3-4-2 Relation between Equatorial Ionospheric Scintillations and Atmospheric Waves from Below - PDF Document

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  1. 3-4-2 Relation between Equatorial Ionospheric Scintillations and Atmospheric Waves from Below OGAWA Tadahiko This paper reviews some results from equatorial GPS ionospheric scintillation observations that have been conducted at Kototabang, Indonesia since January 2003, and briefly discusses relations between scintillation and atmospheric waves from below. GPS scintillations caused by plasma bubbles appeared between 2000 and 0100 LT mainly in equinoctial months, and their activity decreased with decreasing solar activity. The scintillation activity can be related to tro- pospheric disturbances over the Indian Ocean to the west of Kototabang. Scintillation index fluc- tuates with periods of planetary waves, and similar periods are also found in Earth’s brightness temperature variations. Numerical simulations to know behavior of neutral wind in the equatorial thermosphere indicate that planetary waves dissipate rapidly above about 125 km, and short- period atmospheric waves become predominant above 100 km. It is suggested that these atmospheric waves can contribute to the generation of plasma bubbles causing the scintillations. Keywords Equatorial ionospheric disturbance, Ionospheric scintillation, Plasma bubble, GPS, Atmos- pheric wave 1 Preface having periods longer than two days were found to modulate the equatorial mesosphere and thermosphere [10] findings, in addition to the physical quantities (e.g., electric field, conductivity, neutral wind) peculiar to the ionosphere and thermosphere, the roles of AGW and PW in the electrody- namic process of equatorial ionospheric dis- turbances cannot be ignored [12] . Within the bubbles and in the surrounding area are electron density irregularities with various spatial scales. The radio waves of geo- stationary and orbiting satellites passing through these irregularities consequently undergo scintillation. Aboveground scintilla- tion observation conducted using the radio waves of numerous GPS satellites enables the continuous monitoring of bubbles regardless of the weather, and helps to further elucidate the bubble phenomenon. Plasma bubbles—disturbances peculiar to the equatorial ionosphere during the night- time—are generated through the Rayleigh- Taylor plasma instability. The well-known basic physical process and characteristics of bubble generation alone do not allow us to fully understand the electrodynamic process regarding bubbles and equator spread F (see [1] ). The possibility of atmospheric gravi- ty waves (hereafter AGW) propagating through the equatorial thermosphere for the generation of bubbles and spread F has already been pointed out (see [2] – [7] ). Ogawa et al. [8] electron density in the equatorial anomaly (containing bubbles inside) with an east-to- west scale of several hundred to 1,000 km. Moreover, planetary waves (hereafter PW) [11] . And given these [9]discovered a wavy structure of the 433 OGAWA Tadahiko

  2. This paper analyzes the GPS scintillation data accumulated over long periods and gives an overview of the reasons for the daily changes in bubble activity and the causes of initial weak plasma disturbances that develop into bubbles. Note that the contents presented here have already been published in the form of papers in several journals [8] details, please refer to those documents. to determine the drift velocity and direction of electron density irregularities (with a spatial scale of about 350 m) in the ionospheric F layer, that is, the drift velocity and direction of the plasma bubbles that cause GPS scintilla- tions [14] . As an example, the bottom of Fig. 2 shows the temporal changes in GPS signal intensity received on April 1, 2003. The figure at the top shows a horizontal two-dimensional distri- bution of airglow photographed at 22:35 LT with a 630-nm all-sky camera; the several dark regions in thin long patterns extending south to north are plasma bubbles. At 22:15 to 22:40 LT, the signal intensity weakens and there are intense temporal changes in the intensity called scintillations. These scintilla- tions occur when the radio wave propagation path between the GPS and ground passes through the plasma bubbles and proximity thereof having electron density irregularities. The bubble (marked with the white arrow) that developed as far as right above Kototabang caused the most intense scintillation at around 22:35 LT. One indicator often used to represent scin- [13] . For [9] 2 Observation results of GPS scin- tillation 2.1 How to observe GPS scintillation The observation of ionospheric scintilla- tion of radio waves (1.5754 GHz) emitted by GPS satellites orbiting at an altitude of about 20,000 km began in the latter half of January 2003 at Kototabang (latitude 0.20° S, longitude 100.32° E; geomagnetic latitude 10.36° S) located near the geographic equator in West Sumatra, Indonesia [9] [13] the location of Kototabang. Three GPS receivers separated by a distance of about 130 m are arranged in a triangular pattern, with the signal intensity of each receiver being record- ed at a sampling frequency of 20 Hz. A corre- lation analysis of the temporal changes in sig- nals from the three receivers makes it possible [14] . Figure 1 shows Fig.1 Location (★) of Kototabang and the field of view for GPS scintillation observation (Satellite elevation angle ≥ 30° in a circle hav- ing a radius of 520 km centered at an altitude of 300 km right above Kototabang). Fig.2 GPS scintillations caused by plasma bubbles on April 1, 2003 434 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

