Oblique sweep-frequency experiments over a 2000-km North-South subauroral path

The high-frequency limits for radio transmission are calculated from the data of vertical sounding stations. For distances < 2000 km, only the vertical data of the station at the midpoint of the path are used. To check this rule, oblique and vertical sweep-frequency records which were observed under normal and anomalous conditions were compared. If the distribution of ionization is nearly constant along the route, normal conditions are present, and the high-frequency limits calculated from the midpoint station are consistent with the oblique observations. Anomalous conditions are present during winter nights. At this time the critical frequencies may vary over a distance of 500 km by a factor of 2 or more. These steep gradients imply that the maximum observed frequency (MOF) is much higher than the calculated maximum usable frequency (MUF). Additional transmission modes are observed which are explained using rough models. The steep gradients were found to be moving southward during the night. A statistical analysis indicates that the magnitude of the gradient and the extent of this motion is increased with increased magnetic activity.