| 摘要: | 電離層赤道異常(EIA)是低緯度電離層中最顯著的結構，其峰值分佈在±15°N磁緯度的兩團高濃度帶電粒子。形成原因來自磁赤道的北向水平磁場與大氣潮汐中性風產生之東向電場，並令帶電粒子產生E×B向上漂移，位於較高高度的帶電粒子再受到重力與壓力梯度力延著磁場線傳輸，並堆積在磁赤道兩側產生高濃度帶電粒子，形成所謂的「赤道異常峰」帶，這個傳輸過程稱為赤道噴泉效應。中性風、大氣化學成份組成、緯向電場均為影響電離層赤道異常的主要因子。此外，月球重力對於電離層中性風發電機制也伴演一定程度之角色，因此，月相(Lunar phase)變化可以調節E×B向上漂移，進而影響赤道噴泉效應。
本論文利用全球導航衛星系統(GNSS)與全球電離層圖(GIM)全電子含量(TEC)觀察2000至2017年間的電離層赤道異常峰之強度、時間以及緯度對月相之響應，並且研究全日食事件與平流層驟暖事件發生時的電離層月相效應。2000至2017的統計分析顯示月相會以14.76天週期調節電離層赤道異常峰的時間和緯度。新月或滿月時的電離層赤道異常峰頂峰時刻相較於18年的平均時間提早約20–40分鐘; 上弦月或下弦月時的頂峰時刻落後18年平均約20–40分鐘。此外，電離層赤道異常峰之緯度分別在新月或滿月後 2-5 天向極區方向偏移1度，在上弦月或下弦月後 2-5 天則向磁赤道方向偏移1度。14.76天週期的赤道異常峰之頂峰時間和緯度訊號的變化幅度，在地球位於軌道近日點時最為顯著，在地球位於軌道遠日點則最微小，其頂峰時刻和緯度的最大振幅分別達到1小時與2度。此一現象說明太陽與地球軌道的距離與傾角也在電離層赤道異常扮演著關鍵的角色。由於赤道異常峰頂峰時刻以14.76天週期變化，且於新月或滿月日最為領先，這導致全電子含量於08:00太陽當地時間(Solar local time; SLT)之後增強，反之則在 14:00 SLT 之後減弱。相對的，因為頂峰時刻在上弦月或下弦月最為落後，造成全電子含量在08:00 SLT後減弱，在 14:00 SLT 後增強。另一方面，本論文採用2000-2013 年間的全球電離層圖全電子含量和12個平流層驟暖事件來檢驗月相與平流層驟暖事件之間的關係。統計結果顯示，無論月相如何，平流層驟暖都可以進一步令電離層赤道異常頂峰時刻提前0.47小時。然而，由於赤道異常峰頂峰時刻受平流層驟暖調節的幅度不如月相效應顯著，因此電離層全電子含量的增強和減弱特徵受月相主導。平流層驟暖如果不是發生在新月或滿月前後的日子，可能無法造成全電子含量在早上增強。此外，電離層日食效應包含全電子含量的主要衰退（Major depression; MD）、日食前增強（Pre-ascension; PA）、日落後增強（Sunset ascension; SA）和二次衰退（Secondary depression; SD）。本論文以2200多個全球導航衛星系統地面接收站全電子含量觀察2017年8月21日的日全食事件與月相效應。結果顯示，由於月相效應的影響，PA分別在14:00 SLT前後得到增強和抑制；SA和SD分別在14:00–20:00 SLT 期間被抑制和增強；MD在14:00 SLT 之前被低估，在14:00之後被高估。雖然月相效應難以被避免，如果利用接近日食天的日子建立參考值可以大幅降低觀測到的月相效應，並且增強日食訊號。結論上，月相可以顯著調節電離層赤道異常峰的頂峰時刻、緯度位置並主導全電子含量的增減特徵，因此在研究平流層驟暖或日食事件時應考慮電離層月相效應。
;The Equatorial Ionization Anomaly (EIA) is the most pronounced low-latitude ionospheric structure, featuring two dense bands of electron density around ±15°N magnetic latitude straddling the magnetic equator. In this study, the total electron content (TEC) derived from Global Navigation Satellite System (GNSS) and Global Ionosphere Map (GIM) in 2000–2017 are utilized to examine the response of EIA crests strength, appearance time, and latitude to lunar phase, as well as lunar phase effects during the stratospheric sudden warming (SSW) and the total solar eclipse. The 18‐year statistical analysis shows that lunar phases can modulate the appearance time and latitude of the EIA crests with a 14.76-day periodicity. The EIA crests on the new moon or full moon lead those of the 18‐year average by about 20–40 minutes. By contrast, the EIA crests on the first quarter or third quarter lag those of the 18‐year average by about 20–40 minutes. Meanwhile, EIA crests move the furthest poleward and equatorward with 1° latitude 2–5 days after the new moon or full moon and the first quarter or third quarter, respectively. The maximum amplitude of 14.76-day variations in the appearance time extends to 1 hour and the latitude reaches 2°, which appears around the perihelion. By contrast, the minimum amplitude of the 14.76-day variation occurs around the aphelion. Since the appearance time of EIA crests varies with a 14.76-day period, the TEC around the new moon or full moon yields a larger value after 08:00 solar local time (SLT) but a smaller value after 14:00 SLT. On the contrary, around the first quarter or third quarter, TEC decreases after 08:00 SLT and increases after 14:00 SLT. This study further employs the GIM TEC and 12 SSW events during 2000–2013 to examine the relation between lunar phases and SSWs. The statistical analysis shows that SSWs can advance the appearance time of the EIA crests by about 0.47 hours regardless of lunar phases. However, since the modified EIA crest appearance time by the SSWs is smaller than that by the lunar phases, the TEC increases/decreases pattern is dominated by lunar phases. Thus, SSW may not result in an enhancement of TEC in the morning sector, except around the new moon or full moon days. On the other hand, the GNSS TEC is utilized to analyze the 14.76-day signatures in the solar eclipse effects of the major depression (MD), pre-ascension (PA), sunset ascension (SA), and secondary depression (SD) on 21 August 2017. The results show that due to the lunar phase effects, PAs are enhanced and suppressed before and after 14:00 SLT, respectively; SAs and SDs are suppressed and enhanced during 14:00–20:00 SLT, respectively; and MDs have been underestimated before and overestimated after 14:00 SLT. While, a reference nearby the solar eclipse day(s) can minimize the lunar phase effect but enhance solar eclipse signatures. In conclusion, lunar phases can significantly modulate the appearance time, latitude, and pattern of daily TEC of the EIA crests, which should be considered when studying an SSW or a total solar eclipse. |
