Nonlinear evolution of equatorial spread <i>F</i>: 4. Gravity waves, velocity shear, and day‐to‐day variability

<jats:p>Production and evolution of plasma bubbles seeded by gravity waves in the equatorial <jats:italic>F</jats:italic> region are studied under different conditions using a computer simulation. The problem of day‐to‐day variability in the occurrence of equatorial spread <jats:italic>F</jats:italic> (ESF) is discussed. It is shown that gravity waves over a wide range of amplitude and wavelength are a very effective seed mechanism for production of ESF. However, there is the day‐to‐day variability in the occurrence of ESF even if gravity waves are omnipresent. The maximum height of the <jats:italic>F</jats:italic> peak arid the bottomside background electron density gradient can significantly influence production and rise of plasma bubbles. It is found also that the timing of the seed gravity wave is critical to ESF generation. If à gravity wave exists in the <jats:italic>F</jats:italic> region during the upward drifting of the <jats:italic>F</jats:italic> layer, the gravity wave can initiate the Rayleigh‐Taylor instability and result in topside bubbles. In contrast, gravity‐wave‐induced perturbations in a descending ionosphere evolve into large‐scale, wavelike structures but not bubbles. Gravity waves can propagate in any direction, but only the gravity waves propagating with an azimumal angle range of about 10° relative to the zonal direction are expected to be capable of generating ESF bubbles. These factors are all variable from day to day. Their variation must cause variation of resulting ESF. In addition, we show first that in agreement with linear analysis, a velocity shear in the plasma drift can generate plasma structures in the bottomside <jats:italic>F</jats:italic> layer. For reasonable ionospheric parameters, the shear‐determined wavelength of the Rayleigh‐Taylor instability is a few tens of kilometers. The plasma structures organized by velocity shear appear as bottomside spread <jats:italic>F</jats:italic> but cannot evolve into topside bubbles. Velocity shear provides a possible explanation of generation of bottomside spread <jats:italic>F</jats:italic> and secondary plumelike structures on the west wall of large‐scale plasma upwelling.</jats:p>