Five‐station observations of Pc 1 micropulsation propagation

Abstract

An experimental study has been conducted on the characteristics of the propagation of mid‐latitude Pc 1 micropulsations using a five‐station receiving network stretching from New York to Florida and from Maryland to Illinois. All data were telemetered in real time over telephone data lines for centralized recording and analysis. Cross‐correlation techniques were used to determine group velocity and direction of arrival using three different computational methods. Amplitude, spectral, and polarization characteristics are presented for the data analyzed. It was found that the propagation direction tended to coincide approximately with that of the geomagnetic meridian in the vicinity of the network of sites and that the propagation was characterized better by curved wave fronts than by plane wave fronts. No examples of east–west propagation were observed, but this is to some extent a consequence of the mid‐latitude location of the observation sites. Propagation group velocities ranging from 444 to 1523 km/s were observed. Variation in group velocity for a given event over a period of an hour was typically less than 10%. Attenuation rates ranging from 0 to 13 dB/1000 km, with an average of 6.5 dB/1000 km, were observed.

Frequency-dependent polarization characteristics of Pc1 geomagnetic pulsations observed by multipoint ground stations at low latitudes

“…Indeed, Althouse and Davis [1978] also frequently observed that the polarization sense changes repeatedly during a 10 min period. Pope [1964] showed by model calculations that the superposition of a number of waves with different frequencies should result in periodic variations of not only amplitude but also polarization parameters, and that significant effects occur near the beat minimum points.…”

“…Webster and Fraser [1985] calculated the polarization ellipse orientation averaged over time and frequency by the method of Fowler et al [1967] for 28 Pc1 events observed at low latitudes and found no relationship between the orientation and the typical Pc1 frequency. Althouse and Davis [1978] studied 18 Pc1 events observed at five middlelatitude ground stations. They showed that polarization parameters obtained using narrow-band filters often varied significantly over time, and only the time-averaged parameters showed the possible ionospheric source location.…”

Propagation of Alfvén waves through the ionosphere: Dependence on ionospheric parameters

“…In addition, the propagation of compressional waves through the ionospheric waveguide implies that a signal observed on the ground may not be on the same field line as the field-aligned current structure that produced it. Multistation observations of the propagation of Pc 1 signals through the ionospheric waveguide at midlatitudes have been made by Fraser [1975] and Althouse and Davis [1978], who showed that waveguide velocities were approximately in the range 500-2000 km/s. Neudegg et al [ 1995] have shown from an array of ground magnetometers at high latitudes that Pc 1 signals propagate with a typical speed of 450 krn/s over distances of a few hundred kilometers, consistent with the group velocities calculated for propagation in this ionospheric waveguide by Greifinger and Greifinget [1968].…”

The possible statistical relation of Pc1 pulsations to Earthquake occurrence at low latitudes

“…The number of Pc1 pulsations in each T-LT bin is shown in color, and indicates that there is a pronounced tendency of Pc1 pulsations to be observed in the night and dawn hours, due primarily to the larger degree of ionospheric attenuation experienced by Pc1 pulsations as they propagate from high latitudes to lower latitudes on the day side (e.g. Althouse and Davis, 1978;Jacobs, 1970, p. 28). From Fig.…”

“…Manchester, 1966Manchester, , 1968Fraser, 1968;Jacobs, 1970, p. 115), which is centered on the F2 region electron density maximum at ∼350 km (Manchester, 1966). The wave power progressively attenuates and leaks to the ground through the lossy bottomside ionosphere as it propagates from high to low latitudes, the attenuation being most severe during the daytime (Althouse and Davis, 1978), resulting in a nighttime occurrence maximum at low latitudes (e.g. Jacobs, 1970, p. 28), and concomitant minimum during the day.…”