| 摘要: | 電離層電漿濃度赤道異常現象是低緯度電離層中最為顯著的特徵, 其成因主要是因為地球磁場在赤道地區為水平方向, 水平磁場與白天因潮汐產生的東向電場產生E×B向上的電漿漂移, 此向上漂移會將電離層電漿從較低的高度傳輸到較高的高度, 當電漿被帶到較高的高度後因為受到重力跟壓力梯度力的效應, 電漿會延磁力線傳輸或擴散到磁赤道兩旁較高的緯度造成電漿的堆積形成電漿濃度的赤道異常峰, 這種傳輸現象因為狀似噴泉所以被稱為電漿噴泉效應. 因為低緯電離層主要的結構是電漿赤道異常現象, 所以藉由電漿噴泉效應的變化會影響電漿赤道異常進而影響到整個低緯電離層的結構. 電離層赤道峰的電子濃度與位置受因為到下列的影響而改變, (1) 受到中性風延磁力線方向的影響而改變電漿延磁力線方向的傳輸, (2) 因為中性大氣成分的變化造成的化學產生與消失的擾動變化, 以及 (3) 改變東西向電場使得 E×B向上電漿漂移速度的變化. 在太陽寧靜時, 低緯電離層有季節變化. 季節變化可以歸納成以下幾點. (1) 在夏冬至時, 南北半球的赤道異常峰會有不對稱的現象, 在夏半球的異常峰濃度較冬半球的異常峰減低, (2) 在春秋分時, 南北半球的赤道異常峰的電子濃度都較夏冬至時來的大, (3) 地理與地磁赤道的偏移亦會對赤道異常峰的變化造成影響. 在磁暴時, 磁層的能量與動量經由極區下衝的沉降高能粒子以及電場延磁力線映成到高緯電離層產生焦耳熱的方式傳播到電離層. 這些磁暴時產生的大量能量將中性大氣游離並對熱氣層加熱造成電離層的導電性增加並造成中性風與大氣成分的擾動進而影響到電離層的電子濃度結構. 磁暴時的電離層與中性大氣的擾動包括了高緯傳到低緯的電場擾動, 向赤道傳播的赤道方向中性風以及中性大氣氧原子與氮分子, 氧分子分佈的擾動. 因磁暴產生的中性風擾動也會造成電離層電動效應產生擾動電場. 這些電場, 中性風以及中性大氣成份的擾動影響電離層電子濃度甚巨. 本論文利用多種衛星觀測資料, 包括全球定位系統 (GPS) 量測的電離層全電子含量 (TEC), 中華衛星一號的電離層電漿電動效應儀量測的電漿離子濃度與離子漂移, 以及利用NASA TIMED 衛星上的 GUVI 所量得之大氣輝光所推算得中性大氣中的氧原子與氮分子比例 ([O]/[N2]) 來共同觀測磁暴時低緯電離層電漿濃度變化以及電漿漂移與中性大氣擾動對其造成之影響. 觀測結果顯示, 在磁暴開始的初始階段赤道異常峰會移動到較高緯度並伴隨著強大的電子濃度增強, 經過數小時後在慈報的主相過後, 赤道異常峰明顯的減弱, 其後電離層電漿濃度亦有著明顯的變化直到磁暴結束. 赤道異常峰的增強可能是因為磁暴時磁層的電場從極區傳到赤道地區造成強大的電漿向上漂移將電漿傳輸到較大的高度形成較為強大的噴泉效應. 藉由中華衛星一號的電離層電漿電動效應儀的觀測, 此一推論得到證實. 另外, TIMED GUVI 推算的中性大氣氧氮比擾動與電子濃度的減少有著很好的吻合度. 推測電子濃度的減弱除了因為磁暴風擾動產生的電場影響 (disturbance dynamo) 減弱噴泉效應之外, 磁暴產生的氧氮比擾動亦為一重要因素. 除了衛星共同觀測之外, 本論文亦利用了物理理論電離層數值模式模擬低緯電離層于磁暴時之變化, 主要利用的電離層數值模式為Sheffield University Plasmasphere Ionosphere Model (SUPIM) 以及NCAR Thermosphere-Ionosphere Electrodynamic General Circulation Model (TIEGCM). 模擬結果顯示, 造成赤道異常峰的高緯方向移動與電子濃度的強烈增加的原因除了因電場增強產生的增強噴泉效應之外磁暴產生的赤道方向中性風亦扮演著很重要的角色, 赤道方向中性風能夠讓電離層維持在較高的高度, 使得電漿消失的效應減緩而產生電漿的堆積. 此外, 模擬結果亦預測了新的電離層電漿結構的存在以及其產生機制. 數值模式預測了電離層電子洞的結構以及拱形的電子濃度結構. The low latitude ionosphere is unique in that the magnetic field is nearly horizontal, so that zonal electric fields, produced by the neutral wind dynamo during quiet geomagnetic times, can transport the plasma vertically through the E×B drift. This quiet-time vertical drift is upward during the daytime, causing plasma to drift to higher altitudes, from where it diffuses down along magnetic fields to higher latitudes creating two plasma crests on both sides of the magnetic equator. This feature is called the equatorial ionization anomaly (EIA), and the effect of transporting the plasma from the magnetic equator to higher latitudes is described as the fountain effect [Duncan 1960; Wright 1962; Hanson and Moffett 1966; Anderson 1973]. The plasma density and the peak location of the EIA can be modified by changes of: (1) the transport parallel to magnetic field lines through disturbance neutral winds and diffusion; (2) the loss process due to storm produced composition perturbations; and (3) the transport perpendicular to magnetic field lines due zonal electric field perturbations. During the magnetically quiet time, the electron density and the location of EIA peaks in both hemispheres show prominent seasonal variations. They are generally characterized by (1) in solstice, only the EIA peak in the winter hemisphere remains and a comparatively weak EIA density structure appears in the summer hemisphere, (2) in equinox, two EIA peaks are manifest and the overall electron density is larger than in solstice, (3) the offset of the magnetic equator and the geographic equator also has effects in production of the EIA asymmetry. During magnetic storms, magnetospheric energy and momentum are deposited in the