Years Of Solar Activity Research Articles

ТИПИЗАЦИЯ ЗИМ В ИЛЕ АЛАТАУ ПО СНЕЖНОСТИ И ЛАВИННОЙ АКТИВНОСТИ

Snow avalanches affect human economic and recreational activities in the mountains. In some avalanche-active winters, they cause significant damage and loss of life. As a result of many years of research, it was found that avalanches strongly depend on the meteorological and synoptic conditions of each individual winter. The main indicator of the danger of the current season is the total and maximum volumes of avalanches in a particular river basin. It strongly depends on the height and water content of the snow cover and the amount of precipitation. The correlation coefficients are 0.36...0.66. In some avalanche-prone winters (1965...66, 1986...87, 2016...17) in the river basin. In Kishi Almaty, the total volume of avalanches reaches 300...500 thousand m3, and in non-avalanche-hazardous winters it does not exceed 20...30 thousand m3. During the years of minimal solar and cyclonic activity in the studied avalanche basin, the highest snow cover heights and avalanche activity are observed. This is probably due to the fact that winter cyclones produce precipitation in the foothill areas in cold years. And the increase in snowfall and avalanche activity is more associated with spring warming and the beginning of snow melting. Studying the meteorological conditions of avalanche formation helps assess the potential danger of the current season. This is of practical importance for planning the work of tourist sites and operational rescue services in the mountains.

Correlation of Rate of TEC Index and Spread F over European Ionosondes

One of the most popular indices for monitoring the occurrence and intensity of ionospheric L-band irregularities is the Rate of TEC Index (ROTI). Due to low TEC in the mid-latitude ionosphere, ROTI has received significantly less attention than the equatorial and polar ionosphere. On the other hand, spread F is an established ionogram irregularity signature. The present study aims to correlate ROTI and spread F activity over European Digisonde stations for a low-to-moderate solar activity year (2011). With a focus on the latitude-dependent occurrence, the analysis demonstrates that range spread F (RSF) has been identified for all notable ROTI (>0.15 TECU/min) cases which also coincide with MSTID activity over the stations, suggesting induced gravity waves or polarization electric fields as the driving mechanism for enhanced ROTI activity. The diurnal and seasonal features are also presented. Maximum irregularity occurrence was observed around the 45° N from 18:00 to 05:00 UT with the seasonal maximum occurrence in January. Over lower mid-latitude Digisonde stations (latitude < 45° N), the diurnal and seasonal occurrence was observed from 19:00 to 04:30 UT in July.

ED‐AttConvLSTM: An Ionospheric TEC Map Prediction Model Using Adaptive Weighted Spatiotemporal Features

AbstractIn this paper, we propose a novel Total Electron Content (TEC) map prediction model, named ED‐AttConvLSTM, using a Convolutional Long Short‐Term Memory (ConvLSTM) network and attention mechanism based on encoder‐decoder structure. The inclusion of the attention mechanism enhances the efficient utilization of spatiotemporal features extracted by the ConvLSTM, emphasizing the significance of crucial spatiotemporal features in the prediction process and, as a result, leading to an enhancement in predictive performance. We conducted experiments in East Asia (10°N–45°N, 90°E−130°E). The ED‐AttConvLSTM was trained and evaluated using the International GNSS Service TEC maps over a period of six years from 2013 to 2015 (high solar activity years) and 2017 to 2019 (low solar activity years). We compared our ED‐AttConvLSTM with IRI‐2016, COPG, LSTM, GRU, ED‐ConvLSTM and ED‐ConvGRU. The results indicate that our model surpasses the comparison models in forecasting both high and low solar activity years, across most months and UT moments in a day. Moreover, our model exhibits notably superior prediction performance during the most severe phases of a magnetic storm when compared to the comparison models. Subsequently, we then also discuss how the prediction performance of our model is affected by latitude. Finally, we discuss the diminishing performance of our model in multi‐day predictions, demonstrating that its reliability for forecasts ranging from one to 4 days in advance. Beyond the fifth day, there is a pronounced decline in the model's performance.

