# Advances in Ionospheric Space Weather by Using FORMOSAT-7/COSMIC-2 GNSS Radio Occultations
## 3 Equatorial and Low-Latitude Ionosphere
### Wavenumber Four and Plasma Depletion Bay  
In addition to the morphological characteristics, the longitudinal differences in the distribution of EIA electron density have been more intensely examined ever since the demonstration of the existence of the wavenumber-4 pattern in airglow images [90; 91]. The observations demonstrate how the dynamic processes occurring at low altitudes can significantly modulate F-region electron density, providing indisputable evidence for the link between terrestrial weather and ionospheric variations. The absorption of solar radiation by water vapor and ozone drives sun-synchronous atmospheric tides [80; 81; 82], and latent heat release associated with deep tropical convection incites non-migrating tidal variations [83; 92; 93; 94]. These dynamic variations in the lower atmosphere in the form of waves and tides propagate upward, where they grow in amplitude and eventually dissipate in the mesosphere-lower thermosphere (MLT) region around 80-150 km, depositing their energy and momentum and modifying the background temperature and wind [92; 95]. Such tidal interactions modify the E-region dynamo electric fields that drive the equatorial plasma fountain, thus imparting the source variations all the way to F-region altitudes [96; 97]. The potential of F3/C RO observations was fully exploited to analyze the vertical structures and local time evolution of the wavenumber-4 pattern [41; 42; 97], and to identify the contributions of non-migrating tides and stationary planetary waves in modulating the low-latitude plasma distribution by modulating the ionosphere dynamo [98; 99; 100]. Figure 5 (left columns) illustrates the monthly variations of the longitudinal wave pattern at 1500 LT in 2020 at an altitude of 400 km. The modulation of the EIA predominantly displayed a wavenumber-3 pattern during January-June, which then evolved to the wavenumber-4 characteristic until about October, before returning to wavenumber-3 again. Thus, for the local-time period plotted in the figure, the overall EIA pattern throughout the year was dominated by a wavenumber-3 modulation. This monthly pattern generally agrees with seasonal variations of non-migrating oscillations extracted from F3/C measurements, with wavenumber-4 constituents yielding larger amplitudes around September equinox and wavenumber-3 amplitudes being relatively stronger in the solstice months [99; 100]. The seasonal and inter-annual variability in the relative amplitudes of the migrating oscillations in electron density may arise from several factors such as source variations, changes of the mean flow in the MLT region affecting wave dissipation, variations of the zonal mean and diurnal components of ionospheric conductivity or neutral wind that alias with the non-migrating tides, producing child wave modulations, as well as non-linear interactions of migrating tides with local-time variations in the neutral atmosphere, yielding planetary-scale signatures [100]. Figure 4: Local time evolutions of the ionospheric electron density structure at the mid- and low-latitude regions during 0700 LT - 1400 LT over (**a**) \(165^{\circ}\) W, (**b**) \(70^{\circ}\) W, and (**c**) \(50^{\circ}\) W longitude sectors in December 2021. The binning method was same as that for Figure 2. Gray curves represent the magnetic field lines. Most of such earlier studies using F3/C measurements required a larger temporal window (30-90 days over a 2 h interval) and/or integrating the measurements over a given altitude range. The advantage of F7/C2 is the capability to reproduce similar results by combining observations within shorter time intervals, thus offering the opportunity for further investigations to understand variabilities in the factors that modulate the EIA electron density distribution. Though most of the discussion of the low-latitude electron density variations focuses on the influence of E-region dynamo, variations of neutral composition as well as F-region meridional winds by non-migrating tides can also contribute to the EIA modulations [102].