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) at 50 G and 3 mTorr), a single amplitude maximum along the group velocity resonance cone angle was measured that decayed as the distance from the antenna increased. The observed radiation pattern in all cases was consistent with that of a point source in an unbounded plasma, and no global eigenmode resonances were found. As the density was increased, the apparent attenuation of the resonance cone waves increased and they appeared to withdraw into the antenna. At high densities (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.