Variations of Inter‐Hemispheric Field‐Aligned Currents: Observations From Ground Geomagnetic Measurements

AbstractThe east–west component of the geomagnetic field (Y‐component) at the Trivandrum (TRD) station, an equatorial site, is used to investigate the characteristics of Inter‐Hemispheric Field‐Aligned Currents (IHFACs) in the Indian sector during solar cycles 21 and 22. The observations reveal an annual variation of IHFAC in the dawn/noon sector with positive/negative peaks around northern summer. Furthermore, it is noted that IHFAC amplitudes are modulated by solar activity, superimposed with seasonal effects, with consistently higher amplitudes during the northern summer months. The dusk‐time IHFAC also shows a pattern modulated by seasons and solar cycles, with negative peaks around northern summer only during the low solar active years. The seasonal pattern of IHFAC shows a similar trend from April to September, where dawn/noon IHFAC is south/north directed, and for the remaining months, both are northward directed. The dusk‐side IHFAC is mostly south‐directed irrespective of the season. Moreover, it is demonstrated that the seasonal pattern of dawn and noon‐time IHFAC amplitudes show a strong to moderate correlation with amplitudes of Sq asymmetries between two hemispheres, but dusk‐time IHFAC does not follow this trend. The IHFAC directions at TRD follow the Fukushima model for May to September during the dawn‐noon sector for 70%–95% of the days and less than 50% of the days for remaining months. The noon‐dusk sector shows Fukushima model‐type polarity for ∼60% of days in June–July and less than 50% for remaining months. The global analysis of IHFAC, using monthly averages of hourly values from eight observatories spanning Asia, Africa, and America, reveals spatial differences in amplitude and directions. The directions of noon‐time IHFAC in the Indian‐East Asian and East‐African sectors follow similar trends, and the amplitude variations of noon‐time IHFAC can be explained by DE‐3 tidal effects. The IHFAC from the South American and West African sectors show intense southward and northward currents compared to other sites, indicating the influence of the South Atlantic Anomaly (SAA).

Characteristic analysis of the differences between total electron content (TEC) values in global ionosphere map (GIM) grids

Abstract. Using total electron content (TEC) from a global ionosphere map (GIM) for ionospheric delay correction is a common method of eliminating ionospheric errors in satellite navigation and positioning. On this basis, the TEC of a puncture point can be obtained by GIM grid TEC interpolation. However, in terms of grid, only few studies have analyzed the TEC value size characteristics of its four grid points, that is, the TEC difference characteristics among them. In view of this, by utilizing the GIM data from high solar-activity years (2014) and low solar-activity years (2021) provided by CODE (Center for Orbit Determination in Europe), this paper proposes the grid TEC difference as a way of analyzing TEC variation characteristics within the grid, which is conducive to exploring and analyzing the variation characteristics of the ionosphere TEC in the single-station area. The value is larger in high solar-activity years and generally small in low solar-activity years, and the value of high-latitude areas is always smaller than that of low-latitude areas. Specifically, in high solar-activity years, most of the GIM grid TEC internal differences are within 4 TECu (1 TECu = 1016 electrons m−2) in high-latitude and midlatitude regions, while only 78.17 % are in low-latitude regions. In low solar-activity years, the TEC difference values within a GIM grid are mostly less than 2 TECu, and most of them in the high and middle latitudes are within 1 TECu. The main finding of this analysis is that the grid TEC differences are small for most GIM grids, especially in the midlatitudes to high latitudes of low solar years. This means that relevant extraction methods and processes can be simplified when TEC within these GIM grids is needed.

Impact of Solar Activity and ENSO on the Early Summer Asian Monsoon During the Last Millennium

AbstractThe Asian Monsoonal rainfall accounts for the majority of annual regional precipitation in East and South Asia and could be remotely regulated by El Niño‐Southern Oscillation (ENSO). Besides, several paleoclimate records and simulations have indicated solar signals in the Asian Monsoon, implying the impact of solar activity on the regional monsoon precipitation. By conducting multi‐linear regression analysis to the solar irradiance forced single‐forcing experiment in the last millennium, this study presents the comparison of solar and ENSO effects on monsoonal precipitation in South and East Asia during early summer (May–June). Increased total solar irradiance during high solar activity years tends to trigger a favorable environment for developing monsoon onset, leading to more precipitation against ENSO‐related patterns over Southeast and South Asia before peak‐summer (July–August). The result supports reconstructed terrestrial records and underlines considerable influences of the solar cycle on the variation of the Asian Summer Monsoon.

