Indian Journal of Geo-Marine Sciences Vol. 45(4), APRIL 2016, pp. 482-491

Research on the ionospheric VTEC changes during period of typhoon

UTOR

1,2* 3 1 Wang Xinzhi & Yue Dongjie Ke Fuyang 1 School of Geography and Remote Sensing, Nanjing University of Information Science and Technology, Nanjing, 210044 Jiangsu, PR China; 2Jiangsu Key Laboratory of Meteorological Observation and Information Processing，Nanjing University of Information Science and Technology, Nanjing, 210044 Jiangsu, PR China; 3School of Earth Sciences and Engineering, Hohai University, Nanjing, 210098 Jiangsu, PR China. *[E-mail:[email protected]]

Received 13 March 2015; revised 14 August 2015

Present article consists the GPS observation data from the IGS stations of pimo and bjfs to invert the VTEC values of the ionosphere, then use the quarterback method to analyze the ionospheric VTEC changes during the period of typhoon UTOR. The results show that: VTEC values of the pimo station increase significantly during the period of UTOR and reach the critical values of upper limit or cause disturbance. VTEC values of the bjfs station change relatively stable and occasionally reach the critical values of the upper and lower limit, but the disturbance should be independent with the typhoon UTOR.

[Key words: typhoon,ionospheric,VTEC,disturbance]

Introduction processof ionospheric responses to strong Ionosphere is a part of near earth space disturbances in the lower atmosphere, the results environment of human existence and is also showed typhoon could affect the ionosphere2; related to human activities. Researches on The result studied by Liu Yimou showed that f0F2 ionosphere have been an important topic of space of the ionosphere would decreas eduring the time physics and space weather. As an important part of typhoon3; Mao Tian researched the effects of of researches on space weather, ionospheric typhoon "Matsa" on ionospheric Total Electron Vertical Total Electron Content (VTEC) changes Content (TEC) by the observation data from during the period of typhoon has always been a more than 50 GPS stations and found that the hot research topic of many scholars at home and effects could be fully distinguish4; Yu Tao abroad. Shen Changshou et al used the studied the ionospheric changes during the observations collected in 1969, 1970, 1972 and period of 3 typhoons at Xia men in 2007, the

1973 to study the effects of typhoon on the results indicated that the f0F2 was disturbed, and critical frequency of the F2 region and found that the Es and spread-F occurring rates increase typhoon could influence the f0F2; Xiao Saiguan distinctly during the three typhoons approaching used the ionospheric HF Doppler shift data the station5; The study of Chen Hua which used during the periods of two strong typhoons which 2.5°*2.5° grid TEC data from 25 observation occurred in 1988 and 1990 to reveal the detailed stations of China region provided by the Earth WANG et al: IONOSPHERIC VTEC CHANGES DURING PERIOD OF TYPHOON UTOR 483

System Science Data Sharing Network showed ionospheric conditions according to the different that TEC appeared disturbed during the typhoon effects of GPS signals. In order to improve the activities6; A.S.Polyakova used GPS to study the precision of inversed GPS VTEC, it needs several key ionospheric variation of the areas covered by 6 steps such as cycleslip detection and phase smoothing tropical cyclone in Pacific Northwest from pseudo-range, hardware delay solution and the September to November in 2005, the results ionospheric VTEC calculation. Figure1 shows steps indicated that typhoon could make the of data processing procedure, specific calculation ionospheric TEC disturbed7; Bishop used GPS methods are as follows. occultation data to examine the relationship Rinex GPS Data SP3 files between ionospheric TEC and TCs by investigated more than ten tropical cyclones, the Coordinates of Coordinates of Cycle slip dection results found that significant scintillation on TEC GPS Stations GPS satellites could be recorded within 1200 km of a storm Phase smoothing Coordinates of pseudo-range ionospheric pierce points center8. Coupling between typhoon processes

 Spherical harmonics model and the ionosphere has also been discussed by Calculate  IONEX files verification  Single ionospheric model the DCB many other researchers9-13. This paper takes the typhoon UTOR of Ionospheric TEC August, 2013 as the research object, using the

