Nighttime Ionosphere Research Articles

Observations of Strong Range Spread F and Nighttime Ionospheric Enhancement in the Mid‐Latitude Region Under Geomagnetically Quiet Conditions

AbstractSpread F is one of most widely observed nighttime ionospheric irregularities by ionosondes in the equatorial, low‐ and mid‐latitude regions. One type of spread F, known as strong range spread F (backscatter echoes beyond the critical frequency of the F2 layer, foF2), has been observed in the equatorial and low‐latitude regions but not at middle latitudes. This study reported for the first‐time observations of strong range spread F at Zhangye Station (ZHY, Geographical latitude 39.4°N, longitude 100.0°E, Dip Lat 29.6°N) on 29 November 2019 (under quiescent conditions). Observations show that strong range spread F occurred at first during midnight (∼16:30 UT), accompanied with mid‐latitude nighttime ionospheric enhancements in foF2 and total electron content (TEC). Then, it becomes more pronounced when ionospheric collapse occurred. The measurements from satellite revealed that there are large opposite temperature and density gradients (temperature decreased sharply from ∼3100 to ∼2500 K, however, electron density increased sharply from ∼0.3 e5 to ∼1.2 e5 cm−3 around Zhangye Station) during this event. Results suggest that mid‐latitude strong range spread F might be attributed to temperature gradient instabilities.

Peculiar Nighttime Ionospheric Enhancements Over the Asian Sector During the May 2024 Superstorm

AbstractThis study reports a peculiar nighttime ionospheric enhancement in the total electron content (TEC) from the geostationary (GEO) satellites in response to the geomagnetic superstorm of May 2024. The enhancements occurred at low to mid‐latitudes during the storm's recovery phase on 11–12 May, with the TEC values almost twice as high as the quiet ones. Surprisingly, the nighttime ionospheric enhancements sub‐rotated westward with a speed of ∼130.5 m/s. The nighttime TEC enhancements lasted ∼5–7 hr for a given location and persisted over half a day over wide longitude ranges. Meanwhile, ionosonde data from two equatorial ionization anomaly (EIA) stations showed a higher peak height of the F2 layer, and a significant double‐crest EIA structure was observed by both GEO TECs and in situ electron densities from China Seismo‐Electromagnetic Satellite during this period of interest, indicating a possible contribution from disturbance electric fields producing nighttime eastward equatorial electric fields. Nevertheless, it remains a mystery whether the nighttime TEC enhancement and its movement were associated with the wind disturbance dynamo.

A Bayesian Framework for Accurate Determination of the Nighttime Ionospheric Parameters from the ICON FUV Observations

A Bayesian Framework for Accurate Determination of the Nighttime Ionospheric Parameters from the ICON FUV Observations

Virtual Reflection Height of Nighttime Equatorial Ionosphere Estimated With Low‐Frequency Magnetic Sferics Measured in Malacca

AbstractThe return stroke of cloud‐to‐ground (CG) lightning is an impulsive radiator of very low‐frequency/low‐frequency (VLF/LF) electromagnetic signals allowing for the remote sensing of lower ionosphere over large spatial coverage. In this study, we examined the LF magnetic fields measured in Malacca, Malaysia, to probe reflection heights of the lower ionosphere near the equator on three different nights in 2021. The results show that the virtual ionospheric height at nighttime typically ranged from 82.0 to 90.0 km, with a mean value of 85.3 km. Our measurements also revealed significant variations in the virtual ionospheric height across different regions over a spatial scale of about 800 km. The maximum height difference was about 5.0 km. Moreover, the fluctuation characteristics are observed in both estimated ionospheric height and calculated peak reflection ratio during similar periods. This fluctuation may be related to atmospheric gravity waves in the nighttime ionosphere. In addition, we compared the virtual ionospheric height estimated from CG strokes of different polarities, and the results showed that the virtual reflection height for positive CG strokes is lower than that for negative ones.

