Whistler modes excited by magnetic antennas: A review

A review of a deceptively simple topic is presented, i.e., the excitation of whistler modes by antennas. It includes the knowledge of antennas and of the waves and their coupling. This review will show how the research in the last few decades has advanced and become a refined and complex topic which covers nonlinear effects, instabilities, nonuniform fields, whistler modes with orbital angular momentum, wave field topologies, etc. This review is mainly focused on experimental work in laboratory plasmas, but the findings will be related to research on whistler waves in space plasmas, helicon plasma sources in the laboratory, and significant findings of other research groups. This review starts with antenna properties such as radiation patterns, radiation efficiencies, and the topology of the emitted wave packets. Next, the propagation of whistler modes in highly nonuniform ambient magnetic fields will be presented. Even in the linear regime, new phenomena have been discovered such as the reflection of whistler modes from strong magnetic field gradients or the eigenmodes of waves on circular magnetic field lines. Important nonlinear effects of whistler modes are presented. Whistler instabilities will be briefly reviewed although this is a broad topic by itself. Examples of magnetic reconnection in the Hall parameter regime will be shown. This review will also discuss the advances of wave diagnostics in dedicated laboratory plasmas, the difficulties of diagnostics in high power laboratory plasmas, and the limiting multipoint diagnostics in space plasmas.