  3. tillation intensity is an index called S4. It is expressed as S42= (<I2> – <I>2/ <I>2). Here, I denotes the signal intensity and the ensemble average is shown in angle brackets. S4 normal- ly takes fractional values between 0 and 1, and rises with scintillation intensity. Here, the S4 index used was calculated every 10 minutes from the receiver signal of one of the three units. At a low satellite elevation angle, the signal intensity changes due to ionospheric scintillation as well as other causes. Therefore, data obtained at elevation angles exceeding 30° were used. This made it possible to observe scintillations generated on the radio wave propagation path that passes within a circle having a radius of 520 km centered at an altitude of 300 km above Kototabang (Fig. 1). This circle covers geomagnetic latitudes 4° S to 13° S. Plasma bubbles occur in the F layer on the magnetic equator and propagate east- ward as the bubbles gradually develop at high- er altitudes. As evident from Fig. 1, when there is a low upper-limit altitude of bubbles on the magnetic equator, the region is beyond the field of view and bubbles in that region cannot be detected. rectly reflect changes in bubble activity over the equator.) (2) Scintillation activity is high from March to April and from September to October, and occurs from 20:00 to 01:00 LT (13:00 to 18:00 UT). (3) The occurrence of scintillations changes irregularly according to the day, but is more frequent (and with higher S4 values) in March to April than in September to October. The characteristics above agree well with those of plasma bubbles near longitude 100° E as observed by satellite (see [15] ). Plasma bub- bles photographed by an all-sky camera or similar optical equipment are known to exist from after sunset until more than a few hours after midnight. The reason why scintillation only occurs until around midnight is presum- ably because electron density irregularities of the 350 nm scale disappear around midnight. The results of Fig. 3 strongly suggest that the characteristics of bubble activity from after 2.2 Long-term changes and periods in scintillation generation Figure 3 shows the temporal and daily changes in the S4 index observed during a period of about six and a half years from the latter half of January 2003 to mid-June 2009. The vertically long black portion offers no data due to trouble with the receiving system and elsewhere. Moreover, we ignored low S4 values of about 0.2 and lower that may be due to radio wave interference, receiver noise, and other causes. From Fig. 3, we point out the following: (1) Scintillation activity declined each year after 2003, when solar activity was declining, and remained almost still after 2007. (As described above, note that scintillations caused by bubbles other than those developing at high altitudes cannot be observed in Kotota- bang. Therefore, Fig. 3 may not cor- Fig.3 Temporal and daily changes in the S4 index from late January 2003 to mid-June 2009 No measurements were made for the black region. 435 OGAWA Tadahiko

  4. sunset to midnight over Kototabang can be clarified from GPS scintillation observation. As described above, scintillations (there- fore, plasma bubbles) occur irregularly according to the day. In order to examine whether scintillation occurs quite irregularly or with some regularity, long-term S4 data are subjected to wavelet analysis. Figure 4 shows, for 920 days from January 1, 2003 to July 10, 2005, (a) temporal and daily changes in S4, (b) daily changes in S4 deviation (calculated by averaging S4 obtained from 18:00 to 02:00 LT on each day during 920 days, and subtracting this from the average between 18:00 and 02:00 LT on each day), and (c) the results obtained by subjecting these changes in devia- tion to wavelet analysis. Figure 4 (b) is marked with the date of storm commencement (SC). Although several geomagnetic storms may have induced scintillations, no clear cor- relation is seen between scintillations and the onset of geomagnetic storms in the spring and autumn. From Fig. 4 (c), although several spectral peaks are seen between periods of 2 to 30 days in the spring and autumn, these peaks are presumably due to planetary-scale atmospheric waves (PW) and solar activity. Fig.4 (a) Temporal and daily changes in S4 from January 1, 2003 to July 10, 2005 (b) Daily changes in S4 deviation (obtained by averaging S4 from 18:00 to 02:00 LT during the same period, and subtracting this from the average of 18:00 to 02:00 LT on each day) (c) Wavelet analysis of S4 deviation. The thick-line curve represents a 90% confidence level. The cross-hatched portion is unreliable [13] 436 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

