A New Tool for Investigating Complex Ionospheric Structures: Over-the-Horizon High-Frequency Sounding of Ionospheric Irregularities with a Chirp Ionosonde-Direction Finder

The ionosphere is characterized by a complicated inhomogeneous structure, and the scales of ionospheric irregularities vary from several meters to hundreds of kilometers. The irregularities may be caused by atmospheric turbulence as well as ionospheric plasma instabilities of different types. In fact, small-scale irregularities are more intensive in the high-latitude and nocturnal equatorial ionosphere, whereas at midlatitudes the small-scale irregularities component is expressed poorly. The most common inhomogeneous ionospheric structures at middle latitudes are traveling ionospheric disturbances (TIDs), which are the ionospheric responses to acoustic-gravity waves.