Chorus, hiss, and other audio-frequency emissions at stations of the whistlers-east network
Ionospherics, electromagnetic waves originating somewhere in the ionosphere or the magnetosphere of the earth, have been recorded for more than six years in the audio-frequency band at the Whistlers-East chain of ground-based receiving stations which extend approximately along the 0° geomagnetic meridian from Labrador to Antarctica. This has made it possible to study the latitude dependence of: 1) the local time of maximum occurrence of the various types of audio-frequency emissions, 2) the shape of the diurnal occurrence curves, 3) the rate of occurrence, and 4) the effects of geomagnetic disturbances upon the occurrence. Some of the results and conclusions of a comprehensive analysis of these synoptic data are: 1) At middle- and high-latitude stations the occurrence or nonoccurrence of audio-frequency emissions appears to be controlled mainly by the emission processes rather than by the subsequent propagation of the emitted wave through the magnetosphere and the ionosphere. This is contrary to what we have concluded about the occurrence of whistlers. Propagation conditions apparentally do become controlling at low latitudes, and at all latitudes at times of severe geomagnetic disturbances. 2) At middle-latitude stations, audio-frequency emissions are essentially a geomagnetic storm-time phenomenon. At high latitudes, the emissions are more likely to be observed under geomagnetically quiet conditions. 3) At a given station, the correlation between the occurrence of audio-frequency emissions and the level of geomagnetic activity is practically the same for all major types of emissions, although different types of emissions tend to occur at different times of the day. The correlation shows no significant seasonal variation (except possibly at Knob Lake). 4) At some stations, the local time of maximum occurrence of chorus remains surprisingly constant from season to season and from year to year. Moreover, when seasonal data of these stations are averaged over a long period of time, it is found that chorus shows no preference for any season (Dartmouth and Ellsworth). 5) At the two stations of the Whistlers-East network closest to the equator (Bermuda and Port Lockroy), chorus is almost absent in local summer but in local winter the activity is relatively high, displaying a maximum of occurrence appproximately at the same time as at stations more than ten degrees closer to the poles. 6) At high latitudes, on the other hand, more chorus is observed in local summer than in local winter. The local time of maximum occurrence of chorus at high latitudes depends on the level of geomagnetic activity, quiet-day chorus peaking later than disturbed-day chorus. 7) The diurnal variation of hiss also depends on latitude, season, and geomagnetic activity. It peaks near midday in latitudes above Dartmouth but develops a deep midday depression at Dartmouth in local summer and fall and in all seasons in lower latitudes. 8) In the northern hemisphere at sunspot maximum, in the longitude of the Whistlers-East chain, the activity of audio-frequency emissions appears to be highest at a geomagnetic latitude of about 60°. The activity decreases only slowly toward the poles but rapidly below about 50°. As the solar activity has subsided, the region of maximum activity has moved farther north. A corresponding situation is found in the magnetically conjugate region of the southern hemisphere.