Prediction of ionospheric total electron content data using NARX neural network model

Successful prediction of ionospheric total electron content (TEC) data will help in correction of positioning errors in global navigation satellite systems (GNSS) caused by the ionosphere. This research paper proposes a prediction model for ionospheric TEC using a nonlinear autoregressive with exogenous inputs (NARX) neural network that utilizes past TEC data alongwith solar and geomagnetic indices namely F10.7, disturbed storm (Dst), Kp, Ap, and time of the day. We assess the prediction capability of our model at different latitudes during different solar activity years. We compare our results with another NARX model which uses previous TEC data along with time of the day, day of the year and season as exogenous parameters. The results show that for the solar minimum year the TEC prediction accuracy improves by 35.71% and for the solar maximum year it improves by 31.20%. The results using root mean square error (RMSE), mean absolute error (MAE), correlation coefficient, and symmetric mean absolute percentage error (sMAPE) clearly indicate that solar and geomagnetic indices along with time of the day help in enhancing prediction accuracy of TEC across different latitudinal regions during both solar minimum and maximum years.

ОЦЕНКА РАДИАЦИОННОЙ ОБСТАНОВКИ ПРИ КРАТКОВРЕМЕННЫХ ПОЛЕТАХ НА ЛУНУ

Background: The issue of assessing the features of factors affecting the formation of radiation doses of astronauts while in orbit of the Moon and on its surface remains insufficiently studied, which is important for ensuring the anti-radiation safety of astronauts on lunar missions. Purpose: To analyze the factors influencing the formation of the radiation dose of astronauts at the stage of finding the spacecraft in orbit of the Moon and the lander on its surface. Material and methods: The features of the dose load levels on astronauts at the stages of the Moon’s orbit and on the Moon’s surface are analyzed and generalized, calculation methods are used taking into account the orbit of the spacecraft around the Moon, the anti-radiation properties of the materials of the lander and spacesuit and the time spent in them during a short-term lunar mission. Results: The total radiation doses of astronauts for 14 days, calculated according to dosimetric measurements during the years of low solar activity (2009 and 2018‒2019), are 19.5‒23.2 mSv for astronauts staying in a spacecraft in lunar orbit, and from 22,7 to 24,0 mSv for astronauts on the Lunar surface, depending on the mass thickness of the protection at the maximum permissible 250 mSv for 1 month. An increase in the mass thickness of the anti-radiation protection of the lander in the equivalent of aluminum from 1.5 to 3-5 g/cm2 and the lunar spacesuit in the equivalent of aluminum from 0.2 to 0.5‒1 g/cm2 will reduce the total radiation dose of astronauts no more than 1.3 times during a 14-day stay on the surface of the moon. The results indicate that in order to minimize the radiation doses that astronauts receive during a lunar mission, it is important to take into account the forecast of solar activity in order to optimize the launch time of the spacecraft in the «windows» with minimal levels of radiation exposure. Conclusion: When predicting radiation hazard levels for astronauts during a short-term lunar mission, it is necessary to assess the levels of exposure to cosmic ionizing radiation both in the orbit of the Moon, depending on the lunar trajectory of the spacecraft, and on the surface of the Moon, taking into account the time spent in the lunar module and in a spacesuit, as well as levels of solar activity. It is important to take into account the analyzed features of the formation of cosmonauts’ radiation doses while orbiting the Moon and on its surface when predicting the time limits of the lunar mission, anti-radiation protection of astronauts and their compliance with the regulatory limits of exposure.

Active longitudes and the structure of the large-scale magnetic field at solar minimum

We have studied deep minima of 11-year solar activity cycles 13–14, 14–15, 22–23, 23–24, 24–25, using the RGO and USAF/NOAA sunspot group catalogs. All of them have a large number of spotless days. Nonetheless, active longitudes as preferred zones, where sunspots occur, appear at this solar cycle phase. Analysis of synoptic maps and WSO daily magnetograms reflecting the structure of a weak large-scale field shows a non-axisymmetric component of the solar magnetic field. At solar minimum in the structure of the large-scale magnetic field, there are regions of the magnetic field of positive and negative polarity elongated along the meridian and crossing the equator. The most pronounced of them are located in the zone of active longitudes and are often connected with the polar magnetic fields.
 We discuss the possible nature of the meridional structures of the large-scale field during solar minimum. This might be due to giant convection cells with a banana cell structure.