GPS data from IGS stations of pimo and bjfs on Ionospheric VTEC 3rd-12th August to inverse the ionospheric VTEC, references the quarterback method which is Fig.1-Flow chart of data processing commonly used to study the seismic ionospheric disturbances before and after earthquakes, Cycleslip of GPS carrier phase observations contrasts and analyzes the same satellites data is an important influence factor of using GPS observed at the same time by the two observation phase data to inverse ionospheric VTEC. stations during the typhoon activity and Therefore , cycleslip detection is one of the key researches the status of ionospheric VTEC problems that must be solved in order to get the variation during the period of typhoon. high precision ionospheric VTEC. This paper adopts the ionosphere residual error method to Materials and Methods detect the cycleslip and then uses non geometric Ionosphere is adispersive medium. When distance phase observation equation and wide electromagnetic waves travel through the ionosphere, lane model simultaneous to repair it14. the speed of the waves will be changed and the Phase smoothing pseudo-range can changed sizes depend on the frequency of effectively improve the precision of ranging code electromagnetic waves and on the TEC between the pseudo-range and the accuracy of phase satellite and the GPS receiver. The GPS signals pseudo-range. Phase smoothing pseudo-range transmitting from satellites to the receivers will pass has good features that its principle is simple, can through the ionosphere, so they must be influenced by effectively suppress the effects of multipath the dispersion effect of ionospheric. GPS signals noise, does not exist ambiguity and easy to contain two kinds of carrier waves: L1 and L2, their implement. So it is used more and more frequency are separately f1=1575.42 MHz and extensive. This paper uses non-divergence right 15 f2=1227.60 MHz. Therefore, we can inverse the Hatch filter to smooth the phase pseudo-range . 484 INDIAN J. MAR. SCI., VOL. 45, NO. 4 APRIL 2016

Differential Code Biases (DCB) are the In Figure 2, triangle points represent bjfs inter-frequency hardware biases16. If the DCB and pimo stations respectively; the solid lines effects are ignored, the distance errors observed means motion paths of the satellites observed by by the receiver maybe a few meters and the the two GPS stations, the satellites are PRN17, inversed VTEC may even become negative17, so PRN04, PRN11, PRN24 and PRN01 respectively when modeling the ionosphere it is necessary to from left to right; the dashed line stands motion estimate them as additional unknowns18. In this path of typhoon UTOR which moves from west paper, spherical harmonic model and single layer to east. ionosphere model are used to estimate and Table1-Information of typhoon UTOR 19 eliminate the DCB effects . Moreover, we Time Longitude Latitude Wind Date choose 4 orders of the spherical harmonic model （UTC） (°) (°) speed

and 450km of the height of single layer 2013/08/10 18:00:00 127.1 14.2 105 ionosphere model. 2013/08/11 00:00:00 125.9 14.4 115 We can calculate the ionospheric TEC using single layer ionospheric model (SLM) and the 2013/08/11 06:00:00 124.8 15.0 135 differences of phase smoothing pseudo-range. 2013/08/11 12:00:00 123.8 15.4 150 Then the ionospheric VTEC can be calculated by 2013/08/11 18:00:00 122.2 16.0 140 TEC through the following formula: 2013/08/12 00:00:00 120.8 16.5 110 TEC mf  VTEC （1） 2013/08/12 06:00:00 119.2 17.1 100

In formula (1)，mf is the projection function, 2013/08/12 12:00:00 117.9 17.7 100 while height of the single layer ionosphere model 48 o is 450km. N

Results and Discussion o 40 N UTOR is the No.11 super typhoon bjfs generated in southwest sea of Guam on 8 August o 2013, the largest wind power of which is 17 level 32 N and the maximum wind speed reaches 150m/s.

