solar cycle effects on gnss-derived ionospheric total electron content ...

activity, plasmaspheric tails are formed that extend away down to the ionosphere, affecting Earthcommunication (Forster and Jakowski, 2000).Due to the different molecules and atoms in the atmosphere and their differing rates of absorption, aseries of distinct regions or layers of electrons exist. These are denoted by letters D, E, F1, F2 andare usually referred to as the bottom side ionosphere as shown in Figure 2.3. The portion of theupper atmosphere between the F2 layer and the upper boundary is termed the topside ionosphere. Itis in the F2 layer where the maximum electron density usually occurs as a result of the combinedeffect of <strong>solar</strong> EUV radiation and the increase of neutral atmospheric density as the altitudedecreases.The maximum frequency at which a radio wave would be reflected from an ionospheric layer iscalled the critical plasma frequency of the particular layer and is denoted by foD, foE, foF1, andfoF2 according to the designation of the ionospheric layer. The square of a critical frequency islinearly proportional to the maximum electron density of the individual layer, denoted by NmD,NmE, NmF1 and NmF2 respectively. The changes in foD, foE and foF1 are in phase with <strong>solar</strong>variation and foF2 is in anti-phase (Komjathy, 1997). The existence of the D, E, and F1 layers seemsto be primarily controlled by the <strong>solar</strong> zenith angle showing a strong diurnal, seasonal and latitudinalvariation. The diurnal variation of the D, E, and F1 layers also implies that they tend to vanish orgreatly reduce in size at night. The F1 layer disappears in winter when the <strong>solar</strong> zenith angle ishigher than in summer, at which the F1 layer is consistently present. The critical frequency of thelayers follows the 11-year <strong>solar</strong> <strong>cycle</strong> variation caused by the change in the intensity of <strong>solar</strong>radiation. A brief description of individual ionospheric layers is given below. For furtherinformation the reader is referred to Davies (1990), McNamara (1990) and Stubbe (1996).21