Investigation of the Occurrence Characteristics and Possible Origins of Daytime Spread F Irregularities in Low Latitude Region

AbstractA study of occurrence characteristics and morphology of daytime spread F (DSF) irregularities was conducted using ionosonde from 2013 to 2020 over low‐latitude Puer station in the China sector. In this study, a day with two or more consecutive F‐region diffuse echo ionograms within one hour from local sunrise to sunset at an altitude of 250 km is considered a DSF day. The results show that the DSF irregularities observed in different solar activity years have a similar seasonal distribution, manifesting as frequent occurrences in June solstice with a maximum occurrence near sunrise. Furthermore, the morphology of most DSFs on the ionograms is predominantly frequency spread F, with a few observed near sunrise as MSF and RSF. In contrast to in high solar activity years (HSAY, 2013–2015), an interesting phenomenon of DSF irregularities in low solar activity years (LSAY, 2016–2020) is presented. That is, in the late afternoon hours (15–18 LT), DSFs have an unexpectedly high occurrence rate during the December solstice. By analyzing the virtual height variations for iso‐frequency plots of the DSF event occurring in the afternoon, these DSF irregularities were found to be likely associated with daytime medium‐scale traveling ionospheric disturbances (MSTIDs). Compared with the observed characteristics of the nighttime spread F, the fossils of nighttime plasma bubbles irregularities might play a key role in the development of DSF occurring in the early morning. At the same time, the influence of daytime MSTIDs on DSF irregularities occurring in the late afternoon hours cannot be ignored.

Активные долготы и структура крупномасштабного магнитного поля в минимуме солнечной активности

We have studied deep minima of 11-year solar activity cycles 13–14, 14–15, 22–23, 23–24, 24–25, using the RGO and USAF/NOAA sunspot group catalogs. All of them have a large number of spotless days. Nonetheless, active longitudes as preferred zones, where sunspots occur, appear at this solar cycle phase. Analysis of synoptic maps and WSO daily magnetograms reflecting the structure of a weak large-scale field shows a non-axisymmetric component of the solar magnetic field. At solar minimum in the structure of the large-scale magnetic field, there are regions of the magnetic field of positive and negative polarity elongated along the meridian and crossing the equator. The most pronounced of them are located in the zone of active longitudes and are often connected with the polar magnetic fields.
 We discuss the possible nature of the meridional structures of the large-scale field during solar minimum. This might be due to giant convection cells with a banana cell structure.

Effects of Strong Geomagnetic Storms on the Ionosphere and Degradation of Precise Point Positioning Accuracy during the 25th Solar Cycle Rising Phase: A Case Study

Approaching the peak year of the 25th solar activity cycle, the frequency of strong geomagnetic storms is gradually increasing, which seriously affects the navigation and positioning performance of GNSS. Based on the globally distributed GNSS station data and FORMOSAT-7/COSMIC-2 occultation data, this paper explores for the first time the effects of the G4-class geomagnetic storm that occurred on 23–24 April 2023 on the global ionosphere, especially the ionospheric equatorial anomalies and F-layer perturbations. It reveals the precise point positioning (PPP) accuracy degradation during a geomagnetic storm. The results show that the ionospheric rate of total electron content index (ROTI) and near high latitude GNSS phase scintillations index have varying levels of perturbation during geomagnetic storms, with the maximum ROTI and phase scintillations index exceeding 0.5 TECU/min and 0.8, respectively. The equatorial ionization anomaly (EIA) shows an enhanced state (positive ionospheric storms) during geomagnetic storms, and the cause of this phenomenon is most likely the equatorward neutral wind. The variation of the S4 index of the FORMOSAT-7/COSMIC-2 satellite reveals the uplift of the F-layer during geomagnetic storms. During geomagnetic storms, the PPP accuracy degrades most seriously at high latitudes, the maximum MAE exceeds 2.3 m, and the RMS in the three-dimensional (3D) direction exceeds 2.0 m. These investigations can provide case support for space weather and GNSS studies of the impact of geomagnetic storms during peak solar activity years.