The wind is so strong that it influences o 24 N Philippines, Hong Kong, Macao, a large areas of Guang xi, Guang dong and Hai nan province,

o leads rainstorm, floods, landslides and other 16 N disasters to these areas. This paper chooses the pimo IGS stations of pimo in Philippines and bjfs in o o 28 E 96 E o o o 1 Beijing, China to contrast and analyze the 104 E 112 E 120 E ionospheric variations within the influence areas Fig.2-Stations, satellite and path of typhoon UTOR of the typhoon. Table 2 shows information of

typhoon UTOR ( Published by the Weather

Underground Organization ); Figure 2 indicates Ionosphere process is primarily driven by motion paths of the typhoon UTOR and solar and geomagnetic activities, in relatively distributions of the pimo and bjfs stations. calm conditions, the ionosphere usually will not

WANG et al: IONOSPHERIC VTEC CHANGES DURING PERIOD OF TYPHOON UTOR 485

have too big changes over a period of time20. geomagnetic activities on the ionosphere, we Geomagnetic activities are generally global and collect the Dst index (Released by Tokyo almost at the same time. The Disturbance storm Geomagnetic Observatory) and the Kp index time index (Dst index) is always used to describe (Released by USA tmospheric and Oceanic the magnetic field intensity internationally, Bureau) on August 1st-12th, 2013, which are geomagnetic storm may occur when Dst index is shown in figure 3. It can be seen from figure 3 less than -50. The Kp index is three hours global that the Dst indexare all higher than -50 and the magnetic index which is used to describe the Kp index are less than or equal to 4, which intensity of geomagnetic disturbance. In order to indicate that the geomagnetic is quiet in the eliminate the influence of the solar and period of typhoon.

40 A 20 0

st index -20 D -40 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8 8/9 8/10 8/11 8/12 B Date 4

2 p p index k

0 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8 8/9 Date Fig.3-Sketch of dst index and kpindex on August 1st-12th, 2013

In figure 3, the date is divided by UTC time and 11 satellites are closer to the typhoon center. (the date are all divided by UTC time in the Therefore, the paper chooses the satellites of following paper). PRN01 and 11 to study. From table 1, we can see that the wind speed Table2-Satellites observed by pimostation and the ionospheric reaches the maximum and respectively are pierce points at UTC 06:00:00 （unit: degree）

135m/s, 150m/s and 140m/s at UTC 06:00:00, Satellites Longitude Latitude th Longitude Latitude UTC 08:00:00 and UTC 18:00:00 on 11 August No Difference Difference 2013. At the same time, motionpath of the PRN01 121.3912 -3.4088 15.5352 0.5352 typhoon is justin the observation areas of pimo station, so the paper chooses the ionosphere in PRN07 118.7236 -6.0764 14.1565 -0.8435 these three moments to study. We can also see PRN09 116.5172 -8.2828 17.3786 2.3785 from table 1 that the center of the typhoon is PRN11 122.2134 -2.5866 17.7504 2.7504 124.8°E and 15°N at UTC 06:00:00. Table 2 PRN20 123.7079 -1.0921 8.37368 -6.6263 shows the ionospheric pierce points of the satellites received by pimo station at UTC 06:00:00 and the latitude and longitude differences between the ionospheric pierce points The motion cycle of the GPS satellites is 11 and the typhoon center. It can be seen from table hours and 58 minutes and this means that the 2 that the ionospheric pierce points of PRN01 satellites will revisit the same areas every 486 INDIAN J. MAR. SCI., VOL. 45, NO. 4 APRIL 2016

11hours and 58 minutes. According to this 3.3309 degree and the maximum latitude characteristic, the paper analyzes the ionospheric difference is 2.9329 degree. Figure 4 shows the time series changes of the passing areas of longitude and latitude differences. This paper PRN01 and 11 satellites at UTC 06:00:00 using processes the data from the same satellites and 10 days data from 3rd to 12th August 2013. The the same times observed by bjfs station and maximum longitude difference between the compares it with the pimo station. Because bjfs ionospheric pierce points of satellite PRN01 and station is moret han 3000km away from the pimo the typhoon center is -3.3709 degree during the station, so the influence of the typhoon in bjfs period, while the maximum latitude difference is station can be ignored. Because the PRN01 -0.9672 degree. The maximum longitude satellite is not observed in bjfs station, so only difference between the ionospheric pierce points the PRN11 satelliteis analyzed. of satellite PRN11 and the typhoon center is

A pimo PRN01 B pimo PRN11 2 4

0 2

0 -2

-2 -4 -4 8/3 8/5 8/7 8/9 8/11 8/3 8/5 8/7 8/9 8/11 Fig.4-Latitude and longitude differences betweenthe ionosphericpierce points and the typhoon center of PRN01and 11 satellites observed bypimo station at UTC 06:00:00 In figure 4, horizontal axis represents the the latitude differences, ● represents the date and vertical axis represents the latitude and longitude differences. longitude differences (unit: degree); * represents

