energetic electrons which subsequently precipitate from a trapped electron population and has a delayed onset and prolonged duration. An excellent quantitative correlation is obtained between the logarithm of the electron flux and the magnitude of the phase anomaly on the OMEGA‐ALDRA signal. From the local time characteristics of this quantitative relation it is deduced that the electrons with E > 300 keV are the main source of D region ionization responsible for the VLF phase anomaly.

Long term variation of VLF signals over transauroral paths

A part of 1-yr period synoptic representation of NPG and NPM field strength and relative phase are shown in a system of rectangluar coordinates. Sunrise and sunset lines at the 70 km level are involved in the general picture taking into consideration a 30-km screening height. Experimental results are interpreted as far as possible in terms of interferences of mode conversion after Crombie's model ( Crombie, 1964). Profiles of field strength along specific transition lines in the course of days show a quasi-semi-annual amplitude variation with maxima in summer and in winter, and minima in spring and in autumn. From computed solar zenith angles over the wave paths it can be concluded that the ionization rate in D-region required for the formation of the field strength winter maxima do not rely upon the ionizing radiation of the Sun, but on other causes and probably on dynamical and aeronomical effects in the mesosphere. The apparent contradiction between results on LF absorption measurements by Lauter and Nitzsche (1967) and the present computation is to be explained by the height difference between VLF and LF reflecting layers.

Frequency Spread in Ionospheric Radio Propagation

As part of a research program on the time- and frequency-dispersive characteristics of ionospheric radio propagation, techniques were developed for precise measurement of the power spectrum of a received CW signal. Frequency resolution of 0.025 Hz is obtained. This resolution allows the observation of the various propagating modes and, with the aid of oblique-incidence ionograms taken simultaneously, identification of the modes. Occasionally, even the ordinary and extraordinary components of the wave can be separated and identified. Power spectra were derived from measurements made on a midlatitude path and on a transauroral path over a period of one year near the solar cycle minimum. In addition to the plotting of the power spectra, the root-mean-square widths of the spectra (defined as the frequency spread) and the means (or average Doppler shift) were computed. Distributions of these data are presented, and the frequency spread is compared with the magnetic activity, the timedelay spread, and the number of modes.