Spread F and ionospheric F‐region irregularities

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 1 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 max . 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.