We have extended the linear fluid theory of electrojet plasma waves to the region where ion magnetization effects are important. Our general dispersion relation includes the effect of cross‐field and field‐aligned drifts, ion inertia, electron density gradients, and recombination. In the absence of density gradients and recombinational damping, the oscillation frequency at marginal instability is changed by the ion magnetization effects from the ion acoustic frequency, ω = kCs, to the modified ion cyclotron frequency ω² = Ωi² + k²Cs². These upper E region waves can be driven by field‐aligned and/or cross‐field drifts and have the smallest threshold drift velocities at heights where electron‐ion and/or anomalous electron collision frequencies are important. In the upper E region the most unstable wavelengths correspond to k⊥Ri ∼ 1, where Ri is the ion Larmor radius. Electron density gradients can increase or decrease considerably the threshold drift velocity for large‐scale (a few tens of meters and larger) waves. Recombinational damping increases the threshold drift velocity for marginal instability of two‐stream ion cyclotron waves and imposes a threshold drift velocity for the excitation of large‐scale gradient drift waves propagating nearly perpendicular to the magnetic field. The effect of recombination is surprisingly important, even for wavelengths as short as 10–20 m, for altitudes at which νi ∼ Ωi. At these altitudes and above, the effect of even a very small k∥ becomes increasingly important. The theory puts a number of earlier theoretical results together in one framework and provides new results and insights that may explain some puzzling observations such as those of Moorcroft (1979), who sometimes failed to see echoes with the 398‐MHz Homer radar when the electric field measured with the Chatanika radar far exceeded the normal instability threshold.

The Cornell University Portable Radar Interferometer (CUPRI), a 50-MHz Doppler radar system, was operated during May and August/September 1983 on the island of St. Croix (17.7øN, 64.8øW) to study the plasma instabilities associated with nighttime sporadic E layers. Two events, on May 7 and August 22, show evidence of large-amplitude waves, with apparent horizontal wavelengths of 10-12 km and periods of 2-6 min. These apparent wavelengths are upper limits for the true wavelengths. The CUPRI beam was directed over Arecibo, PUerto Rico, and on May 7, concurrent electron density profiles within the CUPRI scattering volume were measured by the Arecibo Observatory's 430-MHz radar. During the stronger event of August 22, radar echoes were received from several altitudes up to 130 km. Large mean Doppler velocities (at times exceeding 250 m/s) were observed during this event, and the power spectra closely resemble those obtained at the magnetic equator during type 1 conditions. We believe (1) that the mid-latitude large-scale waves are generated by the same gradient drift instability mechanism responsible for equatorial large-scale waves and (2) that the type 1 3-m waves can be generated at mid-latitudes with drift velocities well below the sound speed because of the very sharp plasma density gradients associated with metallic ion sporadic E layers.

It is generally accepted that unstable ionospheric plasma waves moving at the ion‐acoustic velocity (“two‐stream” waves) are responsible for the so‐called type 1 VHF radar echoes commonly observed at equatorial and auroral latitudes. These same waves apparently are also the source of type 4 echoes, which have sharply peaked spectra with unusually large Doppler shifts and are seen at auroral latitudes during sufficiently disturbed conditions. But how, exactly, is the observed Doppler shift, or equivalently the ion‐acoustic velocity Cs, related to the electron and ion temperatures? The expression usually quoted, with occasional caveats, is the isothermal result Cs² = K(Te + Ti)/mi. The validity of the isothermal assumption has not been of much concern until recently, when the first simultaneous independent measurements of the temperatures and Cs were made in Scandinavia. We argue here that, in fact, the electrons should usually be treated as adiabatic, with three degrees of freedom, while the ions may or may not be adiabatic (with only one degree of freedom), depending upon the temperatures, the altitude, and the radar frequency. In other words, the ion effects generally should be calculated kinetically. The differences between the two models in the computed wave velocity are substantial (∼20–40%). A comparison between European Incoherent Scatter (EISCAT) temperatures and wave velocities measured with the Cornell University Portable Radar Interferometer (CUPRI) shows good agreement with the model given here.

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During the early morning of February 7, 1983, with a VHF Doppler radar interferometer we observed a discrete auroral arc that was also photographed by an all-sky camera (ASC) at Fort Churchill, Canada.We determined horizontal drift velocities and hence the vector electric fields from the interferometer data.We measured both a weak ambient and stronger arc-associated polarization field (sometimes exceeding 100 mV/m) along the poleward boundary of the arc from where the radar echoes were received. This latter field was in the proper direction (roughly parallel to the electron density gradient) to excite the gradient drift plasma instability. Both the radar and the ASC observed large (20-50 km) structures, probably caused by a Kelvin-Helmholtz instability, along this same poleward edge. The radar measured both type I and type II Doppler spectra, and in addition, we also saw a few examples of a very narrow resonant spectral peak. Because of the Doppler shift (~60-90 Hz) of this feature and the fact that it was associated with velocity shears (which imply a horizontally divergent electric field and field-aligned currents) we believe these latter echoes were from O + electrostatic ion cyclotron (EIC) waves generated at an altitude of about 150 km. But linear kinetic theory indicates that such waves, with lengths as short as 3 m and propagating at the observed radar aspect angle (between k and B) of about 83 ø, could be directly excited only by an unreasonably large field-aligned electron velocity (greater than the electron thermal speed). More realistic velocities could, however, excite 20-m EIC waves propagating at this angle, and if these steepen due to some nonlinear process, they could perhaps explain our observations.