DOI: 10.5061/dryad.xsj3tx9bx
title: The global critical frequency foEs data derived from the FORMOSAT-3/COSMIC S4max for the period 2006 to 2014

description: 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.
description: 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<sub>o</sub>Es in both hemispheres are apparent.
 Ionosondes provide reliable ground-based observations of the
 intensity of Es layers. The ionospheric parameter critical frequency
 f<sub>o</sub>Es (in Hz) is the peak plasma frequency in
 ordinary mode, which is associated with the peak electron concentration of
 the Es layer, N<sub>e</sub> (in m<sup>-3</sup>),
 by the formula
 f<sub>o</sub>Es=8.98N<sub>e</sub><sup>½</sup>. 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<sub>o</sub>Es<sup>2</sup> is defined as (f<sub>o</sub>Es-f<sub>BG</sub>)<sup>2</sup>=a*S4max. f<sub>BG</sub> 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<sub>o</sub>Es and S4max is (f<sub>o</sub>Es-1.2)<sup>2</sup>=13.62*S4max (r=0.40, p&lt;0.01).
description: 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.