Application of the Bagged Trees Technique on Retrieving the Nighttime Ionospheric Peak Density From OI‐135.6 nm Airglow

AbstractThe NASA global‐scale observations of the limb and disk (GOLD) mission is a measurement opportunity to scan the far ultraviolet airglow at ∼134–162 nm over the American Hemisphere since October 2018. The FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission has provided thousands of daily radio occultation soundings in the low‐ and mid‐latitude regions since July 2019. The nighttime OI–135.6 nm emission is mainly through radiative recombination, and the radiance is used to derive the peak electron density. Comparison with corresponding F7/C2 observations demonstrates good correlation in low‐latitudes, while is overestimated near mid‐latitudes in winter, induced by the photoelectrons emanating from magnetically conjugate Hemisphere. The machine learning technique Bagged Trees is implemented to develop an intensity to peak density model training from GOLD and F7/C2 observations. The validation demonstrates that Bagged Trees peak‐density has less influence from conjugate photoelectrons and indicates the power of machine learning techniques for geophysics data processing.

On the possibility of experiments on the excitation of artificial ultra-low and extra-low frequency emissions in the ionosphere by the FENICS installation on the Kola peninsula

A numerical model has been developed to calculate the electromagnetic response in the ionosphere from grounded ultra-low-frequency transmitters of finite length L. Such megatransmitters are the ZEVS installation with a carrier frequency of 82 Hz and the FENICS installation, which can generate artificial emissions at frequencies from fractions of a Hz to a few hundreds of Hz. The amplitude of radiation excited in the upper ionosphere by a grounded horizontal current suspended above a high-resistance earth’s surface has been calculated. The altitude profile of the plasma parameters was reconstructed using the IRI ionospheric model. For the ZEVS transmitter (L = 60 km) powered by a current of 200 A, the simulated amplitudes of the electromagnetic response in the nighttime ionosphere can reach ~60 μV/m, which was confirmed by observations on the DEMETER satellite. According to calculations, the FENICS facility (L = 100 km), powered by a current of 100 A, can generate radiation in the nighttime upper ionosphere with a frequency of 10—100 Hz and an amplitude of up to ~60—70 μV/m. The FENICS facility can be used to excite artificial Pc1 pulsations that could be detected on low-Earth-orbit satellites (e.g., CSES). To create pulsations in the nighttime ionosphere at a frequency of 0.5 Hz with the amplitudes of the magnetic component 1 pT and the electric component 10 μV/m, the current in the FENICS antenna is to be 100 A.

The Influence of Field-Aligned Currents on Electron Density in the Ionosphere: Combined Observations of SWARM Satellites and ESR Radar

The results are presented of combined measurements by the SWARM spacecraft (SC) and European incoherent scatter radar on Svalbard for two events of simultaneous observations: in the nighttime ionosphere during substorm activation on January 9, 2014, and in the daytime ionosphere under quiet conditions on February 5, 2017. Onboard magnetometers of the SWARM SC provide measurements of field-aligned current density over the ionosphere. The radar, which is under the flyby trajectory at this time, measures the vertical distribution of the electron density (Ne). Experiments have shown that, under disturbed nighttime conditions, at the location of the field-aligned current flowing from the ionosphere, the plasma density increases throughout the entire slab of the ionosphere and the change in Ne is in agreement with theoretical estimates. In the daytime quiet ionosphere, Ne increases only in the F layer, but practically does not change in the E layer. The differences may be due to the fact that, in the first case, the carriers of the upward directed current are represented by the entire energy spectrum of auroral electrons of 1–10 keV, and in the second case only by the low-energy part.

Response of the F2-peak electron density and height to plasma depletion bays in the equatorial nighttime ionosphere observed by FORMOSAT-7/COSMIC-2

FORMOSAT-7/COSMIC-2 (F7/C2) satellite cluster, launched in June 2019, performs radio occultation experiments observing the vertical electron density profiles in the low-latitude ionosphere. The F7/C2 F2-peak electron density (NmF2) and height (hmF2) are used to identify and examine the behavior of the plasma depletion bay (PDB) structures in the equatorial nighttime ionosphere. The NmF2 reveals three distinct PDBs within 60°W–15°E, 30−90°E, and 110−160°E during June solstice, while a single PDB appears within 150−60°W during December solstice. All the four PDBs’ features simultaneously appear during March and September equinoxes. Asymmetries in the hmF2 over the PDB regions suggests that the electron density increases (decreases) are related to the NmF2 ascending (descending) in the summer (winter) hemispheres. Agreements between F7/C2 observations and HWM93 results show that the asymmetry of vertical motions driven by the neutral wind is the main causal mechanism for the longitudinal variation of PDBs.

