A thermal mechanism for generation of small-scale irregularities in the ionospheric E region

We present a mechanism for small-scale irregularity generation in the E reg ion due to a thermal instability driven by the neutral wind. The polarizati on electric field induced by the neutral wind causes frictional heating due to ion neutral collisions. In plasma-depleted regions this leads to enhanc ed plasma pressure and plasma is forced out, enhancing the plasma depletion . When heat advection is included, the threshold for instability is lower t han neutral winds commonly observed in the midlatitude E region. Another fa ctor contributing to the instability growth is the dependence of the ion ne utral collisional frequency on ion temperature. This also results in a smal ler threshold neutral wind due to both a reduced ion cooling rate during io n-neutral energy exchange and heat advection. In a standard two-fluid treat ment the thermal theory predicts two preferable altitude ranges for small-s cale irregularity generation: between 93 and 102 km and near 110 km altitud e. We find that the thermal processes may be responsible for the continuous and "thick" continuous echoes observed by the middle and upper atmosphere radar and by the frequency agile radar during the Sporadic E Experiment ove r Kyushu (SEEK) campaign for 3.2-m and 6.1-m field-aligned irregularities, respectively. The theory may explain the SEEK campaign observations of 6.1- m echoes at as high as 150 km altitude if the irregularity elongation is st ronger than that due to ambipolar diffusion. The predicted phase velocities are much smaller than the ion acoustic velocity, which allows us to associ ate the instability with the type 2 radar echoes.