An equatorial bubble:: Its evolution observed in relation to bottomside spread F and to the Appleton anomaly

This is a case study that undertakes the first comprehensive description of the concurrent evolutions of an isolated equatorial bubble, bottomside spread F (BSSF), and the enhanced Appleton anomaly. A principal focus is the dynamics of the intersection of the bubble with the anomaly crest which is the source of maximum scintillation on transionospheric RF propagation. Conditions are equinox at solar maximum. Measurements were by an array of eight ionospheric sounders in the Western Hemisphere with a collective field of view extending along the dip equator for about 1600 km and to 40 degrees north dip latitude that resolved rapid changes in the phenomena yet encompassed their extent and duration. Viewed locally, the bubble was triggered by an upwelling of the bottomside F layer during E x B upward drift that accelerated upward for 15-20 min before the onset of irregularities seen as range spread F (RSF). Viewed macroscopically, this onset was part of continuous front of RSF onset that coincided with E x B reversal and moved westward at the velocity of the terminator so as to remain at 1920 LT, to the east of the bubble as BSSF; within the bubble during the upwelling of the bottomside crest; at the west wall of the bubble at the onset of its eastward drift; and west of the bubble as BSSF. The bubble drifted eastward at 185 m/s and rose upward at less than or equal to 190 m/s intersecting the maximum of the anomaly crest at 2100 LT, 95 min after hubble onset. During this entire period the bubble was in continuous contact with the anomaly profile which was simultaneously increasing but as a function of LT. Thus the intersection was a function of the upward and eastward velocities of the bubble and the westward motion of the anomaly at the velocity of the terminator. The time and place of the intersection is thus dependent on the entire sequence of events and on the differing dependences on UT and LT, and since each can vary, the time and place of maximum scintillation is highly variable. In light of this variability, observations at least as comprehensive as those reported here appear to be necessary to describe the region adequately and therefore to study it further.