Variability and distribution of nighttime equatorial to mid latitude ionospheric irregularities and vertical plasma drift observed by FORMOSAT-5 Advanced Ionospheric Probe in-situ measurements from 2017 – 2020

Irregularities in ionospheric plasma distribution can result in severe scintillation and disruption to the radio frequencies utilized for satellite communications and navigation. In the low and mid latitudes, these irregularities can include Equatorial Plasma Bubbles (EPBs) and Travelling Ionospheric Disturbances (TIDs). EPBs are irregularities manifesting in low latitude nighttime ionosphere plasma density that can extend along magnetic field lines with zonal scales on the order of 100 km or less, while TIDs are propagating wave disturbances. High frequency in-situ measurements of ionospheric plasma aboard spacecraft in Low Earth Orbit (LEO) are a direct measurement of irregularities in plasma density and are therefore valuable for resolving EPB and TID occurrences, variability, and relation to other ionospheric parameters that are believed to play a driving role in the formation of such irregularities. In this study, we utilize observations taken over a three-year period between 2017 and 2020 by the Advanced Ionospheric Probe (AIP) carried aboard the FORMOSAT-5 satellite to examine the spatial, seasonal, and interannual variability of equatorial to mid latitude ionospheric irregularities and vertical ion drift during this time. AIP provides in-situ measurements of ion density and vertical ion drift in the equatorial to mid latitude ionosphere at approximately 720 km altitude with local times between 22:00 – 23:00 local time. Our global scale results resolve distinct and inter-annually recurrent seasonal patterns in the distribution of nighttime ionospheric irregularities and vertical plasma drift during this time. Elevated occurrences of ion density irregularities are resolved along the Equatorial Ionization Anomaly (EIA) latitudes, while notable occurrences with variability consistent with EPBs also observed along the low and equatorial magnetic latitudes. Zonal variability of equatorial irregularities consistent with the signatures of nonmigrating atmospheric tides are observed. It is also notable that the occurrences and geographic distribution of ion density irregularities showed a considerable level of interannual variability, especially at mid latitudes over the South Atlantic and Southern African sectors, which showed much higher levels of irregularities in 2017–––2018, compared to 2019 and 2020. In comparison, the spatial and interannual variation of the co-located vertical ion drifts were much more consistent during the years examined, indicating that the driver for the observed interannual variability in ion density irregularities cannot be attributed to the vertical ion drift at the same time and location of the observations. This highlights the need for in-situ instruments distributed across multiple satellites in different local time zones.

The magnetically controlled ionopause boundary observed by LPW onboard MAVEN within magnetic pile-up region of Mars

We have analysed plasma and magnetic field data obtained from 627 orbits of Mars Atmosphere and Volatile Evolution (MAVEN) that occurred during October 2014, April 2015, September 2017 and May 2018, when the spacecraft was crossing the magnetic pile-up boundary of Mars. Two broad peaks and a drop are observed in this magnetic field region. The magnetic cavity boundary lies between the ionosphere and the magnetic pile-up boundary. A steep ionopause-like boundary is observed in 24 profiles of 627 orbits, when MAVEN was passing from the magnetic pile-up region during the daytime ionosphere in presence of horizontal magnetic field of high strength, while their night side ionosphere did not show such boundary in presence of horizontal magnetic field of low strength. These profiles are unique, which have not been reported earlier. We propose that the horizontal magnetic field can form such ionopause within the magnetic pile-up boundary during the daytime if time and location of the magnetic anomaly coincide with the ion and electron density measurements. The nighttime ionosphere is thought to be produced within the magnetic pile-up boundary due to transportation of plasma from dayside to nightside across the terminator by a horizontal plasma flow velocity as has been predicted by the previous studies.