Wavelet Analysis of Ozone Driving Factors Based on ~20 Years of Ozonesonde Measurements in Beijing

A long-term vertical ozone observational dataset has been provided during 2001–2019 by ozonesonde measurements in Beijing on the North China Plain. Previous studies using this dataset primarily focused on the vertical characteristics of climatological ozone and its variation; however, the driving factors of ozone variation have not been well discussed. In this study, by applying the wavelet analysis method (including continuous wavelet transform and cross wavelet) and sliding correlation coefficients to ~20 years of ozonesonde measurements collected in Beijing, we analyzed the dominant modes of ozone column variability within three height ranges over Beijing (total column ozone: TOT; stratospheric column ozone: SCO; and tropospheric column ozone: TCO). Moreover, we also preliminarily discussed the relationship between these three ozone columns and the El Niño Southern Oscillation (ENSO), Quasi-biennial Oscillation (QBO), and 11-year solar activity cycle. The results revealed that the ozone columns within the three height ranges predominantly adhered to interannual variability patterns, and the short-term variabilities in TOT and SCO may have been related to eruptive volcanic activity. In comparison to the TOT and SCO, the TCO was more susceptible to the forcing influences of high-frequency factors such as pollutant transport. Similar to the results in other mid-latitude regions, strong ENSO and QBO signals were revealed in the interannual ozone column variability over Beijing. The TOT and SCO showed positive anomalous responses to ENSO warm-phase events, and the peak of the ENSO warm phase led the winter peaks of the TOT and SCO by approximately 3–6 months. During the strong cold–warm transition phase in 2009–2012, the TOT and SCO showed a significant positive correlation with the ENSO index. The strong seasonality of the meridional circulation process driven by the QBO led to a significant positive correlation between the QBO index and the TOT and SCO in the interannual cycle, except for two periods of abnormal QBO fluctuations in 2010–2012 and 2015–2017, whereas the TCO showed a time-lagged correlation of approximately 3 months in the annual cycle relative to the QBO due to the influence of the thermodynamic tropopause. In addition, analysis of the F10.7 index and the ozone columns revealed that the ozone columns over Beijing exhibited lagged responses to the peaks of sunspot activity, and there was no obvious correlation between ozone columns and 11-year solar activity cycle. Given the complex driving mechanism of the climatic factors on local ozone variability, the preliminary results obtained in this study still require further validation using longer time series of observational data and the combination of chemical models and more auxiliary data.

Constructing a Regional Ionospheric TEC Model in China with Empirical Orthogonal Function and Dense GNSS Observation

Using Global Navigation Satellite Systems (GNSS) observation data for developing a high-precision ionospheric Total Electron Content (TEC) model is one of the essential subjects in ionospheric physics research and the application of satellite navigation correction. In this study, we integrate the Empirical Orthogonal Function (EOF) method with the TEC data provided by the Center for Orbit Determination in Europe (CODE), and observed by the dense GNSS receivers operated by the Crustal Movement Observation Network of China (CMONOC) to construct a regional ionospheric TEC model over China. The EOF analysis of CODE TEC in China from 1998 to 2010 shows that the first-order EOF component accounts for 90.3813% of the total variation of the ionospheric TEC in China. Meanwhile, the average value of CODE TEC is consistent with the spatial and temporal distribution characteristics of the first-order EOF base function, which mainly reflects the latitude and diurnal variations of TEC in China. The first-order coefficient after EOF decomposition shows an obvious 11-year period and semi-annual variations. The maximum amplitude of semi-annual variation mainly appears in March and October, which is closely associated with the variation in geographical longitude, the semi-annual change of the low-latitude electric field, and the ionospheric fountain effect. The second-order coefficient has an evident annual variation, the minimum amplitude mainly occurs in March, August, and September, and the amplitude values in the high solar activity years are more significant than those in the low solar activity years. The third-order coefficient mainly shows the characteristics of annual variation, and the fourth-order coefficient shows the noticeable semi-annual and annual variations. The third and fourth-order coefficients are both modulated by the solar activity index F10.7. The ionospheric TEC model in China, driven by CMONOC real-time GNSS observation data, can better reflect the latitude, local time and seasonal variation characteristics of ionospheric TEC over China. In particular, it can clearly show the spring and autumn asymmetry of ionospheric TEC in the low latitudes. The root mean square error of the absolute error between the model and the actual observation is mainly distributed around 2.45 TECU (1 TECU = 1016 electrons/m2). The values of the TEC model constructed in this study are closer to the actual observed values than those of the CODE TEC in China.

