Traveling Ionospheric Disturbances Characteristics During the 2018 Typhoon Maria from GPS Observations

remote sensing

Article Traveling Ionospheric Disturbances Characteristics during the 2018 Typhoon Maria from GPS Observations

Yiduo Wen 1,2 and Shuanggen Jin 1,2,3,* 1 School of Remote Sensing and Geomatics Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China; [email protected] 2 Jiangsu Engineering Center for Collaborative Navigation/Positioning and Smart Applications, Nanjing 210044, China 3 Shanghai Astronomical Observatory, Chinese Academy of Science, Shanghai 200030, China * Correspondence: [email protected] or [email protected]; Tel.: +86-25-58235371

Received: 22 January 2020; Accepted: 22 February 2020; Published: 24 February 2020

Abstract: Typhoons often occur and may cause huge loss of life and damage of infrastructures, but they are still difficult to precisely monitor and predict by traditional in-situ measurements. Nowadays, ionospheric disturbances at a large-scale following typhoons can be monitored using ground-based dual-frequency Global Positioning System (GPS) observations. In this paper the responses of ionospheric total electron content (TEC) to Typhoon Maria on 10 July 2018 are studied by using about 150 stations of the GPS network in Taiwan. The results show that two significant ionospheric disturbances on the southwest side of the typhoon eye were found between 10:00 and 12:00 UTC. This was the stage of severe typhoon and the ionospheric disturbances propagated at speeds of 118.09 and 186.17 m/s, respectively. Both traveling ionospheric disturbances reached up to 0.2 TECU and the amplitudes were slightly different. The change in the filtered TEC time series during the typhoon was further analyzed with the azimuth. It can be seen that the TEC disturbance anomalies were primarily concentrated in a range of between 0.2 and 0.2 TECU and mainly located at 135–300 in − ◦ the azimuth, namely the southwest side of the typhoon eye. The corresponding frequency spectrum of the two TEC time series was about 1.6 mHz, which is consistent with the frequency of gravity waves. Therefore, the upward propagating gravity wave was the main cause of the traveling ionospheric disturbance during Typhoon Maria.

Keywords: typhoon; GPS; TEC; ionospheric disturbance; gravity wave

1. Introduction Solar and geomagnetic activities together with geohazards (e.g., earthquakes and typhoons) may cause the oscillations of electron densities in the ionosphere. Numerous studies have shown a close relationship between intense solid-Earth activities (such as volcanos, earthquakes, tsunamis, or typhoons) and ionospheric disturbances [1–4]. As typical and complex strong weather systems in the lower atmosphere, typhoons can excite gravity waves and propagate to the upper layers of the ionosphere, causing several disturbances [3]. As early as 1958, Bauer [5] studied the response of the ionosphere during hurricane transit and found that the ionospheric F2-layer critical frequency (foF2) increased during hurricane transit, and the frequency reached the maximum value when it was closest to the observation station. Huang et al. [6] used a high-frequency (HF) Doppler detector to observe 15 typhoons on Taiwan Island from 1982 to 1983. As a result, ionospheric disturbances excited by acoustic-gravity waves triggered by two strong typhoons were detected. Sato [7] observed the small-scale wind disturbance produced by Typhoon Kelly through middle and upper atmosphere

Remote Sens. 2020, 12, 746; doi:10.3390/rs12040746 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 746 2 of 16

(MU) radar for 60 h. Rice et al. [8] used four ionospheric detectors to observe ionospheric disturbances caused by Typhoon Mellor from 9–11 October 2009. Tao et al. [9] used the ionospheric frequency map data of the Xiamen Ionospheric Monitoring Station to study the change of the ionosphere during three typhoons that landed near Xiamen in 2007 and found that the ionospheric foF2 was disturbed by the three typhoons, but the disturbance was the opposite to the declined foF2 caused by the typhoon landing [10]. Therefore, more analysis and studies are needed. Furthermore, the disturbance propagation direction, period, and speed do not clearly follow the typhoon [11]. Nowadays, denser Global Positioning System (GPS) observations provide important data for the study of ionospheric variations, allowing the continuous monitoring of large-scale ionospheric disturbances caused by typhoons [11–14] and earthquakes [15–17]. The characteristics of traveling ionospheric disturbances (TIDs) are consistent with the theory of gravity waves. Song et al. [11] used Chinese GPS station data to study two ionospheric disturbances caused by Typhoon Chan-hom and found the periods and speeds of two medium-scale traveling ionospheric disturbances (MSTIDs) to be 56 min, 268 m/s and 45 min, 143 m/s, respectively, which may have been caused by the acoustic-gravity wave excited by the super typhoon propagating upward in the ionosphere. Due to the complexity of the typhoon mechanism, it is still difficult to know the mode and pattern of ionospheric disturbances triggered by different typhoons. More cases studies are needed to investigate ionospheric disturbance characteristics and understand the coupling process between typhoons and the ionosphere. The Category-5 Super Typhoon Maria occurred in 2018 and caused serious loss of life and economic loss, but traditional observations were not sufficient to predict or help understand this typhoon. In this paper, the traveling ionospheric disturbance following the 2018 Typhoon Maria passing through Taiwan is investigated and analyzed using the dense GPS network data in Taiwan. Furthermore, the disturbance mode and characteristics (e.g., disturbance period, velocity and direction, etc.) are studied and discussed. In Section2 data and methods are described, Section3 presents the results and discussion, and finally conclusions are given in Section4.

