Equatorial Plasma Depletions Research Articles

Investigating Equatorial Plasma Depletions through CSES-01 Satellite Data

Ionospheric plasma density irregularities, which are one of the primary sources of disturbance for the Global Navigation Satellite System, significantly impact the propagation of electromagnetic signals, leading to signal degradation and potential interruptions. In the equatorial ionospheric F region after sunset, certain plasma density irregularities, identified as equatorial plasma bubbles, encounter optimal conditions for their formation and development. The energy spectra of electron density fluctuations associated with these irregularities exhibit a power-law scaling behavior qualitatively similar to the Kolmogorov power law observed in fluid turbulence theory. This intriguing similarity raises the possibility that these plasma density irregularities may possess turbulent characteristics. In this study, we analyzed electron density, temperature, and pressure data obtained from the China Seismo-Electromagnetic Satellite (CSES-01) to delve into the spectral properties of equatorial plasma depletions in the ionospheric F region at an altitude of about 500 km. This research marks the first exploration of these properties utilizing CSES-01 data and focuses on 14 semi-orbits that crossed the equator after midnight (01:00–03:00 LT), characterized by a geomagnetic quiet condition (Kp < 1). The analysis of electron temperature, density and pressure within equatorial plasma depletions revealed power-law scaling behavior for all the selected parameters. Notably, the spectral index values of these parameters are different from each other. The significance of these findings in terms of investigating plasma depletions via magnetic field signatures, as well as their relationship to the occurrence of Rayleigh–Taylor convective turbulence, is examined and discussed.

An Empirical Model of the Occurrence Rate of Low Latitude Post‐Sunset Plasma Irregularities Derived From CHAMP and Swarm Magnetic Observations

AbstractThe prediction of post‐sunset equatorial plasma depletions (EPDs), often called ionospheric plasma bubbles, has remained a challenge for decades. In this study, we introduce the Ionospheric Bubble Probability (IBP) model, an empirical model to predict the occurrence probability of EPDs derived from 9 years of CHAMP and 9 years of Swarm magnetic field measurements. The model predicts the occurrence probability of EPDs for a given longitude, day of year, local time and solar activity, for the altitude range of about 350–510 km, and low geographic latitudes of ±45°. IBP has been found to successfully reconstruct the distribution of EPDs as reported in previous studies from independent data. IBP has been further evaluated using 1‐year of untrained data of the Ionospheric Bubble Index (IBI). IBI is a Level 2 product of the Swarm satellite mission used for EPD identification. The relative operating characteristics (ROC) curve shows positive excursion above the no‐skill line with Hanssen and Kuiper's Discriminant (H&KSS) score of 0.52, 0.51, and 0.55 at threshold model output of 0.16 for Swarm A, B, and C satellites. Additionally, the reliability plots show proximity to the diagonal line with a decent Brier Skill Score (BSS) of 0.249, 0.210, and 0.267 for Swarm A, B, and C respectively at 15% climatological occurrence rate. These tests indicate that the model performs significantly better than a no‐skill forecast. The IBP model offers compelling glimpses into the future of EPD forecasting, thus demonstrating its potential to reliably predict EPD occurrences. The IBP model is publicly available.

A Synoptic‐Scale Wavelike Structure in the Nighttime Equatorial Ionization Anomaly

AbstractBoth ground‐ and satellite‐based airglow imaging have significantly contributed to understanding the low‐latitude ionosphere, especially the morphology and dynamics of the equatorial ionization anomaly (EIA). The NASA Global‐scale Observations of the Limb and Disk (GOLD) mission focuses on far‐ultraviolet airglow images from a geostationary orbit at 47.5°W. This region is of particular interest at low magnetic latitudes because of the high magnetic declination (i.e., about ‐20°) and proximity of the South Atlantic magnetic anomaly. In this study, we characterize an exciting feature of the nighttime EIA using GOLD observations from October 5, 2018 to June 30, 2020. It consists of a wavelike structure of a few thousand kilometers seen as poleward and equatorward displacements of the EIA‐crests. Initial analyses show that the synoptic‐scale structure is symmetric about the dip equator and appears nearly stationary with time over the night. In quasi‐dipole coordinates, maxima poleward displacements of the EIA‐crests are seen at about ± 12° latitude and around 20 and 60° longitude (i.e., in geographic longitude at the dip equator, about 53°W and 14°W). The wavelike structure presents typical zonal wavelengths of about 6.7 × 103 km and 3.3 × 103 km. The structure's occurrence and wavelength are highly variable on a day‐to‐day basis with no apparent dependence on geomagnetic activity. In addition, a cluster or quasi‐periodic wave train of equatorial plasma depletions (EPDs) is often detected within the synoptic‐scale structure. We further outline the difference in observing these EPDs from FUV images and in situ measurements during a GOLD and Swarm mission conjunction.