  5. 3 Relation between GPS scintilla- tion and tropospheric convec- tion Figure 5 compares the daily changes in S4 (shown in Fig. 4) and Tbb during the period from March 1 to April 30, 2003. In the Tbb plot in the left figure, the x-axis is east longi- tude, with Kototabang at 100.32° E. For Tbb, the average between latitude 2° N to 2° S and that between 00:00 and 24:00 UT are plotted. From this figure, for example, from day 80 to day 90, a low-temperature region at not more than 250 K can be seen moving eastward over the Indian Ocean. Unlike the situation over the Indian Ocean, one reason for the irregular temperature distribution near Kototabang is presumably the effect of high mountains on the island of Sumatra. Comparing the left fig- ure with the right figure shows that scintilla- tion also occurs not only when Tbb is low over the Indian Ocean (with high cloud top) but also when it is high (with low cloud top), thereby suggesting that some kind of tropos- pheric disturbance contributes to the genera- tion of scintillation. For a detailed discussion of the correlation between Tbb and S4 over the The preceding chapter suggested that upward-propagating PW generated from tro- pospheric disturbance are related to the gener- ation of equatorial GPS scintillation. This chapter examines this possibility in more detail. Tropospheric disturbance on the equa- tor is caused by temporal and spatial changes in rain, clouds, atmospheric pressure, convec- tion, and other factors. The parameter used here to represent the degree of disturbance is the cloud-top temperature (Tbb) observed by a geostationary meteorological satellite. In gen- eral, as tropospheric convection becomes active (or inactive) for various reasons, the cloud top rises (or lowers), and Tbb lowers (or rises). As tropospheric convection becomes active, AGW and PW having various periods are presumably intensely excited, and then propagate upwards through the stratosphere. Fig.5 (Left) Longitudinal and daily changes in Tbb averaged between 2°N and 2°S, and between 00:00 and 24:00 UT from March 1 (day 60) to April 30 (day 120), 2003 (Right) Temporal and daily changes in the S4 index during the same period. The white-line curve represents daily changes in S4 averaged between 18:00 and 02:00 LT. [13] 437 OGAWA Tadahiko

  6. Indian Ocean, refer to Ogawa et al. [9] Long-term changes in the S4 wavelet spec- trum were shown in Fig. 4. Here we note the period from February to May 2003 and com- pare the wavelet spectrum of S4 and Tbb. The top row in Fig. 6 shows the S4 spectrum dur- ing this period and the bottom row indicates the Tbb spectra at four points (80° , 85° , 90° and 95° E) over the equator (0° N). The Tbb spectra at the different points show several peaks having periods of 3 to 16 days, and cor- responding peaks are also seen in the S4 spec- trum. Figure 7 shows the spectral intensity of S4 and Tbb obtained from Fig. 6. The Tbb peaks are near the periods of 5, 7, and 14 days, as are the S4 peaks. However, the peaks near the periods of 2 to 3 days and 25 days seen in S4 are not present in Tbb. As a result, Figs. 6 and 7 suggest that PW from the troposphere hav- ing periods longer than 5 days contribute to the generation of scintillations. The periods of PW called normal mode Rossby waves are 2, 5, 10, and 16 days [16] , and several periods indicated above are close to these periods. [13] . Fig.6 Wavelet analyses of S4 deviation [see Fig. 4 (c)] and Tbb at four points on the equator The thick-line curve represents a 90% confidence level. The cross-hatched portion is unreliable. Fig.7 Periodgram (period versus spectral density) of S4 deviation and Tbb obtained from Fig. 6 The smooth curve represents a 95% confidence level. 438 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