2. Date and Method

2.1. Typhoon Information On 25 June 2018, an atmospheric disturbance formed in the Marshall Islands sea area. On 4 July, the United Typhoon Warning Center upgraded it to a tropical storm, naming it “Maria”. On the afternoon of 5 July, the United Typhoon Warning Center upgraded it to the typhoon. Finally, it was upgraded to a super typhoon on the morning of 6 July, with a maximum wind speed of 60 m/s and a minimum air pressure of 925 hPa in the center. After 8 July, Maria’s convection weakened gradually, and its intensity gradually decreased. On 9–10 July, Maria landed in the eastern sea area of Taipei with a maximum wind speed of 50 m/s and a minimum air pressure of 935 hPa in the center. The Central Meteorological Bureau issued an oﬀshore and land-based typhoon warning. Maria formed strong convective weather in Taiwan and on the morning of 11 July, Maria reached Lianjiang county, Fujian Province. The tracks of the typhoon are shown in Figure1 from 0:00 UTC (10 July) to 6:00 UTC (11 July) with the colorful circles. The blue, yellow, orange, purple, and red circles represent the tropical storm, severe tropical storm, typhoon, severe typhoon, and super typhoon, respectively. When Typhoon Maria landed on the eastern sea area of Taiwan, the super typhoon was changed to severe typhoon until reaching the Fujian ocean coast, which lasted from 9:00–24:00 UTC on 10 July 2018 and then got weaker and weaker. Remote Sens. 2020, 12, 746 3 of 16

continuous GPS network in Taiwan was constructed. In this study, the data from about 150 GPS stations were obtained from the Central Meteorological Bureau (CWB) with a sampling rate of 30s, which were used to calculate the TEC time series. In Figure 1, the red triangles represent the GPS stations in Taiwan used in this study. In this way, the traveling ionospheric disturbance following RemoteTyphoon Sens. 2020 Maria, 12, 746 passing over the eastern sea area of Taiwan was investigated from dense3 ofGPS 16 observations in Taiwan.

FigureFigure 1.1. GPSGPS receivers(triangles)receivers(triangles) andand thethe tracktrack ofof TyphoonTyphoonMaria(colorful Maria(colorful circles). circles). 2.2. GPS Observations 2.3. Methods The island of Taiwan is located at the subduction zone between the Philippine Sea Plate, Pacific When the ionosphere is affected by severe atmospheric activities such as typhoons, the total Plate, and Eurasian Plate, which has the fastest convergence in the world and is often subject to various electron content (TEC) will be disturbed. The ionospheric TEC can be derived from GPS dual- hazards, e.g., earthquakes and typhoons. In order to monitor and predict hazards, a dense continuous f 1575.42 MHz f 1227.60 MHz GPSfrequency network observation in Taiwan ( was1 constructed. Inand this2 study, the data). from Vertical about TEC 150 GPS (VTEC) stations is usually were obtainedcalculated from based the Central on an ionosphere Meteorological single-layer Bureau (CWB) model with with a samplingthe assumption rate of 30s, that which all electrons were used are toconcentrated calculate the in TECthe thin time shell series. at a Infixed Figure height.1, the To red get triangles VTEC, it representis necessary the to GPS use stationsa mapping in Taiwanfunction usedto project in this the study. slant In TEC this (STEC) way, the into traveling the vertical. ionospheric Normally, disturbance the ionosphere following shell Typhoon height Maria is set passing as 250 overkm for the converting eastern sea STEC area ofto TaiwanVTEC as was follows investigated [14,18,19] from dense GPS observations in Taiwan. 2 2 2.3. Methods f1 f 2 STEC( L  L ()( N b  N b ))  (1) 40.3(f2 f 2 ) 1 2 1 1 1 2 2 2 L When the ionosphere is a1ffected 2 by severe atmospheric activities such as typhoons, the total electron content (TEC) will be disturbed. The ionospheric TEC can be derived from GPS dual-frequency observation ( f1 = 1575.42MHz and f2 = 1227.60MHz). Vertical TECRsin (VTEC) z   is usually calculated based VTEC STEC *cos arcsin (2) on an ionosphere single-layer model with the assumption that all electrons  are concentrated in the thin R H   shell at a fixed height. To get VTEC, it is necessary to use a mapping function  to project the slant TEC (STEC)where intoL is thethe vertical.carrier phase Normally, observation, the ionosphere is the shell signal height wavelength, is set as 250N km is for the converting ambiguity, STECb is the to VTECinstrument as follows biases [14 for,18 carrier,19] phase,  is other residual error, H is the height of the ionosphere shell,