First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS

At some frequencies, Earth’s ionosphere may significantly impact satellite communications, Global Navigation Satellite Systems (GNSS) positioning, and Earth Observation measurements. Due to the temporal and spatial variations in the Total Electron Content (TEC) and the ionosphere dynamics (i.e., fluctuations in the electron content density), electromagnetic waves suffer from signal delay, polarization change (i.e., Faraday rotation), direction of arrival, and fluctuations in signal intensity and phase (i.e., scintillation). Although there are previous studies proposing GNSS Reflectometry (GNSS-R) to study the ionospheric scintillation using, for example TechDemoSat-1, the amount of data is limited. In this study, data from NASA CYGNSS constellation have been used to explore a new source of data for ionospheric activity, and in particular, for travelling equatorial plasma depletions (EPBs). Using data from GNSS ground stations, previous studies detected and characterized their presence at equatorial latitudes. This work presents, for the first time to authors’ knowledge, the evidence of ionospheric bubbles detection in ocean regions using GNSS-R data, where there are no ground stations available. The results of the study show that bubbles can be detected and, in addition to measure their dimensions and duration, the increased intensity scintillation (S4) occurring in the bubbles can be estimated. The bubbles detected here reached S4 values of around 0.3–0.4 lasting for some seconds to few minutes. Furthermore, a comparison with data from ESA Swarm mission is presented, showing certain correlation in regions where there is S4 peaks detected by CYGNSS and fluctuations in the plasma density as measured by Swarm.

Multi-Instrumental Observations of Early Morning Equatorial Plasma Depletions During the 2017 Memorial Weekend Storm

We investigate an equatorial plasma depletion (EPD) event in Japan near the early morning sector (04:30–7:30 LT) during the recovery phase of a geomagnetic storm occurred on Memorial weekend (May 28) 2017 by using multi-instrumental measurements, including ground-based Global Navigation Satellite Systems (GNSS) network, ionosonde stations, and space-based Swarm and Defense Meteorological Satellite Program (DMSP) missions. The detailed 2-D evolution of the early morning EPD is well observed using GNSS ROTI maps with high temporal-spatial resolution. Moreover, ROTI data from BeiDou Geostationary Earth Orbit satellites are used to detect the EPD signature for the first time. The dawn EPD signature was also observed at the topside ionosphere by in situ electron density (Ne) measurements and ROTI results by space-borne Swarm Alpha (A), Charlie (C), and DMSP F17 satellites, indicating that the equatorial depletions arose to higher altitudes. We suggest that both the eastward disturbance dynamo electric field and overshielding electric field near the early morning sector created a favorable condition for the growth of the Rayleigh–Taylor (R-T) instability, and thus result in the generation of the dawn EPD.

A Statistical Study on the Climatology of the Equatorial Plasma Depletions Occurrence at Topside Ionosphere During Geomagnetic Disturbed Periods

AbstractUsing the in situ measurements of ROCSAT‐1 satellite during 181 geomagnetic storms happened from July 1999 to June 2004, a superposed epoch analysis of the equatorial plasma depletions (EPDs) occurrence is conducted. At postsunset hours (1800–2200 LT), the EPDs occurrence is enhanced shortly at the storm onset, but afterward, a long‐last suppression dominates. The EPDs occurrence at Midnight (2200–0200 LT) generally shares a similar pattern to that at postsunset hours. The occurrence at predawn (0200–0400) gradual increases near storm onset and reach its maximum at 6–9 hr and decays until 18 hr. For a given longitude at postsunset/midnight, the EPDs occurrence tends to be suppressed or promoted when the EPDs do or do not prevail. The disturbed vertical plasma drift generally determines the inhibition/promotion of the EPDs occurrence at postsunset/predawn. However, for predawn EPDs occurrence, the plasma vertical drift cannot well explain the longitudinal variation. The continuous observations from consecutive orbits of ROCSAT‐1 are carefully compared and the result suggests that the geomagnetic storm‐induced additional predawn EPDs are preferred to be the longer‐lived developed EPDs rather than fresh EPDs. In addition, a possible mechanism concerning the background plasma density enhancement which might be related with the energetic electrons induced nighttime ionization is proposed.

