LANGMUIR TURBULENCE IN THE AURORAL IONOSPHERE .1. LINEAR-THEORY

Intense bursts of Langmuir waves with electric fields of 50-500 mV/m h ave been frequently observed at altitudes > 500 km in the auroral iono sphere. These bursts are driven by 20 eV - 4 keV field-aligned electro ns, which are embedded in an approximately isotropic nonthermal tail o f scattered electrons. The Langmuir bursts are often observed at altit udes where the ionosphere is moderately magnetized (OMEGA(e) almost-eq ual-to omega(pe). Both the moderate magnetization and the scattered el ectrons 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 (OME GA(e) < omega(pe)) or supercritical (OMEGA(e) > omega(pe)). The correc t dispersion and damping can account for the observed polarization of the Langmuir waves, which is very nearly parallel to the geomagnetic f ield. Inferred properties of the linear instability driven by the fiel d-aligned electrons are discussed. The linear dispersion and damping d erived here provide the basis for a nonlinear turbulence study describ ed in a companion paper (Newman et al., this issue).