Ionospheric irregularities

Ionospheric irregularities are small‐scale structures in the ionospheric plasma density generally oriented so that the plasma density variations occur rapidly across the geomagnetic field but slowly (or not at all) along the geomagnetic field. On the practical side, they play an important role in satellite communication systems [ Utlaut , 1974]. The scale length of ionospheric irregularities can range from a few meters to several kilometers across the magnetic field. A vast literature of radio wave studies has produced a fairly complete morphological picture of their occurrence pattern [ Herman , 1966; Aarons and Allen , 1971]. Briefly, they occur in the auroral and polar cap F region almost all the time [ Aarons , 1973], in the auroral E region in times of geomagnetic activity [ Greenwald et al. , 1973], in the equatorial E region [ Balsley and Farley , 1973], and in the equatorial F region at night [ Farley et al. , 1970]. Ionospheric irregularities occur at temperate latitudes only in the nighttime F region during times of geomagnetic activity [ Herman , 1966]. Often the occurrence of F region irregularities is called ‘spread F ,’ a term derived from the appearance of ionograms under conditions when irregularities occur. Ionospheric irregularities have also been generated artificially by barium plasma releases [ Rosenburg , 1971] and radio wave heating experiments [ Utlaut and Cohen , 1971].