Abstract
Abstract. Previous climatological investigations of ionospheric irregularity occurrence in the equatorial ionosphere have utilized in situ measurements of plasma density to identify the presence of an irregularity. Here we use the Morlet wavelet and C/NOFS to isolate perturbations in meridional ion drifts and generate irregularity occurrence maps as a function of local time, longitude, season, and solar activity. For the low solar activity levels in 2008, the distributions identified by velocity perturbations follow normalized density perturbation (ΔN/N) maps with large occurrences after midnight into dawn over all longitudes. The velocity and normalized density occurrence maps contract in both local time and longitude with increasing solar activity. By 2011 irregularities are confined to particular longitudes expected by alignment and a few hours of local time after sunset. The variation in the occurrence of the late night irregularities with solar activity is consistent with the presence of gravity wave seeding.
Highlights
Irregularities in the plasma number density are a common feature of the equatorial ionosphere that can be responsible for significant disruption to radio communication and navigation systems
This instability is dependent upon the ion-neutral collision frequency; an upward vertical E×B ion drift, which raises the height of the ionosphere into lower neutral densities, is thought to play an important role in irregularity formation (Sultan, 1996)
We find at moderate levels of solar activity that irregularity formation after sunset is consistent with the upward drift at sunset
Summary
Irregularities in the plasma number density are a common feature of the equatorial ionosphere that can be responsible for significant disruption to radio communication and navigation systems. The use of sines and cosines gives the Morlet wavelet decomposition an interpretation similar to the Fourier transform; the use of a Gaussian time window provides for greater specificity in time This wavelet has been used by Stoneback et al (2013) to determine a scale-limited variance in ion density. The variation in altitude for C/NOFS leads to measured variations in ion density that should not be confused with irregularity formation; the wavelet variance is limited here to a maximum scale size of 100 km. C j=0 sj where A is the variance at a given time, Pj is the wavelet power at size sj , δt is the time between samples, δj is the dyadic spacing between scale sizes (0.25), and C is a constant dependent upon the wavelet choice This equation is used to obtain the variance in plasma density ( N) and the meridional ion drift ( V ). The presented seasonal averages may be approximately translated into geographic longitudes by adding −72◦ to the listed apex longitudes
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