  7. 4 Numerical simulation of atmos- pheric waves conduct numerical simulations intended to study the behavior of equatorial atmospheric waves, thereby examining what kind of waves can exist in the thermosphere. For details of the KUGCM, refer to other documents (see [17] Because east-west neutral winds are important for the generation of bubbles (see [19] ), only the east-west winds are examined here. Figure 8 (a) shows the frequency spectra of east-west winds at altitudes of 90, 100, 150, and 200 km as calculated from simulation data for one year and obtained at certain points near the equator (0.4° N, 100° E), and Fig. 8 (b) shows those obtained during the last 10 days of March; Fig. 8 (a) plots spectra with The preceding chapter (based on an analy- sis of S4 and Tbb data) indicated the possibility of long-period atmospheric waves from the bottom layer controlling the generation of scintillations (plasma bubbles). However, it is very difficult to prove from scintillation obser- vations and other observation methods of the ionosphere and thermosphere alone that these atmospheric waves exist at thermospheric and ionospheric altitudes. Here, the Kyushu Uni- versity General Circulation Model (KUGCM) developed by Kyushu University was used to [18] ). Fig.8 Frequency spectra of east-west winds near Kototabang as obtained from numerical simu- lation data throughout the year and during the last ten days of March [13] (a) Periods of 1 to 100 days, (b) periods of 0.5 to 12 hours. Given the very high intensity, the components for one day and half a day are omitted. 439 OGAWA Tadahiko

  8. periods of 1 to 100 days and Fig. 8 (b) plots those with periods of 0.5 to 12 hours. As is evident from Fig. 8 (a), waves having a period of about one day or more attenuate quickly at altitudes above 100 km, and do not exist at 150 km or higher. Conversely, the amplitudes of waves having periods of 0.5 to 3 hours (Fig. 8 (b)) increases with altitude, and the waves can propagate up to 400 km as described below. Based on the simulation data for one year obtained at 2.8° N and longitude 0° to 360° , Fig. 9 shows how the amplitudes of Fig.9 Power spectra of atmospheric waves at 2.8°N having periods longer than 2 days as obtained from numerical simulation data spanning one year [13] The peaks near zero frequency are due to the seasonal and yearly changes in waves. 440 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

  9. westward-propagating PW (negative frequen- cy) having periods longer than 2 days and those of eastward-propagating Kelvin waves (positive frequency) attenuate with altitude. As described above, waves having periods of 2 to 20 days attenuate at 125 km or higher, but several wave components exist at up to 125 km. Particularly prominent are Kelvin waves having an east-west wave number (K) of 1 to 3 and a period of 2 days, and PW having a K of 1 and a period of 6 days. Other waves pre- sent include PW having a K of 2 and a period of 4 days, PW having a K of 1 and periods of 5, 10, and 16 days, and PW having a K of 3 to 4 and a period of 2 to 2.5 days. Figure 7 shows the waves having periods of 2.5, 5, 8, 14, and 25 days. These periods are partially consistent with the simulation results in Fig. 9. More- over, it is also pointed out that PW having a periods longer than 2 days modulate the equa- torial mesosphere and thermosphere [10] We note that the simulation data at alti- tudes of 150 to 350 km in March reveal that east-west winds at altitudes above 200 km over Kototabang are eastward (with a maxi- mum wind velocity of about 80 m/s around [11] . Fig.10 Cross section of longitudes and altitudes of east-west winds (with periods of 1 to 4 hours) every 40 minutes as obtained from numerical simulation data on March 21 [13] The solid lines (broken lines) are eastward. The vertical broken line indicates the longitude of Kototabang. 441 OGAWA Tadahiko

