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 (f_oEs) data. The global distribution of f_oEs with a high spatial resolution shows a strong seasonal variation in f_oEs 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 f_oEs values from ionosondes near the instrumental detection limits corresponding to minimum frequencies in the range 1.28–1.60 MHz.

The global hourly ionospheric critical frequency of Es layer data were derived, based on the parameters of Es layers from GNSS-RO satellite measurements and ground-based ionosonde observations. The hourly coincident events were analysed using FORMOSAT-3/COSMIC RO data and ionosonde data from 25 low-to middle-latitude stations during the period 2006–2014. The relation between the two types of measurement was used to derive a high-resolution global map of the intensity of Es layers, in which the effects of the earth's magnetic field, the diurnal and semi-diurnal tides on the latitude/longitude distribution of f_oEs in both hemispheres are apparent.

Ionosondes provide reliable ground-based observations of the intensity of Es layers. The ionospheric parameter critical frequency f_oEs (in Hz) is the peak plasma frequency in ordinary mode, which is associated with the peak electron concentration of the Es layer, N_e (in m^-3), by the formula f_oEs=8.98N_e^½. The S4 index derived from satellite measurements, is defined as the standard deviation of signal intensity fluctuations normalized by average intensities. The S4max index is the maximum value of the amplitude scintillation S4 index in the GPS/GNSS RO signals. Large S4max values are associated with strong vertical gradients in the ionospheric electron density.

Since the S4max is related to the fluctuations of GPS/GNSS RO signals caused by large vertical gradients in the Es layer, the magnitude of S4max has a linear dependence on the electron density (Whalen, 2009). Therefore, the relationship between S4max and f_oEs² is defined as (f_oEs-f_BG)²=a*S4max. f_BG is the background frequency of the ambient electron density in the absence of Es layers, which is 1.2 MHz estimated from the low detection threshold of Es layers by ionosondes. The relation between f_oEs and S4max is (f_oEs-1.2)²=13.62*S4max (r=0.40, p<0.01).

Twelve columns represent S4max, foEs(MHz), Year,  DOY(Day of Year), Ut_time(h), local_time(h), mag.local_time(h), Altitude(km), latitude(°), longitude(°), mag.latitude(°), and mag.longitude(°) respectively.