A new strategy for ionospheric remote sensing using the 130.4/135.6 nm airglow intensity ratios

We demonstrate here that global-scale determination of a key ionospheric parameter, the peak height of the F2 region (hmf2), can be obtained by making a simple ratio measurement of the atomic oxygen 130.4 nm and 135.6 nm emission in the far ultraviolet nightglow with a nadir-viewing system such as a pair of photometers suitable for flight on a cubesat. We further demonstrate that measurements from an altitude that is within the typical range of nighttime hmf2≃250-450 km can provide the ratios that are needed for the hmf2 retrieval. Our study is conducted mostly through numerical simulations, using radiative transfer models of the two emissions coupled with empirical models of the atmosphere and ionosphere. Modeling results show that the relationship between hmf2 and the intensity ratio is sensitive to the altitude from which the emissions are observed, primarily due to the distinctly different degrees of resonant scattering of the two emissions in the atmosphere. A roughly quadratic relationship can be established for observations from an orbit of ~400 km, which enables hmf2 retrieval. A parametric analysis indicates that the relationship can be affected by the ambient atmospheric conditions through resonant scattering and O2 absorption. For typical nighttime conditions with hmf2≃250-450 km, retrieval of hmf2 from synthetic observations shows that the typical errors are only a few kilometers (up to~20 km), depending on the accuracy of the ambient conditions predicted by the empirical models. Our findings pave the way for the use of the 130.4/135.6 intensity ratios for global-scale monitoring of the nighttime ionosphere at mid-to-low latitudes.

A Statistical Study of Nighttime Ionospheric NmF2 Enhancement at Middle‐to‐High Latitudes in the Northern Hemisphere

AbstractThe electron density in the ionosphere usually continuously decreases during nighttime due to the vanishing of sunlit ionization. However, it sometimes increases unexpectedly, which has been called nighttime ionospheric enhancement. Previous researches studied this type of phenomenon mainly in the low‐to‐middle latitude regions. Here, we investigate the nighttime ionospheric NmF2 enhancement at middle‐to‐high latitudes in the northern hemisphere (NH) during solar cycles 23–24 by analyzing the ionosonde observations at Chilton (51.1°N, 359.4°E, geomagnetic 48.5°N), Juliusruh (54.6°N, 13.4°E, 51.7°N), Tromsø (69.9°N, 19.6°E, 66.9°N), and Qaanaaq (77.5°N, 290.8°E, 84.3°N), as well as comparing with the Empirical Canadian High Arctic Ionospheric Model (E‐CHAIM). The observations show that the nighttime NmF2 enhancement occurs mainly in the winter solstice months (November–February), and the occurrence rate and relative amplitude of the enhancement are inversely related to solar activity. The observed results are in good agreement with the results obtained from E‐CHAIM. Furthermore, the model shows the spatial distribution of the enhancement at the middle‐to‐high latitudes in NH in winter, most obvious in geomagnetic latitude between 50°N and 65°N, and have longitudinal minima centered at 30°W and maxima centered at 120°W and 75°E. From these results, this unique phenomenon at middle latitude is explained probably by the plasma flux from the higher altitudes (e.g., topside ionosphere and plasmasphere) diffusing downward into the ionospheric F2 layer. However, at polar regions, under the influence of the abundant high‐density structures such as polar cap patches, auroral blob, particle precipitation, etc., the F region plasma density enhancement becomes more complicated.

Hemispherically Asymmetric Evolution of Nighttime Ionospheric Equatorial Ionization Anomaly in the American Longitude Sector

AbstractThe Global‐scale Observation of Limb and Disk (GOLD) mission observed distinct post‐sunset, hemispherically asymmetric evolution of the equatorial ionization anomaly between 40° and 50°W on 19 November 2018, with the southern crest shifting poleward but the northern crest remaining in the same latitude region. The Whole Atmosphere Community Climate Model‐eXtended captured this phenomenon. Diagnostic analysis revealed that this asymmetric evolution was due to the hemispheric asymmetry in plasma transport by E × B drifts and neutral winds. In the Northern hemisphere, northward neutral wind transport dominated around sunset (20 UT near 45°W), causing the crest to be at lower altitudes and decay faster. After pre‐reversal enhancement (PRE) occurred at 20 UT in the northern hemisphere, E × B and neutral wind transport largely balanced each other, and the crest remained in the same latitude region. In the southern hemisphere EIA crest, PRE occurred around 1 hr later than the one in the north. The northward neutral wind transported the plasma to higher altitudes with less recombination. E × B transport augmented the neutral wind transport after 21 UT, lifting and pushing the crest poleward. The magnetic field configuration (large declination) and the resultant occurrence time difference in PRE between 40°W and 50°W not only play critical roles in the asymmetric evolution of the two crests, but also makes it possible for this phenomenon to be observed in the GOLD field‐of‐view.

