Sounding of sporadic E layers from CSES radio occultation and comparing with ionosonde measurements

GNSS radio occultation (RO) plays an important role in ionospheric electron density inversion and sounding of sporadic E layers. As the China's first electromagnetic satellite, China Seismo Electromagnetic Satellite (CSES) has collected the RO data from both GPS and BDS-2 satellites since March 2018. In this study, we extracted the carrier to noise density ratio (CNR) data of CSES and calculated the standard deviation of normalized CNR. A new criterion is developed to determine the Es events, that is when the mean value of the absolute value of the difference between the normalized CNR is greater than 3 times of the standard deviation. The statistics show that sporadic E layers have strong seasonal variations with highest occurrence rates in summer season at middle latitudes. It is also found that the occurrence height of Es is mainly located at 90–110 km, and the period of local time 15:00–18:00 is the high incidence period of Es. In addition, the geometric altitudes of a sporadic E layer detected in CSES radio occultation profiles and the virtual heights of a sporadic E layer obtained by the Wuhan Zuo Ling Tai (ZLT) ionosonde show four different space-time matching criterions. Our results reveal that there is a good agreement between both parameters which is reflected in the significant correlation.

Tidal signatures in sporadic E occurrence rates - migrating and nonmigrating components, and comparison with neutral wind shear

<p>In the lower ionospheric E region, shallow regions of high electron density are found, which are called sporadic E (ES) layers. ES layers consist of thin clouds of accumulated ions. They occur mainly at middle latitudes, and they are most frequently found during the summer season. ES are generally formed at heights between 90 and 120 km. At midlatitudes, their occurrence can be described through the wind shear theory. According to this theory, ES formation is due to interaction between the metallic ion concentration, the Earth’s magnetic field, and the vertical shear of the neutral wind. Here, we analyze ES occurrence rates (OR) obtained from ionospheric radio occultation measurements by the FORMOSAT-3/COSMIC constellation. To derive information on ES from RO, we use the Signal-to-Noise ratio (SNR) profiles of the GPS L1 phase measurements. If large SNR standard deviation values occur that are concentrated within a layer of less than 10 km thickness, we assume that the respective SNR profile disturbance is owing to an ES layer.</p><p>Midlatitude ES are found to be mainly connected with a migrating diurnal and semidiurnal component. Especially at high latitudes of the southern hemisphere, nonmigrating components such as a diurnal westward wave 2 and a semidiurnal westward wave 1 are also visible. Near the equator, a strong diurnal eastward wavenumber 3 component and a semidiurnal eastward wavenumber 2 component are found in summer and autumn. Terdiurnal and quarterdiurnal components are weaker than the diurnal and semidiurnal ones. We discuss seasonal and global distributions of migrating and nonmigrating components, and their relation to neutral wind shear derived from ground-based observations and numerical modeling.</p>

Sporadic E morphology based on COSMIC radio occultation data and its relationship with wind shear theory

The S4max data retrieved from the Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements during 2007 to 2015 is adopted to investigate the global distribution and seasonal variation of the sporadic E (Es) layers in the present work. The long-term and short-term global Es occurrence maps are presented and the spatial and temporal distributions of Es occurrence rates (ORs) are further confirmed and studied. The International Geomagnetic Reference Field model (IGRF12) is used to calculate the horizontal intensity and inclination of the Earth’s magnetic field. The analysis shows that the Earth’s magnetic field is one of the fundamental reasons for the global distribution of the Es layers. In addition, the Horizontal Wind Field model HWM14 and the IGRF12 model were employed to calculate the vertical ion convergence (VIC) to examine the role of neutral wind shear in the global distribution of the Es ORs. The results reveal that the middle latitude distribution of simulated vertical concentration of Fe+ is similar to that of Es ORs, which indicates that the VIC induced by the neutral wind shear is an important factor in determining the geographical distribution, summer maximum (or winter minimum) and diurnal characteristics of Es ORs in middle latitudes. The new findings mainly include the following two aspects: (1) in summer over mid-latitudes, VIC peaks in the morning and afternoon to evening, which explains the semidiurnal behavior of Es ORs; (2) VIC reaches its minimum value in low-altitude (100 km) areas, which is the reason for the significant decrease in Es ORs in low-altitude areas. The disagreements between the VIC and Es ORs indicate that other processes, such as the meteor influx rate, the ionospheric electric fields and atmospheric tides, should also be considered as they may have an important impact on the variation of Es layers. Graphical Abstract

Comparison of Seasonal foEs and fbEs Occurrence Rates Derived from Global Digisonde Measurements

A global climatology of sporadic-E occurrence rates (ORs) based on ionosonde measurements is presented for the peak blanketing frequency, fbEs, and the ordinary mode peak frequency of the layer, foEs. ORs are calculated for a variety of sporadic-E frequency thresholds: no lower limit, 3, 5, and 7 MHz. Seasonal rates are calculated from 64 Digisonde sites during the period 2006–2020 using ionograms either manually or automatically scaled with ARTIST-5. Both foEs and fbEs ORs peak in the Northern Hemisphere during the boreal summer, with a decrease by roughly a factor of 2–3 in fbEs rates relative to foEs rates without a lower threshold on the sporadic-E intensity. This ratio of foEs to fbEs OR increases with increasing sporadic-E intensity, up to a factor of 5 for the 7 MHz threshold. An asymmetry is observed with the Southern Hemisphere peaks during the austral summer, with slightly lower rates compared with the Northern Hemisphere during the boreal summer. A drastic decrease in ORs is observed for the higher intensity thresholds, such that the fbEs occurrence rates for 7 MHz are nearly zero during most locations and seasons. These updated occurrence rates can be used for future statistical comparisons with GPS radio occultation-based sporadic-E occurrence rates.