On the Balance Between Plasma and Magnetic Pressure Across Equatorial Plasma Depletions

AbstractIn magnetized plasmas such as the ionosphere, electric currents develop in regions of strong density gradients to balance the resulting plasma pressure gradients. These currents, usually known as diamagnetic currents decrease the magnetic pressure where the plasma pressure increases, and vice versa. In the low‐latitude ionosphere, equatorial plasma depletions (EPDs) are well known for their steep plasma density gradients and adverse effect on radio wave propagation. In this paper, we use continuous measurements of the magnetic field and electron density from the European Space Agency's Swarm constellation mission to assess the balance between plasma and magnetic pressure across large‐scale EPDs. The analysis is based on the magnetic fluctuations related to diamagnetic currents flowing at the edges of EPDs. This study shows that most of the EPDs detected by Swarm present a decrease of the plasma pressure relative to the ambient plasma. However, EPDs with high plasma pressure are also identified mainly in the vicinity of the South Atlantic magnetic anomaly. From the electron density measurements, we deduce that such an increase in plasma pressure within EPDs might be possible by temperatures inside the EPD as high as twice the temperature of the ambient plasma. Due to the distinct location of the high‐pressure EPDs, we suggest that a possible heating mechanism might be due to precipitation of particle from the radiation belts. This finding corresponds to the first observational evidence of plasma pressure enhancements in regions of depleted plasma density in the ionosphere.

Interhemispheric field-aligned currents at the edges of equatorial plasma depletions

A comprehensive description of electromagnetic processes related to equatorial plasma depletions (EPDs) is essential for understanding their evolution and day-to-day variability. Recently, field-aligned currents (FACs) flowing at both western and eastern edges of EPDs were observed to be interhemispheric rather than anti-parallel about the dip equator, as suggested by previous theoretical studies. In this paper, we investigate the spatial and temporal variability of the FACs orientation using simultaneous measurements of electron density and magnetic field gathered by ESA’s Swarm constellation mission. By using empirical models, we assess the role of the Pedersen conductance in the preference of the FACs to close either in the northern or southern magnetic hemisphere. Here we show that the closure of the FACs agrees with an electrostatic regime determined by a hemispherical asymmetry of the Pedersen conductance. That is, the EPD-related FACs close at lower altitudes in the hemisphere with the highest conductivity. The evidence of this conclusion stands on the general agreement between the longitudinal and seasonal variability of both the conductivity and the FACs orientation.

Edge Plasma Enhancements of Equatorial Plasma Depletions Observed by All‐Sky Imager and the C/NOFS Satellite

AbstractEquatorial plasma depletions (EPDs) appearing at nighttime low latitudes were widely studied before. The edge plasma enhancements of equatorial plasma depletions (EPEEPDs) were repeatedly observed by the all‐sky imager and the Communications/Navigation Outage Forecasting System (C/NOFS) satellite. However, the two‐dimensional structures of EPEEPDs have not been well addressed. In this study, the airglow observations from one all‐sky imager showed that the EPEEPDs occurred only in the east and west edges of EPDs but not at the poleward edge as earlier reported from the in situ measurements of low Earth orbit satellites. Four‐year observations (2012–2015) from both our all‐sky imager and the C/NOFS satellite have been analyzed. Compared to the airglow images, the in situ observation from C/NOFS showed that the edge plasma enhancements appeared to be less prominent with respect to the background, but both showed that the plasma enhancement was a high‐incidence phenomenon, and the average incidence reached about 82% during days with EPDs observed. The occurrence of EPEEPDs showed similar characteristic as EPDs, such as the longitudinal dependences. The zonal extension of EPEEPDs showed different scale characteristics at different altitudes; its zonal extent varied with its altitudinal dependence. Four‐year observations from C/NOFS satellite showed that the occurrence of EPEEPDs peaked during 20:00–22:00 LT. The result from all‐sky imager showed that the relative intensity enhancement appeared contemporaneously, with maximal relative intensity exceeding 90%. We suggest that the generation mechanism of EPEEPDs is possibly related to the polarized electric field of EPDs. The plasma is more likely to be redistributed and exhibits as depletion and enhancement.

