Seasonally Discrepant Long‐Term Variations of the F2‐Layer Due To Geomagnetic Activity and Modulation to Linear Trend

Abstract

AbstractIonospheric long‐term variation has attracted wide attention since the upper atmosphere cooling due to increasing greenhouse gas was suggested. Observations of the lower ionosphere have indicated that cooling effect, whereas the F2 layer long‐term variation still remains controversial as multiple driving sources may cause it. It is crucial for investigating the F2 layer long‐term variation to distinguish contributions of different sources. foF2 records covering both long‐term increasing and decreasing phases of geomagnetic activity were selected for this study. foF2 long‐term component related to those increasing and decreasing phases is separable from the linear trend due to increasing greenhouse gas; it was identified and investigated. Geomagnetic activity induced long‐term component was found to be seasonally discrepant. With long‐term increasing geomagnetic activity, foF2 increases in winter while decreases in summer at middle and low latitudes; it decreases at higher latitudes while turns to increases with decreasing latitudes in equinox. In addition to the geomagnetic activity induced component, the linear component is dominated by long‐term decreasing trend, which is in line with the increasing greenhouse gas. Long‐term evolution of frequent weak geomagnetic activity was suggested to be primarily responsible for the seasonally discrepant long‐term variation of foF2 related to geomagnetic activity; it may change thermospheric composition by modulating average circulation. Moreover, geomagnetic activity has a decreasing trend as a whole. That may lead to misleading correlation between foF2 long‐term variation and geomagnetic activity if linear trend is dominant, and cause significant deviation when estimating greenhouse gas related decreasing trend using linear regression.