Electrostatic ion cyclotron waves in a plasma with an ion beam and counterstreaming bulk electrons: Waves in the zero‐frequency band

Abstract

Electrostatic ion cyclotron (EIC) waves driven by electron currents and ion beams at frequencies near the harmonics of the ion‐cyclotron frequency have been extensively studied in the literature. Recently, however, EIC waves with nearly zero‐frequency in an ion‐beam‐plasma system were seen in numerical simulations. In the simulations, the excitation mechanism was the resonant excitation of the beam‐ion‐cyclotron modes by the relative drift of the target ions. These waves occurred at frequencies near zero in the frame of reference of the target ions because of the downward doppler shift. It is shown here that the counterstreaming background electrons, in addition to the target ions, make an important contribution to the growth rate of the EIC waves in the zero‐frequency band defined by |ωr| < Ωi, where ωi and Ωi are the wave and ion cyclotron frequencies, respectively. The growth mechanism is the Landau interaction of the negative energy (slow) ion‐cyclotron‐beam modes with the electrons. Since the counterstreaming of electrons (upward currents) and ion beams along auroral magnetic field lines is common in the region above parallel electric fields, this growth mechanism may be of relevance to satellite observations of waves and turbulence near zero frequency. It is shown that if the observed narrow‐banded zero‐frequency waves are those seen in the simulations, then the electron currents must be sufficiently weak or the current‐carrying electrons must be sufficiently hot. In that event, the relatively cold ion beams are the source of free energy for exciting the EIC waves. On the other hand, in the presence of sufficiently large currents or cold current‐carrying electrons, the narrow‐banded feature of the zero‐frequency waves is destroyed. Both isotropic and anisotropic ion beams were considered. For cold isotropic ion beams in a warm background plasma, the banded feature of the waves, which is an important characteristic of EIC waves, disappears and the excited waves become broadband. The banded feature appears if the isotropic beam temperature becomes comparable to that of the background plasma temperature or if the beam is anisotropic with the perpendicular temperature much larger than the parallel temperature.