Experimental and theoretical first approach to ƒH plasma resonance from a relaxation sounding rocket experiment

Abstract

The various plasma resonance phenomena observed in the ionosphere as relaxation signals near the characteristic frequencies ƒN, ƒT (plasma and upper hybrid frequencies), and n ƒH (electron gyrofrequency harmonics) have been correctly interpreted as oblique echoes. However, the ƒH case is not yet understood. The EIDI 3 relaxation sounding rocket experiment has provided original results regarding the plasma resonances. These results provide grounds to approach the ƒH problem. Indeed, the spectral features of the ƒH resonance signal look like pure sine‐wave ones; their frequency location is found to be very close to the gyrofrequency deduced from the earth's magnetic field model (relative frequency shift of the order of 10−4). The duration of the ƒH resonances is very short and their amplitude level very weak in the EIDI 3 ionospheric flight conditions. Moreover, a well‐defined dependence of this level on the orientation of the dipole antenna with respect to the earth's magnetic field has been pointed out, as well as an increase of the signal level when the antenna length is doubled. To interpret theoretically these experimental results, a brief review is made of the various modes of principal waves propagating close to ƒH. Their frequency range, their wave vector orientation, and the EM field polarization are examined with respect to the corresponding experimental features in order to discard the disagreeing kinds of waves. The ordinary wave mode gives good agreement with the EIDI 3 results. A first‐approach study of the interest of this mode for explaining the ƒH resonance phenomenon provides good grounds for its further study.