Langmuir turbulence in the auroral ionosphere: 1. Linear theory

Intense bursts of Langmuir waves with electric fields of 50‐500 mV/m have been frequently observed at altitudes >500 km in the auroral ionosphere. These bursts are driven by 20 eV ‐ 4 keV field‐aligned electrons, which are embedded in an approximately isotropic nonthermal tail of scattered electrons. The Langmuir bursts are often observed at altitudes where the ionosphere is moderately magnetized (Ω ≈ ωpe). Both the moderate magnetization and the scattered electrons have a major influence on the linear dispersion and damping of Langmuir waves. In particular, the linear dispersion is topologically different depending on whether the magnetic field is subcritical (Ω < ωpe) or supercritical (Ωe > ωpe). The correct dispersion and damping can account for the observed polarization of the Langmuir waves, which is very nearly parallel to the geomagnetic field. Inferred properties of the linear instability driven by the field‐aligned electrons are discussed. The linear dispersion and damping derived here provide the basis for a nonlinear turbulence study described in a companion paper (Newman et al., this issue).