On the generation of large wave parallel electric fields responsible for electron heating in the high‐latitude E region

The sources of parallel electric fields ( E ∥ ) to explain E region electron heating measurements made by incoherent scatter radars have not been resolved. This paper considers the effect of electron density gradients parallel to the geomagnetic field (∇ ∥ n ) on the Farley‐Buneman instability and the resulting change in wave δE ∥ . The dispersion relation including the ∇ ∥ n effect is derived and solved for a set of wave vectors at marginal stability. It is shown that ∇ ∥ n with scales of several kilometers enable propagation of 50 m or longer wavelength waves at large angles (up to 5°) from perpendicularity to the geomagnetic field. The fact that kilometric scales of ∇ ∥ n were previously shown to occur during strong electrojet and that they coincide statistically well with the altitudes of strongest electron heating support the role of these long‐wavelength waves in electron heating. Furthermore, unlike the gradient‐drift instability, the ∇ ∥ n effect contributes to the growth of the waves with the proper k ∥ sign regardless of the direction of the convection electric field E c ; this makes the ∇ ∥ n effect eligible as a source of electron heating, resulting in the consistent increase of electron temperature with the magnitude of E c . Moreover, it is shown that the inclusion of the ∇ ∥ n effect reduces the discrepancy between the theoretically required perpendicular electric field turbulence (to raise the electron temperature to the measured values) and the in situ rocket measurements.