Beat wave generation experiment for study of VLF whistler wave interactions with ionospheric plasmas and radiation belts

Abstract

We have been conducting controlled studies of VLF whistler wave interactions with “inner” radiation belts at Arecibo, Puerto Rico and with “outer radiation belts” at Gakona, Alaska. Whistler waves at 40.75 kHz can be launched from a Naval transmitter code-named NAU at Arecibo, while HF amplitude-modulated electrojet current can radiate at selected VLF wave frequencies at Gakona, Alaska. A new technique has been recently developed in our Gakona experiments, which does not require a VLF transmitter or the presence of electrojet current. It relies on the operation of two HF heaters to generate two X-mode waves at a difference frequency in the VLF range. For example, the beat wave at 6.5 kHz can be generated by a heater wave at 4.2 MHz with the other at 4.2065 MHz. A series of whistler waves generated by the aforementioned beat wave technique at 6.5, 7.5, 8.5, 9.5, 11.5, 15.5, 22.5, 28.5, and 40.50 kHz, respectively, were successfully detected by our VLF receiving system, deployed a few miles away from the HF heater. It is interesting to notice that the intensity of VLF waves does not monotonically decrease with the frequency. We are currently working on this subject theoretically, after analyzing some data, to examine optimum conditions for generating VLF whistler waves by beat waves. Whistler wave interactions with ionospheric plasmas can result in energization of ionospheric electrons via a four-wave interactions process and a direct acceleration process additively. Monitored by an incoherent scatter radar, the energized electrons streaming along the geomagnetic field lead to enhanced plasma lines. It is expected that, only beat wave-generated whistler waves with frequencies exceeding lower hybrid resonance frequency can produce enhanced plasma lines in the ionospheric F region via a four wave interaction process. Contrastingly, whistler wave interactions with energetic charged particles in radiation belts result in enhanced plasma lines in the ionospheric E region. The development and demonstration of beat wave technique to generate VLF whistler waves has further extended our capability to investigate whistler wave interactions with ionospheric plasmas and radiation belts.