Abstract : Ionosphere is being used by numerous communication systems to reflect radio signals over long distances. The critical frequency (foF2) is the preventive frequency at or lower which a radio wave is reflected by ionosphere in HF radio propagation. The atmosphere can be divided vertically into zones based on changes that occur with altitude as Troposphere, Stratosphere, Mesosphere, Ionosphere and Thermosphere. The Earth’s ionosphere exhibits complex variabilities with varying spatial and temporal scales. The ionosphere is also restricted by the solar zenith angle, manifesting diurnal, annual and semiannual variations. In the low latitudes, the thermospheric wind and electric field, in conjunction with Earth’s magnetic field, play an important role in the redistribution of plasma. With regard to the low-latitude ionosphere, variabilities in the F- region have been given special attention due to their direct effects on communication / navigation systems. A large number of observational and modeling studies have been made to understand F region parameters, such as the critical frequency of the F2 layer, foF2 and the corresponding height, hmF2, in terms of variabilities in solar flux, magnetic field, neutral dynamical/ electro dynamical parameters.

Key Words-- Ionospheric Variations, Critical frequency, FoF2 and hmF2, Solar ionizing flux, low-latitude, Ionospheric parameters, Troposphere, Stratosphere, Mesosphere, Ionosphere, Thermosphere.

| DOI: 10.17148/IARJSET.2021.8833