A pimo PRN01 B pimo PRN11

52 52 50 50 48 48 46

44 46 8/3 8/5 8/7 8/9 8/11 8/3 8/5 8/7 8/9 8/11

D pimo PRN01/11 bifs PRN11 C bjfs PRN11 35 50

30 40

25 30

20 20 8/3 8/5 8/7 8/9 8/11 8/3 8/5 8/7 8/9 8/11 Fig.5-VTEC diagram of PRN01and 11satellites observed by pimo station and PRN11 satellite observed by bjfs station at UTC 06:00:00 In figure 5, horizontal axis represents the PRN01 satellite observed by pimo station, * date and vertical axis represents VTEC values represents VTEC values of PRN11 satellite (units: TECU); ● represents VTEC values of observed by bjfs station and ▲ represents

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VTEC values of PRN11 satellite observed by bjfs changes should not be affected by the typhoon. In station; The dotted line represents the upper and other times, the VTEC values are in the normal lower limit of the VTEC values. range. In figure 5D, the VTEC change curves of Figure 5 shows the VTEC changes of the PRN01 are in good agreement with PRN11, the PRN01 and 11 satellites observed by pimo values of PRN11 satellite observed by bjfs station and the PRN11 satellite observed by bjfs station are overall smaller compared to that of station at UTC 06:00:00 of 3rd-12th August 2013. pimo station. In order to better analyze the VTEC disturbances This section chooses satellites of PRN04 of the ionosphere, the paper also uses the and 17 and uses the same method that has been quarterback method (oretical of the quarterback used in 3.3 section to analyze the ionospheric method can be seen from ref. 12) to conduct the changes at UTC 12:00:00. The maximum VTEC anomaly detection. It can be seen from longitude difference between the ionospheric figure 5A that the VTEC values of PRN01 pierce points of PRN04 satellite and the typhoon satellite have a larger increase than before and center is -1.5016 degree, while the maximum even exceed the upper limit on 11th-12th August. latitude difference is 4.2830 degree. Meanwhile, It also can be seen from figure 5B that the VTEC the maximum longitude difference between the values of PRN11 satellite also have a larger ionosphericpierce points of PRN17 satellite and increase than before andreach the critical value the typhoon center is -0.4452 degree, the on 11th August, while the values exceed the upper maximum latitude difference is -3.8461 degree. limit on 12th August. In figure 5C, the VTEC Figure 6 shows the latitude and longitude values of PRN11 satellite observed bybjfs station differences. reach the critical value on 4th August, but the

A pimo PRN04 B pimo PRN17 2 6

4 0 2 -2 0

-2 -4 -4 8/3 8/5 8/7 8/9 8/11 8/3 8/5 8/7 8/9 8/11 Fig.6-Latitude and longitude differences between the ionospheric pierce points and the typhoon center of PRN04 and 17satellites observed bypimo station at UTC 12:00:00 In figure 6, horizontal axis represents the the latitude differences, ● represents the date, vertical axis represents the latitude and longitude differences. longitude differences (unit: degree); * represents

