Comparative studies on solar activity variations of NmF2 at the Arctic and Antarctic Stations

Solar activity dependences of the ionospheric F-2 peak electron density (NmF2) are investigated and compared based on long-term observations at the Arctic and Antarctic stations of Tromso, Longyearbyen, and Zhongshan. It is found that NmF2 increases linearly with solar activity indices at most of the time at all three stations, illustrating that the solar radiation is the primary factor in the formation of the ionosphere. Also, there exits an apparent "saturation effect" at Zhongshan in the Southern Hemisphere summer, but exits an "amplification effect" at Tromso and Longyearbyen in the Northern Hemisphere winter. The linear relationship between NmF2 and solar activity indices F10.7P is the strongest at Tromso, and it is the weakest at Longyearbyen. For the diurnal variations, NmF2 is the most sensitive to solar activity variations around local noon at Tromso, around magnetic noon at Longyearbyen, and between the two at Zhongshan. It is mainly subjected to the different proportion of photoionization and plasma convection between the three stations, which is controlled by their geographic latitude and geomagnetic latitude. For its annual variations, NmF2 is the is the most sensitive to solar activity variations in winter at Tromso, and it is more sensitive in equinoxes than in summer. At Zhongshan and Longyearbyen, NmF2 is the most sensitive to solar activity in equinoxes. The sensitivity is the lowest in summer at Zhongshan, but the lowest in winter at Longyearbyen. The annual variations of sensitivity of NmF2 to the solar activity are mainly due to the differences, in photoionization and chemical compositions at the three stations. At magnetic midnight in winter of both Tromso and Zhongshan, NmF2 has the lowest sensitivity to solar activity and the linear relationship between NmF2 and F10.7P is the worst. And so is it during 12-16UT at Zhongshan in winter. Different from the other two stations, NmF2 still has a strong sensitivity to solar activity before magnetic midnight in winter at Longyearbyen, which is attributed to the transpolar transport of EUV ionization from day to night.