Publikationsdatum:
2013-02-27
Beschreibung:
[1] Summer midnightand afternoonVLF wave generation comparison experiments were conducted on July 25 and July 27, 2011, respectively, using two CW HF X-mode waves with eleven VLF frequency differences from 2 to 21.5 kHz. The background magnetic variations were at comparable levels. During the afternoonexperiment, the D region absorption was significant and increasing. The number of the ionogram echoes decreased during the afternoonexperiment. VLF signals were detected from 2 to 7.6 kHz in both experiments, showing an inverse frequency dependence of intensity, although signal intensity (except at 5.5 kHz) detected during the midnight experiment was stronger than the corresponding afternoonintensity before observing a decrease of the number of ionogram echoes. However, VLF signals from 11.5 to 21.5 (except at 19.6 kHz) were also generated in the afternoonexperiment concurrent with a decrease of the O-mode ionosonde echoes from 2 to 4 MHz. The concurrence of a decrease of the afternoon ionogram echoes, the unexpected generation of VLF waves at higher frequencies, and the increasing D region absorption throughout the experiment may be explained by the generation of large scale density irregularities, which scatter the ionosonde signals as well as couple with the modulated electrojet to generate whistler waves. A theoretical formulation of the coupling mechanism for the whistler wave generation is presented.
Print ISSN:
0148-0227
Thema:
Geologie und Paläontologie
,
Physik
