||Study of Medium Scale Traveling Ionospheric Disturbance in Low-Latitude Ionosphere
||Department of Earth Sciences
low latitude ionosphere
medium scale traveling ionospheric disturbance (MSTID)
中尺度電離層移行擾動(Medium Scale Traveling Ionospheric Disturbance, 簡稱 MSTID)為一種常見的電離層擾動現象，具有100 - 500公里的波長以及30-300公尺/秒的速度。MSTID較常在中緯度地區(如日本)被觀測到，利用日本高密度GPS接收站觀測統計MSTID之特徵，發現在夏季夜晚以及冬季全日有著較高的發生機率，並且MSTID在太陽活動極小期之活躍度有著大於太陽活動極大期的特性。此外MSTID在白天與夜晚也有著不同的傳播特性，白天的MSTID波前方向為東西向、傳播方向為南北方向，而在夜晚則有著西北-東南方向的特殊波前並由東北往西南方向的傳播。在過去，因為其擾動波的特性，夜晚的MSTID一般以電離層F層帕金斯電漿不穩定(Perkins Instability)機制解釋其生成，但該機制的不穩定發生率所需時間(數天)遠高於觀測結果(數十分鐘)。近年來，發現電離層散塊E層(Sporadic E, Es)不穩定機制(Es Layer Instability)產生的擾動波前與F層帕金斯電漿不穩定有著相同的排列方向，藉由Es層不穩定機制產生之電場映射至F層，可大幅縮短不穩定發生率所需時間，而與觀測吻合。此外近年來更多的觀測證據顯示MSTID可以傳播至低緯度地區，顯示MSTID相關研究仍有許多可發展空間，特別是低緯度MSTID之研究較無相關文獻，因此本研究將著重於MSTID於低緯度赤道異常地區(台灣地區)之統計分析。為統計三年期間(2013年至2015年)台灣GPS觀測資料，本研究利用互相關(Cross Correlation)及二維快速傅立葉轉換(2DFFT)建立電腦程式自動判斷MSTID傳播之系統，統計結果發現在低緯度地區的春夏季以及冬季全日有著較高的發生機率，其中日間的發生率為夏季較高(72%)，夜間的發生率則為冬季較高(79%)，進一步將MSTID分為南北傳播方向統計發現向北傳播的MSTID常發生在春、夏季，並以夏季日間之發生率較高(57%)，向南傳播的MSTID則通常發生在春、冬季，並以冬季夜間之發生率較高(74%)。統計結果也顯示在太陽活動較小的年份中(2013年及2015年) MSTID事件發生次數多於太陽活動較大的年份(2014年)。
關鍵字：低緯度電離層; 中尺度電離層移行擾動(MSTID); GPS觀測; 自動化判定
Study of Medium Scale Traveling Ionospheric Disturbance in Low-Latitude Ionosphere
Author’s Name: Yu-Ting Chou
Advisor’s Name: Charles C. H. Lin
Department of Earth Sciences, National Cheng Kung University
This thesis statistically studies the medium scale traveling ionosphere disturbance (MSTID) using the network of ~100 GPS receivers in Taiwan. Previous studies showed that MSTID with wavelengths of 100~500 km and phase velocities of 30-300 m/s is commonly observed in the middle latitude ionosphere. The daytime MSTID with wavefronts aligned in east-west direction propagates in north-south directions, while the nighttime MSTID with the wavefronts aligned in the northwest-southeast (NW-SE) direction propagates along northeast-southwest (NE-SW) direction. It is more frequently occurred in nighttime than in daytime and during solar minimum than maximum. To statistically study the MSTID features in the low latitude equatorial ionization anomaly region of ionosphere, an autonomous MSTID detection algorithm is developed for detection of its occurrences and direction of propagation by using both the two-dimensional fast Fourier transform (2D-FFT) and the cross correlation analyses. By applying the autonomous algorithm on the GPS observations around Taiwan (Geographic 20°N-30°N, 115°E-125°E) during 2013-2015, statistical analyses of MSTID is then performed. Results show that, the daytime MSTID appeared most frequently in local summer (72%, average of 2013-2015) and the nighttime MSTID appears more frequently in local winter with appearance rate of 79% (average of 2013-2015). The occurrence rate of northward propagation (57%, average of 2013-2015) is higher than the southward propagation (45%, average of 2013-2015) in daytime during summer, while the occurrence rate of southward propagation (74%, average of 2013-2015) is higher than the northward propagation (48%) in nighttime during winter. The statistical results in this thesis show that the nighttime MSTID in low latitude ionosphere is quite different from that in the middle latitudes, where MSTID is considered to be stronger in summer than in winter. The occurrence rate of MSTID in the low latitude ionosphere may as well be related to the coupled Es layer and Perkins instability that is highly related to the wind shear in the E-region altitude. The wind shear variations derived from the empirical NRL horizontal wind model (HWM) are applied to interpret the seasonal dependences of MSTIDs propagation and occurrence. It suggests that the background wind might play an important role affecting the MSTID occurrence.
