Ionospheric Disturbances Investigation By Using Ground-based GNSS Observations From Dense CORS

Since the electromagnetic signals traveling between space and the Earth have to be passing through the ionosphere,satellite navigation and communications systems are substantially disturbed by ionospheric perturbations.As it is well known,the ionospheric variations affect the Global Navigation Satellite System（GNSS）signals which can lead to large positioning errors and a discontinuity in a receiver’s phase lock on a GNSS signal which is defined as cycle slip.Consequently,monitoring ionospheric variability has great significance for safe and accurate navigation.Acoustic and gravity waves generated in the lower atmosphere through natural and anthropogenic hazards have become an important source of variability in the upper atmosphere.The amplitudes of these waves could substantially grow,propagate up,and then cause significant variability in the upper atmosphere.Indeed,the space weather represented in geomagnetic activities and solar radiation plays a crucial role in the dynamic regime of the ionosphere.However,according to the evolution of the atmosphere‐ionosphere coupling theory,the acoustic gravity waves（AGWs）could largely be associated with some powerful meteorological disturbances that further lead to some significant ionospheric perturbations.Many researchers have conducted several experiments to distinguish that AGWs could be associated with some powerful meteorological disturbances.In order to distinguish whether the space weather impacts ionospheric variations,meticulous observations of the geomagnetic and solar radiation indices should be investigated to ensure that the possible ionospheric disturbances are induced by the hazards rather than by other random events.In the present study,the Disturbance storm-time（Dst）and Kp“geomagnetic indices”,solar wind speed（Vsw）,and the solar radio flux F10.7 cm“solar indices”,are used in the analysis.An increasing number of satellites and ground-based monitoring stations with the rapid development of space-based navigation systems have led to great breakthroughs in ionospheric applications.Since GNSS observations provide global coverage with high spatial and temporal resolutions,low costs,near-real-time,and continuity compared with that of the traditional ionospheric sounding tools such as Ionosonde,Shortwave（ULF/LF）radio signal,and topside sounders.GNSS observations have been preferred over the last few decades as a remotely sensed tool to monitor the ionospheric disturbances associated with hazards.GNSS provides global coverage of Total Electron Content（TEC）data with high temporal and spatial resolutions.By using GNSS observations,the TEC values could be calculated over a ground GNSS station then the ionospheric response to a specific event could be investigated.GNSS signals encompass three L‐band frequencies(L₁,L_2,and L₅)for the carrier phase and pseudo‐range.These multiple bands are mainly used to eliminate the ionospheric effect on GNSS signals and/or to derive the number of free electrons in the ionosphere.The TEC is a well-known parameter for GNSS observation and it describes the number of free electrons along the line of sight（LOS）between receivers to GNSS satellites.The carrier‐to‐code leveling（CCL）technique was utilized to calculate the TEC parameter.CCL is carried out for each continuous arc where the CCL degrades the pseudo‐range noise,and it eliminates potential ambiguity influence,as well as retains the high precision in the carrier‐phase.However,it is not easy to select the proper algorithm that can effectively distinguish the ionospheric disturbances wave of interest from a group of waves,such as signal trend and noise.Besides,the detrending method may have a weakness based on skips detectable ionospheric disturbance and/or filter out lower-frequency variations where the ionospheric variation values are a function based on time,location,and strength of hazards.So,some innovative and cutting-edge approaches were proposed in this work to avoid the defects of common detrending methods.Furthermore,using different detrended fluctuations analysis techniques could increase the verification chances of the detectable span for perturbation duration.In order to investigate the ionospheric variations associated with different hazards,three recent case studies have been selected at mid-and low-latitude regions.In the first part of this thesis,in Chapter three（3）,I focused on the short-term irregular disturbances through a strong thunderstorm on March 2^nd,2018 in Wuhan City,Hubei,China by using GNSS observations from dense Hubei Continuously Operating Reference Stations（HB-CORS）with a sampling rate of 1s.I have investigated the ionospheric response to a strong thunderstorm over a mid-latitude region through the“Multi-step numerical difference”（MSND）technique to get the variations in STEC sequences.Three time-intervals of 30,90,and 300 seconds are used to choose the best time interval to be utilized in the current study.The detected TEC variations were compared with a non-lightning day（DOY 59）to check their significance.The