Equatorial Low Latitude Research Articles

Multifractal characteristics of the low latitude equatorial ionospheric E–F valley region irregularities

Ionospheric irregularities of the E–F valley region are relatively less explored with in situ experiments enabling to capture local fine structures. Here, we present the multifractal analysis of electron density fluctuations in the E–F valley region, obtained from a rocket experiment performed at equatorial low latitude station, Alcântara, Brazil to explore scaling structures in the plasma irregularities. The multifractal spectrum is validated with a analytical model that mimics the energy distribution in a turbulent cascade using probabilistic weights. We report the nature of the E–F valley region irregularities to be multifractal, asymmetric, intermittent and non–homogeneous. The multifractal measures show transition of the influence from smaller to larger fluctuations as the rocket approaches the F layer base, consolidating earlier observations. By identifying the nature of the irregularities, we explore the possible cause for a wide variation reported in the spectral indices. Our analysis demonstrates the usability of the multifractal approach in studying the nonlinear fluctuations observed from the E-F valley region in situ data.

Investigation of the Influence of Distant Parameters on Horizontal Component of Ground Magnetic Field.

The influence of distant parameters on the horizontal component of magnetic field was investigated for two years within two locations in Africa. The measure of storm occurrence (Dst-index) was used to select successful storm days. The effect of distant parameters on residual field was investigated using filter analysis. This study was considered on both time domain and frequency domain. The results showed very close correspondence of rapid changes in amplitudes between residual H–component and the selected IMF parameters especially the solar wind velocity, proton density and Bz. The correlation analysis between the distant parameters and residual H–component completely revealed effective dependence of the depression of residual field on the selected parameters. The value of the correlation coefficient (r) with solar wind velocity, proton density and Bz showed significant values of the range of 0.5 and above. This is direct evidence that Solar wind velocity, proton density and Bz are more effective in causing geomagnetic fluctuations at equatorial low latitude stations.

Analysis of Ionospheric Scintillations of NavIC L5 and S -band Signals over Low Latitude Indian Region

Abstract The NavIC (Navigation with Indian Constellation) system’s performance degrades significantly in equatorial low latitude region due to ionospheric effects. Ionospheric scintillation (IS) is one of the major problems to NavIC system. IS occurs due to the random fluctuations in amplitude and phase of the signal. It also depends on solar cycle, latitude, seasons, time and elevation angle of the satellite. Not much significant amount of work is reported on ionospheric scintillation effects on NavIC signals. In this paper, ionospheric scintillation effects on NavIC L5 band (1176.42 MHz) and S1 band (2492.08 MHz) signals due to (1B,1C,1D,1E,1F,1G) are investigated. Amplitude scintillation index S4 is computed based on Carrier to Noise ratio (C/N0) measurements from ISRO IGS (IRNSS/GPS/SBAS) receiver for Kurnool station (Lat:15°47′49″N, Lon:78°4′39″E) and Hyderabad (Lat:17°23′31″N, Long: 78°19′10″E) station. The analysis is carried out for Kurnool station (June and September months) and Hyderabad station for Equinox Months (March & September) and Solstice Months (June & December) for the year 2017. IS effects of L5 and S1 band signals are analysed. It is observed that scintillation effects are severe for L5 band compared to S1 band signals. This work will be helpful to improve the user position accuracy.

Performance evaluation of neural network TEC forecasting models over equatorial low-latitude Indian GNSS station

Global Positioning System (GPS) services could be improved through prediction of ionospheric delays for satellite-based radio signals. With respect to latitude, longitude, local time, season, solar cycle and geomagnetic activity the Total Electron Content (TEC) have significant variations in both time and space. These temporal and spatial TEC variations driven by interplanetary space weather conditions such as solar and geomagnetic activities can degrade the communication and navigation links of GPS. Hence, in this paper, performance of TEC forecasting models based on Neural Networks (NN) have been evaluated to forecast (1-h ahead) ionospheric TEC over equatorial low latitude Bengaluru (12.97∘N,77.59∘E), Global Navigation Satellite System (GNSS) station, India. The VTEC data is collected for 2009–2016 (8 years) during current 24th solar cycle. The input space for the NN models comprise the solar Extreme UV flux, F10.7 proxy, a geomagnetic planetary A index (AP) index, sunspot number (SSN), disturbance storm time (DST) index, solar wind speed (Vsw), solar wind proton density (Np), Interplanetary Magnetic Field (IMF Bz). The performance of NN based TEC forecast models and International Reference Ionosphere, IRI-2016 global TEC model has evaluated during testing period, 2016. The NN based model driven by all the inputs, which is a NN unified model (NNunq) has shown better accuracy with Mean Absolute Error (MAE) of 3.15 TECU, Mean Square Deviation (MSD) of 16.8 and Mean Absolute Percentage Error (MAPE) of 19.8% and is 1–25% more accurate than the other NN based TEC forecast models (NN1, NN2 and NN3) and IRI-2016 model. NNunq model has less Root Mean Square Error (RMSE) value 3.8 TECU and highest goodness-of-fit (R2) with 0.85. The experimental results imply that NNunq/NN1 model forecasts ionospheric TEC accurately across equatorial low-latitude GNSS station and IRI-2016 model performance is necessarily improved as its forecast accuracy is limited to 69–70%.