ionosphere/thermosphere through auroral particle precipitation and ionospheric plasma convection driven by electric fields mapped from the magnetosphere. Intense auroral particle precipitation heats the thermosphere, ionizes the neutral gas, and increases the conductivity of the ionosphere. The increased conductivity combined with the magnetospheric electric field produces Joule heating in the ionosphere/thermosphere, which is the major energy source during storms. Heating of the thermosphere drives equatorward wind surges and causes an upwelling at high latitudes which carries heavier neutrals upward and increases the mean molecular mass. In addition to the thermospheric responses, the ionospheric electric field disturbances are observed at middle and low latitudes on different time scales. They result from both prompt penetration of time-varying magnetospheric fields from high latitudes to low latitudes and longer time lasting disturbance wind dynamo effects. In this study, the GPS derived total electron content (TEC), drift measurements from the ROCSAT-I at 600 km, and far ultraviolet airglow measured by the Global Ultraviolet Imager (GUVI) carried aboard the NASA TIMED satellite are utilized for observing the disturbance of the low latitude ionosphere during the magnetic storms. Observations from GPS-TEC often show that the equatorial ionization anomaly (EIA) expanded to much higher latitude with a great enhancement in the density during the early stage of the magnetic storm compared with quiet time. Following the expansion of the EIA, suppression of the EIA is often observed several hours after the storm onset. The derived ExB drifts measured from the Ionospheric Plasma and Electrodynamics Instrument (IPEI) onboard the ROCSAT-I show strong upward/poleward E×B drifts during the EIA expansions and downward/equatorward E×B drifts during the suppression. The [O]/[N2] inferred from the ratio of the 135.6 nm and LBH emissions from the GUVI provides information of storm-time composition perturbations which often result in negative ionospheric effect, i.e. reduced of the plasma density due to the magnetic storm. Theoretical models, the Sheffield University Plasmasphere Ionosphere Model (SUPIM) and the NCAR Thermosphere-Ionosphere Electrodynamic General Circulation Model (TIEGCM), are used to examine the relative importance of the ionospheric drivers in changing the EIA morphology during both magnetically quiet and disturbed periods. Model results show that the summer to winter meridional neutral winds produce the trans-equatorial transport of the plasma, resulting in seasonal asymmetry of the EIA peaks during the magnetically quiet period. Poleward expansion of the EIA peaks and strong increased EIA peak densities observed by the GPS TEC during the early stage of the magnetic storm are simulated and examined by the model. Simulation results show that the storm-produced equatorward meridional neutral wind plays a role in maintaining the ionospheric layer at higher altitude, where the recombination loss is smaller and the plasma is able to accumulate. Combing the upward/poleward E×B drifts with the equatorward neutral wind, poleward expansion of EIA peaks and very high EIA peak densities are simulated by the model during the early stage of the storm. Additionally, new features in the topside ionosphere, such as storm-time electron density hole and density arch are predicted by the model simulations. |