R is the Earth radius, and Z is the2 2 elevation angle of the satellite. f1 f2 TheSTEC characterization= of( L ionospheric1 L2 + λ1( N disturbances1 + b1) λ2 ( mainlyN2 + b2 ) reflects + εL) the relative TEC(1) 40.3( f 2 f 2) − − change value, not the absolute 1TEC,− 2 so the relative TEC value is calculated by GPS carrier phase measurement. Cycle slip is an important error" in obtaining !# high-precision TEC from GPS R sin(z) measurements. Therefore, beforeVTEC calculating= STEC thecos STEC,arcsin in this study a second-order time-difference(2) R + H phase ionosphere residual (STRIP) algorithm ∗was used to take into account the cycle slip [20]. After whereobtainingL is the the carrier VTEC phase time observation,series, the trendλ is the term signal in wavelength,the TEC seriesN is themust ambiguity, be removedb is the to instrumentextract the biasesionospheric for carrier disturbance, phase, ε iswhich other is residual mainly error, causedH isby the the height ionospheric of the ionosphere background shell, changeR is theand Earth sub- radius, and z is the elevation angle of the satellite. The characterization of ionospheric disturbances mainly reflects the relative TEC change value, not the absolute TEC, so the relative TEC value is calculated by GPS carrier phase measurement. Cycle slip is an important error in obtaining high-precision TEC from GPS measurements. Therefore, before Remote Sens. 2020, 12, 746 4 of 16 calculating the STEC, in this study a second-order time-difference phase ionosphere residual (STRIP) algorithm was used to take into account the cycle slip [20]. After obtaining the VTEC time series, Remote Sens. 2020, 12, 746 4 of 16 the trend term in the TEC series must be removed to extract the ionospheric disturbance, which is mainlyionospheric caused point by the (SIP) ionospheric movement. background The Butterworth change and filter sub-ionospheric defines the maximally point (SIP) flat movement. filter in the Thepassband Butterworth that rolls filter off defines towards the zero maximally in the stopband flat filter [21], in the which passband is used that to rollsfilter otheff towards trend term zero of in the theTEC stopband series. [Figure21], which 2 shows is used examples to filter of the TEC trend disturbances term of the TECdetected series. by Figurestation2 DAJNshows and examples satellite ofPRN06 TEC disturbances filtered by detectedButterworth by stationfilters at DAJN different and satellitecutoff frequencies. PRN06 filtered The bycutoff Butterworth frequency filters range at of diGPSfferent data cuto withff frequencies. 30s interval The samples cutoff frequency was 1-15mHz. range As of GPS shown data in with Figure 30s 2, interval the TEC samples disturbance was 1–15amplitude mHz. As was shown largest in when Figure the2, the passband TEC disturbance frequency amplitudewas 1-3mHz, was so largest the fourth-order when the passband zero-phase frequencyButterworth was band-pass 1–3 mHz, filter so the with fourth-order cut-off frequencies zero-phase of Butterworth 1 and 3mHz band-pass was used filterto extract with the cut-o mostff frequenciesobvious ionospheric of 1 and 3 mHzdisturbance. was used to extract the most obvious ionospheric disturbance.

FigureFigure 2. 2.Total Total electron electron content content (TEC) (TEC) time time series series filtered filtered by by di ffdifferenterent cuto cutoffff frequencies frequencies observed observed by by stationstation DAJN DAJN and and satellite satellite PRN06 PRN06 on on 10 July July 10, 2018. 2018.

3.3. Results Results and and Discussions Discussions

3.1.3.1. Ionospheric Ionospheric Disturbance Disturbance Characteristics Characteristics AboutAbout 150 150 GPS GPS continuous continuous stations’ stations’ data in data Taiwan in wereTaiwan collected, were whichcollected, were which used to were investigate used to ionosphericinvestigate disturbances ionospheric during disturbances the 2018 Typhoon during the Maria. 2018 It Typhoonwas found Maria. that not It all was stations found and that satellites not all observationsstations and showed satellites significant observations traveling showed ionospheric significant disturbances. traveling The ionospheric ionospheric disturbances. disturbances The wereionospheric mainly detected disturbances with a were distance mainly of less detected than about with 400 a distance km between of less the than SIPs about and the 400 typhoon km between eye andthe by SIPs GPS and satellites the typhoon with highereye and elevations by GPS satellites angles. with Most higher interestingly, elevations two angles. significant Most ionospheric interestingly, disturbancestwo significant were ionospheric found during disturbances the period ofwere 10–12 found UTC, during which the was period the stage of 10-12 of severe UTC, typhoon which was with the a windstage speedof severe of abouttyphoon 50 mwith/s. There a wind was speed no significantof about 50m/s. anomaly There the was rest no of significant the time. No anomaly disturbance the rest wasof foundthe time. in theNo ionospheredisturbance when was found the typhoon in the ionosphere moved away. when the typhoon moved away. FigureFigure3 shows 3 shows each each sub-ionospheric sub-ionospheric point point (SIP) (SIP) trajectory trajectory obtained obtained by by observing observing each each GPS GPS satellitesatellite passing passing over over station station PUS2 PUS2 at at the the eastern eastern seas seas area area of of Taiwan Taiwan during during the the period period of of 10–12 10-12 UTC, UTC, 10July July 10, 2018. 2018. The The blue blue asterisk asterisk indicates indicates the the typhoon typhoon eye, eye, which which is constantlyis constantly moving, moving, the the magenta magenta squaresquare is is the the location location of of station station PUS2, PUS2, and and the the color color bar bar shows shows the the time time of of the the typhoon typhoon change. change. From From FigureFigure3 it 3 canit can be be seen seen that that during during the the period period of 10–12of 10-12UTC— UTC—the the stage stage of of severe severe typhoon—there typhoon—there are are manymany visible visible satellites, satellites, and and the SIPs the ofSIPs PRN02 of PRN02 andPRN06, andPRN06, etc. are etc. located are nearlocated the neartyphoon the eye. typhoon During eye. During its passage through the severe typhoon stage, the significant ionospheric disturbances are found at SIPs closer to the typhoon eye.