488 INDIAN J. MAR. SCI., VOL. 45, NO. 4 APRIL 2016

A pimo PRN04 B pimo PRN17 45

40 30

35

30 25

25

20 20 8/3 8/5 8/7 8/9 8/11 8/3 8/5 8/7 8/9 8/11

C bjfs PRN04 D bjfs PRN17 30 35

30

25 25

20

20 15 8/3 8/5 8/7 8/9 8/11 8/3 8/5 8/7 8/9 8/11 Fig.7-VTEC diagram of PRN04and 17 satellites observed by pimoand bjfs stations at UTC 12:00:00 In figure 7, horizontal axis represents the satellite observed by bjfs station are in normal date and vertical axis representsVTEC values areas. (units: TECU); ● represents VTEC values of In Figure 7A the VTEC values from pimo PRN04 satellite observed by pimo station and * station have a larger increase than before on 10th represents VTEC values of PRN17 satellite -11th August while the value is normal on 12th observed by the same station; ▼ represents August. This is because that the PRN04 observed VTEC values of PRN04 satellite observed by by pimo satellite is far away from the typhoon bjfs station and ▲ represents VTEC values of route at UTC 06:00:00 12th August. PRN17 satellite observed by the same station; This section also chooses the satellites of The dotted line represents the upper and lower PRN04 and 17 and uses the same method that has limit of the VTEC values. been used in 3.3 section to analyze the Figure 7 shows the VTEC changes of ionosphere at UTC 18:00:00. The maximum PRN04 and 17 satellites from pimo and bjfs longitude difference between the ionospheric stations at UTC 12:00:00. It can be seen from pierce points of PRN24 satellite and the typhoon Figure 7A that the VTEC values of PRN04 center is -0.3940 degree, while the maximum satellite from pimo stations have a larger increase latitude difference is -0.9403 degree. Figure 8 than before on 10th -11th August and exceed the shows the latitude and longitude differences. upper limit, while the values return to the normal th pimo PRN24 areas on 12 August. Figure 7B shows that the 2 VTEC values of PRN17 satellite from pimo 1 station also have a larger increase than before on 10th -11th August and exceed the upper limit, then 0

the values decline sharply and exceed the lower -1 limit. Figure 7C shows that the VTEC values of -2 PRN04 satellite from bjfs station exceed the 8/3 8/5 8/7 8/9 8/11 upper limit on 4th August, reach critical values on 6th August, and exceed theupper and lower limit, Fig.8-Latitude and longitude differences between the while the values are in normal areas in other ionospheric pierce points and the typhoon center of PRN24 times. In Figure 7D, the VTEC values of PRN17 satellite observed by pimo station at UTC 18:00:00

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In figure 8, horizontal axis represents the the latitude differences, ● represents the date, vertical axis represents the latitude and longitude differences. longitude differences (unit: degree); * represents

A pimo PRN24 B bjfs PRN24 22 16 21 14 20 12 19 10 18 8/3 8/5 8/7 8/9 8/11 8/3 8/5 8/7 8/9 8/11

Fig.9-VTEC diagram of PRN24 satellite observed by pimoand bjfs stations atUTC 18:00:00

In figure 9, horizontal axis represents the date and vertical axis representsVTEC values Conclusion (units: TECU); ● represents VTEC values of This paper uses the GPS data observed by PRN24 satellite observed by pimo station and * pimo and bjfs stations to inverse the ionospheric represents VTEC values of PRN17 satellite VTEC values and also uses the quarterback observed by bjfs station. The dotted line method to analyze the ionospheric VTEC represents the upper and lower limit of the VTEC disturbances at UTC 06:00:00, UTC 12:00:00 values. Figure 9 shows the VTEC changes of and UTC 18:00:00 during the period of typhoon PRN24 from pimo and bjfs stations at UTC UTOR. The ionospheric VTEC values observed 18:00:00. It can be seen from Figure 9A that the by pimo station increase significantly and reach VTEC values of PRN24 satellite from pimo the upper limit or cause disturbance. The station have a larger increase than before on ionospheric VTEC values observed bybjfs 11th-12th August and exceed the upper limit. station are relatively stable and occasionally Figure 9B shows that the VTEC values of reach critical values of the upper or lower limit. PRN24 satellite observed by bjfs station reach The VTEC values observed by bjfs station at upper limit on 6th August, and reach lower limit UTC 12:00:00 on 4th August shows disturbance, on 6th August, while the values are in normal while the time is far away from the typhoon areas in other times. activity and the distance between the station and In figure 9A the VTEC values from pimo typhoon path center is large, so the disturbance station have a larger increase than before on 11th may have no relationship withthe typhoon. It -12th August while the value is normal on 10th needs to further analyze the reason which caused August. This is also because that the PRN24 the disturbance. Because this paper takes only a observed by pimo satellite is far away from the typhoon as an example to analyze the typhoon route at UTC 18:00:00 10th August ionospheric VTEC changes, meanwhile the 2013. factors that cause the distance are uncertainty as well, so it need further sustained and in-depth studies to research the disturbances of the 490 INDIAN J. MAR. SCI., VOL. 45, NO. 4 APRIL 2016

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