Dusk-to-nighttime enhancement of mid-latitude iNm/iF2 in local summer: inter-hemispheric asymmetry and solar activity dependence

摘要

In this paper ionosonde observations in the East Asiaa??Australia sector were collected to investigate dusk-to-nighttime enhancement of mid-latitude summer iNm/iF2 (maximum electron density of the F2 layer) within the framework of iNm/iF2 diurnal variation. iNm/iF2 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 iNm/iF2 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. ihm/iF2 (peak height of the F2 layer) diurnal variations show that daytime ihm/iF2 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.