First tomographic observations of the Midlatitude Summer Nighttime Anomaly over Japan

Abstract

Recently, a chain of digital beacon receivers has been established over Japan, mainly for the tomographic imaging of the ionosphere. These receivers are installed at Shionomisaki (33.45°N, 135.8-E), Shigaraki (34.85°N, 136.1°E), and Fukui (36.06°N,136°E), which continuously track the Low Earth Orbiting Satellites (LEOS), and the simultaneous line-of-sight Total Electron Content (TEC) data are used for tomographic reconstruction. In the images obtained during July 2008, it is seen that the nighttime electron densities exceed the daytime values on almost all days over latitudes >33-34°N. On several days, these northern latitudes show enhanced electron densities compared to the low-latitude region during nighttime. These are the prominent features of the "Midlatitude Summer Nighttime Anomaly (MSNA)" that is recently observed in the northern hemisphere and is considered similar to the nighttime Weddell Sea Anomaly (WSA). This is the first study of the MSNA using tomographic technique and found its significant day-to-day variability. The ionosonde data from Wakkanai (45.4°N, 141.7° E), ground-based GPS TEC observations using the GEONET, CHAMP in situ electron density measurements, and Formosat3/COSMIC (F3/C) occultation measurements are also used to confirm the presence of MSNA over this region and to examine its variability. It is seen that, in general, during the local summer period, electron density over the northern latitudes is highest at ∼2000-2100 LT and the latitudinal enhancement in electron density also begins to appear around the same time, which continues to exist even at later hours. Copyright 2009 by the American Geophysical Union.