Low-latitude Ionospheric Irregularities Research Articles

On the low-latitude ionospheric irregularities under geomagnetically active and quiet conditions using NavIC observables: A spectral analysis approach

Ionospheric irregularities and associated scintillations under geomagnetically active/quiet conditions have detrimental effects on the reliability and performance of space- and ground-based navigation satellite systems, especially over the low-latitude region. The current work investigates the low-latitude ionospheric irregularities using the phase screen theory and the corresponding temporal Power Spectral Density (PSD) analysis to present an estimate of the outer irregularity scale sizes over these locations. The study uses simultaneous L5 signal C/No observations of NavIC (a set of GEO and GSO navigation satellite systems) near the northern crest of EIA (Indore: 22.52∘N, 75.92∘E, dip: 32.23∘N) and in between the crest and the dip equator (Hyderabad: 17.42∘N, 78.55∘E, dip: 21.69∘N). The study period (2017–2018) covers disturbed and quiet-time conditions in the declining phase of the solar cycle 24. The PSD analysis brings forward the presence of irregularities, of the order of a few hundred meters during weak-to-moderate and quiet-time conditions and up to a few km during the strong event, over both locations. The ROTI values validate the presence of such structures in the Indian region. Furthermore, only for the strong event, a time delay of scintillation occurrence over Indore, with values of 36 min and 50 min for NavIC satellites (PRNs) 5 and 6, respectively, from scintillation occurrence at Hyderabad is observed, suggesting a poleward evolution of irregularity structures. Further observations show a westward propagation of these structures on this day. This study brings forward the advantage of utilizing continuous data from the GEO and GSO satellite systems in understanding the evolution and propagation of the ionospheric irregularities over the low-latitude region.

Correlation between rate of TEC index and positioning error during solar flares and geomagnetic storms using navigation with Indian constellation receiver measurements

Abstract The real-time position accuracy of the Global Navigation Satellite System (GNSS)/Navigation with Indian Constellation (NavIC) receiver is limited by the dynamic behavior of the ionosphere, particularly in adverse conditions like solar flares and geomagnetic storms. The NavIC satellites broadcast dual coherent radio beacon signals on L5 (1,164.5 MHz) and S (2,472.5 MHz) bands for providing position, velocity, and timing services in all weather conditions. The Total Electron Content (TEC) and Rate of TEC Index (ROTI) are the potential indicators for characterizing the ionosphere and its irregularities. In this research work, the TEC and ROTI are computed from the code and carrier phase observations of the NavIC receiver located at Kurnool low latitude station (15.79° N, 78.07° E) with geomagnetic coordinates (7.30° N, 151.65° E). This paper presents a statistical study of TEC, ROTI, and the correlation between ROTI and NavIC positioning error during highly intense solar flares (X9.3 and X2.2) and geomagnetic storm conditions. Compared to quiet days mean TEC, the enhancement is 3 TECU due to X9.3 flares, and the maximum peak of TEC on storm day (September 8, 2017) is 80.92 TECU. Moreover, the correlation coefficient between ROTI and position error is 0.76 on a quiet day (September 4, 2017), 0.54 on an intense solar flares day (September 6, 2017), and 0.24 on a storm day (September 8, 2017), this indicates positional accuracy degradation on a geomagnetic storm day. The outcome of this research work would be helpful for investigating characteristics of the northern low latitude ionospheric irregularities and, in turn, useful for developing suitable ionospheric nowcasting/prediction models for GNSS applications.

The Short-Term Prediction of Low-Latitude Ionospheric Irregularities Leveraging a Hybrid Ensemble Model

The Short-Term Prediction of Low-Latitude Ionospheric Irregularities Leveraging a Hybrid Ensemble Model

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.

Event Analysis of the Influence of Strong Tropospheric Convection in Summer on Low-latitude Ionospheric Irregularities in East Asia

利用海南台站和东南亚地区的多种地基和天基观测手段，对2014年7月28日夜间观测到的东亚低纬F区不规则体事件的时空变化及其物理过程进行分析。结果表明，海南台站观测到了罕见的长时间持续的F区电离层不规则体，不同手段观测到的电离层不规则体存在明显的形态差异。不同台站观测到的电离层不规则体活动存在明显的差异。海南台站经度区南北异常峰附近的TEC起伏活动在日落后至午夜附近明显增强，在午夜后明显减弱。C/NOFS卫星轨迹午夜后逐渐接近于磁赤道，且处于较低高度上，几乎总会观测到弱等离子体扰动/泡的发生，与该区域地基观测的弱电离层不规则体活动存在明显的联系。SWARM卫星在黎明海南台站附近经度区仍观测到较强的赤道异常双峰结构，且西侧异常峰区附近仍存在明显的等离子体密度耗空/泡结构。海南台站西侧磁赤道区附近（中南半岛）强对流活动（MCC）激发的重力波种子扰动对东亚低纬区等离子体泡及准周期结构的产生发挥了重要作用。