changes in d TEC have also been analyzed during the non-lightning day on the 28th of February 2018（DOY 59）and compared with the lightning day.Our findings indicated that the d TEC was insignificant in comparison to the one that was on the day of the event.Three satellites G06,G17,and G28 have detected the ionospheric response over HB-CORS stations.Owing to G28 IPPs paths were passing close to the storm over Wuhan City Center.There were detectable ionospheric disturbances over many CORS stations through satellite G28 compared with other satellites.Whereas the other two satellites G17 and G06 have IPPs paths passing far from Wuhan Center that makes it away from lightning activity over Wuhan.As well as the longest period associated with the lightning time was by the G28 satellite which gave more chances to detect the ionospheric disturbance during and after the lightning.This experiment was implemented under the meticulous observations of the solar‐terrestrial environment and geomagnetic storm indices to ensure that the possible ionospheric disturbances have been induced by lightning rather than by other random events.The detectable ionospheric disturbances were found over only 12 CORS stations.Therefore,I applied two more detrending methods,six order polynomials fitting（PF）and a proposed superior approach“one-week average difference”（AD）,to observe the influence of thunderstorm activities on ionospheric perturbation over six CORS stations.Accordingly,using different detrended fluctuations analysis techniques could increase the verification chances of the detectable span for perturbation duration.The analyzed results showed that MSND has the lowest performance.Meanwhile,the fitted TEC data with the 6^th order polynomials technique presented an improvement and a discrepancy related to MSND.The proposed AD technique accomplished the best performance related to the TEC disturbances and was compared with the other two techniques.Based on the results of the three detrending methods,further studies are therefore necessary to evaluate more ways with different phenomena,investigating various case studies for further improvement could lead to a fast and efficient algorithm for detecting of duration and frequency of ionospheric perturbations can be proposed,where it will be more suitable for certain phenomena and regions.Then,the spherical harmonic expansion based on a single-layer model has been used to model the spatial distribution of STEC.I studied the 2D spatial distribution of TEC disturbances by creating regional ionosphere maps（RIMs）based on the HB-CORS observations with a high spatial resolution of 0.2 x 0.4 degrees in latitude and longitude,respectively,and a temporal resolution of 2 hours to coincide with global ionosphere maps（GIMs）.The VTEC ionospheric variations over the Hubei Province area were detected by anomaly maps of the event day represent differences from the mean value of the previous 5 days.This case study aims to provide a better understanding of the ionospheric disturbance over a mid-latitude region and the reasons for different responses over different stations by associating with the occurrence of acoustic and gravity waves during a strong thunderstorm.The findings obtained from this case study provide a good opportunity to understand the level of a thunderstorm’s effect on TEC variation in a mid-latitude region and endorse other theoretical and modeling studies on TEC responses to a large thunderstorm.The findings showed that ionospheric disturbed electrons can be generated with various rates and different velocities through lightning influences based on the distance and direction of each CORS station from the Wuhan city center.The ionosphere showed a positive response up to 2 TECU,especially in the northern part of Hubei Province where the maximum VTEC amplitude variations coincided with the thunderstorm influence.The findings are tentatively interpreted by lightning-induced electric fields and acoustic gravity waves.Our research provided a robust method over mid-latitude regions for detecting ionospheric disturbance associated with lightning through the STEC sequences than the previous studies.In the second part of this thesis,in Chapter four（4）,typhoon Mangkhut in September 2018 have been investigated for the first time,by applying the slant TEC（STEC）and vertical TEC（VTEC）sequences derived from GNSS observations collected from Hong Kong CORS（HK-CORS）and international GNSS service（IGS）.Firstly,the ionospheric observations derived from six GNSS monitoring stations located in Hong Kong and three IGS stations are used for investigation.Savitzky-Golay smoothing filter is employed to de-trend STEC observations,and the S transform spectrum analysis method is used to convert d STEC in the time domain to its frequency domain for ionospheric disturbances detection.Since most significant ionospheric disturbances happened in the daytime in autumn/winter and at nighttime in spring/summer,I selected nighttime GNSS observations to demonstrate that the ionospheric disturbances might occur during the period of typhoon Mangkhut.I investigated the ionospheric