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The F2-peak plasma frequency (foF2) and the height of the F2 peak (hmF2) are two of the most important parameters for any ionospheric model, as well as radio propagation studies and applications. In this study, we have developed empirical models to capture the most significant variations of foF2 and hmF2. The derived empirical models (referred to as the USTC models within this study) are specified through global ionosonde and reanalysis data based on the International Reference Ionosphere (IRI) Consultative Committee on International Radio (CCIR) method and Constellation Observindg System for Meteorology, Ionosphere, and Climate (COSMIC) observations based on the empirical orthogonal function analysis, respectively. The USTC models are validated against the IRI CCIR model prediction. The comparison results revealed that the empirical foF2 model performs better in capturing the foF2 variations than the IRI CCIR model, which can overcome the underestimation of the IRI CCIR model at low latitudes. Although the IRI CCIR model overestimation at middle latitudes is addressed by the empirical hmF2 model, the visible differences between the model predictions and ionosonde observations still exist at low latitudes, which could be attributed to the significant difference between COSMIC and ionosonde hmF2 measures. [ABSTRACT FROM AUTHOR]

It has previously found that at Sanya (109.6°E, 18.3°N) a post-midnight enhancement in the F2-layer peak electron density (NmF2) is often accompanied by the increase in the bottom side F layer electron density (Ne) and a reduction in the top side F layer Ne. We conduct a case study to explore the complete spatial coverage of the post-midnight enhancements of electron density in the low latitude ionosphere. The total electron content (TEC) data are retrieved from the Beidou Geostationary Earth Orbit (GEO) satellite signals recorded by a network of Global Navigation Satellite System (GNSS) receivers in Asia-Australia. The F2-layer peak parameters and the altitudinal profiles of Ne are measured by the ionosonde, and incoherent scatter radar at Sanya (109.6°E, 18.3°N). The GEO-TEC does not always completely follow the enhancement of NmF2. The enhancements may peak roughly simultaneously or with significant time shifts in the west-east direction, even in the GEO-TEC series recorded by a GNSS receiver in an event, which indicates possibly the enhancement of different longitudinal scales. Further, in the cases the enhancement is clustered in a limited area. The low latitude enhancements show a hemispheric asymmetry; they occur mainly in the northern hemisphere and usually have no correspondingly magnetic conjugate symmetrical features in the southern hemisphere. The hemispheric asymmetry of the enhancements highlights that both the field-aligned plasma transport and electric fields are possibly responsible for regulating the limited spatial presence of post-midnight enhancements in the low latitude ionosphere. [ABSTRACT FROM AUTHOR]

Given that the ionosphere is strongly determined by the thermosphere and its state depends on thermospheric parameters, we propose a new method to extract exospheric temperature (Tex) from electron density (Ne) profiles based on the relationship between the variations in Tex and Ne profiles established through simulation. Ne profiles and corresponding Tex from the Millstone Hill incoherent scatter radar (ISR) observations are used to test the method. ISR Ne profiles are used for Tex retrieval and ISR Tex is used to make a comparison with Tex calculated by model and retrieved Tex. The results show that the retrieved Tex effectively captures diurnal, anomalous and short-period variations. The relative deviation between the retrieved-observed Tex is approximately 2%, which is significantly improved compared with the Mass Spectrometer Incoherent Scatter model, especially under disturbed conditions. This result confirms that thermospheric temperature variation can be deduced from ionospheric profiles and our method can be considered a useful tool to obtain Tex from Ne profiles. [ABSTRACT FROM AUTHOR]

Using the electron density profiles from Mars Global Surveyor Radio Occultation observations, the ionospheric longitudinal wave-3 (WN3) structure with different frequencies is investigated. The semidiurnal component is the dominant component for the entire Mars ionosphere profile at northern high latitudes. Both diurnal and semidiurnal components manifest double peaks in altitudinal variations in amplitude, which are 5-15 km lower than the ionospheric M1 and M2 peaks. The phase generally increases with altitude and shows a rapid rate of increase around the M2 peak. To interpret the altitudinal variation in amplitude and phase, we then derive the tidal coupling equation to elucidate the impact of atmospheric tides on the Mars ionosphere. Under the assumption of photochemical equilibrium, the tidal coupling equation indicates that the log density gradient of electrons (1) critically modulates the ionospheric response to atmospheric tides. In the observations, a positive correlation between the profile and the amplitude of the WN3 structure at altitude is found. The solar longitude variation in the peaks also agrees with the vertical movement of the amplitude peaks of the WN3 structure along the solar longitude. The rapid phase increase corresponds to the abrupt decrease in around the M2 peak. These observational results agree with the tidal coupling equation prediction, thereby supporting that the interpretation of in modulating the altitudinal variation in the WN3 structure is reasonable. [ABSTRACT FROM AUTHOR]

Ionospheric day-to-day variability is essential for understanding the space environment, while it is still challenging to properly quantify and forecast. In the present work, the day-to-day variability of F2 layer peak electron densities (NmF2) is examined from both observational and modeling perspectives. Ionosonde data over Wuhan station (30.5°N, 114.5°E; 19.3°N magnetic latitude) are compared with simulations from the specific dynamics Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (SD-WACCM-X) and the Thermosphere-IonosphereElectrodynamics General Circulation Model (TIEGCM) in 2009 and 2012. Both SD-WACCM-X and TIEGCM are driven by the realistic 3 h geomagnetic index and daily solar input, and the former includes self-consistently solved physics and chemistry in the lower atmosphere. The correlation coefficient between observations and SD-WACCM-X simulations is much larger than that of the TIEGCM simulations, especially during dusk in 2009 and nighttime in 2012. Both the observed and SD-WACCM-X simulated day-to-day variability of NmF2 reveal a similar day-night dependence in 2012 that increases large during the nighttime and decreases during the daytime, and shows favorable consistency of daytime variability in 2009. Both the observations and SD-WACCM-X simulations also display semiannual variations in nighttime NmF2 variability, although the month with maximum variability is slightly different. However, TIEGCM does not reproduce the day-night dependence or the semiannual variations well. The results emphasize the necessity for realistic lower atmospheric perturbations to characterize ionospheric day-to-day variability. This work also provides a validation of the SD-WACCM-X in terms of ionospheric day-to-day variability. [ABSTRACT FROM AUTHOR]

Aurora provides an essential diagnostic to spatial and temporal variations of terrestrial space environment and is also an important proxy of solar and geomagnetic activities. Contemporary auroral observations have just continued for more than half a century. Visual auroral phenomena recorded in historical books provide key clues to understand the solar and geomagnetic activities in the long history prior to modern era. In this study, we compiled a new auroral catalog from ancient Korean historical books, including 2013 auroral records with day-level resolution from 1012 to 1811 CE, especially for the records searched from the Seungjeongweon Ilgi. The number of auroral records in this new catalog is greatly enlarged compared with previous lists. The occurrence of the aurora in the new catalog is generally consistent with previous data sets. This extended data set provides valuable support for various studies related to solar-terrestrial space weather and ancient climates in the past millennium. [ABSTRACT FROM AUTHOR]