A.L. Nulsen, I.H. Cairns, D.B. Melrose, P.A. Robinson
School of Physics, University of Sydney, Sydney, Australia
Lower hybrid waves can resonate with both electrons and ions, allowing energy to be transfered from the perpendicular motions of ions to the parallel motions of electrons or vice versa. Lower hybrid waves are proposed to play major roles in many heliospheric environments, from the chromosphere to the outer heliosheath. Magnetic reconnection sites are particularly relevant due to the bulk flow of plasma across the magnetic field and production of fast electrons travelling along the magnetic field. Most treatments of lower hybrid waves include either electromagnetic or warm-plasma effects but not both. Here a new analytic dispersion relation that includes both effects is derived. This is compared with numerical dispersion curves from a fully electromagnetic code. Very good agreement is obtained over significant ranges in wavenumber and plasma parameters except where ion magnetization effects become important. It is shown that ion magnetization effects are unexpectedly strong, and are the main source of damping of lower hybrid waves in a thermal plasma. These effects result in the lower hybrid mode being reduced to a series of perturbations to the ion Bernstein modes at both low and high frequencies for high ion temperatures. These effects are directly relevant to the hotter regions of the heliosphere, including the chromosphere, corona and Earth's magnetotail and plasmasheet.