Abstract
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 and 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.
Highlights
The ionospheric sporadic E (Es) layer has a significant impact on the global positioning system (GPS)/global navigation satellite system (GNSS) signals
Patches of ionization within Es layers form as a result of the vertical ion convergence driven by vertical shears in the zonal neutral wind and meridional neutral wind
The yellow dots show the distribution of 2638 FORMOSAT-3/COSMIC radio occultation (RO) events in 24 h on 25 December 2007
Summary
The FORMOSAT-3/COSMIC mission is a low-Earth-orbit (LEO) constellation of six microsatellites launched from Vandenberg Air Force Base in April 2006 [57]. Six FORMOSAT-3/COSMIC satellites initially followed the same orbit at approximately 512 km and subsequently orbited the Earth at 800 km. The computed detrended S4max data occurring between 90 and 130 km altitude over a 9-year period from 2006 to 2014 were used to study the intensity of Es layers. The FORMOSAT-3/COSMIC can provide 1500–2500 RO measurements per day and a total of approximately 5.8 million S4max profiles were used to study the occurrence and intensity of Es layers during the study period. S4max observations made within a region of 5° × 5° geographical latitudes and longitudes square centred on each ionosonde station were used. A total of 26 863 h coincident events were analysed using the FORMOSAT-3/COSMIC S4max data and the ionosonde data from 25 stations
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