Equinoctial asymmetry in ionosphere over Indian region during 2006–2013 using COSMIC measurements

In this study, the ionospheric equinoctial asymmetry (EA, defined as different ionospheric behaviour in the two equinoxes) is investigated in the maximum electron density (NmF2) and the corresponding altitude (hmF2) over the Indian region (∼5–30° N geog. lat., 60–100° E geog. long.) during quiet geomagnetic conditions (Ap≤20) and varying phase of solar activity (2006–2013, F10.7 ∼65–192 sfu) using COSMIC observations. Both NmF2 and hmF2 exhibited strong solar-activity dependence and which increased with the increase in solar activity throughout the day. We found EA in NmF2 is not just a daytime phenomenon but also occurs in morning and nighttime during high solar-activity. The EA in NmF2 over equatorial-low latitude is found solar-dependant in nighttime and morning (weaker/stronger in low-/high-solar activity) whereas stronger/weaker in noontime in low-/high-solar activity years (higher NmF2 in vernal in 2007–2009, 2013) respectively. Whereas for hmF2, EA is stronger in nighttime and weaker in morning for all years over both equatorial and low-latitudinal regions and weaker/moderate in low-/high-solar activity for noontime respectively. We found F10.7P and Dst index in the equinoxes playing a dominant role throughout the day and the chemical compositions (O, N2 and neutral mass densities) also a responsible factor for the EA in NmF2 and hmF2 in the morning and nighttime respectively.

Characters of the Pc3-4 Magnetic Pulsations at Middle and Low Latitudes: Preliminary Geomagnetic Results From Chinese Meridian Project

利用新建成的子午工程地磁台站数据,对比分析了地磁平静期间(2011年3月20-27日)和磁暴期间(2011年9月25日至10月1日)Pc3-4地磁脉动的时空分布特征及其对行星际条件的响应.数据分析结果表明,中低纬度(1.3<<em>L</em><2.3,<em>L</em>为磁壳参数)的Pc3-4地磁脉动在这两个时期内的分布存在明显的晨昏不对称性,在昼侧前出现明显的Pc3-4地磁脉动并与行星际上游波动密切相关,其振幅增强可能与太阳风动压脉冲相关,高速太阳风更易导致Pc3-4地磁脉动;而对于近赤道低纬(<em>L</em><1.3)区域,无论是在地磁平静期还是磁暴期均未能观测到Pc3-4地磁脉动,Pc3-4地磁脉动存在明显的纬度效应.

Mathematical modeling of plasma drifts over equatorial low latitude regions

This paper presents a mathematical model to simulate ionospheric plasma drifts at equatorial low latitude regions by coupling of E- and F-regions. The governing non-linear differential equations (of elliptic and parabolic nature) are solved numerically through finite-difference schemes and obtained neutral winds and electric fields. The temperature and electron density profiles are generated utilizing MSIS-86 atmospheric model. The continuity equation is employed to obtain night-time E-region density profile using measured ionograms at Trivandrum (India). The computed vertical and zonal plasma drifts are comparable with measured Jacamarca plasma drifts with little variations during noon and evening times. The plasma drifts at Trivandrum (8.5° N, 76.5° E, dip 0.5° N) are compared with those of Jicamarca (12° S, 76.9° W, dip 2° N). Neutral wind simulations of present model agree well with those of horizontal wind model (HWM-93). The post-sunset enhancement and its reversal are also discussed.

Response of equatorial‐low latitude ionosphere to sudden expansion of magnetosphere

The response of the equatorial‐low latitude nightside ionosphere to the geomagnetic negative sudden impulse(si−) on March 15, 1993 is studied using recordings of Doppler velocity of ionospheric echoes at vertical incidence at Kodaikanal(10.2°N) and Doppler shift of standard HF signals on oblique paths at Lunping(25.0°N) and Kure(34.25°N). The si− at 1541 UT is characterised by a simple decrease of H‐field at low latitude stations widely distributed in longitude, and by a double‐pulse structure at mid and high latitude stations on the dayside. The usual downward drift of F region plasma during the premidnight hours over Kodaikanal near the dip equator abruptly increased for ≈ 2.5 min coincident with the first pulse of the si− and immediately reversed direction to upward. Recordings at Lunping and Kure also showed short‐lived Doppler frequency deviations simulataneous with those at Kodaikanal and of the same polarity and sequence. These observations constitute the first and direct experimental evidence for vertical plasma motions due to si− associated electric fields in the nighttime equatorial‐low latitude ionosphere. The case study supports the view that si− can be explained by the physical model of sc/si+ with a reversal in the direction of the global current systems responsible for the groundlevel magnetic field variations.