disturbances during the typhoon from three different aspects,e.g.,ionospheric disturbances occurred by d STEC sequences of the satellites observed by a station,d STEC sequences by other stations located in the same region,and the sequences by some IGS stations far away from the typhoon were selected to verify whether the ionospheric disturbances were induced by typhoon Mangkhut.The analysis results and some conclusions are shown in the following part.The experimental results show that the typhoon can cause ionospheric disturbances one day before landfall.STEC results shown that there are obvious ionospheric disturbances with a distance of about 1050 and 230 km away from the typhoon eye before and after typhoon landfall,which is related to the power of the typhoon.The magnitude of the disturbances before typhoon landfall is about two times larger than that on landing day,which is related to the power of the typhoon.Meanwhile,the disturbances detected from the de-trended STEC sequences for the same satellites’observations collected from other stations located in Hong Kong occurred nearly simultaneously.The magnitude of disturbance reaches its maximum about 1300 km away from the typhoon eye and then decreased.The typhoon-induced disturbances almost disappear farther than 2100 km from the typhoon eye.RIMs were created through the HK-CORS and IGS data around the area of the Mangkhut typhoon.RIMs were utilized to analyze the ionospheric response to typhoon under the meticulous observations of the solar‐terrestrial environment and geomagnetic storm indices.Ionospheric VTEC time sequences over the maximum wind speed points（maximum spots）are detected by three methods:interquartile range method（IQR）,the enhanced average difference（EAD）,and range of ten days（RTD）during the super typhoon Mangkhut.VTEC series were implemented over the maximum wind speed points of the typhoon.I investigated the highest amplitude ionospheric variations,along with the atmospheric anomalies,such as the sea‐level pressure,Mangkhut’s cloud system,and the meridional and zonal wind during the typhoon.The atmospheric anomalies were retrieved from National Oceanic and Atmospheric Administration‐Physical Sciences Laboratory（NOAA‐PSL）to analyze their possible impacts on ionospheric response through the anomaly maps of the sea level pressure,zonal wind,and meridional wind.Meanwhile,infrared satellite snapshots of typhoon cloud for selected time instances have been collected from the cooperative Institute for Meteorological Satellite Studies/the University of Wisconsin‐Madison to monitor the typhoon’s eye movement with respect to the point of maximum wind speed.The new findings indicated significant ionospheric variations over the maximum spots during this powerful tropical cyclone within a few hours before the extreme wind speed,when the sustained wind speed in the typhoon had a second‐highest value on September 13,2018（DOY 256）.As a new finding,the highest VTEC variation amplitude could happen before the max wind speed that may be based on the few difference between the highest and second-highest wind speeds of about 10 km/h.Moreover,the ionosphere showed a positive response where the maximum VTEC amplitude variations coincided with the cyclone rainbands or typhoon edges rather than the center of the storm.These variations may be a result of the production of the Global Electric Circuit（GEC）due to the thunderstorm and electrified convective cloud activities at the rainbands of the typhoon.The vertical conduction current produced from the GEC flows upward from the thunderstorm cloud to the ionosphere where it endorses severe variations in the electron concentrations.The sea‐level pressure tends to decrease around the typhoon periphery,and the highest ionospheric VTEC amplitude was observed when the low‐pressure cell covers the largest area.The possible mechanism of the ionospheric response is based on strong convective cells that create gravity waves over tropical cyclones.Moreover,the critical change state in the meridional wind happened on the same day of maximum ionospheric variations on the256^th day of the year（DOY 256）.This comprehensive analysis suggests that the meridional winds and their resulting waves may contribute in one way or another to upper atmosphere‐ionosphere coupling which was suggested for the first time in this thesis.In the third part of this thesis,in Chapter five（5）,I focused on the Beirut Port Explosion as an anthropogenic event on August 4,2020,about 2,750 tonnes of ammonium nitrate（NH4NO3）had been stored unsafely at a warehouse in the port,were the reason for the blast.As it is widely known,Beirut is an ancient capital of Lebanon located on the east coast of the Mediterranean Sea,also Beirut is known as Paris of Middle East,Beirut citizens about 2 million,and scarred by civil war.The deadly explosion of the port has infected the socio-economic backbone of Beirut city and it became paralyzed.According to the experts,the Beirut Port explosion is considered one of the most severe non-nuclear manmade blasts in history.The STEC time series were derived through IGS and Turkish National