Solar cycle variations of the equatorial ionospheric anomaly in total electron content in the Asian region

Abstract

Daily equatorial ionospheric anomaly (EIA) contour charts in total electron content (TEG) were obtained by receiving simultaneously two coherent radio signals transmitted from the U.S. Navy Navigation Satellite System satellite at Lunping Observatory (25.00°N, 121.17°E) from September 1985 to December 1994. The latitude, occurrence time, and strength of the most developed EIA crest obtained from daily TEC contour charts have been used to study the solar cycle variations of EIA in the Asian region. No significant solar cycle effect can be seen in the occurrence time and latitude of the most developed EIA crest. Seasonally, the winter crest appears larger and earlier than the summer crest, and the summer crest appears in lower latitude than other seasons. These seasonal changes are mainly accounted for by the effect of daytime meridional wind. The strength of EIA crest increases with the increasing of solar activity and exhibits winter anomaly with the winter strength larger than the summer one Positive correlation is found between the 12‐month smoothed EIA crest strength and sunspot number. No saturation effect can be seen during the high solar activity period. The 12‐month smoothed EIA crest strength exhibits a hysteresis variation about the solar cycle variation. Monthly mean TEC contour charts for the latitude from 15°N to 35°N are also obtained from those daily contour charts. By using linear regression analysis, the monthly mean TEC I at a given month is linearly correlated with smoothed sunspot number R by a formula I = A + BR. These results can be used to construct the monthly EIA contour chart in TEC for any given sunspot number.