Formation of Ionospheric Density Irregularities: Recent Advances in Observation and Theory

Ionospheric density irregularities are an important part of space weather disturbances since they are responsible for scattering of radio signals propagating through the ionosphere. They routinely produce strong echoes observed by coherent radars operating at HF, VHF, and UHF as well as for scintillation in GPS/GNSS L-band. In this paper, we review recent advances in observation and theory of ionospheric irregularities focusing on their observations with the global Super Dual Auroral Radar Network (SuperDARN) at high latitudes and development of a unified theoretical formalism describing formation of irregularities throughout the entire lower ionosphere. Gradients in the ionospheric plasma density that are needed to directly generate decameter irregularities seen by SuperDARN are estimated for different altitudes and experimental verifications of the suggested direct generation are presented. It is shown that the transitional “valley” region near 120 km in altitude has a distinctly different generation regime where the generation is largely independent of the gradients at short wavelengths (<100 m) whereas at long wavelengths (>100 m) it is strongly controlled by the gradients. Implications for space weather modeling are discussed.