Using GNSS radio occultation data to derive critical frequencies of the ionospheric sporadic E layer in real time

The small-scale electron density irregularities in the ionosphere have a significant impact on the interruptions of Global Navigation Satellite System (GNSS) navigation and the accuracy of GNSS positioning techniques. The sporadic ionospheric E (Es) layer significantly contributes to the transient interruptions of signals (loss of lock) for GNSS tracking loops. These effects on the GNSS radio occultation (RO) signals can be used to derive the global location and intensity of Es layers as a complement to ground-based observations. Here we conduct statistical analyses of the intensity of Es layers, based on the scintillation index S4max from the FORMOSAT-3/COSMIC during the period 2006-2014. In comparison with simultaneous observations from an ionosonde network of five low-to-middle latitude ionosondes, the S4max indices from COSMIC, especially the small values, are linearly related to the critical frequency of Es layers (foEs). An accumulated period of less than one hour is required to derive the short-term variations in real-time ionospheric Es layers. A total of 30.22%, 69.57% and 98.13% coincident hourly foEs values have a relative difference less of than 10%, 30% and 100%. Overall, the GNSS RO measurements have the potential to provide accurate hourly observations of Es layers. Observations with S4max<0.4 (foEs<3.6 MHz), accounting for 66% of COSMIC S4 measurements, have not been used fully previously, as they are not easily visible in ground-based ionosonde data

Derivation of global ionospheric sporadic E critical frequency (foEs) data from the amplitude variations in GPS/GNSS radio occultations

The ionospheric sporadic E (Es) layer has a significant impact on the Global Positioning System (GPS)/Global Navigation Satellite System (GNSS) signals. These influences on the GPS/GNSS signals can also be used to study the occurrence and characteristics of the Es layer on a global scale. In this paper, 5.8 million radio occultation (RO) profiles from the FORMOSAT-3/COSMIC satellite mission and ground-based observations of Es layers recorded by 25 ionospheric monitoring stations and held at the UK Solar System Data Centre at the Rutherford Appleton Laboratory and the Chinese Meridian Project were used to derive the hourly Es critical frequency (foEs) data. The global distribution of foEs with a high spatial resolution shows a strong seasonal variation in foEs with a summer maximum exceeding 4.0 MHz and a winter minimum between 2.0–2.5 MHz. The GPS/GNSS RO technique is an important tool that can provide global estimates of Es layers, augmenting the limited coverage and low frequency detection threshold of ground-based instruments. Attention should be paid to small foEs values from ionosondes near the instrumental detection limits corresponding to minimum frequencies in the range 1.28–1.60 MHz

An empirical model of the ionospheric sporadic E layer based on GNSS radio occultation data

The intense plasma irregularities within the ionospheric sporadic E (Es) layers at 90–130 km altitude have a significant impact on radio communications and navigation systems. As a result, the modeling of the Es layer is very important for the accuracy, reliability, and further applications of modern real-time global navigation satellite system precise point positioning. In this study, we have constructed an empirical model of the Es layer using the multivariable nonlinear least-squares-fitting method, based on the S4max from Constellation Observing System for Meteorology, Ionosphere, and Climate satellite radio occultation measurements in the period 2006–2014. The model can describe the climatology of the intensity of Es layers as a function of altitude, latitude, longitude, universal time, and day of year. To validate the model, the outputs of the model were compared with ionosonde data. The correlation coefficients of the hourly foEs and the daily maximum foEs between the ground-based ionosonde observations and model outputs at Beijing are 0.52 and 0.68, respectively. The model can give a global climatology of the intensity of Es layers and the seasonal variations of Es layers, although the Es layers during the summer are highly variable and difficult to accurately predict. The outputs of the model can be implemented in comprehensive models for a description of the climatology of Es layers and provide relatively accurate information about the global variation of Es layers

Impact of sudden stratospheric warmings on the neutral density, temperature and wind in the MLT region

In this study, the neutral density and horizontal wind observed by the four meteor radars, as well as the temperature measured by the Microwave Limb Sounder (MLS) onboard the Aura satellite are used to examine the response of neutral density, wind, and temperature in the MLT region to the stratospheric sudden warmings (SSWs) during 2005 to 2021 in the Northern Hemisphere. The four meteor radars include the Svalbard (78.3°N, 16°E) and Tromsø (69.6°N, 19.2°E) meteor radars at high latitudes and the Mohe (53.5°N, 122.3°E) and Beijing (40.3°N, 116.2°E) meteor radars at middle latitudes. The superposed epoch analysis results indicate that: 1) the neutral density over Svalbard and Tromsø at high latitude increased at the beginning of SSWs and decreased after the zonal mean stratospheric temperature reached the maximum. However, the neutral density over Mohe at midlatitudes decreased in neutral density at the beginning of SSW and increase after the zonal mean stratospheric temperature reached the maximum. 2) The zonal wind at high latitudes show a westward enhancement at the beginning of SSWs and then shows an eastward enhancement after the stratospheric temperature reaches maximum. However, the zonal wind at midlatitudes shows an opposite variation to at high latitudes, with an eastward enhancement at the onset and changing to westward enhancements after the stratospheric temperature maximum. The meridional winds at high and midlatitudes show a southward enhancement after the onset of SSW and then show a northward enhancement after the stratospheric temperature maximum. 3) In general, the temperature in the MLT region decreased throughout SSWs. However, as the latitudes decrease, the temperature cooling appears to lag a few days to the higher latitudes, and the degree of cooling will decrease relatively