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The background ionospheric conditions shaped by sunset electrodynamic processes are responsible for the development of equatorial plasma bubble (EPB)/equatorial spread F (ESF) irregularities of the post-sunset ionosphere. Distinct conditions exist for the EPB/ESF development also at later hours of the night. The plasma instability growth leading to EPB generation is dependent on the basic precursor conditions defined by the well-known parameters: the evening prereversal enhancement in vertical plasma drift (PRE), wave structure in plasma density and polarization electric field required to initiate/seed the instability, and the F layer bottom side density gradient, as a significant factor in controlling the growth rate. Competing roles of the zonal versus meridional thermospheric winds additionally control their development. Statistical as well as case studies have addressed aspects of the EPB development and occurrence under different geophysical conditions, generally focusing attention on any one of the above specific parameters. Little is known regarding the relative importance of concurrent presence of the precursor parameters (mentioned above) in shaping a given event. A large degree of day-to-day variability in these parameters arise from different sources of forcing, such as upward propagating atmospheric waves, and magnetic disturbance time electric fields in the form of prompt penetration and disturbance dynamo electric fields that often contribute to the widely observed short-term variabilities in EPB development and dynamics. In this paper, we will present and discuss some important aspects of the EPB/ESF short-term variability, focusing attention on their enhanced development, or suppression and, wherever possible, highlighting also the relative roles of the precursor parameters in such variability.

Day-to-day and short-term variabilities in the equatorial plasma bubble/spread F irregularity seeding and development