Remote Sens. 2020, 12, 746 5 of 16 its passage through the severe typhoon stage, the signiﬁcant ionospheric disturbances are found at SIPsRemote closer Sens. 2020 to the, 12, typhoon746 eye. 5 of 16

Figure 3. Sub-ionosphericSub-ionospheric point point (SIP) (SIP) trajectory trajectory observed observed over over station station PUS2 PUS2 during during the the period period of 10–1210-12 UTC,UTC, 10 July July 2018.10, 2018 The. blueThe asteriskblue asterisk indicates indicates the typhoon the typhoon eye, the eye, magenta the magenta square issquare the location is the oflocation station of PUS2, station and PUS2, the colorand the bar color shows bar the shows time ofthe the time typhoon of the typhoon change. change.

Each GPS station in the area has a similar geometry, because all stations are located in a limited region, so satellites tracked by PUS2 can also be tracked by by more more than than 100 100 stations stations simultaneously. simultaneously. The TEC residuals after using the Butterworth filterfilter to eliminate the SIP motion trend and ionospheric background showed significantsignificant disturbances during Typhoon Maria.Maria. Figure4 4 shows shows thethe filteredfiltered TECTEC map from 10:41 to 11:24 UTC. The cross marker in Figure4 4 indicates indicates thethe typhoontyphoon eye,eye, thethe colorcolor barbar indicates thethe rangerange of of TEC TEC change, change, and and the the colored colored solid solid points points show show the position the position of the of SIP the at aSIP certain at a time.certain According time. According to Figure to4, twoFigure significant 4, two significant ionospheric ionospheric disturbances disturbances caused by Typhoon caused by Maria Typhoon were clearlyMaria were observed. clearly At observed. 10:41 UTC, At the 10:41 ionospheric UTC, the anomaly ionospheric was detected anomaly for was the detected first time for (W the # 1), first the time TEC range was between 0.15 and 0.15 TECU, and the disturbance period was about 10 min (Figure4a–c). (W # 1), the TEC range− was between -0.15 and 0.15 TECU, and the disturbance period was about 10 Atmin 11:12 (Figure UTC, 4a–c). the anomalyAt 11:12 wasUTC, detected the anomaly for the was second detected time for (W the # 2), second and the time disturbance (W # 2), and period the wasdisturbance about 12 period min (Figure was about4d–f). 12 Positive min (Figure and negative 4d–f). Positive disturbances and negative were located disturbances in the southwestwere located of thein the typhoon southwest eye, of spreading the typhoon southeast. eye, spreading southeast.

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Figure 4. Cont.

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FigureFigure 4. 4.The Change change of the of thefiltered filtered TEC TEC from at 10:41 10:41 to UTC 11:24 ( aUTC.), 10:46 The UTC W#1 ( band), 10:51 W#2 UTCare shown (c), 11:12 in (a UTC–c) and (d–f), respectively. The cross marker indicates the typhoon eye, the color bar represents the (d), 11:18 UTC (e) and 11:24 UTC (f). The W#1 and W#2 are shown in (a–c) and (d–f), respectively. amplitude of TEC changes, and the colored solid points show the position of SIP. The cross marker shows the typhoon eye, the color bar represents the amplitude of TEC changes, and the colored solid circle points show the position of SIP. Figure 5 shows the tracks of the sub-ionospheric point (SIP) observed by satellite PRN02 at stationFigure FKD25 shows and thesatellite tracks PRN06 of the sub-ionosphericat station DAJN pointfrom (SIP)10:00 observedto 12:00 UTC, by satellite the filtered PRN02 TEC at stationtime FKD2series and diagram satellite of PRN06 the two at stations, station DAJN the change from 10:00of elevation to 12:00 angle UTC, and the filtereddistance TEC between time seriesthe typhoon diagram eyes and the SIP. The top two graphs show the SIP track of two stations from 10:00-12:00 UTC. The of the two stations, the change of elevation angle and distance between the typhoon eyes and the SIP. black cross indicates the typhoon eye, the red dots show the positions of the two aforementioned The top two graphs show the SIP track of two stations from 10:00–12:00 UTC. The black cross indicates stations, and the blue line represents the SIP track. Both tracks were located in the southwest of the the typhoon eye, the red dots show the positions of the two aforementioned stations, and the blue line typhoon eye and moved from southeast to northeast. From the middle two panels, two periodic wave represents the SIP track. Both tracks were located in the southwest of the typhoon eye and moved forms of ionospheric disturbances were observed at 10:40 and 11:06 UTC, respectively, instead of the from southeast to northeast. From the middle two panels, two periodic wave forms of ionospheric typical N-shaped waves. The amplitude of the disturbance observed by satellite PRN02 at station disturbances were observed at 10:40 and 11:06 UTC, respectively, instead of the typical N-shaped FKD2 was slightly smaller than that from satellite PRN06 at station DAJN. The bottom panel shows waves.that the The elevation amplitude range of the of disturbance satellite PRN02 observed observed by satellite by station PRN02 FKD2 at was station 86.7-87.1°, FKD2 was while slightly the smallerelevation than range that from of satellite satellite PRN06 PRN06 observed at station by DAJN. station The DAJN bottom was panel 83.5-87.8°. shows that Therefore, the elevation the rangeionospheric of satellite disturbances PRN02 observed were mainly by station detected FKD2 by was GPS 86.7–87.1 with higher◦, while elevations the elevation angles range and of with satellite a PRN06distance observed of less bythan station about DAJN400 km was between 83.5–87.8 the ◦SIP. Therefore, and the typhoon the ionospheric eye. In addition, disturbances the background were mainly detectedTEC may by have GPS affected with higher the magnitude elevations anglesof the ionospheric and with a disturbance distance of lesscaused than by about the typhoon. 400 km between the SIP and the typhoon eye. In addition, the background TEC may have affected the magnitude of the ionospheric disturbance caused by the typhoon.

