The results of ionospheric observations by means of a variety of techniques are summarized to show that magnetic field-aligned irregularities exist in the ionosphere. The irregularities appear in patches having horizontal dimensions up to 1000 km in the east-west direction and several hundred kilometers in the north-south direction. Size of the irregularities is typically about I km transverse to the magnetic field and elongated along the field line. Irregularities associated with spread F occur in the altitude range from 250 km up to the height of F .... Correlations between ionogram spread F and other ionospheric observations indicate that spread echoes are closely related to the field-aligned irregularities. A critique of theories explaining why the irregularities cause spread echoes is given. Statistical studies of the geographic and temporal variations in spread-F occurrence are summarized. The fundamental question of how irregularities causing spread F are produced in the ionosphere is discussed. Now at Lowell Technological Institute Research Foundation, Lowell, Massachusetts. 255 256 JOHN R. HERMAN rather well known. These variations and their correlations are summarized in section 4.Two types of irregularities are differentiated in the literature: large patches (hereaoe•er re•erred to as large-scale irregularities), which may have a horizontal exten• up to several hundred kilometers, and individual field-aligned irregularities (small scale), whose sizes transverse •o the magnetic field line are on the order ooe I km. It appears that the large patches are in actuality a collection of small-scale irregularities, and spread F will be seen as long as the patch is over an observing station. Observational evidence connecting the two types of irregularities is discussed in section 5.One ooe the important problems in a complete theory ooe spread F is the explanation of how the field-aligned irregularities are produc,ed. The explanation could, ooe course, be applicable to the phenomena ooe radio star and satellite scintillations as well. An acceptable irregularity production mechanism must be compatible with the observed •emporal and geographical variations in spread-F occurrence frequency and known correlations with o•her geophysical phenomena, and it must permit irregularity sizes that allow spreading in range, fr, equency of the ionosonde echo, or both. The observational evidence suggests that the mechanisms for irregularity production may be different in equatorial regions than in polar regions, so that a single model would not necessarily have. to explain spread F in both regions. In section 6 prevalent theories of irregularity formation are discussed.After a fairly short section of summary and conclusions (section 7), a rather complete bibliography is given. Some pertinent papers which might have been missed in this paper may be found in the references for previous reviews, including those by Little et al. [1956], Dagg [1957a], Glover [1960], Calvert [1962], Singleton [1962/], and Clemesha and Wright [1964]....

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