Computer experiments on a three-wave coupling in association with microwave power transmission in space plasma

We studied a three-wave coupling process occurring in microwave power trans mission (MPT) experiment in the ionospheric plasma by performing computer e xperiments with one-dimensional electromagnetic PIC (Particle-In-Cell) mode l. In order to examine the spatial variation of the coupling process, we co ntinuously emitted intense electromagnetic wave from an antenna located at a simulation boundary. In the three-wave coupling, a low-frequency electros tatic wave is excited as the consequence of a nonlinear interaction between the forward propagating pump wave and backscattered one. In the computer e xperiments, low-frequency electrostatic bursts are discontinuously observed in space. The discontinuity of the electrostatic bursts is accounted for b y the local electron heating due to the bursts and associated modification of the wave dispersion relation. In a case where the pump wave propagates a long the geomagnetic field B-ext, several bursts of Langmuir waves are obse rved. Since the first burst consumes a part of the pump wave energy, the pu mp wave is weakened and cannot trigger the three-wave coupling beyond the r egion where the burst occurs. Since the dispersion relation of the Langmuir wave is variable due to the local electron heating by the burst, the coupl ing condition eventually becomes unsatisfied and the first interaction beco mes weak. Another burst of Langmuir waves is observed at a different region beyond the location of the first burst. In the case of perpendicular propa gation, the upper hybrid wave, one of the mode branches of the electron cyc lotron harmonic waves, is excited. Since the dispersion relation of the upp er hybrid wave is less sensitive to the electron temperature, the coupling condition is not easily violated by the temperature increase. As a result, the three-wave coupling periodically takes place in time and eventually the transmission ratio of the microwaves becomes approximately 20% while almos t no attenuation of the pump waves is observed after the first electrostati c burst in the parallel case.