Multi-spectral optical imaging of the spatiotemporal dynamics of ionospheric intermittent turbulence

Equatorial plasma depletions have significant impact on radio wave propagation in the upper atmosphere, causing rapid fluctuations in the power of radio signals used in telecommunication and GPS navigation, thus playing a crucial role in space weather impacts. Complex structuring and self-organization of equatorial plasma depletions involving bifurcation, connection, disconnection and reconnection are the signatures of nonlinear evolution of interchange instability and secondary instabilities, responsible for the generation of coherent structures and turbulence in the ionosphere. The aims of this paper are three-fold: (1) to report the first optical imaging of reconnection of equatorial plasma depletions in the South Atlantic Magnetic Anomaly, (2) to investigate the optical imaging of equatorial ionospheric intermittent turbulence, and (3) to compare nonlinear characteristics of optical imaging of equatorial plasma depletions for two different altitudes at same times. We show that the degree of spatiotemporal complexity of ionospheric intermittent turbulence can be quantified by nonlinear studies of optical images, confirming the duality of amplitude-phase synchronization in multiscale interactions. By decomposing the analyses into North-South and East-West directions we show that the degree of non-Gaussianity, intermittency and multifractality is stronger in the North-South direction, confirming the anisotropic nature of the interchange instability. In particular, by using simultaneous observation of multi-spectral all-sky emissions from two different heights we show that the degree of non-Gaussianity and intermittency in the bottomside F-region ionosphere is stronger than the peak F-region ionosphere. Our results are confirmed by two sets of observations on the nights of 28 September 2002 and 9 November 2002.

Statistics of Durations and Spacings of Equatorial Plasma Depletions Detected by the C/NOFS Planar Langmuir Probe

AbstractThe Planar Langmuir Probe onboard the Communication/Navigation Outage Forecasting System satellite monitored ionospheric plasma densities and their irregularities with high resolution almost seamlessly for approximately five years since May 2008. Planar Langmuir Probe data from two different years (1 October 2008 to 30 September 2009 and 5 June 2012 to 4 June 2013) were selected for analysis. The first data set corresponds to solar minimum conditions, and the second one is as close to solar maximum conditions of solar cycle 24 as possible, with basis on their continuous availability. The results from a level‐crossing analysis applied to detrended evening‐time data from a fixed altitude range show how depletion durations and spacings between consecutive depletions are statistically distributed as functions of solar activity and longitude sector. In quantitative terms, they show that (i) the fewer depletions observed in the solar‐minimum year are also shorter, shallower, and closer together than those detected in the more active one and (ii) great similarities exist between structures of depletion durations and spacings between consecutive depletions observed in the South American and African longitude sectors, which are more noticeable than those associated with in the Indian‐Asian longitude sector.

Climatology of the Occurrence Rate and Amplitudes of Local Time Distinguished Equatorial Plasma Depletions Observed by Swarm Satellite

AbstractIn this study, we developed an autodetection technique for the equatorial plasma depletions (EPDs) and their occurrence and depletion amplitudes based on in situ electron density measurements gathered by Swarm A satellite. For the first time, comparisons are made among the detected EPDs and their amplitudes with the loss of Global Positioning System (GPS) signal of receivers onboard Swarm A, and the Swarm Level‐2 product, Ionospheric Bubble Index (IBI). It has been found that the highest rate of EPD occurrence takes place generally between 2200 and 0000 magnetic local time (MLT), in agreement with the IBI. However, the largest amplitudes of EPD are detected earlier at about 1900–2100 MLT. This coincides with the moment of higher background electron density and the largest occurrence of GPS signal loss. From a longitudinal perspective, the higher depletion amplitude is always witnessed in spatial bins with higher background electron density. At most longitudes, the occurrence rate of postmidnight EPDs is reduced compared to premidnight ones; while more postmidnight EPDs are observed at African longitudes. CHAMP observations confirm this point regardless of high or low solar activity condition. Further by comparing with previous studies and the plasma vertical drift velocity from ROCSAT‐1, we suggest that while the F region vertical plasma drift plays a key role in dominating the occurrence of EPDs during premidnight hours, the postmidnight EPDs are the combined results from the continuing of former EPDs and newborn EPDs, especially during June solstice. And these newborn EPDs during postmidnight hours seem to be less related to the plasma vertical drift.

