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

Some results of five years of whistler observations from Labrador to Antarctica

Some interesting curves result when whistler data are averaged over a long period of time to smooth out random short-term variations in whistler generation and propagation. Local season is such a strong factor in determining the shape of the diurnal curves of observed whistler activity that even in the case of the north-south "Whistlers-East" chain of audio-frequency receiving stations, diurnal curves of the northern- and southern-hemisphere stations tend to be similar not during the same period of the year but during the same local season at the point of observation. A basic form of the diurnal curves appears to be one which is symmetrical about local midnight with a deep minimum at local noon and relatively high nighttime activity showing maxima at about 2000, 2400 and 0400 hours local time. Depending on local season and location of the station, one or more of these peaks may be absent or enhanced. It is concluded that the shape of the diurnal curves is determined largely by the conditions of whistler propagation rather than of generation. The curves presented should therefore be useful in predicting the behavior of man-made signals propagating in the "whistler mode" through the ionosphere and the magnetosphere of the earth. Some difficulties are pointed out with the prevalent idea that the marked depression in the occurrence of whistler-mode signals in the daytime is primarily the result of absorption of these waves in passing through the D layer. On the other hand, caution is advised against neglecting ionospheric factors, other than D-region absorption, in deference to the role of field-aligned ducts of enhanced ionization in the magnetosphere. Finally, some results are presented which show that, in addition to a latitude variation of the dependence of whistler rate upon the K p index of geomagnetic disturbance, there is, especially at the lower latitude stations, a seasonal variation of these curves as well

Advances in Plasmaspheric Wave Research with CLUSTER and IMAGE Observations

International audienceThis paper highlights significant advances in plasmaspheric wave research with Cluster and Image observations. This leap forward was made possible thanks to the new observational capabilities of these space missions. On one hand, the multipoint view of the four Cluster satellites, a unique capability, has enabled the estimation of wave characteristics impossible to derive from single spacecraft measurements. On the other hand, the Image experiments have enabled to relate large-scale plasmaspheric density structures with wave observations and provide radio soundings of the plasmasphere with unprecedented details. After a brief introduction on Cluster and Image wave instrumentation, a series of sections, each dedicated to a specific type of plasmaspheric wave, put into context the recent advances obtained by these two revolutionary missions