| 摘要: | 利用福爾摩沙衛星三號 (FORMOSAT-3/COSMIC, F3/C) 電離層掩星觀測電子密度廓線研究2007年至2010年期間中緯度電子密度增強 (Midlatitude Electron Density Enhancement, MEDE) 之日變化、月變化、年變化。研究結果顯示MEDE具有地方時間、季節、太陽活動、地磁緯度效應。夜晚至清晨期間之MEDE較為明顯，其中以0500LT （local time, 當地時間） 最為顯著，而白天則不容易觀測到MEDE現象。春秋分期間，南北半球MEDE在各個經度上都會明顯出現；冬夏至期間則在冬季半球較為顯著，可能是夏季半球威德海異常 (Weddell Sea Anomaly) 和鄂霍次克海異常 (Okhotsk Sea Anomaly) 之電子密度異常增加，使得MEDE現象較不明顯。MEDE之分布以地磁緯度±40°N附近最為明顯。電離層F2層層峰電子密度一般位於250～350公里高，MEDE現象會隨著高度之增加而更加明顯，推測上層電離層電漿動力機制扮演一定的角色。電離層電子密度正比於太陽活動，MEDE現象於高太陽活動期間卻相對不明顯。分析0200~0500 LT不同經度之緯度-高度剖面電子密度，並將各個高度之電子密度歸一化，可以探討MEDE緯度-高度剖面隨時間變化，以及磁偏角和赤道偏移效應。比對福衛三號電子密度廓線觀測與水平風場模式 (Horizontal Wind Model, HWM 14) 結果，發現MEDE形成主要是電漿層電漿向下擴散，但受到中性風影響，在不同的經度形成不同的密度分布。;The research investigates the diurnal, monthly, and annual variations of Midlatitude Electron Density Enhancement (MEDE) from 2007 to 2010 using FORMOSAT-3/COSMIC (F3/C) GPS occultation electron density profiles. The results indicate that MEDE exhibits effects related to local time, season, solar activity, and geomagnetic latitude. MEDE is more pronounced during nighttime to early morning, with the most significant occurrence at 0500 LT (local time), whereas it is less observable during the daytime. During the equinoxes, MEDE is evident in both hemispheres across various longitudes; during solstices, it is more prominent in the winter hemisphere. This may be due to the anomalous increase in electron density over the Weddell Sea and Okhotsk Sea in the summer hemisphere, making MEDE less noticeable. The distribution of MEDE is most evident around geomagnetic latitudes of ±40°N. The F2 layer peak electron density typically occurs at altitudes of 250-350 km, with the MEDE phenomenon becoming more pronounced at higher altitudes, suggesting the involvement of upper ionospheric plasma dynamics. The ionospheric electron density is proportional to solar activity; however, MEDE is relatively less apparent during high solar activity periods. By analyzing the electron density profiles from 0200 to 0500 LT at different longitudes and normalizing the electron density at various altitudes, we can explore the temporal variation of the latitude-altitude profile of MEDE, as well as the effects of magnetic declination and equatorial offset. Comparing the F3/C electron density profiles with the Horizontal Wind Model (HWM 14) results reveals that the formation of MEDE is primarily due to the downward diffusion of plasmaspheric plasma, influenced by neutral winds, resulting in different density distributions at various longitudes. |