Longitudinal Thin Structure of Equatorial Plasma Depletions Coincidently Observed by Swarm Constellation and all‐Sky Imager

AbstractThe lower pair satellites of Swarm mission, flying side‐by‐side and separated by 1.4° in longitude (about 150 km), usually observed equatorial plasma depletions (EPDs) showing quite different structures, and sometime even only one satellite observed EPD. In this study, we provided 6‐h continuous observations of EPDs on the night of 23–24 September 2014, from an all‐sky imager located at Fuke (geographic:19.5°N,109.1°E), south of China. From the airglow images the EPDs were found with longitudinal extensions of about 50 km and all tilted from northwest to southeast direction. We further checked the in situ electron density simultaneously measured by the Swarm lower pair satellites and found the differences of Swarm in situ electron densities explained well by the longitudinally thin structure of EPDs observed from the all‐sky imager. During later periods the bifurcation and merging were observed by the airglow images, and it was the first time to report both processes in the evolution of one EPD. The bifurcation was first observed at the higher‐latitude part, and then observed at lower latitudes of EPD. The subbranches generated through bifurcation showed even thinner longitudinal extension of about 20–30 km, and later the subbranches started to merge with each other, forming a really complicated mesh of depleted regions.

Improved characterization and modeling of equatorial plasma depletions

This manuscript presents a method to identify the occurrence of Equatorial Plasma Bubbles (EPBs) with data gathered from receivers of Global Navigation Satellite System (GNSS). This method adapts a previously existing technique to detect Medium Scale Travelling Ionospheric Disturbances (MSTIDs), which focus on the 2nd time derivatives of total electron content estimated from GNSS signals (2DTEC). Results from this tool made possible to develop a comprehensive analysis of the characteristics of EPBs. Analyses of the probability of occurrence, effective time duration, depth of the depletion and total disturbance of the EPBs show their dependence on local time and season of the year at global scale within the latitude belt from 35°N to 35°S for the descending phase of solar cycle 23 and ascending phase of solar cycle 24, 2002–2014. These results made possible to build an EPBs model, bounded with the Solar Flux index, that simulates the probability of the number of EPBs and their characteristics expected for a representative day at given season and local time (LT). The model results provided insight into different important aspects: the maximum occurrence of bubbles take place near the equatorial anomaly crests, asymmetry between hemispheres and preferred longitudes with enhanced EPBs activity. Model output comparisons with independent observations confirmed its soundness.

On the direction of the Poynting flux associated with equatorial plasma depletions as derived from Swarm

AbstractMagnetic and electric field observations from the European Space Agency Swarm mission are used to report the direction of electromagnetic energy flux associated with equatorial plasma depletions. Contrary to expectations, the observations suggest a general interhemispheric Poynting flux rather than concurrent flows at both hemispheres toward or away from the equator. Of high interest is a particular behavior noticed over the region with the largest variation in the magnetic declination. This is a Poynting flux flowing mainly into the southern magnetic hemisphere about between 60°W and 30°E and into the northern magnetic hemisphere between 110°W and 60°W. The abrupt change in the flow direction at 60°W is suggested to be caused by an asymmetry between the hemispheres on the ionospheric conductivity, likely due to the influence of thermospheric winds and the presence of the South Atlantic Anomaly.

