UTOR. Jiangsu, PR China; Science and Technology, Nanjing, Jiangsu, PR China;

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1 Indian Journal of Geo-Marine Sciences Vol. 45(4), PRIL 16, pp Research on the ionospheric VTEC changes during period of typhoon UTOR Wang Xinzhi 1,* & Yue Dongjie 3 Ke Fuyang 1 1 School of Geography and Remote Sensing, Nanjing University of Information Science and Technology, Nanjing, 144 Jiangsu, PR China; Jiangsu Key Laboratory of Meteorological Observation and Information Processing,Nanjing University of Information Science and Technology, Nanjing, 144 Jiangsu, PR China; 3 School of Earth Sciences and Engineering, Hohai University, Nanjing, 198 Jiangsu, PR China. *[ wangxinzhi@16.com] Received 13 March 15; revised 14 ugust 15 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 Ionosphere is a part of near earth space environment of human existence and is also related to human activities. Researches on ionosphere have been an important topic of space physics and space weather. s an important part of researches on space weather, ionospheric Vertical Total Electron Content (VTEC) changes during the period of typhoon has always been a hot research topic of many scholars at home and abroad. Shen Changshou et al used the observations collected in 1969, 197, 197 and 1973 to study the effects of typhoon on the critical frequency of the F region and found that typhoon could influence the f F ; Xiao Saiguan used the ionospheric HF Doppler shift data during the periods of two strong typhoons which occurred in 1988 and 199 to reveal the detailed processof ionospheric responses to strong disturbances in the lower atmosphere, the results showed typhoon could affect the ionosphere ; The result studied by Liu Yimou showed that f F of the ionosphere would decreas eduring the time of typhoon 3 ; Mao Tian researched the effects of typhoon "Matsa" on ionospheric Total Electron Content (TEC) by the observation data from more than 5 GPS stations and found that the effects could be fully distinguish 4 ; Yu Tao studied the ionospheric changes during the period of 3 typhoons at Xia men in 7, the results indicated that the f F was disturbed, and the Es and spread-f occurring rates increase distinctly during the three typhoons approaching the station 5 ; The study of Chen Hua which used.5 *.5 grid TEC data from 5 observation stations of China region provided by the Earth

2 WNG et al: IONOSPHERIC VTEC CHNGES DURING PERIOD OF TYPHOON UTOR 483 System Science Data Sharing Network showed that TEC appeared disturbed during the typhoon activities 6 ;.S.Polyakova used GPS to study the ionospheric variation of the areas covered by 6 tropical cyclone in Pacific Northwest from September to November in 5, the results indicated that typhoon could make the ionospheric TEC disturbed 7 ; ishop used GPS occultation data to examine the relationship between ionospheric TEC and TCs by investigated more than ten tropical cyclones, the results found that significant scintillation on TEC could be recorded within 1 km of a storm center 8. Coupling between typhoon processes and the ionosphere has also been discussed by many other researchers This paper takes the typhoon UTOR of ugust, 13 as the research object, using the GPS data from IGS stations of pimo and bjfs on 3 rd -1 th ugust to inverse the ionospheric VTEC, references the quarterback method which is commonly used to study the seismic ionospheric disturbances before and after earthquakes, contrasts and analyzes the same satellites data observed at the same time by the two observation stations during the typhoon activity and researches the status of ionospheric VTEC variation during the period of typhoon. Materials and Methods Ionosphere is adispersive medium. When electromagnetic waves travel through the ionosphere, the speed of the waves will be changed and the changed sizes depend on the frequency of electromagnetic waves and on the TEC between the satellite and the GPS receiver. The GPS signals transmitting from satellites to the receivers will pass through the ionosphere, so they must be influenced by the dispersion effect of ionospheric. GPS signals contain two kinds of carrier waves: L1 and L, their frequency are separately f 1 = MHz and f =17.6 MHz. Therefore, we can inverse the ionospheric conditions according to the different effects of GPS signals. In order to improve the precision of inversed GPS VTEC, it needs several key steps such as cycleslip detection and phase smoothing pseudo-range, hardware delay solution and the ionospheric VTEC calculation. Figure1 shows steps of data processing procedure, specific calculation methods are as follows. Spherical harmonics model Single ionospheric model Rinex GPS Data Cycle slip dection Phase smoothing pseudo-range Calculate the DC Ionospheric TEC Ionospheric VTEC Coordinates of GPS Stations SP3 files Coordinates of GPS satellites Coordinates of ionospheric pierce points Fig.1-Flow chart of data processing IONEX files verification Cycleslip of GPS carrier phase observations is an important influence factor of using GPS phase data to inverse ionospheric VTEC. Therefore, cycleslip detection is one of the key problems that must be solved in order to get the high precision ionospheric VTEC. This paper adopts the ionosphere residual error method to detect the cycleslip and then uses non geometric distance phase observation equation and wide lane model simultaneous to repair it 14. Phase smoothing pseudo-range can effectively improve the precision of ranging code pseudo-range and the accuracy of phase pseudo-range. Phase smoothing pseudo-range has good features that its principle is simple, can effectively suppress the effects of multipath noise, does not exist ambiguity and easy to implement. So it is used more and more extensive. This paper uses non-divergence right Hatch filter to smooth the phase pseudo-range 15.