Investigation of the Post‐Sunset Extra Electron Density Peak Poleward of the Equatorial Ionization Anomaly Southern Crest

AbstractThe Global‐scale observation of limb and disk mission observed an extra electron density (Ne) peak after sunset at approximately 30°S near 40°W on 4 November 2019, which is poleward and immediately next to the southern equatorial ionization anomaly (EIA) crest. This Ne peak is different from previously reported mid‐latitude peaks that occur at all local times. The Whole Atmosphere Community Climate Model‐eXtended captures this phenomenon. Model diagnostic analysis reveals that the decrease of Ne and hmF2 between 15° and 25°S makes Ne near 30°S appear as an extra density peak relative to the southern EIA crest. Transport by poleward meridional wind decreases Ne between 15° and 25°S. Moreover, the upward E × B drifts due to pre‐reversal enhancement lift the plasma between the dip equator and 15°S but do not affect Ne much between 15° and 25°S with a low drift speed. Comparison with days without the extra peak shows the importance of E × B drift latitudinal variations on the extra peak formation. This study provides new insights into the dynamic variability of the nighttime ionosphere.

First Lidar Profiling of Meteoric Ca+ Ion Transport From ∼80 to 300 km in the Midlatitude Nighttime Ionosphere

AbstractWe report a world record of lidar profiling of metallic Ca+ ions up to 300 km in the midlatitude nighttime ionosphere during geomagnetic quiet time. Ca+ measurements (∼80–300 km) were made over Beijing (40.42°N, 116.02°E) with an Optical‐Parametric‐Oscillator‐based lidar from March 2020 through June 2021. Main Ca+ layers (80–100 km) persist through all nights, and high‐density sporadic Ca+ layers (∼100–120 km) frequently occur in summer. Thermosphere‐ionosphere Ca+ (TICa+) layers (∼110–300 km) are likely formed via Ca+ uplifting from these sporadic layers. The lidar observations capture the complete evolution of TICa+ layers from onset to ending, revealing intriguing features. Concurrent ionosonde measurements show strong sporadic E layers developed before TICa+ and spread F onset. Neutral winds can partially account for observed vertical transport but enhanced electric fields are required to explain the results. Such lidar observations promise new insights into E‐ and F‐region coupling and plasma inhomogeneities.

Low-latitude plasma blobs above Africa: Exploiting GOLD and multi-satellite in situ measurements

Low-latitude plasma blobs are localized density enhancements of electron density that are occasionally observed in the night-time tropical ionosphere. Two-dimensional (2D) imaging of this phenomenon has been rare and frequently restricted to Central/South America, which is densely covered with ground-based airglow imagers and Global Navigation Satellite System (GNSS) receivers. In Africa, on the contrary, no 2D image of a blob has been reported. Here we present two low-latitude blob events above Africa, one in the Northern summer and the other in winter, in the 2-dimensional Far-UltraViolet (FUV) images from the Global-scale Observations of the Limb and Disk (GOLD) mission. Additionally, multiple satellites (four spacecraft per event) on the Low-Earth-Orbit (LEO) encountered the blob events, some within the GOLD images and some outside. The LEO data support the robustness of GOLD observations and bridge time gaps between the consecutive images. Properties of the two blob events above Africa generally support the conclusions in a previous case study for Central/South America. Plasma therein exhibited higher O+ fraction and faster ion flow toward outer L-shells than the ambient. The blobs were conjugate to locally intensified Equatorial Ionization Anomaly crests without conspicuous equatorward-westward propagation. Our results demonstrate the usefulness of GOLD and multiple LEO satellites in monitoring the ionosphere above Africa, which is a fascinating laboratory of low-latitude electrodynamics but still waiting for more observatories to be deployed.

