Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar

Annual variations of the ionosphere and thermosphere studied with the middle and upper atmosphere (MU) radar during the solar maximum period 1988–1992 show that the well‐known seasonal anomaly in the electron density Ne exists only during daytime and at altitudes near the ionospheric peak and below. The observations also reveal the existence of equinoctial asymmetries in the ionosphere and thermosphere, with the asymmetry in the ionosphere changing its character with altitude during daytime. In the bottomside ionosphere the electron density Ne is slightly greater in September equinox than in March equinox. At higher altitudes the asymmetry reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 100%. The electron temperature Te exhibits equinoctial asymmetries almost opposite those in Ne. The ion temperature Ti shows a weak asymmetry, in phase with the asymmetry in Ne. The field‐aligned and field‐perpendicular plasma velocities V ‖ and V ⊥ are also different in the two equinoxes. In the thermosphere the neutral wind and composition show consistent equinoctial asymmetries. The meridional component of the daytime poleward wind velocity (U θ ) derived from the field‐parallel plasma velocity is weaker in March equinox than in September equinox by up to 20 m s −1 , and the values of the daytime [O]/[N 2 ] ratio obtained from MSIS‐86 are larger in September equinox than in March equinox by about 20%. Model calculations carried out by incorporating the measured V ⊥ and U θ into the Sheffield University plasmasphere‐ionosphere model that uses MSIS‐86 for neutral atmosphere show that the equinoctial asymmetries in the ionosphere arise mainly from the corresponding asymmetries in the thermosphere, with major contributions from neutral winds and minor contributions from composition.