3 484 INDIN J. MR. SCI., VOL. 45, NO. 4 PRIL 16 Differential Code iases (DC) are the inter-frequency hardware biases 16. If the DC effects are ignored, the distance errors observed by the receiver maybe a few meters and the inversed VTEC may even become negative 17, so when modeling the ionosphere it is necessary to estimate them as additional unknowns 18. In this paper, spherical harmonic model and single layer ionosphere model are used to estimate and eliminate the DC effects 19. Moreover, we choose 4 orders of the spherical harmonic model and 45km of the height of single layer ionosphere model. We can calculate the ionospheric TEC using single layer ionospheric model (SLM) and the differences of phase smoothing pseudo-range. Then the ionospheric VTEC can be calculated by TEC through the following formula: TEC mf VTEC (1) In formula (1),mf is the projection function, while height of the single layer ionosphere model is 45km. In Figure, triangle points represent bjfs and pimo stations respectively; the solid lines means motion paths of the satellites observed by the two GPS stations, the satellites are PRN17, PRN4, PRN11, PRN4 and PRN1 respectively from left to right; the dashed line stands motion path of typhoon UTOR which moves from west to east. Date Table1-Information of typhoon UTOR Time (UTC) Longitude ( ) Latitude ( ) Wind speed 13/8/1 18:: /8/11 :: /8/11 6:: /8/11 1:: /8/11 18:: /8/1 :: /8/1 6:: /8/1 1:: o N Results and Discussion UTOR is the No.11 super typhoon generated in southwest sea of Guam on 8 ugust 13, the largest wind power of which is 17 level and the maximum wind speed reaches 15m/s. The wind is so strong that it influences Philippines, Hong Kong, Macao, a large areas of Guang xi, Guang dong and Hai nan province, leads rainstorm, floods, landslides and other disasters to these areas. This paper chooses the IGS stations of pimo in Philippines and bjfs in eijing, China to contrast and analyze the ionospheric variations within the influence areas of the typhoon. Table shows information of typhoon UTOR ( Published by the Weather Underground Organization ); Figure indicates motion paths of the typhoon UTOR and distributions of the pimo and bjfs stations. 4 o N 16 o N 4 o N 3 o N 96 o E 14 o E bjfs pimo 11 o E 1 o E Fig.-Stations, satellite and path of typhoon UTOR Ionosphere process is primarily driven by solar and geomagnetic activities, in relatively calm conditions, the ionosphere usually will not 18 o E

4 WNG et al: IONOSPHERIC VTEC CHNGES DURING PERIOD OF TYPHOON UTOR 485 have too big changes over a period of time. Geomagnetic activities are generally global and almost at the same time. The Disturbance storm time index (Dst index) is always used to describe the magnetic field intensity internationally, geomagnetic storm may occur when Dst index is less than -5. The Kp index is three hours global magnetic index which is used to describe the intensity of geomagnetic disturbance. In order to eliminate the influence of the solar and Dst index kp index geomagnetic activities on the ionosphere, we collect the Dst index (Released by Tokyo Geomagnetic Observatory) and the Kp index (Released by US tmospheric and Oceanic ureau) on ugust 1 st -1 th, 13, which are shown in figure 3. It can be seen from figure 3 that the Dst indexare all higher than -5 and the Kp index are less than or equal to 4, which indicate that the geomagnetic is quiet in the period of typhoon /1 8/ 8/3 8/4 8/5 8/6 8/7 8/8 8/9 8/1 8/11 8/1 Date 4 8/1 8/ 8/3 8/4 8/5 8/6 8/7 8/8 8/9 Date Fig.3-Sketch of dst index and kpindex on ugust 1st-1th, 13 In figure 3, the date is divided by UTC time (the date are all divided by UTC time in the following paper). From table 1, we can see that the wind speed reaches the maximum and respectively are 135m/s, 15m/s and 14m/s at UTC 6::, UTC 8:: and UTC 18:: on 11 th ugust 13. t the same time, motionpath of the typhoon is justin the observation areas of pimo station, so the paper chooses the ionosphere in these three moments to study. We can also see from table 1 that the center of the typhoon is 14.8 E and 15 N at UTC 6::. Table shows the ionospheric pierce points of the satellites received by pimo station at UTC 6:: and the latitude and longitude differences between the ionospheric pierce points and the typhoon center. It can be seen from table that the ionospheric pierce points of PRN1 and 11 satellites are closer to the typhoon center. Therefore, the paper chooses the satellites of PRN1 and 11 to study. Table-Satellites observed by pimostation and the ionospheric pierce points at UTC 6:: (unit: degree) Satellites No Longitude Longitude Difference Latitude Latitude Difference PRN PRN PRN PRN PRN The motion cycle of the GPS satellites is 11 hours and 58 minutes and this means that the satellites will revisit the same areas every