  10. midnight) during the nighttime from sunset to sunrise due to diurnal tide, and westward at 80 m/s maximum during the daytime [see 13 in detail]. The eastward wind at around sunset plays an important role in increasing the east- ward electric field in the equatorial F layer, which triggers the generation of plasma bub- bles [19] . In Fig. 8 (b), it was pointed out that the higher the altitude, the more important the short-period AGW with periods of 0.5 to 12 hours. In particular, AGW with periods of 1 to 4 hours are used to examine east-west winds that appear in the sky near Kototabang (0.4° N). Figure 10 shows the temporal changes every 40 minutes in east-west winds at 10:40 to 16:00 UT (17:40 to 23:00 LT at Kototabang) as based on simulation data from March 21. Each figure plots the contours of wind velocity by using the east longitude and altitude coordinates, with the solid lines repre- senting eastward winds and the broken lines westward winds. At altitudes of 120 to 300 km, the regions of eastward and westward winds are both 300 to 1,000 km in the longitu- dinal direction, and have scales of 30 to 100 km in the altitudinal direction. Moreover, both regions generally move eastward at about 100 Fig.11 (a) Distribution of 135.6 nm airglow observed by the IMAGE satellite, and (b) geomag- netic-conjugate plasma bubbles photographed by an all-sky camera at Shigaraki, Japan, and by one in (c) Darwin, Australia [9] The circle in white line represents the field of view of the all-sky cameras at both Shigaraki and Darwin. 442 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

  11. m/s while changing shape, and almost agree with temporal changes in east-west winds in the sky above Kototabang as mentioned above. The wind velocity in each region also changes with time and reaches a maximum of about 100 m/s. The wave structure at altitudes of 120 km or higher (as shown in Fig. 10) is mainly due to ion drag and molecular viscosi- ty, which also affect the upward propagation of PW in the thermosphere. Figure 11 shows multiple geomagnetic- conjugate plasma bubbles, separated east-west between each other by 200 to 250 km, appear- ing within corrugated structures of F layer electron density. The structures have east-west scales of several hundred to 1,000 km and move eastward at about 100 m/s [8] supports the simulation results shown in Fig. 10. As described at the beginning of this paper, many researchers have pointed out that AGW propagating through the equatorial ther- mosphere are related to the generation of bub- bles and to wavy structure at the bottom of the F layer (see [2] – [7] , [20] ). (2) Scintillation occurs mainly between 20:00 and 01:00 LT, but its possible occurrence changes day by day. As a result of wavelet analysis of the long- term S4 index, the generation of scintil- lation is found to be related to PW propagating from the bottom layer and having periods longer than 2 days. This fact is also supported by wavelet analy- sis of Tbb, an index of equatorial tro- pospheric disturbance. (3) As a result of numerical simulations using the KUGCM to determine the characteristics of atmospheric waves in the thermosphere near the equator, it is found that long-period PW and Kelvin waves do not propagate to an altitude higher than 120 km, but short-period AGW exist even at altitudes higher than 120 km. AGW form a structure of east-west winds having a scale of 100 to 1,000 km and moving eastward at about 100 m/s. The behavior of such east-west winds is presumably closely related to the generation of bubbles around sunset and their eastward movement after generation. (4) As described above, PW and Kelvin waves cannot propagate to altitudes where bubbles are present. Supposing that electrical fields generated at alti- tudes of the E layer by these atmos- pheric waves are transported to alti- tudes of the F layer along the geomag- netic field line, the effect of PW and Kelvin waves appears in scintillation activity (bubbles). As such, it is considered beyond doubt that AGW and PW propagating from the bottom layer are related to the electrodynamic process of the equatorial ionosphere, but more research is needed regarding the detailed inter- action process between neutral atmospheric waves and plasma, and for elucidating the triggering process in bubble generation by AGW, along with other issues. Moreover, it will become important in the future to conduct simulations that fuse the neutral atmosphere [9] , which 5 Summary This paper briefly discussed equatorial ionospheric disturbances based on long-term observation of the GPS scintillation phenome- non and numerical simulation. As described at the beginning, most of the contents covered here have already been reported in journals, and the text given here is a simple review of those papers. The main results are as follows: (1) The generation of scintillation above Kototabang is closely related to the presence of equatorial plasma bubbles, and scintillation observation is an effective means of elucidating the bub- ble phenomenon. Scintillation activity since 2003 has been declining with a decline in solar activity. Such activity is asymmetric between the spring and autumn. These characteristics agree with the characteristics of plasma bub- bles observed, for example, by satel- lites. 443 OGAWA Tadahiko

