Magnetic antenna excitation of whistler modes. IV. Receiving antennas and reciprocity

Abstract

Antenna radiation patterns are an important property of antennas. Reciprocity holds in free space and the radiation patterns for exciting and receiving antennas are the same. In anisotropic plasmas, radiation patterns are complicated by the fact that group and phase velocities differ and certain wave properties like helicity depend on the direction of wave propagation with respect to the background magnetic field B 0. Interference and wave focusing effects are different than in free space. Reciprocity does not necessarily hold in a magnetized plasma. The present work considers the properties of various magnetic antennas used for receiving whistler modes. It is based on experimental data from exciting low frequency whistler modes in a large uniform laboratory plasma. By superposition of linear waves from different antennas, the radiation patterns of antenna arrays are derived. Plane waves are generated and used to determine receiving radiation patterns of different receiving antennas. Antenna arrays have radiation patterns with narrow lobes, whose angular position can be varied by physical rotation or electronic phase shifting. Reciprocity applies to broadside antenna arrays but not to end fire arrays which can have asymmetric lobes with respect to B 0. The effect of a relative motion between an antenna and the plasma has been modeled by the propagation of a short wave packet moving along a linear antenna array. An antenna moving across B 0 has a radiation pattern characterized by an oscillatory "whistler wing." A receiving antenna in motion can detect any plane wave within the group velocity resonance cone. The radiation pattern also depends on loop size relative to the wavelength. Motional effects prevent reciprocity. The concept of the radiation pattern loses its significance for wave packets since the received signal does not only depend on the antenna but also on the properties of the wave packet. The present results are of fundamental interest and of relevance to loop antennas in space.

Comparison of electric dipole and magnetic loop antennas for exciting whistler modes

“…Next, we demonstrate that the rf magnetic field is that of a propagating whistler mode. Although this has been shown many times for a circular loop, 30,31 it will now be confirmed to hold also for an elongated loop antenna. Figure 3 displays three snapshots of one magnetic field component B z ðy; zÞ for four consecutive time steps, each a quarter rf period apart.…”

Rotating Alfvén waves in rotating plasmas

“…For this we consider two CK potentials describing two Alfvén modes with opposite OAM content in the rotating frame : These transform in the CK potentials in the laboratory frame : as a result of the rotational Doppler shift . These Alfvén CK potentials can be driven by a multicoil antenna similar to that used to study Whistler–Helicon modes (Stenzel & Urrutia 2014, 2015 a , b , c ; Urrutia & Stenzel 2015, 2016; Stenzel & Urrutia 2018; Stenzel 2019). The radial field pattern is then a superposition of and Bessel amplitudes , where is associated with the radial modulation of the antenna currents (Rax & Gueroult 2021).…”