5 486 INDIN J. MR. SCI., VOL. 45, NO. 4 PRIL 16 11hours and 58 minutes. ccording to this characteristic, the paper analyzes the ionospheric time series changes of the passing areas of PRN1 and 11 satellites at UTC 6:: using 1 days data from 3 rd to 1 th ugust 13. The maximum longitude difference between the ionospheric pierce points of satellite PRN1 and the typhoon center is degree during the period, while the maximum latitude difference is degree. The maximum longitude difference between the ionospheric pierce points of satellite PRN11 and the typhoon center is degree and the maximum latitude difference is.939 degree. Figure 4 shows the longitude and latitude differences. This paper processes the data from the same satellites and the same times observed by bjfs station and compares it with the pimo station. ecause bjfs station is moret han 3km away from the pimo station, so the influence of the typhoon in bjfs station can be ignored. ecause the PRN1 satellite is not observed in bjfs station, so only the PRN11 satelliteis analyzed. pimo PRN1 4 pimo PRN Fig.4-Latitude and longitude differences betweenthe ionosphericpierce points and the typhoon center of PRN1and 11 In figure 4, horizontal axis represents the date and vertical axis represents the latitude and longitude differences (unit: degree); * represents satellites observed bypimo station at UTC 6:: the latitude differences, represents the longitude differences. pimo PRN pimo PRN C bjfs PRN11 5 D pimo PRN1/11 bifs PRN Fig.5-VTEC diagram of PRN1and 11satellites observed by pimo station and PRN11 satellite observed by bjfs station at UTC In figure 5, horizontal axis represents the date and vertical axis represents VTEC values (units: TECU); represents VTEC values of 6:: PRN1 satellite observed by pimo station, * represents VTEC values of PRN11 satellite observed by bjfs station and represents

6 WNG et al: IONOSPHERIC VTEC CHNGES DURING PERIOD OF TYPHOON UTOR 487 VTEC values of PRN11 satellite observed by bjfs station; The dotted line represents the upper and lower limit of the VTEC values. Figure 5 shows the VTEC changes of the PRN1 and 11 satellites observed by pimo station and the PRN11 satellite observed by bjfs station at UTC 6:: of 3 rd -1 th ugust 13. In order to better analyze the VTEC disturbances of the ionosphere, the paper also uses the quarterback method (oretical of the quarterback method can be seen from ref. 1) to conduct the VTEC anomaly detection. It can be seen from figure 5 that the VTEC values of PRN1 satellite have a larger increase than before and even exceed the upper limit on 11 th -1 th ugust. It also can be seen from figure 5 that the VTEC values of PRN11 satellite also have a larger increase than before andreach the critical value on 11 th ugust, while the values exceed the upper limit on 1 th ugust. In figure 5C, the VTEC values of PRN11 satellite observed bybjfs station reach the critical value on 4 th ugust, but the pimo PRN4-4 changes should not be affected by the typhoon. In other times, the VTEC values are in the normal range. In figure 5D, the VTEC change curves of PRN1 are in good agreement with PRN11, the values of PRN11 satellite observed by bjfs station are overall smaller compared to that of pimo station. This section chooses satellites of PRN4 and 17 and uses the same method that has been used in 3.3 section to analyze the ionospheric changes at UTC 1::. The maximum longitude difference between the ionospheric pierce points of PRN4 satellite and the typhoon center is degree, while the maximum latitude difference is 4.83 degree. Meanwhile, the maximum longitude difference between the ionosphericpierce points of PRN17 satellite and the typhoon center is degree, the maximum latitude difference is degree. Figure 6 shows the latitude and longitude differences pimo PRN17 Fig.6-Latitude and longitude differences between the ionospheric pierce points and the typhoon center of PRN4 and 17satellites observed bypimo station at UTC 1:: In figure 6, horizontal axis represents the date, vertical axis represents the latitude and longitude differences (unit: degree); * represents the latitude differences, represents the longitude differences.