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a FigureFigure 5. 5.The The tracks tracks of of thethe SIPSIP observed by ( satellite) satellite PRN02 PRN02 at stationat station FKD2 FKD2 (a and) and (b satellite) satellite PRN06 PRN06 at stationat station DAJN DAJN (b) duringduring thethe periodperiod 10:00–12:0010:00-12:00 UTC,UTC in the the ﬁltered upper TEC panel, time the series filtered at stationsTEC time FKD2 series for PRN02diagram (c) of and the station two stations DAJN foris shown PRN06 in (d(c),,d and) in the middle change panel, of elevation and the angle change and of distance elevation between angle theand typhoon distance eyes between and the the SIP typhoon at FKD2 eyes (e )and and the DAJN SIP is (f ).shown in (e,f) in the bottom panel.

SevenSeven di differentlyfferently distributeddistributed stations stations were were selected selected to toclearly clearly show show the TEC the TECdisturbances. disturbances. The Thecharacteristics characteristics of ofthe the SIP SIP tracks, tracks, the the occurrence occurrence time time of of the the perturbation perturbation sequence, sequence, and and the the amplitude amplitude ofof the the disturbance disturbance observed observed by by the the satellite satellite PRN06PRN06 werewere allall analyzed. As As shown shown in in Figure Figure 6,6, the the upper upper panelpanel shows shows the the SIP SIP tracks tracks mapmap ofof thethe satellitesatellite PRN06 at seven seven different different stations stations (different (different colors). colors). TheThe red red pentagon pentagon representsrepresents the position of of the the typhoon typhoon eyes eyes in intime. time. The The bottom bottom panel panel presents presents the thefiltered filtered TEC TEC disturbance disturbance sequences sequences of of the the seven seven stations stations as as a a function function of of time. time. It It clearly clearly shows shows significant TEC disturbances at seven stations when the typhoon passed through the eastern sea area significant TEC disturbances at seven stations when the typhoon passed through the eastern sea area of of Taiwan on July 10, 2018. In addition, it is interesting to see that the disturbance amplitude at the Taiwan on 10 July 2018. In addition, it is interesting to see that the disturbance amplitude at the SPAO SPAO station closer to the typhoon eye was smaller, while the JLUT station that is farther from the typhoon eye had a larger TEC disturbance.

Remote Sens. 2020, 12, 746 9 of 16 station closer to the typhoon eye was smaller, while the JLUT station that is farther from the typhoon eyeRemote had Sens. a larger2020, 12 TEC, 746 disturbance. 9 of 16

FigureFigure 6. 6. TheThe trackstracks ofof SIPSIP atat sevenseven selectedselected stationsstations inin thethe upup panelpanel andand ﬁlteredfiltered TECTEC timetime seriesseries changeschanges at at selected selected stations stations in in the the bottom bottom panel panel when when the the typhoon typhoon passed passed through through the easternthe eastern sea area sea ofarea Taiwan of Taiwan on 10 Julyon July 2018. 10, 2018. 3.2. Disturbance Propagation Velocity 3.2. Disturbance Propagation Velocity InIn order order to to further further know know the the propagation propagation velocity velocity of the of traveling the traveling ionospheric ionospheric disturbance disturbance caused by Typhoon Maria, a linear ﬁtting was performed according to the position of where the TEC disturbance caused by Typhoon Maria, a linear fitting was performed according to the position of where the TEC wasdisturbance at its maximum was at during its maximum the 10:00–12:00 during UTC the period 10:00-12:00 on 10 UTC July 2018, period and on the July propagation 10, 2018, speeds and the of propagation speeds of the two disturbances were estimated. Figure 7 shows the time series of the filtered TEC by PRN02 and PRN06 at all stations from 10:00-12:00 UTC, which clearly illustrates the linear relationship between the disturbance propagation time and the distance between the typhoon eyes and the SIP changes. The velocity of ionospheric disturbance was about 118.09m/s from PRN02

Remote Sens. 2020, 12, 746 10 of 16 the two disturbances were estimated. Figure7 shows the time series of the ﬁltered TEC by PRN02 and PRN06 at all stations from 10:00–12:00 UTC, which clearly illustrates the linear relationship between the disturbance propagation time and the distance between the typhoon eyes and the SIP changes. Remote Sens. 2020, 12, 746 10 of 16 The velocity of ionospheric disturbance was about 118.09 m/s from PRN02 and about 186.17 m/s from andPRN06. about The 186.17m/s velocities from of bothPRN06. disturbances The velocities were of in both the disturbances range of the gravity-wavewere in the range propagation of the gravity- speed, waveso the propagation two traveling speed, ionospheric so the two disturbances traveling wereionospheric mainly causeddisturbances by the were gravity mainly waves caused being by excited the gravityby Typhoon waves Maria. being excited by Typhoon Maria.