Equatorial broad plasma depletions associated with the evening prereversal enhancement and plasma bubbles during the 17 March 2015 storm

AbstractBroad plasma depletions (BPDs) in the equatorial F region represent plasma depletions whose longitudinal and latitudinal scales are much greater than those of normal plasma bubbles. This study investigates the characteristics and origin of BPDs using the coincident ionospheric observations by the Communication/Navigation Outage Forecasting System, Defense Meteorological Satellite Program, and Swarm satellites during the 2015 St. Patrick's Day (17 March) storm. Two types of BPDs were detected before midnight during the main phase of the storm. One type of BPDs showed a gradual plasma density variation (Type 1), and the other type of BPDs showed a steep density gradient (Type 2) at the walls of BPDs. The Type 1 BPDs were detected with no signature of plasma bubbles nearby, whereas the Type 2 BPDs were accompanied by bubbles. The formation of the Type 1 BPDs is attributed to the uplift of the bottomside of the F region above the satellite altitude by the action of storm‐induced electric fields. The steep walls of Type 2 BPDs are associated with the ionospheric uplift and the spatial discontinuity of the ionosphere produced by bubbles. The detection of BPDs that are more than 15° wide in latitude by the polar orbit Swarm satellites arises from the elongation of bubbles along the magnetic field lines and the alignment of the elongation with the plane of the orbit.

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

Equatorial plasma depletions observed by the DMSP F13 satellite near dawn during geomagnetic disturbances in a solar minimum

We report on in situ measurements of equatorial plasma depletions in the dawn sector from the DMSP F13 satellite during magnetic disturbances which occurred within the solar minimum period of 1995–1996. Examples are given when DMSP F13 observed prominent pre-sunrise depletions associated with weak magnetic disturbances, while the most severe magnetic storms of the period showed only small amplitude depletions of plasma density. Our results indicate that the depletions were observed within about two third of the magnetically perturbed days with the negative maximum excursions of SYM-H less than −40nT at any season and at various longitudes. Commonly, the depletions appear following the negative SYM-H peaks in the recovery phase of the magnetic disturbances. We discuss the roles of corotating evening depletions and near-dawn effects of the prompt penetration overshielding and the disturbance dynamo eastward electric fields as sources for the reported depletions.

Radio‐tomographic images of postmidnight equatorial plasma depletions

AbstractFor the first time, equatorial plasma depletions (EPDs) have been imaged in the longitude‐altitude plane using radiotomography. High‐resolution (~10 km) reconstructions of electron density were derived from total electron content (TEC) measurements provided by a receiver array in Peru. TEC data were obtained from VHF/UHF signals transmitted by the Coherent Electromagnetic Radio Tomography (CERTO) beacon on the C/NOFS satellite. EPDs generated premidnight were observed near dawn. On one night, the bubble densities were highly reduced, 100–1000 km wide, and embedded within a layerlike ionosphere. Three nights later, the EPDs exhibited similar features but were embedded in a locally uplifted ionosphere. The C/NOFS in situ instruments detected a dawn depletion where the reconstruction showed lifted EPDs, implying that the postmidnight electric fields raised sections of ionosphere to altitudes where embedded/reactivated fossil EPDs were detected as dawn depletions. Satellites flying under domelike distortions of the ionosphere may observe these distortions as broad plasma decreases (BPDs).

Equatorial broad plasma depletions associated with the enhanced fountain effect

AbstractBroad plasma depletions (BPDs), plasma depletions whose longitudinal width is over several hundred kilometers, have been detected in the equatorial F region by low‐earth‐orbit satellites during both magnetically quiet and magnetically disturbed periods. A few hypotheses were suggested to explain the creation of BPDs, but the underlying mechanism of this phenomenon is still under debate. We investigate the origin of BPDs by analyzing the simultaneous in situ and optical observations of the ionosphere on 30 May 2003 (Kp = 8+), 24 April 2012 (Kp = 7−), and 31 October 2012 (Kp = 0+). BPDs on 30 May 2003 were detected by the Republic of China Satellite‐1 at an altitude of 600 km, and BPDs on the other days were detected by the Communication/Navigation Outage Forecasting System satellite near an altitude of 400 km. Our results show that the detection of BPDs is closely associated with background ionospheric morphology; BPDs are detected on the days when the equatorial ionization anomaly (EIA) is intense and the crests of the EIA have moved poleward. Measurements of upward plasma motion support the existence of ionospheric uplift at BPD locations. These observations and the detection of BPDs near the magnetic equator lead to the interpretation that the satellite detection of BPDs during those 3 days is likely related to the uplift of the F peak height above the satellite orbits.