Transitions from electrostatic to electromagnetic whistler wave excitation

At frequencies below the electron cyclotron and above the lower hybrid frequency in a magnetoplasma, the refractive index is anisotropic and shows resonances at certain angles caused by electron inertia. For a radiating antenna in a plasma, the short wavelengths near this resonance angle may contribute to the radiation pattern of the antenna. A series of experiments is reported, in which waves were excited using a small 1-cm-diam electrostatically coupled antenna into a preformed plasma, densities (n(e)) from 10(15) to 10(18) m(-3) and magnetic fields from 30 to 60 G. Maps of the wave amplitude and phase were made within the plasma by scanning the position of a b-dot probe. At low densities (e.g., n(e)5x10(16) m(-3)) the radiation pattern was characterized by a monotonic increase in wave phase in the axial direction, a central maximum for B-z, and off-axis maxima for B-x and B-y which are consistent with the propagation of m=0 helicon waves, and no evidence of resonance cone structure. This change in the radiation pattern is reproduced numerically in a homogeneous plasma model including an electrostatically coupled antenna with the same geometry as that used in the experiment. (C) 2004 American Institute of Physics.