FigureFigure 7. 7. AA graph graph of offiltered filtered TEC TEC with withthe time. the The time. dashed The blackdashed line black indicates line the indicates fitted perturbation the fitted perturbationpropagation speed, propagation and the speed, colored and bars the indicates colored the bars magnitude indicates of the the VTEC magnitude disturbance. of the VTEC disturbance. Figure8 shows the detailed characteristics of the TEC anomalies detected by PRN06. The top panel showsFigure the SIP8 shows tracks the of all detailed stations characteristics from 10:00–12:00 of UTC.the TEC The anomalies blue and red detected dots indicate by PRN06. the position The top of panelthe typhoon shows the eye SIP during tracks the of typhoon all stations movement, from 10:00-12:00 and the solid UTC. blue The line blue indicates and red the dots SIP indicate tracks. Most the positionSIP tracks of the TEC typhoon were distributed eye during at the the typhoon south of movement, the typhoon and eye the and solid moved blue southeast. line indicates The the middle SIP tracks.panel showsMost SIP the tracks relationship TEC were between distributed the average at the filtered south of TEC the and typhoon the distance eye and between moved the southeast. typhoon Theeyes middle and the panel SIP from shows 10:00–12:00 the relationship UTC. It between can be concluded the average that filtered the maximum TEC and value the distance of average between filtered theTEC typhoon appeared eyes near and to the 10:45 SIP UTC. from Finally, 10:00-12:00 the bottom UTC. It panel can be shows concluded the travel that time-distance the maximum diagram value of of averagePRN06 withfiltered a fitting TEC appeared speed of aboutnear to 186.17 10:45 m UTC./s. Finally, the bottom panel shows the travel time‒ distance diagram of PRN06 with a fitting speed of about 186.17m/s.

Remote Sens. 2020, 12, 746 11 of 16 Remote Sens. 2020, 12, 746 11 of 16

FigureFigure 8. 8.Detailed Detailed features features of of the the ionospheric ionospheric disturbancesdisturbances detected detected by by PRN06 PRN06 when when Typhoon Typhoon Maria Maria passedpassed the the eastern eastern sea sea area area of Taiwanof Taiwan on 10on July July 2018. 10, 2018. The topThe panel top panel shows shows the SIP the tracks SIP tracks of all stationsof all fromstations 10:00–12:00 from 10:00-12:00 UTC. The middleUTC. The panel middle shows panel a graph shows of thea graph average of the ﬁltered average TEC filtered and distance, TEC and and thedistance, bottom paneland the shows bottom a graphpanel ofshows the ﬁltereda graph TECof the time filtered series TEC with time the series time. with the time.

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3.3.3.3. DisturbanceDisturbance AzimuthAzimuth VariationVariation FigureFigure9 shows9 shows the the change change of the of ﬁlteredthe filtered TEC withTEC the with azimuth, the azimuth, while the while color the represents color represents di ﬀerent distancesdifferent distances between thebetween typhoon the typhoon eye and theeye SIP.and Itthe can SIP. be It seen can be that seen the that disturbance the disturbance anomalies anomalies were mainly concentrated from 0.2–0.2 TECU and mainly located at 135–300 in the azimuth, namely were mainly concentrated from− -0.2-0.2TECU and mainly located at 135-300°◦ in the azimuth, namely thethe southwestsouthwest direction direction of of the the typhoon typhoon eye, eye, which which is is consistent consistent with with the the conclusion conclusion drawn drawn in in Figure Figure4. Additionally,4. Additionally, it is it interesting is interesting to know to know that thethat farther the farther the station the station was from was thefrom typhoon the typhoon eye, the eye, larger the thelarger ionospheric the ionospheric disturbance disturbance was, although was, although more observations more observations are needed are to test needed and understand to test and theunderstand processing the in processing the future. in the future.

FigureFigure 9. 9Change. Change of of ﬁltered filtered TEC TEC with with the theazimuth azimuth andand thethe distance between the the typhoon typhoon eye eye and and the SIP. the SIP.