Permanent GNSS Network-Active（CORS-TR）to detect possible ionospheric response to the blast.There was a large initial explosion,followed by a series of smaller blasts,about～30 seconds,a colossal explosion has happened,a supersonic blast wave radiating through Beirut City.As a result of the chemistry behind ammonium nitrate’s explosive,a mushroom cloud was sent into the air.Savitzky-Golay smoothing filter is employed to de-trend STEC observations,and the corresponding Morlet wavelet transform is used to convert time domain to frequency domain for ionospheric disturbances detection.To verify STEC disturbances are not associated with space weather,so the Dst,Vsw,Kp,and F10.7 indices were investigated before,during,and after the explosion.The steady state of space weather before and during the event indicated that the observed variations of TEC sequences were caused by the ammonium nitrate explosion.I have focused on the missing ionospheric response in the northern part during the explosion based on STEC sequences.The TEC ionospheric disturbances in terms of TECU were coincident in space and time with the Beirut Explosion for the current particular case study.The satellite G22 has detected the ionospheric response over IGS and CORS-TR stations.Owing to G22 IPPs paths were passing close to Beirut Explosion.As well as the longest continuous arcs associated with the blast time were by the G22 satellite which gave more chances to detect the ionospheric disturbance during and after the blast.The research findings showed that the ionosphere response to blast through with two-time arrivals,the first arrival was after the blast within few minutes and it has low amplitude,meanwhile,the second time arrival for the ionospheric variations was after more than 2hrs from the explosion time with higher amplitude than that of the first one.The findings acquired from this work provide an opportunity to understand the level of Beirut blast impact on ionosphere variation and endorse other theoretical and modeling studies on TEC responses to massive explosions.The results showed ionospheric variations with different strength and different time arrival,I suggest that the acoustic and gravity waves generated through the explosion could be the reason for different time arrival of the detected ionospheric disturbances over GNSS ground-based stations.To summarize,this thesis brings insight into the lithosphere-atmosphere-ionosphere coupling（LAIC）of areas that were previously poorly constrained by insufficient algorithms,the low spatial resolution（2.5 x 5 degrees in latitude and longitude）based on GIMs from the Center for Orbit Determination in Europe（CODE）and temporal resolution（30s）of GNSS data mostly and IGS network stations and where I used multi-GNSS data with a high rate（1s）,and dense CORS stations to reveal local changes of the ionosphere and create high-resolution（0.2 x 0.4 degrees in latitude and longitude）RIMs.The coupling evolution of the LAIC is greatly enriched by the newly acquired knowledge on thunderstorms,tropical cyclones,and anthropogenic hazards.I investigated the ionospheric response by using ground-based GNSS observations from IGS dense CORS stations during strong thunderstorms over Wuhan city,tropical cyclones over the northwest Pacific Ocean,and the Beirut Port Explosion.This study investigated in order to enhance understanding of the physical mechanism which disturb ionosphere during thunderstorm,tropical cyclones.In order to avoid weakness in common de-trending techniques for one or more of these reasons:（1）skips detectable disturbance,（2）time step could be relevant to the detectable period,（3）filter out lower-frequency variations.I proposed three de-trending techniques for analyzing and detecting ionospheric variations.One over the mid-latitude regions and the other two for the low-latitude regions.Besides,I used the other four common de-trending methods and clarify their defects.In this thesis,multiple analysis algorithms could help increase the opportunity to study the ionosphere dynamic regime.The variation of space weather indices indicated that the ionospheric conditions were not contributed by solar and geomagnetic activities.The divisions of the thesis show how satellite-GNSS data effectively used as a remotely sensed tool to monitor the ionosphere.The contributions of the selected areas on low and mid-latitudes during different events,especially in poorly covered by IGS station studies will help us to better understand the process of LAIC.Besides,my research findings provide a good opportunity to understand the level of effect during thunderstorms,typhoons,and giant surface explosions on STEC/VTEC variation in study regions and endorse other theoretical and modeling studies on TEC responses to the selected events.Eventually,in this thesis,innovative strategies are introduced,which significantly increase the verification chances of the detectable span for ionospheric perturbation duration based on some innovative and cutting-edge approaches were proposed in this work to avoid the defects of common detrending methods.Besides,the thesis brings insight into the LAIC.