Detection of Low-Latitude Ionospheric Irregularities From GNSS Observations

The equatorial and low-latitude regions, from 20°N to 20°S, frequently experience strong ionospheric disturbances which affect the signals from global navigation satellite systems (GNSS). The satellite signals propagating through ionospheric irregularities undergo scattering and dispersion, resulting in the amplitude and phase scintillation of the signal. The signals from some of the satellites may encounter plasma irregularities along their path, causing them to be corrupted. In this paper, a fast Fourier transform (FFT)-based irregularity detecting algorithm called FFT averaging ratio (FAR) algorithm and an enhanced FAR algorithm have been implemented as postprocessing techniques to identify the satellite PRNs which were corrupted by the ionospheric irregularities. GNSS data was collected using a Novatel GPStation-6 receiver located at Koneru Lakshmaiah University ( $16^{\circ} 10^{\prime}{\rm N}/80^{\circ} 62^{\prime}{\rm E}$ ) in Guntur, Andhra Pradesh, in the southern part of India during the solar maximum year of 2013. FAR algorithm detected 991 events and enhanced FAR detected 954 events in 2013. Most of the disturbances were detected in the months of vernal equinox. The number of disturbances detected in the autumnal equinox months was small due to missing data over 46% of the total number of days in September and October. The summer solstice months showed very few events. A large number of disturbances were detected during the winter solstice month of December which is 43% higher than that of July. The outcome of this research work is compliant with the existing ionospheric climatological model for low-latitude regions.

Interferometry observations of low-latitude E-region irregularity patches using the Sanya VHF radar

Interferometry plays an important role in revealing fine-scale structures of ionospheric irregularity. By placing an additional receiving array of four 5-element Yagi antennas in the north of the main East-West array of the Sanya VHF radar, multiple interferometry baselines with components parallel and perpendicular to the magnetic meridian are formed. These baselines allow us to study the three-dimensional (3D) behavior of low-latitude ionospheric field-aligned irregularity (FAI) over Sanya. Using multiple non-collinear receiving baselines, an experiment for which the Sanya VHF radar operated as an interferometer was performed on 10 July 2013. Ionospheric E-region FAI echoes with periods of several minutes were observed during 0745-0915 UT; mean Doppler velocity was around −30 m/s with spectral widths of about 50 m/s. The interferometry results show fine-scale structures of E-region FAI with a zonal scale size of 15 km or less. In addition, we found that the periodic variations of echo intensity shown in radar range-time-intensity (RTI) maps were produced by spatially separated E-region irregularity patches. The patches drifted westward with a velocity of about 40–60 m/s and could be associated with propagating gravity waves. These results provide a clearer picture of the characteristics of FAI and thus help to investigate the generation and dynamics of low-latitude ionospheric irregularities in the Chinese sector.

Is Space Weather Different Over Africa, and If So, Why? An AGU Chapman Conference Report

Key Points Chapman Conference Meeting Report

The first time observations of low-latitude ionospheric irregularities by VHF radar in Hainan

Sanya VHF radar (18.4°N, 109.6°E, dip latitude 12.8°N) at Hainan Island is the first coherent backscatter radar for sounding low-latitude ionospheric irregularities in the mainland of China. In this paper, we present the first results of low-latitude ionospheric E and F region irregularities using the radar data during the period from February 2009 to March 2010. The Doppler velocity of radar echoes from E region field aligned irregularities (FAIs) was about several tens of meters per second, while the Doppler spectral width was appreciably larger than the velocity, and could reach one hundred meters per second, indicating that the observed E region FAIs belonged to type 2 irregularities. The observations of range time intensity (RTI) maps of FAIs showed that E region irregularities most often occurred at night within the altitude range 85–115 km, and were rarely observed at afternoon hours. The percentage occurrence of E region FAIs maximized during spring months (Feb.–May) with a peak value over 80%. The heights at which the strongest echo related FAIs occurred were mainly around 100 km, lower than h’Es and the difference is mostly 10–20 km. December solstice seemed to be the minimum period of occurrence when the FAI echoes were commonly detected at a narrow altitude range 90–100 km. Moreover, simultaneous radar and GPS observations during spread F events in the pre-midnight hours of solar minimum revealed that significant GPS L band scintillations coincided with the appearance of F region plasma plume structures, which extended up to 450 km in altitude.

On postmidnight low-latitude ionospheric irregularities during solar minimum: 1. Equatorial Atmosphere Radar and GPS-TEC observations in Indonesia

[1] Using the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36°S dip latitude), it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere (MU) radar in midlatitudes (29.3°N dip latitude). Utilizing the rapid beam-steering capability of the EAR, the spatial structure of the postmidnight irregularities is clearly presented for the first time. It is found that they usually propagate westward and can be categorized into two types. One shows sharp upwelling plumes near local midnight, which should not be a mere passage of fossil plasma bubbles. The other has successive tilted structures which have the same orientation as medium-scale traveling ionospheric disturbances typically observed at midlatitudes. We suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector. The displacement between geographic and magnetic equators may also be important for seasonal/longitudinal variation of the postmidnight irregularities.

