Reexamining the Longitudinal Distributions of Post-sunset Quiettime Equatorial Ionospheric Irregularity Occurrences During Solstices with Respect to the Vertical Drift and Density Variations

Abstract:

The longitudinal distributions of post-sunset quiettime equatorial ionospheric irregularity occurrences in two solstices are examined to understand the irregularity occurrences with respect to the vertical drift velocity and density variations. It is found that high irregularity occurrences are clustered at longitudes where background mean vertical drift velocities are positive and densities are high. Conversely, longitudes of practically nil irregularity occurrences are located in the longitude sectors where mean vertical drift velocities are downward and densities are low. The results further confirm the fact that high post-sunset ionosphere with positive vertical drift velocities are keys to the irregularity occurrences. However, high positive vertical drift velocities and densities are not necessarily located at longitudes of good sunset terminator alignment with the magnetic flux tube, or vice versa during solstices. Scatters found in the vertical drift velocities around the dip equator seem to indicate the existence of large variations in the vertical drift velocities that are sufficient to provide perturbations for the irregularity occurrences. The need of outside agitation from direct gravity wave seeding for the irregularity occurrences seems unnecessary.