  12. with the plasma process. Last but not least, the author thanks Dr. Yuichi Otsuka (Solar-Terrestrial Environment Laboratory, Nagoya University) for providing the latest data about GPS scintillation, and Dr. Yasunobu Miyoshi (Department of Earth and Planetary Sciences, Kyushu University) for offering simulation data. The author wishes to acknowledge both persons for their valuable assistance. References 01 M. A. Abdu, "Outstanding Problems in the Equatorial Ionosphere-Thermosphere Electrodynamics Rele- vant to Spread F," Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 63, pp. 869–884, 2001. 02 J. Röttger, "Travelling Disturbances in the Equatorial Ionosphere and Their Association with Penetrative Cumulus Convection," Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 39, pp. 987–998, 1977. 03 J. Rötger, "Equatorial Spread-F by Electric Fields and Atmospheric Gravity Waves Generated by Thun- derstorms," Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 43, pp. 453–462, 1981. 04 M. C. Kelley, M. F. Larsen, C. LaHoz, and J. P. McClure, "Gravity Wave Initiation of Equatorial Spread F: A Case Study," Journal of Geophysical Research, Vol. 86, pp. 9087–9100, 1981. 05 S. Singh, F. S. Johnson, and R. A. Power, "Gravity Wave Seeding of Equatorial Plasma Bubbles," Journal of Geophysical Research, Vol. 112, pp. 7399–7410, 1997. 06 C. S. Lin, T. J. Immel, H. C. Yeh, S. B. Mende, and J. L. Burch, "Simultaneous Observations of Equatorial Plasma Depletion by IMAGE and ROCSAT- 1 Satellites," Journal of Geophysical Research, Vol. 110, A06304, doi: 10.1029/2004JA010774, 2005. 07 R. T. Tsunoda, "On the Enigma of Day - to - Day Variability in the Equatorial Spread F," Geophysical Research Letters, Vol. 32, L08103, doi: 10.1029/2005GL022512, 2005. 08 T. Ogawa, E. Sagawa, Y. Otsuka, K. Shiokawa, T. J. Immel, S. B. Mende, and P. Wilkinson, "Simultane- ous Ground- and Satellite-Based Airglow Observations of Geomagnetic Conjugate Plasma Bubbles in the Equatorial Anomaly," Earth Planets Space, Vol. 57, pp. 385–392, 2005. 09 T. Ogawa, T., Y. Otsuka, K. Shiokawa, A. Saito, and M. Nishioka, "Ionospheric Disturbances Over Indonesia and Their Possible Association With Atmospheric Gravity Waves From the Troposphere," Journal of the Meteorological Society of Japan, Vol. 84A, pp. 327–342, 2006. 10 H. Takahashi, L. M. Lima, C. W. Wrasse, M. A. Abdu, I. S. Batista, D. Gobbi, R. A. Buriti, and P. P. Batista, "Evidence on 2-4 Day Oscillations of the Equatorial Ionosphere h'F and Mesospheric Airglow Emissions," Geophysical Research Letters, Vol. 32, L12102, doi: 10.1029/2004GL022318, 2005. 11 M. A. Abdu, P. P. Batista, I. S. Batista, C. G. M. Brum, A. J. Carrasco, and B. W. Reinisch, "Planetary Wave Oscillations in Mesospheric Winds, Equatorial Evening Prereversal Electric Field and Spread F," Geophysical Research Letters, Vol.33, L07107, doi: 10.1029/2005GL024837, 2006. 12 J. Laˇ stoviˇ cka, "Forcing of the Ionosphere by Waves From Below," Journal of Atmospheric and Solar-Ter- restrial Physics, Vol. 68, pp. 479–497, 2006. 13 T. Ogawa, T., Y. Miyoshi, Y. Otsuka, T. Nakamura, and K. Shiokawa, "Equatorial GPS Ionospheric Scintil- lations Over Kototabang, Indonesia and Their Relation to Atmospheric Waves From Below," Earth Plan- ets Space, Vol. 61, pp. 397–410, 2009. 14 Y. Otsuka, Y., K. Shiokawa, and T. Ogawa, "Equatorial Ionospheric Scintillations and Zonal Irregularity Drifts Observed With Closely -Spaced GPS Receivers in Indonesia," Journal of the Meteorological Soci- ety of Japan, Vol. 84A, pp. 343–351, 2006. 15 L. C. Gentile, W. J. Burke, and F. J. Rich, "A Global Climatology for Equatorial Plasma Bubbles in the Topside Ionosphere," Annales Geophysicae, Vol. 24, pp. 163–172, 2006. 444 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

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