This paper presents monthly variations of the midlatitude summer nighttime anomaly (MSNA) of the ionosphere for the first time by global observations of the FORMOSAT‐3/COSMIC (F3/C) mission. The anomaly is characterized by the greater nighttime (1800 LT ∼ 0200 LT) ionospheric electron density than during daytime (0800 ∼ 1800 LT) at middle latitudes during months around June and December solstices. The anomaly shown during December solstice was known as the Weddell Sea Anomaly (WSA) occurring around the Antarctica and nearby the Pacific Ocean. This paper demonstrates that the WSA‐like feature also exists in the Northern Hemisphere and is most prominent near the Northeast Asia, Europe/Africa, and Central Pacific longitudes around June solstice. In both hemispheres, the anomalies with similar electron density characteristics and variations caused by the similar mechanism prompts us to name this phenomenon the MSNA. The monthly F3/C observations indicate that the anomaly appears as the most prominent structure of the global ionosphere around midnight hours.

[1] The ionospheric Weddell Sea Anomaly (WSA) was first reported more than five decades ago based on ionosonde data near the Antarctica peninsula. The WSA is an ionospheric structure characterized by a larger nighttime electron density than daytime density. Recent satellite observations indicate that the WSA can extend from South America and Antarctica to the central Pacific. The major physical mechanisms that have been suggested for the WSA formation are an equatorward neutral wind, an electric field, the photoionization, and the downward diffusion from the plasmasphere. On the basis of the theoretical modeling performed in this study using the SAMI2 model, an equatorward neutral wind is identified as the major cause of the WSA, while the downward flux from the plasmasphere provides an additional plasma source to enhance or maintain the density of the anomalous structure.

[1] In this paper, modifications of the ionospheric tidal signatures during the 2009 stratospheric sudden warming (SSW) event are studied by applying atmospheric tidal analysis to ionospheric electron densities observed using radio occultation soundings of FORMOSAT-3/COSMIC. The tidal analysis indicates that the zonal mean and major migrating tidal components (DW1, SW2 and TW3) decrease around the time of the SSW, with 1.5-4 hour time shifts in the daily time of maximum around EIA and middle latitudes. The typical ionospheric SSW signature: a semi-diurnal variation of the ionospheric electron density, featuring an earlier commencement and subsidence of EIA, can be reproduced by differencing the migrating tides before and during the SSW period. Our results also indicate that the migrating tides represent $80% of the ionospheric tidal components at specific longitudes, suggesting that modifications of the migrating tides may be the major driver for producing ionospheric changes observed during SSW events, accounting for greater variability than the nonmigrating tides that have been the focus of previous studies. Citation:

[1] In this paper, the total electron content (TEC) of the global ionosphere map (GIM) is used to detect seismoionospheric anomalies associated with the 12 January 2010 M7 Haiti earthquake, and an ionospheric model is applied to simulate the detected anomalies. The GIM temporal variation shows that the TEC over the epicenter significantly enhances on 11 January 2010, 1 day before the earthquake. The latitude-time-TEC (LTT) plots reveal three anomalies: (1) the northern crest of equatorial ionization anomaly (EIA) moves poleward, (2) the TECs at the epicenter and its conjugate increase, and (3) the TECs at two dense bands in the midlatitude ionosphere of 35°N and 60°S further enhance. The spatial analysis demonstrates that the TEC enhancement anomaly appears specifically and persistently in a small region of the northern epicenter area. The simulation well reproduces the three GIM TEC anomalies, which indicate that the dynamoelectric field of the ionospheric plasma fountain might have been perturbed by seismoelectric signals generated around the epicenter during the earthquake preparation period.

[1] This paper is the first study to employ a three-dimensional physics-based ionosphere model, SAMI3, coupled with the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) and Global Scale Wave Model to simulate the mesospheric and lower thermospheric tidal effects on the development of midlatitude summer nighttime anomaly (MSNA). Using this coupled model, the diurnal variation of MSNA electron densities at 300 km altitude is simulated on both June solstice (day of year (DOY) 167) and December solstice (DOY 350) in 2007. Results show successful reproduction of the southern hemisphere MSNA structure including the eastward drift feature of the southern MSNA, which is not reproduced by the default SAMI3 runs using the neutral winds provided by the empirical Horizontal Wind Model 93 neutral wind model. A linear least squares algorithm for extracting tidal components is utilized to examine the major tidal component affecting the variation of southern MSNA. Results show that the standing diurnal oscillation component dominates the vertical neutral wind manifesting as a diurnal eastward wave-1 drift of the southern MSNA in the local time frame. We also find that the stationary planetary wave-1 component of vertical neutral wind can cause diurnal variation of the summer nighttime electron density enhancement around the midlatitude ionosphere.