3.4.3.4. DisturbanceDisturbance SpectrumSpectrum CharacteristicsCharacteristics TheThe filteredfiltered TECTEC timetime seriesseries werewere usedused furtherfurther toto analyzeanalyze thethe amplitude,amplitude, period,period, andand velocityvelocity ofof thethe traveling traveling ionospheric ionospheric disturbance disturbance caused caused by the by gravity the gravity wave that wave was that excited was by excited the 2018 by Typhoon the 2018 MariaTyphoon in the Maria time in domain, the time but domain it mainly, but reflected it mainly the reflected amplitude the amplitude changes of changes the TEC of series. the TEC For series. this reason,For this short-time reason, short-time Fourier transform Fourier transform was used towas convert used to disturbance convert disturbance signal from signal the time from domain the time to thedomain frequency to the domain. frequency The domain. spectrum The diagram spectrum of the diagram disturbance of the series disturbance of PRN02 series for stationof PRN02 FKD2 for andstation PRN06 FKD2 for and station PRN06 DAJN for station are shown DAJN in are Figure shown 10. in The Figure filtered 10. VTECThe filtered time series VTEC and time the series change and inthe distance change betweenin distance the between typhoon the eye typhoon and the eye SIP areand shown the SIP in are the shown left panel in the (blue left andpanel green), (blue andand thegreen), spectrograms and the spectrograms of the two station of the disturbance two station sequences disturbance are shown sequences in the are right shown panel. in Thethe attenuationright panel. ofThe the attenuation fourth-order of band-passthe fourth-order Butterworth band-pass filter Butterworth in the passband filter ofin 1–3the mHzpassband was small,of 1-3mHz and twowas obvioussmall, and ionospheric two obvious disturbances ionospheric were disturbances extracted. Itwere can beextracted. clearly observedIt can be fromclearly Figure observed 10 that from at theFigure time 10 when that anat the abnormal time when value an occurred abnormal in thevalue time occurred series, thein the frequency time series, peak the of thefrequency corresponding peak of timethe corresponding on the spectrum time diagram on the increased spectrum significantly, diagram increased and the significantly, corresponding and spectrum the corresponding of the two VTECspectrum disturbance of the time two series VTEC was disturbance about 1.6 mHz. time Generally, series was when about the 1.6 atmospheric mHz. Generally, wave was when coupled the withatmospheric the ionosphere wave was in the coupled gravity-wave with the mode, ionosphere its signal in the frequency gravity-wave was 0.1–2.9 mode, mHz, its signal and thefrequency center frequencywas 0.1-2.9mHz, of the two and disturbances the center in frequency Figure 10 was of the within two the disturbances frequency range in Figure of gravity 10 was waves within [14, 16 the]. Therefore,frequency itrange can beof concludedgravity waves that [14,16] the. gravityTherefore, waves it can excited be concluded by the typhoon that the Mariagravity were waves the excited main causeby the of typhoon the traveling Maria ionospheric were the main disturbances. cause of the traveling ionospheric disturbances.

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Remote Sens. 2020, 12, 746 13 of 16

Figure 10. Spectrum diagrams of the TEC disturbance series of (a,b) the PRN02 satellite for station Figure 10. TEC disturbance time series in blue and the distance between SIPs and Typhoon eye in green forFKD2 station and FKD2 (c,d) andthe PRN06 PRN02 satellite satellite for (a )station and for DAJN. station DAJN and PRN06 satellite (c), and Spectrum Figure 10. Spectrum diagrams of the TEC disturbance series of (a,b) the PRN02 satellite for station diagrams of TEC disturbance time series for station FKD2 and PRN02 satellite (b) and for station DAJN FKD2 and (c,d) the PRN06 satellite for station DAJN. 3.5.( dDiscussion) and PRN06 satellite (d). Solar and geomagnetic activity are the primary factors affecting ionospheric changes [22]. The 3.5.3.5. Discussion Discussion Kp index and Dst index are used to describe the level of geomagnetic activity. The Kp index representsSolarSolar and andthe geomagnetic globalgeomagnetic geomagnetic activity activity disturbance are are the the primary primary index, factors factors with aaﬀ affectingtimeecting resolution ionospheric ionospheric of 3 changesh. changes Generally, [22 [22].]. when The The Kp indexKpthe Kp index and index Dst and indexis less Dst than are index used four, are to the used describe geomagnetic to describe the level activity the of geomagneticlevel is considered of geomagnetic activity. as a quiet activity.The period. Kp index The The KpDst represents index represents the global geomagnetic disturbance index, with a time resolution of 3 h. Generally, when theindicates global geomagnetic the degree of disturbance disturbance index, of the with Earth’s a time surface resolution magnetic of 3field h. Generally, near the equator. when the The Kp time index the Kp index is less than four, the geomagnetic activity is considered as a quiet period. The Dst index isresolution less than four,is 1 h, the and geomagnetic it is generally activity believed is that considered when the as Dst a quietindex period.is higher The than Dst -50, index geomagnetic indicates indicates the degree of disturbance of the Earth’s surface magnetic field near the equator. The time theactivity degree is of calm. disturbance The F10.7 of thecm Earth’ssolar irradiance surface magnetic is another ﬁeld indicator near the of equator.solar activity. The time It is resolution generally is resolution is 1 h, and it is generally believed that when the Dst index is higher than -50, geomagnetic 1 h,considered and it is generally that when believed the F10.7 that index when is less the Dstthan index 100, the is higher level of than solar -50, activity geomagnetic is low. The activity Kp index is calm. activity is calm. The F10.7 cm solar irradiance is another indicator of solar activity. It is generally Theand F10.7 Dst cmindex solar were irradiance downloaded is another from indicatorhttp://isgi.unistra.fr/ of solar activity. and F10.7 It is generally index was considered downloaded that from when consideredhttp://data.cma.cn/data/. that when the Figure F10.7 index11 shows is less the than Kp 100,index, the Dst level index, of solar and activity F10.7 index is low. for The a total Kp index of 20 theand F10.7 Dst indexindex iswere less downloaded than 100, the from level http://isgi.unistra.fr/ of solar activity is and low. F10.7 The index Kp index was anddownloaded Dst index from were downloadeddays before from and http: after//isgi.unistra.fr Typhoon Maria’s/ and F10.7 maximum index was wind downloaded speed. It can from be http: seen//data.cma.cn that solar / anddata /. http://data.cma.cn/data/.geomagnetic activity during Figure the 11 typhoonshows the activity Kp index, were Dst in index, a calm and period, F10.7 index so these for a ionospheric total of 20 Figure 11 shows the Kp index, Dst index, and F10.7 index for a total of 20 days before and after Typhoon daysdisturbances before andwere aftermainly Typhoon caused by Maria’s the typhoon. maximum wind speed. It can be seen that solar and Maria’sgeomagnetic maximum activity wind during speed. the It can typhoon be seen activity that solar were and in geomagnetic a calm period, activity so theseduring ionospheric the typhoon activitydisturbances were in were a calm mainly period, caused so these by the ionospheric typhoon. disturbances were mainly caused by the typhoon.