Key words: low latitude ionosphere; medium scale traveling ionospheric disturbance (MSTID); GPS observations; autonomous detection
The traveling ionospheric disturbances (TIDs) are known to be triggered by the gravity waves launched by disturbances due to magnetospheric and lower atmospheric forces. Among TIDs, the disturbances with the horizontal wavelength of 100-300 km are classified as the medium scale TIDs (MSTIDs) [c.f. Hunsucker, 1982; Otsuka et al., 2013]. MSTID is commonly observed, especially by GPS total electron conten (TEC) observations, in the middle latitudes and the past statistical studies show that it has high occurrence rate in nighttime in local summer [Tsugawa et al., 2007; Kotake et al., 2007]. Additionally, the recent study reported by Hernández-Pajares et al. (2012) show that the northward MSTIDs appear more frequently than the southward MSTIDs in summer, while the southward MSTIDs appear more frequently than northward MSTIDs in winter.
The generation mechanism of MSTID was previously explained various mechanisms during daytime and nighttime. The daytime MSTIDs with wavefronts aligned in east-west direction propagate in north-south directions and are explained by the gravity wave effects. On the other hand, the nighttime MSTIDs with the wavefronts aligned in the northwest-southeast (NW-SE) direction propagate along northeast-southwest (NE-SW) direction and are explained by coupling of Perkins and Es (sporadic E) layer instabilities, as the Perkins instability has very low growth rate alone. The Es layer instability is mainly produced by the effect of wind shear in ionosphere E region. The wind shear produces the polarization electric fields in the Es and the electric fields are mapped along magnetic field lines to the F region to accelerate the Perkins instability.
Previous studies showed that MSTID is most commonly observed in the middle latitude ionosphere with the wavefronts in the northwest-southeast (NW-SE) direction, wavelengths of 100~500 km, phase velocities of 30-300 m/s and propagation direction of northeast-southwest (NE-SW). Because most of the statistical studies were in the mid-latitudes, but the characteristics of MSTID might be related to the location for various local times and seasons. Especially for the low-latitude ionospheric where the dynamics is much more complex, as there are several disturbance sources from lower atmosphere could affect the low-latitude ionosphere.
This thesis studies the medium scale traveling ionosphere disturbance (MSTID) statistically at the low-latitude equatorial ionization anomaly (EIA) region of ionosphere using the network of ~100 GPS receivers in Taiwan during 2013-2015. To statistically study the MSTID features, an autonomous MSTID detection algorithm is developed for detection of its occurrences and direction of propagation by using both the two-dimensional fast Fourier transform (2D-FFT) and the cross correlation analyses.
We develop an autonomous MSTID detection algorithm to automatically detect the MSTID over Taiwan by using the two-dimension fast Fourier transform (2D-FFT) and cross correlation method. The algorithm is applied on the GPS data around Taiwan (Geographically 20°-30°N, 115°-125°E) to statistically analyze MSTID occurrence and propagation directions in the low latitude ionosphere EIA during 2013-2015. Due to the limited coverage of GPS observation in the zonal direction around Taiwan, the north-south propagation direction is addressed and the east-west direction is omitted.
We apply the high-pass filter on the raw GPS-TEC data to extract data with periods less than 1-hour. The filtered TEC are then interpolated to the fixed grid as function of time (30 s resolution) and latitude (0.25° resoution). As the period of MSTID is normally in between 20-60 min, the maps of TEC perturbations are organized hourly. It is noted that we select continuous data last more than 40 minutes for each latitudes of the map and the data gap less than 1-hour are substituted by zero value. Then, the time-latitude TEC map is further analyzed by using the 2-D FFT and the frequency-wavenumber matrix is obtained. The positive (negative) wavenumber represents northward (southward) propagation of the disturbances. The northward (southward) propagation of MSTID map could be obtained by performing the inverse 2-D FFT on the data of positive (negative) wavenumber. Finally, we perform cross correlation calculation on the each of the northward/southward MSTID maps. The cross correlation is performed for each of the latitudes in respect to its adjacent latitudes (higher and lower neighboring latitudes). The cross correlation is performed in every other latitudes, and totally 18 sets of cross correlation coefficients will be obtained for one MSTID map. If 7 out of 18 sets of cross correlation coefficients has a value larger than 0.55, the MSTID event is then identified.
The abovementioned autonomous detection algorithm is performed for all GPS-TEC data during 2013-2015. There are two statistical definitions are applied in this study:
1. Statistic-1: We count the MSTID events detected by each of GPS satellites and calculate the occurrence rate by dividing the total number of detected MSTID events by the total observable satellites within the 1-hour period.