Validation of the European Ionosonde Service nowcasting foF2 maps over the eastern Mediterranean

The Cyprus ionosonde (Digisonde) has been operating since 2009. Therefore it has been monitoring ionospheric conditions over the eastern Mediterranean region for more than 14 years. It is the only ionosonde operating in the area and it does not contribute to any existing European database and subsequently has not been utilised in the frames of any Space Situational Awareness (SSA) program. In the frames of the European Space Agency (ESA) activity “Potential Enhancements in Ionospheric Monitoring under SSA (PEIMSSA)” a quantitative validation of the the European Ionosonde Service (EIS) was undertaken. This validation is based on 8 years of manually scaled F-layer critical frequency (foF2) data that was compared with the corresponding interpolated values from EIS nowcasting foF2 maps over Europe. The aim of the study was to quantify EIS accuracy over the eastern Mediterranean with respect to manually scaled foF2 data from the Cyprus Digisonde. The findings of this study suggest that there are systematic underestimation-overestimation features that appear in all years considered. The most notable feature is an overestimation of EIS over Cyprus in the interval 00:00–03:00 UT, from October to March and an underestimation from April to August. An underestimation is also systematically observed in the interval, 14:00–16:00 UT from April to December and 22:00–23:00 UT in September to October. This overestimation-underestimation pattern is evident throughout all years under investigation with a higher mean % difference discrepancy in low than in high solar activity years. During disturbed days we observe, for all years, EIS slightly overestimating Cyprus foF2 values.

Seasonal morphology and solar activity dependence analysis of mid-latitude post-midnight enhancement using Global Ionospheric Map

In the mid-latitude ionosphere, the anomalous enhancement during nighttime is defined by an increase in electron density of the F2 region and/or total electron content. Studying this phenomenon is important because it enriches our understanding of the complex physical processes responsible for ionosphere formation. Furthermore, long-term investigations can reveal patterns and trends in ionospheric behavior, enabling more accurate predictions and improvements in ionosphere models. In this paper, we have investigated the mid-latitude post-midnight enhancement (MPE) utilizing Global Ionosphere total electron content (TEC) Maps (GIMs) during Low Solar Activity (LSA) years 2009, 2020 and High Solar Activity (HSA) year 2014. The investigation is done in the Northern and Southern Hemispheres during post-midnight hours (0–6 LT) focusing on the occurrence and strength of the MPE according to its seasonal morphology, in addition to the MPE occurrence dependence on the Interplanetary Magnetic Field (IMF) and the influence of the F10.7 on the strength of MPE. The findings indicate that the MPE occurrence and strength are more frequent and stronger during the winter solstice and depend on season, longitude, latitude, and level of solar activity. The MPE occurrence rate and strength reduce when solar activity is higher. The MPE occurrence rate shows a correlation with the IMF's By and Bz components. The negative correlation between F10.7 and MPE strength emphasizes solar activity's impact on MPE.

A Reconstruction Method for Ionospheric foF2 Spatial Mapping over Australia

To improve the accuracy of predicting the ionospheric critical frequency of the F2 layer (foF2), a reconstruction method for the spatial map of the ionospheric foF2 based on modified geomagnetic dip coordinates is proposed. Based on the strong correlation between the ionospheric foF2 and geomagnetic coordinates, the variation function of ionospheric distance is built. In the end, the spatial map of the ionospheric foF2 is predicted by solving the Kriging equation. The results show that the regional characteristics of the ionospheric foF2 analyzed by the proposed method are consistent with the observations. Compared with the reconstructed value of foF2 using traditional geographic coordinates, the root-mean-square error (RMSE) in high solar activity years decreased by 0.43 MHz, and the relative RMSE decreased by 5.48%; The RMSE decreased by 0.35 MHz during low solar activity which is 5.99% lower to relative RMSE. The research results provide support for high-frequency communication frequency selection.

