Abstract
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.
Highlights
Variations of solar extreme ultraviolet (EUV) irradiance are important for the ionosphere and thermosphere
These nonlinear total electron content (TEC)-EUV variations can be presented with cubic functions of EUV
In terms of the cubic TEC-EUV fitting constrained with the term of zero TEC at zero EUV, we qualitatively discussed the features of the ionosphere under extremely low solar EUV (ELSE) conditions
Summary
Variations of solar extreme ultraviolet (EUV) irradiance are important for the ionosphere and thermosphere. Solar EUV is well correlated with the sunspot number, an important solar activity proxy, during normal solar cycles; EUV decreases as the sunspot number declines. Based on the sunspot historical record, Eddy (1976) justified the existence of an extremely low solar activity period known as the Maunder Minimum (1645 to 1715 A.D.) during which virtually no sunspots were observed. It is unclear how solar EUV and the ionosphere varied during the extended extremely low solar activity periods such as the Maunder Minimum. Some researchers have attempted to derive the EUV irradiance during these extreme periods by solar irradiance models (e.g., Lean et al 2011); the results reveal that EUV irradiance potentially
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