An investigation of quasi-periodic VLF emissions

Quasi-periodic VLF emissions (QP's) observed at Eights, Byrd, and Polar Plateau Stations in Antarctica have been investigated. Periods of most QP's range from about 10 sec to about 2 min. At each of the stations QP's were generally found when the observing station was on the daylight side of the earth. Two general types of QP's can be identified on the basis of spectral structure and associated geomagnetic micropulsation (GP) activity. One type, which is characterized by an emission frequency that increases without interruption during one quasi-period, lacks association with GP activity. When this type of QP occurs, either there is no concurrent GP activity or the QP period is entirely different from that of the accompanying GP. The VLF emissions may be either diffuse or discrete or a combination of both. The period often varies with the average VLF signal strength, becoming longer for weaker emissions. The second type of QP exhibits an association with GP activity. The upper frequency limit of the emission band often fluctuates in synchronism with changes in the signal strength. This type of QP has a diffuse spectral structure and is referred to here as low-frequency hiss. Such QP's often occur simultaneously with chorus and/or periodic emissions that may or may not show QP variations. Quasi-periodic low frequency hiss associated with geomagnetic micropulsations does not generally occur simultaneously at a pair of magneto-conjugate Stations. QP's of the second type have lower emission frequencies (0.3–1.5 kHz) than the first type (1.5–6 kHz).