Effects of tidal forcing, conductivity gradient, and active seeding on the climatology of equatorial spread F over Kwajalein

Plasma structure in nighttime equatorial F layer, referred to as equatorial spread F (ESF), displays climatology whose seasonal variation depends on longitude. At longitudes where ESF favors equinoxes, times when maxima occur can be predicted in terms of the day of year, when E region sunset is simultaneous in conjugate hemispheres (i.e., sunset nodes). Aside from occurrences around equinoxes, there are only three longitudes where ESF also occurs during a solstice; one is the central Pacific region. Here ESF activity is strong during the June solstice, when solar activity is high. To understand this puzzling behavior, ESF climatology over the Kwajalein Atoll was compared with properties of the postsunset rise (PSSR) of the F layer and seeding activity in the troposphere. The key findings are as follows: (1) Maxima in PSSR velocity (V-PSSR) are better aligned with equinoxes than with sunset nodes; hence, seasonal pattern of V-PSSR, not only sunset nodes, should be included in interpretation of ESF climatology. (2) The source of V-PSSR during solstice appears to differ from that during equinoxes. (3) Equinoctial maxima in V-PSSR could be related to a semiannual variation in equatorial electrojet strength and its contribution to polarization of the F region dynamo current. (4) Enhanced V-PSSR during the June solstice is interpreted in terms of tidal forcing with a wave number of 2. (5) Displacements of maxima in ESF climatology from maxima in V-PSSR are shown to be consistent with deep convective activity.