Instantaneous amplitude of low-latitude ionospheric irregularities probed by ROCSAT-1, DEMETER, and FORMOSAT-7/COSMIC-2

The occurrence probability of plasma irregularities has long been used to quantify that of ionospheric scintillations. We use the Hilbert-Huang transform to examine the occurrence probability and the instantaneous total amplitude of plasma irregularities in the low-latitude nighttime ionosphere observed by the low-inclination satellites, ROCSAT-1 during 1999–2004 and FORMOSAT-7/COSMIC-2 (F7C2) in 2020, as well as a high-inclination satellite, DEMETER, during 2006–2010. Generally, the longitudinal distribution of the occurrence probability is similar to that of the total amplitude, except that the probability of ion density irregularity observed by F7C2 and DEMETER yields nonphysical anomalous enhancements over the South American sector during May–August of 2007–2010 and 2020, respectively. S4 scintillations observed by F7C2 show that the anomalous enhancement results from the low ambient density in calculating the occurrence probability during the low solar activity year of 2020. We suggest the instantaneous total amplitude as a better indicator for describing ionospheric plasma irregularities.

Ionospheric Topside Diffusive Flux and the Formation of Summer Nighttime Ionospheric Electron Density Enhancement Over Millstone Hill

AbstractIonospheric F‐region electron density is anomalously higher in the evening than during the daytime on many occasions in the summer in geomagnetic mid‐latitude regions. This unexpected ionospheric diurnal variation has been studied for several decades. The underlying processes have been suggested to be related to meridional winds, topside influx arising from sunset ionospheric collapse, and other factors. However, substantial controversies remain unresolved. Using a numerical model driven by the statistical topside O+ diffusive flux from the Millstone Hill incoherent scatter radar data, we provide new insight into the competing roles of topside diffusive flux, neutral winds, and electric fields in forming the evening density peak. Simulations indicate that while meridional winds, which turn equatorward before sunset, are essential to sustain the daytime ionization near dusk, the topside diffusive flux is critically important for the formation and timing of the summer evening density peak.

The nighttime ionospheric response and occurrence of equatorial plasma irregularities during geomagnetic storms: a case study

Recent studies revealed that the long-lasting daytime ionospheric enhancements of Total Electron Content (TEC) were sometimes observed in the Asian sector during the recovery phase of geomagnetic storms (e.g., Lei (J Geophys Res Space Phys 123: 3217–3232, 2018), Li (J Geophys Res Space Phys 125: e2020JA028238, 2020). However, they focused only on the dayside ionosphere, and no dedicated studies have been performed to investigate the nighttime ionospheric behavior during such kinds of storm recovery phases. In this study, we focused on two geomagnetic storms that happened on 7–8 September 2017 and 25–26 August 2018, which showed the prominent daytime TEC enhancements in the Asian sector during their recovery phases, to explore the nighttime large-scale ionospheric responses as well as the small-scale Equatorial Plasma Irregularities (EPIs). It is found that during the September 2017 storm recovery phase, the nighttime ionosphere in the American sector is largely depressed, which is similar to the daytime ionospheric response in the same longitude sector; while in the Asian sector, only a small TEC increase is observed at nighttime, which is much weaker than the prominent daytime TEC enhancement in this longitude sector. During the recovery phase of the August 2018 storm, a slight TEC increase is observed on the night side at all longitudes, which is also weaker than the prominent daytime TEC enhancement. For the small-scale EPIs, they are enhanced and extended to higher latitudes during the main phase of both storms. However, during the recovery phases of the first storm, the EPIs are largely enhanced and suppressed in the Asian and American sectors, respectively, while no prominent nighttime EPIs are observed during the second storm recovery phase. The clear north–south asymmetry of equatorial ionization anomaly crests during the second storm should be responsible for the suppression of EPIs during this storm. In addition, our results also suggest that the dusk side ionospheric response could be affected by the daytime ionospheric plasma density/TEC variations during the recovery phase of geomagnetic storms, which further modulates the vertical plasma drift and plasma gradient. As a result, the growth rate of post-sunset EPIs will be enhanced or inhibited.

Features of the Spatial Distribution of a Beam of Whistler-Range Radiation Incident on the Nighttime Ionosphere from above

Features of the Spatial Distribution of a Beam of Whistler-Range Radiation Incident on the Nighttime Ionosphere from above