Magnetic antenna excitation of whistler modes. I. Basic properties

Abstract

Properties of magnetic loop antennas for exciting electron whistler modes have been investigated in a large laboratory plasma. The parameter regime is that of large plasma frequency compared to the cyclotron frequency and signal frequency below half the cyclotron frequency. The antenna diameter is smaller than the wavelength. Different directions of the loop antenna relative to the background magnetic field have been measured for small amplitude waves. The differences in the topology of the wave magnetic field are shown from measurements of the three field components in three spatial directions. The helicity of the wave magnetic field and of the hodogram of the magnetic vector in space and time are clarified. The superposition of wave fields is used to investigate the properties of two antennas for small amplitude waves. Standing whistler waves are produced by propagating two wave packets in opposite directions. Directional radiation is obtained with two phased loops separated by a quarter wavelength. Rotating antenna fields, produced with phased orthogonal loops at the same location, do not produce directionality. The concept of superposition is extended in a Paper II to generate antenna arrays for whistlers. These produce nearly plane waves, whose propagation angle can be varied by the phase shifting the currents in the array elements. Focusing of whistlers is possible. These results are important for designing antennas on spacecraft or diagnosing and heating of laboratory plasmas.

A generalized two-fluid picture of non-driven collisionless reconnection and its relation to whistler waves

“…In the negative x region, the propagation direction (mainly þy) is against the background field (mainly Ày), so the field helicity is spatially right-handed, and similarly in the positive x region, the propagation direction (mainly þy) is along the background field (mainly þy), so the field helicity is spatially left-handed as it should be. 46 If we assume that the initial field line (t ¼ 0) is the background field and that the final field line (t ¼ 180jx ce j À1 ) results from a perturbation to this background at the transition region, the final state is consistent with whistler wave propagation towards the outflow region. The fact that the central perturbation strength is about twice the propagated perturbation supports this idea.…”

“…However, the field helicity is spatially left-handed for propagation along B 0 and right-handed for propagation against B 0 (see detailed analysis by Ref. 46). Therefore, one has to be careful when identifying spatial right-handed circular polarization as whistler waves, as the direction of propagation must be specified with respect to B 0 .…”

Helicons in Unbounded Plasmas

“…In spite of a vast body of literature on helicons no measurements of magnetic field lines have been reported, partly because in-situ field line measurements are impossible in solid state plasmas or space plasmas. This Letter describes field line measurements in large laboratory plasmas free of boundary effects [7,8]. Helicons, contrary to their name, have only a subset of helical field lines while most lines are transverse as in circularly polarized whistler modes.…”

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

“…The present approach to determine the radiation resistance is from first principles, i.e., measuring the power radiated and loop current. More commonly, the antenna impedance is measured externally and the input power is determined from its real part, P in ¼ I 2 rms R in . 8,27,28 However, the input power into the antenna is not only due to radiation but also due to antenna losses, dissipation in the sheath, excitation of electrostatic modes, heating, and ionization.…”

“…1 When loops are used to excite waves, they exhibit a radiation pattern which usually differs from that in vacuum. 2,3 One may therefore expect that loops used to receive waves also have radiation patterns different from those in vacuum. 4 This paper addresses the reciprocity between receiving and transmitting radiation patterns for different types of magnetic antennas operating in the regime of low frequency whistler modes.…”