7 488 INDIN J. MR. SCI., VOL. 45, NO. 4 PRIL pimo PRN4 pimo PRN C bjfs PRN4 35 D bjfs PRN Fig.7-VTEC diagram of PRN4and 17 satellites observed by pimoand bjfs stations at UTC 1:: In figure 7, horizontal axis represents the date and vertical axis representsvtec values (units: TECU); represents VTEC values of PRN4 satellite observed by pimo station and * represents VTEC values of PRN17 satellite observed by the same station; represents VTEC values of PRN4 satellite observed by bjfs station and represents VTEC values of PRN17 satellite observed by the same station; The dotted line represents the upper and lower limit of the VTEC values. Figure 7 shows the VTEC changes of PRN4 and 17 satellites from pimo and bjfs stations at UTC 1::. It can be seen from Figure 7 that the VTEC values of PRN4 satellite from pimo stations have a larger increase than before on 1 th -11 th ugust and exceed the upper limit, while the values return to the normal areas on 1 th ugust. Figure 7 shows that the VTEC values of PRN17 satellite from pimo station also have a larger increase than before on 1 th -11 th ugust and exceed the upper limit, then the values decline sharply and exceed the lower limit. Figure 7C shows that the VTEC values of PRN4 satellite from bjfs station exceed the upper limit on 4 th ugust, reach critical values on 6 th ugust, and exceed theupper and lower limit, while the values are in normal areas in other times. In Figure 7D, the VTEC values of PRN17 satellite observed by bjfs station are in normal areas. In Figure 7 the VTEC values from pimo station have a larger increase than before on 1 th -11 th ugust while the value is normal on 1 th ugust. This is because that the PRN4 observed by pimo satellite is far away from the typhoon route at UTC 6:: 1 th ugust. This section also chooses the satellites of PRN4 and 17 and uses the same method that has been used in 3.3 section to analyze the ionosphere at UTC 18::. The maximum longitude difference between the ionospheric pierce points of PRN4 satellite and the typhoon center is degree, while the maximum latitude difference is degree. Figure 8 shows the latitude and longitude differences. 1-1 pimo PRN4 - Fig.8-Latitude and longitude differences between the ionospheric pierce points and the typhoon center of PRN4 satellite observed by pimo station at UTC 18::

8 WNG et al: IONOSPHERIC VTEC CHNGES DURING PERIOD OF TYPHOON UTOR 489 In figure 8, horizontal axis represents the date, vertical axis represents the latitude and longitude differences (unit: degree); * represents the latitude differences, represents the longitude differences. pimo PRN bjfs PRN4 18 Fig.9-VTEC diagram of PRN4 satellite observed by pimoand bjfs stations atutc 18:: In figure 9, horizontal axis represents the date and vertical axis representsvtec values (units: TECU); represents VTEC values of PRN4 satellite observed by pimo station and * represents VTEC values of PRN17 satellite observed by bjfs station. The dotted line represents the upper and lower limit of the VTEC values. Figure 9 shows the VTEC changes of PRN4 from pimo and bjfs stations at UTC 18::. It can be seen from Figure 9 that the VTEC values of PRN4 satellite from pimo station have a larger increase than before on 11 th -1 th ugust and exceed the upper limit. Figure 9 shows that the VTEC values of PRN4 satellite observed by bjfs station reach upper limit on 6 th ugust, and reach lower limit on 6 th ugust, while the values are in normal areas in other times. In figure 9 the VTEC values from pimo station have a larger increase than before on 11 th -1 th ugust while the value is normal on 1 th ugust. This is also because that the PRN4 observed by pimo satellite is far away from the typhoon route at UTC 18:: 1 th ugust 13. Conclusion This paper uses the GPS data observed by pimo and bjfs stations to inverse the ionospheric VTEC values and also uses the quarterback method to analyze the ionospheric VTEC disturbances at UTC 6::, UTC 1:: and UTC 18:: during the period of typhoon UTOR. The ionospheric VTEC values observed by pimo station increase significantly and reach the upper limit or cause disturbance. The ionospheric VTEC values observed bybjfs station are relatively stable and occasionally reach critical values of the upper or lower limit. The VTEC values observed by bjfs station at UTC 1:: on 4 th ugust shows disturbance, while the time is far away from the typhoon activity and the distance between the station and typhoon path center is large, so the disturbance may have no relationship withthe typhoon. It needs to further analyze the reason which caused the disturbance. ecause this paper takes only a typhoon as an example to analyze the ionospheric VTEC changes, meanwhile the factors that cause the distance are uncertainty as well, so it need further sustained and in-depth studies to research the disturbances of the