On postmidnight low-latitude ionospheric irregularities during solar minimum: 2. C/NOFS observations and comparisons with the Equatorial Atmosphere Radar

[1] A detailed comparison between the observations of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite and the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36°S dip latitude) on the postmidnight irregularities is presented. The zonal and meridional E × B drift velocities measured by the vector electric field instrument on the C/NOFS are consistent with the westward propagation of backscatter echoes and the line-of-sight Doppler velocities observed with the EAR, respectively. The plasma density depletions are observed in the postmidnight sector for several consecutive orbits, which suggests the depletions grow slowly during the premidnight period and reach the spacecraft altitude around local midnight. The convergence of the equatorward wind which could be responsible for the midnight temperature maximum may produce a preferable condition for the growth of the Rayleigh-Taylor instability around midnight. Electric field fluctuations of medium-scale traveling ionospheric disturbances may play an important role in seeding the instability. Both equatorial and midlatitude-type plasma instabilities could be operational at the EAR latitude sector, which together would foster a high occurrence of postmidnight irregularities during solar minimum.

Vertical <i>E</i> × <i>B</i> drift velocity variations and associated low-latitude ionospheric irregularities investigated with the TOPEX and GPS satellite data

Abstract. With a well-selected data set, the various events of the vertical E × B drift velocity variations at magnetic-equator-latitudes, the resultant ionospheric features at low-and mid-latitudes, and the practical consequences of these E × B events on the equatorial radio signal propagation are demonstrated. On a global scale, the development of a equatorial anomaly is illustrated with a series of 1995 global TOPEX TEC (total electron content) maps. Locally, in the Australian longitude region, some field-aligned TOPEX TEC cross sections are combined with the matching Guam (144.86° E; 13.59° N, geographic) GPS (Global Positioning System) TEC data, covering the northern crest of the equatorial anomaly. Together, the 1998 TOPEX and GPS TEC data are utilized to show the three main events of vertical E × B drift velocity variations: (1) the pre-reversal enhancement, (2) the reversal and (3) the downward maximum. Their effects on the dual-frequency GPS recordings are documented with the raw Guam GPS TEC data and with the filtered Guam GPS dTEC/min or 1-min GPS TEC data after Aarons et al. (1997). During these E × B drift velocity events, the Port Moresby (147.10° E; - 9.40° N, geographic) virtual height or h'F ionosonde data (km), which cover the southern crest of the equatorial anomaly in the Australian longitude region, show the effects of plasma drift on the equatorial ionosphere. With the net (D) horizontal (H) magnetic field intensity parameter, introduced and called DH or Hequator-Hnon-equator (nT) by Chandra and Rastogi (1974), the daily E × B drift velocity variations are illustrated at 121° E (geographic) in the Australian longitude region. The results obtained with the various data show very clearly that the development of mid-latitude night-time TEC increases is triggered by the westward electric field as the appearance of such night-time TEC increases coincides with the E × B drift velocity reversal. An explanation is offered with the F-region dynamo theory and electrodynamics, and with the ionospheric-plasmaspheric coupling. A comparison is made with the published model results of SUPIM (Sheffield University Plasmasphere-Ionosphere Model; Balan and Bailey, 1995) and experimental results of Park (1971), and the good agreement found is highlighted.Key words. Ionosphere (electric fields; equatorial ionosphere; mid-latitude ionosphere)

First results of all-sky imaging from India

A CCD based all-sky (180° field view) airglow imaging system was operated from a low latitude station, Kolhapur (16.8° N, 74.2° E; 10.6° N dip lat.) in India to map the thermospheric motion and to study the low latitude ionospheric irregularities by measuring the nightglow signatures at 630 nm. In general, all-sky imaging techniques offer broad, instantaneous coverage (2.5 million sq km area at 150° field of view at 300 km height) of the spatial and temporal characteristics of the airglow features from the selected layers of the thermosphere and ionosphere system. In addition to the all-sky camera, photometer and radio scintillation observations together with ionospheric soundings were carried out from Kolhapur to verify the signature of small scale as well as large scale size F-region irregularities. Initial results from these measurements display good examples of north-south motion of the thermosphere with apparent drift speeds of 143–200 m/s during geomagnetically quiet nights. The north-south aligned (>1000 km) bands of plasma depletions or bubbles with east-west dimension of 50–250 km moving with drift speed of 140 m/s towards east have been observed. The zonal component of plasma drift speed matches well with the eastward component of the neutral speed computed using Hedin’s recent model. However, the meridional components of plasma drift speed are generally higher than the components of the neutral wind speed computed using the model. The results confirm the earlier quantitative investigations of equatorial depletion characteristics from Brazil and elsewhere.

The size of low-latitude ionospheric irregularities determined from observations of discrete sources of different angular diameters

Several methods have been used to measure the horizontal dimensions of ionospheric irregularities that cause the scintillation of radio signals propagating through the terrestrial ionosphere. A frequently used technique is to measure the size of the shadow pattern on the ground with spaced receivers using a very strong radio source (usually Cassiopeia A or Cygnus A) or a satellite as the source of radio frequency energy. The technique described in this paper is to utilize the differences in the angular diameter of various discrete radio sources to determine irregularity size. The phenomenon, by which a radio source of small angular diameter scintillates while one of large angular diameter does not, is directly analogous to the optical portion of the electromagnetic spectrum where stars scintillate but planets do not.