VLF wave generation by beating of two HF waves in the ionosphere

Abstract

[1] Theory of a beat-wave mechanism for very low frequency (VLF) wave generation in the ionosphere is presented. The VLF current is produced by beating two high power HF waves of slightly different frequencies through the nonlinearity and inhomogeneity of the ionospheric plasma. Theory also shows that the density irregularities can enhance the beat-wave generation. An experiment was conducted by transmitting two high power HF waves of 3.2 MHz and 3.2 MHz + f, where f = 5, 8, 13, and 2.02 kHz, from the HAARP transmitter. In the experiment, the ionosphere was underdense to the O-mode heater, i.e., the heater frequency f 0 > foF2, and overdense or slightly underdense to the X-mode heater, i.e., f 0 < fxF2 or f 0 ≥ fxF2. The radiation intensity increased with the VLF wave frequency, was much stronger with the X-mode heaters, and was not sensitive to the electrojet. The strongest VLF radiation of 13 kHz was generated when the reflection layer of the X-mode heater was just slightly below the foF2 layer and the spread of the O-mode sounding echoes had the largest enhancement, suggesting an optimal setting for beat-wave generation of VLF waves by the HF heaters.

Excitation of guided ELF-VLF waves through modification of the F2 ionospheric layer by high-power radio waves

“…Comparison of the data presented in Figs. [3][4][5] shows that an inhomogeneous waveguide duct forms in the disturbed magnetic tube. The characteristic size of the plasma density duct along the satellite trajectory is about 90 km.…”

“…At present, several kilometer long space cable systems [1] and active plasma antennas formed in the ionospheric plasma by high frequency (HF) radio emission from either ground based facili ties [2,3] or flying vehicles [4] are assumed to be the most promising for controlled excitation of LF emis sion. In [5], the possibility of excitation of LF emis sion by the beatings of two or several HF radio waves was also considered.…”

Ionospheric very low frequency transmitter

“…The frequency dependencies of the daytime/nighttime radiation amplitudes are quite similar, but the nighttime radiation is much stronger than the corresponding one generated in the daytime experiment, consistent with the facts that the D region absorption reduced the heater wave intensity significantly in the daytime experiment. Moreover, the nighttime radiation was generated in the underdense situation attained with relatively low heater frequency, which were identified in the previous experiment 23 to be the favorable beat wave generation conditions.…”

“…Let T e ¼ T e0 þ hdT e i þ Á Á ÁÁ Á Á and hjV pe j 2 i ¼ Wexp(Àix 1 t) þ c.c., the induced temperature perturbation hdT e i, which oscillates at the beat frequency x 1 , is obtained to be hdT e i ¼ ið2m=3Þ½ el =ðx 1 þ idÞ W expðÀix 1 tÞ þ c:c:; (23) where el ¼ ei þ en is the elastic collision frequency and…”

“…In the present work, a beat wave modulation mechanism 23,24 is explored for possibly establishing a reliable virtual ionospheric transmitter for VLF communications. It employs transmissions with slightly different frequencies from the two sub arrays of the HF transmitter divided equally.…”