9 49 INDIN J. MR. SCI., VOL. 45, NO. 4 PRIL 16 ionosphere with more typhoons cases from different regions and different levels. cknowledgements This research work was supported by the open project of Jiangsu Key Laboratory of Meteorological Observation and Information Processing (KDXS149), the National Natural Science Foundation of China (413436, ), the research project of Surveying Mapping and Geoinformation of Jiangsu Province (JSCHKY158). uthors are grateful to the Tokyo geomagnetic station for providing the Dst magnetic index data and to the merican atmospheric and Oceanic dministration for providing the Kp index data and to the Weather Underground Organization for providing Related information about typhoon UTOR Reference 1 Shen Changshou. The correlations between the typhoon and the f F of ionosphere [J]. Chin. J.Space Sci. (in Chinese), (198) Xiao S G,Hao YQ,Zhang D H, et al. case study on whole response processes of the ionosphere to typhoons [J]. Chinese J.Geophys. (in Chinese), 49(6) Liu Yimou, Wang Jingsong, Xiao Zuo, Suo Yueheng. possible mechanism of typhoon effects on the ionospheric F layer [J]. Chin. J.Space Sci. (in Chinese), 6(6) Mao T, Wang J S, Yang G L, et al. Effects of typhoon Matsa on ionospheric TEC [J]. Chinese Sci ull (in Chinese), 54(9) Yu Tao, Wang Yungang, MaoTian, et al. case study of the variation of ionospheric parameter during typhoon in Xiamen [J]. cta Meteorologica Sinica (in Chinese), 68(1) Chen Ye. Disturbance response analysis of ionospheric TEC account for typhoon event[d]. Nan Jing: Nan Jing University of information science and technology, 1, Master degree thesis. 7.S.Polyakova, N.P. Perevalova. Comparative analysis of TEC disturbance over tropical cyclone zones in the North-West Pacific Ocean [J]. dvance in space research, 5 (13) Luo Li. The theory and practical research of ionosphere affection in GPS surveying [D]. Nan Chang: Jiang Xi University of science and technology, 7, Master degree thesis. 9 ishop, R.L., P.R., Straus. Characterizing Ionospheric Variations in the Vicinity of Hurricanes and Typhoons Using GPS Occultation Measurements [C], GU Fall Meeting, San Francisco, December11 st -15 th, 6. 1 Kazimirovsky, E., Herraiz, M., De La Morena,.. Effects on the ionospheredue to phenomena occurring below it. Effects on the ionosphere due tophenomena occurring below it [J]. Surveys in Geophysics, 4 (3) Sun, L., Wan, W., Ding, F., Mao, T. Gravity wave propagation in the realisticatmosphere based on a three-dimensional transfer function model [J]. nnals of Geophysics, 5(7) Xu, G., Wan, W., She, C., Du, L. The relationship between ionospheric total electron content (TEC) over East sia and the tropospheric circulation around the Qinghai-Tibet Plateau obtained with a partial correlation method [J]. dvances in Space Research, 4 (8) Liu, Y.M., Wang, J.S., Suo, Y.C. Effects of typhoon on the ionosphere [J]. dvanced Geosciences, 9(6) Liu Changjian. Study on modeling method and model quality control of ionosphere based on GNSS [D]. Zheng Zhou: PL information engineering university, 11, Ph.D. thesis. 15 Sardon E, Ruis, Zarraoa N. Estimation of the transmitter and receiver differential biases and the ionospheric total electron content from Global Positioning System observations [J]. Radio Sci., 9(1994) Mannucci,.J., Wilson,., Yuan, D., Linqwister, U., Runge, T. global mapping technique for GPS-derived ionospheric total electron content measurements [J]. Radio Sci., 33(1998)

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