Derivation of global ionospheric Sporadic E critical frequency ( f _o Es) data from the amplitude variations in GPS/GNSS radio occultations

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

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

“…The IPS5A ionosonde appears to be measuring some abnormal low values near the detection threshold, which makes the Canberra data did not match for low f o Es values. From a global-scale investigation of ionosonde parameters of Es layers, such abnormally high occurrences of foEs values could also be found near the instrumental detection limits within the frequency range 1.28-1.60 MHz (Yu et al 2020). It indicates that f o Es values are determined less reliably from ionosondes in the low frequency, as a likely result from the influence of ambient ionizations within the background E layers to characterize the intensity of weak Es layers.…”

“…The S4max is the maximum value of S4 index, denoting an amplitude scintillation index in the GNSS RO signals. Large S4 indices occurring at altitudes of 90-130 km are related to the occurrence of Es layers (Yue et al 2015a;Yu et al 2019aYu et al , 2020. In this study, the intensity of Es layers, represented by the continuous 24-h recording of S4max in RO data from the COSMIC satellites, was compared with the critical frequency of the Es layers, f o Es, observed with ground-based ionosondes.…”

“…On the basis of COSMIC S4max observations, 66% of Es layers have a f o Es < 3.6 MHz (S4max < 0.4), which is not easily visible in ionosonde data. Global critical frequency f o Es data have been derived based on the FORMOSAT-3/COSMIC RO measurements correlated with ground-based ionosonde data, augmenting the limited coverage and low-frequency detection threshold of ground-based instruments (Yu et al 2020). Mathews (1998) proposed that the Es layer is considered to be sporadic due to instrumental limitation rather than physical property.…”

“…The maximum values of the amplitude scintillation S4 index (S4max) derived from 50 Hz L1 amplitude measurements, which occur at the altitude of Es, can be used as a proxy of the intensity of Es layers (Yue et al 2015a). Recently, Yu et al (2019aYu et al ( , 2020 present an investigation into the global climatology of the relative intensities of Es layers, based on the S4 index of the FORMOsa SATellite mission-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FOR-MOSAT-3/COSMIC). Furthermore, the successful launch of six satellites on a SpaceX Falcon Heavy rocket for the FORMOSAT-7/COSMIC-2 (COSMIC-2) mission will soon enable improved capability of global ionospheric Es layer observations with the high quality and quantity of GNSS RO measurements (Hsu et al 2018).…”

A Signature of 27 day Solar Rotation in the Concentration of Metallic Ions within the Terrestrial Ionosphere

“…The ionospheric sporadic E (Es) layer at altitudes between 90 and 130 km is of particular interest in ionospheric investigations, because it is a thin layer composed of terrestrial metallic ions of meteoric origin near the boundary between the upper atmosphere (containing the ionosphere) and the lower neutral atmosphere (Cai et al 2019;Yu et al 2019a). The plasma irregularities in Es layers can significantly affect signals from the Global Positioning System (GPS)/Global Navigation Satellite System (GNSS) by causing a temporary loss of phase lock between the two frequencies in use to eliminate ionospheric propagation effects (Yue et al 2016;Yu et al 2020). Diurnal and semidiurnal atmospheric tides are known to dominate the short-term variations in Es layers (Mathews & Bekeny 1979).…”