[1] Summer midnight and afternoon VLF wave generation comparison experiments were conducted on 25 July and 27 July 2011, respectively, using two CW HF X-mode waves with 11 VLF frequency differences from 2 to 21.5 kHz. The background magnetic variations were at comparable levels. During the afternoon experiment, the D region absorption was significant and increasing. The number of the ionogram echoes decreased during the afternoon experiment. 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 afternoon intensity 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 afternoon experiment 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.
[1] Results of experiment conducted at Gakona, Alaska, using intensity-modulated HF heating waves of 3.2 MHz to generate geomagnetic pulsations and large-scale fieldaligned density irregularities (FAI), are reported. The echo traces of o-and x-mode sounding pulses from 3.53 to 4 MHz were recorded during heater on/off periods. The ionograms showed that only x-mode echo traces were significantly affected by the HF heater. The returns from 3.53 to $3.8 MHz disappeared during heater on period and reappeared after heater was turned off. Ray tracings are performed to explore the effect of FAI on the backscattering of o-and x-mode sounding pulses. The drastic difference between the FAI effects on o-and x-mode backscattering trajectories provides a theoretical interpretation of the observation. Geomagnetic pulsations as large as 2.5 nT were also recorded by a Fluxgate Magnetometer. Thermal filamentation of the HF heater leads to the simultaneous generation of FAI and geomagnetic pulsations.
[1] The effects on the ionosphere of powerful O-mode and X-mode HF pump waves, modulated 3 minutes on and 1 minute off, were explored. The experiments were monitored using the digisonde and magnetometer located at the HAARP facility. The results show that the virtual heights of the O-mode sounding echoes shifted down/up as the O/X mode heater was turned on; the ionosphere also moved downward/upward accordingly. Enhanced spread-F was also observed in O-mode heater-on periods. Heater-induced magnetic pulsation was observed. Its intensity increased progressively in the heater on/off sequence and X-mode heater was more effective than O-mode heater in the generation of magnetic pulsation. In the last X-mode heater-on period, when the magnetic pulsation reached the highest level, pc 3 pulsations, with increasing intensity were also observed. Citation: Kuo, S., W.-T. Cheng, A. Snyder, P. Kossey, and J. Battis (2010), Contrasting O/X-mode heater effects on O-mode sounding echo and the generation of magnetic pulsations, Geophys. Res. Lett.,
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