The rapid increase of electron temperature in the early morning hours at low latitudes is a well‐known ionospheric phenomenon called morning overshoot. In this study, we extensively investigate the dependence of morning overshoot on local time, season, latitude/longitude/altitude, and magnetic activity. The electron temperature and density data set used in this study are obtained from (1) the Swarm constellation at two different altitudes of 470 and 520 km with identical payloads and (2) the Floating Potential Measurement Unit onboard International Space Station at an altitude of 400 km. Based on the data between 2014 and 2019, the main findings of this study are as follows: (1) on a global average, morning overshoot generally weakens with decreasing altitudes. (2) Morning overshoot is stronger around the dip equator than at midlatitude regions. As latitude increases, the overshoot decreases gradually and shifts to later local times. (3) In off‐equatorial regions the overshoot is stronger in the winter than in the summer hemisphere, especially at higher altitudes. (4) Lastly, the morning overshoot shows multiday oscillations, which are negatively correlated with plasma density and affected by geomagnetic activity.

Previously, all-sky airglow images observed at Shigaraki (34.9° N, 136.1° E), Japan, during 2004 and2005 were analyzed in relation to those observed at Mt. Bohyun (36.2° N, 128.9° E) for a comparison of their gravity wave characteristics (Kim et al. 2010). By applying the same selection criteria of waves and cloud coverages as in the case of Mt. Bohyun all-sky images, we derived apparent wavelengths, periods, phase velocities, and monthly occurrence rates of gravity waves at Shigaraki in this study. The distributions of wavelengths, periods, and speeds derived for Shigaraki were found to be roughly similar to those for Mt. Bohyun. However, the overall occurrence rates of gravity waves at Shigaraki were 36% and 34% for OI 557.7 nm and OH Meinel band airglow layers, respectively, which were significantly higher than those at Mt. Bohyun. The monthly occurrence rates did not show minima near equinox months, unlike those for Mt. Bohyun. Furthermore, the seasonal preferential directions that were clearly apparent for Mt. Bohyun were not seen in the wave propagation trends for Shigaraki. These differences between the two sites imply different origins of the gravity waves near the Korean peninsula and the Japanese islands. The gravity waves over the Japanese islands may originate from sources at various altitudes; therefore, wind filtering may not be effective in causing any seasonal preferential directions in the waves in the airglow layers. Our analysis of the Shigaraki data supports recent theoretical studies, according to which gravity waves can be generated from in situ sources, such as mesosphere wind shear or secondary wave formation, in the mesosphere.

No abstract

We present for the first time the characteristics of upper atmospheric horizontal winds over the Korean Peninsula. Winds and their variability are derived using four-year measurements by the Korea Astronomy and Space Science Institute (KASI) meteor radar. A general characteristic of zonal and meridional winds is that they exhibit distinct diurnal and seasonal variations. Their changes indicate sometimes similar or sometimes different periodicities. Both winds are characterized by either semi-diurnal tides (12 hour period) and/or diurnal tides (24 hour period) from 80–100 km. In terms of annual change, the annual variation is the strongest component in both winds, but semi-annual and ter-annual variations are only detected in zonal winds.

A new 40.8 MHz coherent scatter radar was built in Daejeon, South Korea (36.18°N, 127.14°E, dip latitude: 26.7°N) on 29 December 2009 and has since been monitoring the occurrence of field‐aligned irregularities (FAIs) in the northern middle latitudes. We report on the occurrence climatology of the F region FAIs as observed by the Daejeon radar between 2010 and 2014. The F region FAIs preferentially occur around 250–350 km at 18:00–21:00 local time (postsunset FAI), around 350–450 km near midnight (nighttime FAI), around 250–350 km before sunrise (presunrise FAI), and around 160–300 km after 05:00 local time (postsunrise FAI). The occurrence rates of nighttime and presunrise FAIs are maximal during summer, though the occurrence rates of postsunset and postsunrise FAIs are maximal during the equinoxes. FAIs rarely occur during local winter. The occurrence rate of F region FAIs increases in concert with increases in solar activity. Medium‐scale traveling ionospheric disturbances (MSTIDs) are known as an important source of the F region FAIs in middle latitudes. The high occurrence rate of the nighttime FAIs in local summer is consistent with the high occurrence rate of MSTIDs in that season. However, the dependence of the FAI activity on the solar cycle is inconsistent with the MSTID activity. The source of the F region FAIs in middle latitudes is an open question. Our report of different types of FAIs and their occurrence climatology may provide a useful reference for the identification of the source of the middle latitude FAIs.