Figure 11. Cont.

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FigureFigure 11. 11.The The Kp (a index) Kp index, (a), Dst (b index) Dst index (b) and and F10.7 (c) indexF10.7 index (c) from from June June 29 to29 July to July 18 (DOY 18 (DOY 180-200), 180-200), 2018. 2018. 4. Conclusions 4. Conclusions In this paper, the traveling ionospheric disturbance following Severe Typhoon Maria passing throughIn thethis eastern paper, seathe traveling area of Taiwan ionospheric are studied disturbance and analyzed following using Severe dense Typhoon GPS observationsMaria passing in Taiwan.through The the spatial-temporal eastern sea area of ionospheric Taiwan are disturbance studied and characteristics, analyzed using propagation dense GPS observations speed, azimuth in changes,Taiwan. and The spectrum spatial‒temporal analysis wereionospheric investigated. disturbance The results characteristics, show that propagation under the conditions speed, azimuth of calm solarchanges, and geomagnetic and spectrum activity, analysis two were ionospheric investigated. disturbances The results were show clearly that under observed the conditions during 10:00–12:00 of calm UTCsolar on and 10 Julygeomagnetic 2018, in theactivity, southwest two ionospheric sector of the disturbances typhoon eye, were propagating clearly observed to the southeastduring 10:00- with the12:00 periods UTC ofon 10 July and 10, 12 2018, min, in respectively. the southwest The sector amplitude of the typhoon of the firsteye, propagating ionospheric to disturbance the southeast was slightlywith the smaller periods than of that 10 and of the 12 secondmin, respectively. disturbance. The Furthermore, amplitude of the the linear first fittingionospheric of the time-distancedisturbance was slightly smaller than that of the second disturbance. Furthermore, the linear fitting of the time‒ diagram shows that the two disturbances propagate at speeds of 118.09 and 186.17 m/s, respectively, distance diagram shows that the two disturbances propagate at speeds of 118.09 and 186.17m/s, which are in the range of the gravity-wave propagation speed. The analysis of the azimuth change respectively, which are in the range of the gravity-wave propagation speed. The analysis of the of the disturbance confirms the directivity of the ionospheric disturbance propagation. The TEC azimuth change of the disturbance confirms the directivity of the ionospheric disturbance disturbance anomalies were mainly located at 135–300 in the azimuth, namely the southwest side of propagation. The TEC disturbance anomalies were mainly◦ located at 135-300° in the azimuth, namely the typhoon eye. The corresponding frequency spectrum of the two TEC disturbance time series was the southwest side of the typhoon eye. The corresponding frequency spectrum of the two TEC about 1.6mHz, which is consistent with the frequency of gravity waves and further confirms that these disturbance time series was about 1.6mHz, which is consistent with the frequency of gravity waves twoand traveling further ionosphericconfirms that disturbances these two traveling were mainly ionospheric caused disturbances by the gravity were waves mainly excited caused by theby the 2018 Typhoongravity Maria.waves Inexcited the future, by the more 2018 casesTyphoon will beMaria. investigated In the future, to better more understand cases will thebe investigated physical process to andbetter mechanism understand of ionospheric the physical anomalies process and during mechanism typhoons. of ionospheric anomalies during typhoons.

AuthorAuthor Contributions: Contributions:S.J. S.J. and and Y.W. Y.W. conceived conceived and designed and designed the experiments the experiments and Y.W. and performed Y.W. performed the experiments the andexperiments analyzed the and data. analyzed S.J. and the Y.W.data. contributed S.J. and Y.W. to contributed the writing to of the the writing paper. Allof the authors paper. have All authors read and have agreed read to theand published agreed to version the published of the manuscript. version of the manuscript.

Funding:Funding:This This work work was was supported supported byby the the National National Natural Science Foundation Foundation of of China‒German China-German Science Science Foundation (NSFC-DFG) Project (Grant No. 41761134092) and the Jiangsu Province Distinguished Professor Foundation (NSFC‒DFG) Project (Grant No. 41761134092) and the Jiangsu Province Distinguished Professor Project (Grant No. R2018T20). Project (Grant No. R2018T20).

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Acknowledgments: The authors would like to thank the Central Meteorological Bureau (CWB), Taiwan, China for providing GPS data. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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