Abstract
Abstract. In this paper ionosonde observations in the East Asia–Australia sector were collected to investigate dusk-to-nighttime enhancement of mid-latitude summer NmF2 (maximum electron density of the F2 layer) within the framework of NmF2 diurnal variation. NmF2 were normalized to two solar activity levels to investigate the dependence of the dusk-to-nighttime enhancement on solar activity. The dusk-to-nighttime enhancement of NmF2 is more evident at Northern Hemisphere stations than at Southern Hemisphere stations, with a remarkable latitudinal dependence. The dusk-to-nighttime enhancement shows both increasing and declining trends with solar activity increasing, which is somewhat different from previous conclusions. The difference in the dusk-to-nighttime enhancement between Southern Hemisphere and Northern Hemisphere stations is possibly related to the offset of the geomagnetic axis from the geographic axis. hmF2 (peak height of the F2 layer) diurnal variations show that daytime hmF2 begins to increase much earlier at low solar activity level than at high solar activity level at northern Akita and Wakkanai stations where the dusk-to-nighttime enhancement is more prominent at low solar activity level than at high solar activity level. That implies neutral wind phase is possibly also important for nighttime enhancement.
Highlights
IntroductionFor the ionosphere over a given location, solar irradiance arriving at the ionosphere varies on different timescales, such as the regular diurnal (local time), seasonal and solar cycle timescales, which certainly drives the variations of the ionosphere on these timescales
The ionization in the ionosphere is primarily caused by solar irradiance
Photochemical processes are dominative at ionospheric E layer height; diurnal variation feature of the E layer electron density is basically consistent with the diurnal trend of the Chapman theory (e.g., Yue et al, 2006)
Summary
For the ionosphere over a given location, solar irradiance arriving at the ionosphere varies on different timescales, such as the regular diurnal (local time), seasonal and solar cycle timescales, which certainly drives the variations of the ionosphere on these timescales. It is significant for understanding and modeling the ionosphere to study ionospheric variations on these different timescales. Photochemical processes are dominative at ionospheric E layer height; diurnal variation feature of the E layer electron density is basically consistent with the diurnal trend of the Chapman theory (e.g., Yue et al, 2006).
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