Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion - Rishbeth, H; Müller-Wodarg, ICF; Zou, L; Fuller-Rowell, TJ; Millward, GH; Moffett, RJ; Idenden, DW; Aylward, AD (ANNALES GEOPHYSICAE-ATMOSPHERES HYDROSPHERES AND SPACE SCIENCES, 2000)