2. Statistic-2: We count the MSTID events occurred in the specific spatial area (geographic 20-30oN, 115-125oE) during the 1-hour period. The monthly average of occurrence rate is calculated in every 2-hour period.
In general, Statitic-2 is more suitable to identify the statistical results of the MSTID phenomena, and we discuss the results based on Statistic-2.
RESULTS AND DISCUSSIONS
By applying the two-dimension fast Fourier transform (2D-FFT) and cross correlation method to the algorithm on the GPS observations in the equatorial ionization anomaly region around Taiwan (Geographic 20°N-30°N, 115°E-125°E) during 2013-2015, statistical analyses of MSTID is performed. The results show that, MSTID appears more frequently in local winter during nighttime with appearance rate of 79%, and it is most frequently appeared in daytime of summer (72%). The occurrence rate of northward propagation (57%) is higher than the southward propagation (45%) during daytime in summer and the occurrence rate of southward propagation (74%) is higher than the northward propagation (48%) during nighttime in winter. The statistical results of the nighttime MSTID in low-latitude ionosphere is quite different from that in the mid-latitudes, where MSTID is considered to be stronger in summer than in winter. Additionally, the number of MSTID event seems to be related to the solar activity showing the smallest occurrence rate of MSTID on 2014 where the Sun is more active than 2013 and 2015.
Our results also show that the daytime occurrence rate is higher than nighttime during summer, whereas the nighttime occurrence rate is higher than daytime during winter. According to the empirical horizontal wind model-2014 (HWM-14), the northward meridional background wind around ~200-300 km altitude is weaker in summer and becomes stronger in winter. The weaker northward wind in summer may allow more upward propagation of gravity waves and the stronger wind in winter may filter out more gravity waves to prohibit its upward propagation. The background wind filtering of the gravity wave could explain the larger/smaller daytime occurrence of MSTID during summer/winter, as the daytime MSTID is more related to the gravity wave.
The nighttime MSTID movements may be related to the movement of Es layer instability triggered by the E-region wind shear. According to HWM-14, the results show that the wind shear around E-region (100-110 km altitudes) propagates southward/northward with time and its velocity coincides with MSTID speed. It suggests that the movement of MSTID may be related to the movement of the wind shear.
The main findings of the statistics for the MSTID occurrences, direction of propagations and the background wind effects are summarized as follows.
Statistical results of the MSTID occurrence near Taiwan region in 2013-2015:
1. The high occurrence rates (>70%) appear in both daytime and nighttime for spring and summer during 2013-2015.
2. The least occurrence rate (<60%) occurs during autumn (September-November) in 2014 and 2015, expect for 2013 with a high occurrence rate (>80%).
3. The high occurrence rates (>80%) appear in nighttime for winter during 2013-2015 but the high occurrence rates also appear in daytime of 2013.
4. The daytime occurrence rate is higher than nighttime during summer, except July 2015. The nighttime occurrence rate is higher than daytime during winter solstice months, for the entire three years statistics.
5. The number of MSTID event seems to be related to the solar activity showing the smallest occurrence rate of MSTID on 2014 where the Sun is more active than 2013 and 2015.
Statistics of northward and southward propagations:
1. The northward propagation of MSTID occurs more frequently in spring and summer. Especially, the number of northward event decreases substantially in daytime during winter.
2. The southward propagation of MSTID occurs more frequently in spring and winter. Especially, the number of event increases obviously in nighttime during winter.
The background wind effects according to HWM-14 simulations:
1. Gravity wave filtering could explain the daytime results for larger occurrence rate in summer than in winter.
2. The movements of the wind shear around E-region (100-110 km altitudes) that is important to the Es Layer Instability are in agreement with the movement of MSTID.
Extended Abstract III
第1章 緒論 1
1.1 電離層簡介 1
1.1.1 電離層分層 1
1.1.2 電漿漂移 3
1.1.3 赤道噴泉效應 5
1.2 Rayleigh-Taylor Instability 8
1.3 中尺度電離層移行擾動之生成 9
1.4 MSTID在中緯度地區之統計結果 17
1.5 研究動機與目的 20
第2章 觀測與分析方法 21
2.1 GPS觀測原理 21
2.2 濾波器 27
2.3 使用之分析工具 29
2.3.1 Cross Correlation 29
2.3.2 二維快速傅立葉轉換(2D-FFT) 31
2.4 Horizontal Wind Model 33
第3章 資料處理及分析步驟 34
3.1 分析資料步驟及流程圖 34
3.2 GPS資料處理 36
3.3 分析方法之演進 39
3.3.1 互相關(Cross correlation)步驟說明 39
3.3.2 二維快速傅立葉轉換(2DFFT)及互相關(Cross Correlation)步驟 51
第4章 研究結果 66
第5章 結果與討論 88
第6章 結論 108
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