摘要：It is an interesting topic how the ionosphere varies when solar extreme ultraviolet (EUV) irradiance decreases far below normal levels.When extrapolating the total electron content (TEC)-EUV relation, significantly negative TECs at the zero solar EUV point are obtained, which indicates that TEC-EUV variation under extremely low solar EUV (ELSE) conditions does not follow the TEC-EUV trend during normal solar cycles.We suggest that there are four types of nonlinear TEC-EUV variations over the whole EUV range from zero to the solar maximum level.The features of the ionosphere under ELSE conditions were investigated using the TEC extrapolated with cubic TEC-EUV fitting.With the constraint of zero TEC at zero EUV, the cubic fitting takes not only observations but also the trend of the ionosphere (only an extremely weak ionosphere can exist when EUV vanishes) into account.The climatology features of TEC under ELSE conditions may differ from those during normal solar cycles at nighttime.Ionospheric dynamic processes are supposed to still significantly affect the ionosphere under ELSE conditions and induce this difference.With solar EUV decreasing, global electron content (GEC) should vary largely in accordance with the GEC-EUV trend during normal solar cycles, and the seasonal fluctuation of GEC declines, owing to the contraction of the ionosphere.
关键词：Ionosphere ;Total electron content ;Nonlinear variation
