Oscillating two-stream instability in ionospheric heating experiments

It is shown that oscillating two-stream instability (OTSI) can be excited in sizable spatial region below the reflection height of the O-mode heating wave in mid-latitude ionospheric heating experiments. Near the reflection height, the threshold field of OTSI increases drastically with the oblique propagation angle of its Langmuir sidebands. Such dependence limits the angular distribution of Langmuir sidebands in narrow cones around the geomagnetic field. Therefore, obliquely propagating Langmuir sidebands of OTSI prefer to be excited in their matching height regions, where sidebands satisfy the local dispersion relation and their matching height locations move downward from the reflection height as oblique propagation angles increase. (C) 2002 American Institute of Physics.