NUMERICAL STUDY FOR THE SEMI-ANNUAL VARIATION OF ELECTRIC FIELDS AT THE MID-AND LOW-LATITUDES

With a theoretical model of ionospheric electric fields, the yearly variation of ionospheric electric fields at mid- and low-latitudes under the condition of low solar and quiet geomagnetic activities were studied. The main results are as follows: when the ionospheric electric fields are calculated individually in the northern or southern hemisphere without including the influence of the other hemisphere （uncoupled）, the electric fields in both hemispheres vary in an obvious annual pattern with an anti-phase; when the electric field was calculated simultaneously in both hemispheres with the influence of the other hemispheres （coupled） included, the electric field varies in a predominant semi-annual pattern. Moreover, quite good consistency was found between the behaviors of semi-annual variation calculated by our theoretical electric field model and an empirical model of Richmond. A simple circuit with the coupling between the northern and southern hemispheres was introduced to interpret the semi-annual variation of electric field. Results from the circuit model reveal that the coupled electric field would show a pronounced semi-annual variation due to the ’coupling effect’ along the magnetic field lines between two hemispheres, whereas the uncoupled electric field have an annual variation in respective hemisphere.