The VLF-induced ionospheric perturbations detected by DEMETER during solar minimum

The VLF-induced ionospheric heating events from the powerful transmitter NWC are collected at extremely low solar activity years of 2008–2009, and their spatial and temporal distribution features are investigated. The spatial distribution of heating signals exhibits clear north-direction inclination of 2–3°, and covering area within 300 km radius in longitude and latitude. Plasma parameters show the obvious negative correlations between decreasing Ne and increasing Te, and positive correlations of the increasing ion density and ion temperature during the VLF heating time. The ions move upwards to cause the perturbations in ULF electric field due to mathop{V}limits^{rightharpoonup} times mathop{B}limits^{rightharpoonup}. The VLF heating events easily occur at seasons with low plasma density, which illustrates that low plasma environment plays an important role in VLF-induced ionospheric heating. Typical three enhanced VLF signals are obtained, one at 19.8 kHz with the largest amplitude, one at the broadened frequency band of 19.8 ± 300 Hz with one order of amplitude decreasing, and one between 19.5 kHz and lower hybrid frequency with at least two orders of amplitude reductions relative to that at 19.8 kHz but still one order increasing relative to non-heating region. These quasi electrostatic waves during VLF heating may not be from the VLF whistler conversion, but the lightning-whistler waves are enhanced when they pass through the plasma cavity formed during VLF heating process.

Ionospheric scintillation characteristics over Indian region from latitudinally-aligned geodetic GPS observations

Ionospheric scintillations pose one of the biggest threats to Satellite-Earth communication links in the communication and navigation systems services whose occurrence characteristics could be explained through the strength of fading (amplitude scintillation index; S4) and the rapidity of the fades (decorrelation time; \U0001d70f0). In the present work, we analyzed the S4 index from a latitudinal array of three geodetic global positioning system (GPS) stations along the Indian longitude sector during the descending phase of the 24th solar cycle. The results show predominant occurrences of scintillations during the post-sunset and nighttime periods indicating the level of scintillations close to equatorial, beyond anomaly crest, and near mid-latitude locations. The strong amplitude scintillations during 2014 and 2015 and their lowest magnitudes during 2016 and 2017 reflect the solar activity dependence of the scintillation occurrences. The directional distribution and 2-dimensional surface sky plots of the S4 index substantiate the occurrence of intense scintillation being more prevalent towards the equatorial location, whereas the weak and moderate scintillations are perceived towards the higher low latitude stations. The occurrences of intense scintillations are confirmed in high solar active years and equinox season at all stations, while the equatorial station presents relatively higher occurrences in winter followed by summer. The summer season witnessed intense occurrences even under moderate to low solar activity years (2016–2017), reflecting the post-midnight occurrences due to meridional thermospheric winds. Moreover, maximum percentage occurrences of weak scintillations in all seasons are evident towards the poleward side, far away from the anomaly crest. The amplitude scintillation frequency during the period is further compared with the solar and geomagnetic indices to substantiate the analysis drawn from the variations. The correlation coefficients between the scintillation index occurrence rate with the solar and geomagnetic in-dices at three GNSS stations provide valuable insights into the relationship between solar activity and scintillation. The results show that the strength and direction of the correlation can vary significantly depending on the specific solar index and location. Thus, emphasize further studies on scintillation occurrences for developing effective forecasting and mitigation models over the Indian low-latitude region.

Daily Plasmaspheric TEC Variations From COSMIC GPS Observations Based on RBF Neural Network‐Kriging Method

AbstractThe low Earth orbit (LEO) satellite provides valuable direct observations for scientific investigation of the plasmasphere, while the plasmaspheric total electron content (PTEC) with a high temporal resolution cannot be precisely estimated due to fewer LEO satellites. In this paper, a novel joint method of radial basis function neural network—Kriging (RBF‐Kr) method is designed to construct the daily PTEC model using the Constellation Observing System for Meteorology, Ionosphere and Climate Global Positioning System observations during the low (2009) and high (2013) solar activity years. Compared with the original RBF method, the RBF‐Kr method reduces the mean absolute error and root mean square error from 0.77 to 0.60 TEC unit (TECU) and 0.99 to 0.80 TECU, respectively. The correlation coefficient (Corr) increased from 0.90 to 0.94. Furthermore, daily PTEC variations show that the PTEC at low latitudes is evenly distributed during equinox periods. The South American‐Atlantic Ocean sector has a peak and trough in PTEC during the December and June solstices. A certain symmetrical distribution of PTEC is observed in the latitudinal direction, and the symcenter moves toward the summer hemisphere. The duration of extremal PTEC at 60°W is observed, which lasted up to more than 80 days around the December solstice. An obvious correlation between the solar flux and PTEC is found with up to 0.86, indicating that daily PTEC variations are mainly related to solar activities.