Browsing by Author "Luan, Xiaoli"
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Item Restricted A numerical study of nighttime ionospheric variations in the American sector during 28–29 October 2003(American Geophysical Union, 2016-08-17) Chen, Xuetao; Lei, Jiuhou; Wang, Wenbin; Burns, Alan G.; Luan, Xiaoli; Dou, XiankangVariations of nighttime F2 peak height (hmF2) over the American sector during the 28–29 October 2003 storm period were investigated using the National Center for Atmospheric Research Thermosphere‐Ionosphere Electrodynamics Global Circulation Model. The model was generally able to reproduce the ionospheric variations on 28–29 October 2003 observed by the ionosondes. A series of controlled model simulations were subsequently undertaken to examine the effects of electric fields and neutral winds on the ionosphere. The numerical experiments suggest that the dramatic nighttime increase of hmF2 on the storm day 29 October is mainly caused by traveling atmospheric disturbances (TADs) from the high latitudes of the Northern Hemisphere. However, the electric field plays an important role in causing the elevation of hmF2 in the equatorial region. The prompt penetration electric field (PPEF) associated with the southward component of the interplanetary magnetic field (Bz) is westward on the nightside, whereas when Bz reverses and becomes northward, the PPEF is westward in the premidnight and turns to eastward in the postmidnight. These PPEFs greatly affect the low‐latitude ionosphere during storm time. On 28 October, even though the Bz disturbance was weak with a short duration of southward Bz, the TADs from the Southern Hemisphere can propagate to the Northern Hemisphere and result in the corresponding oscillations in the nightside hmF2.Item Restricted Contribution of the topside and bottomside ionosphere to the total electron content during two strong geomagnetic storms(American Geophysical Union, 2016-02-12) Zhu, Qingyu; Lei, Jiuhou; Luan, Xiaoli; Dou, XiankangIn this study, the ionospheric observations from ionosondes, GPS receivers, and incoherent scatter radars (ISR) at low and middle latitudes were used to investigate the contribution of the bottomside and topside ionosphere to the total electron content (TEC) during the September 2005 and December 2006 geomagnetic storms. It was found that the contribution of the bottomside TEC below F2 peak (BTEC) to the ionosonde ionospheric TEC (ionosonde ITEC), namely, BTEC/ITEC was almost constant during both quiet and storm times, while the ratio of BTEC to GPS TEC (i.e., BTEC/GPS‐TEC) underwent obvious diurnal variations at all stations. The BTEC/GPS‐TEC during the positive phase was similar to that during quiet time, regardless of the formation mechanisms of the observed positive phases. Moreover, our analysis revealed that the ISR calculated BTEC/ITEC during positive ionospheric phases was comparable to that during quiet time. This suggests that the positive phases in these two events mainly occurred around the F2 peak height. There were large differences between the calculated BTEC/ITEC from the ISR observations and BTEC/GPS‐TEC during the negative phase or at night when the plasmasphere possibly contributed significantly to the TEC in the relative sense. Although the absolute changes of the topside TEC were larger than the bottomside TEC at low and middle latitudes associated with the larger topside effective ionospheric thickness, unlike the October 2003 superstorms, the relative changes of the topside TEC to the quiet time reference in these two strong storms were not greater than the changes of the bottomside TEC and peak density NmF2.Item Restricted Response of the topside and bottomside ionosphere at low and middle latitudes to the October 2003 superstorms(American Geophysical Union, 2015-07-14) Lei, Jiuhou; Zhu, Qingyu; Wang, Wenbin; Burns, Alan G.; Zhao, Biqiang; Luan, Xiaoli; Zhong, Jiahao; Dou, XiankangIonospheric observations from the ground‐based GPS receiver network, CHAMP and GRACE satellites and ionosondes were used to examine topside and bottomside ionospheric variations at low and middle latitudes over the Pacific and American sectors during the October 2003 superstorms. The latitudinal variation and the storm time response of the ground‐based GPS total electron content (TEC) were generally consistent with those of the CHAMP and GRACE up‐looking TEC. The TECs at heights below the satellite altitudes during the main phases were comparable to, or even less than, the quiet time values. However, the storm time CHAMP and GRACE up‐looking TECs showed profound increases at low and middle latitudes. The ground‐based TEC and ionosonde data were also combined to study the TEC variations below and above the F2 peak height (hmF2). The topside TECs above hmF2 at low and middle latitudes showed significant increases during storm time; however, the bottomside TEC below hmF2 did not show so obvious changes. Consequently, the bottomside ionosphere made only a minor contribution to the ionospheric positive phase seen in the total TEC at low and middle latitudes. Moreover, at middle latitudes F2 peak electron densities during storm time did not have the obvious enhancements that were seen in both the ground‐based and topside TECs, although they were accompanied by increases of hmF2. Therefore, storm time TEC changes are not necessarily related to changes in ionospheric peak densities. Our results suggest that TEC increases at low and middle latitudes are also associated with effective plasma scale height variations during storms.