Yearly variations of global plasma densities in the topside ionosphere at middle and low latitudes
Yearly variations of global plasma densities in the topside ionosphere at middle and low latitudes
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DOI：
10.1029/2007ja012283
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发表时间：
2007-07
影响因子：
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通讯作者：
Libo Liu;B. Zhao;W. Wan;S. Venkartraman;Man‐Lian Zhang;X. Yue
中科院分区：
文献类型：
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作者：
Libo Liu;B. Zhao;W. Wan;S. Venkartraman;Man‐Lian Zhang;X. Yue
[1]利用1996-2005年美国国防气象卫星（DMSP）在0930和2130 LT观测的10年总离子密度（Ni）数据，分析了磁纬60°S ~ 60°N上电离层等离子体密度的年际变化。结果表明，840 km处DMSP Ni有较强的年际变化.在大多数纬度地区，平均Ni的年分量占主导地位，在北方半球的6月至日和南半球的12月至日附近达到最大值。相反，季节异常（最大值Ni在冬至前后）存在于北方赤道带。此外，在两个至日的镍的差异是不对称的磁赤道，通常是在南半球高于北方半球。共轭平均的Ni在十二月至日比在六月至日大得多。这种年不对称性受太阳活动的影响，具有纬向和经向结构。年度不对称的经度效应取决于当地时间，在晚上部门比在上午部门更强。随着太阳活动的增加，冬至差异和年度不对称性更加明显。年不对称性不仅出现在太阳活动周期的上升阶段，也出现在太阳活动周期的下降阶段。因此，两个至日之间的太阳条件差异并不能解释Ni的不对称性。中性氧[O]的浓度，从NRLMSIS模式提供的，显示出类似的模式的年度和半球的不对称性。此外，HWM模式中性风的影响也与Ni的变化模式有关。因此，考虑到顶侧电离层的主要过程，[O]和热层风率的变化应有助于在840公里高度Ni的年度不对称性。
[1] In this paper, the 10-year (1996–2005) measurements of total ion density (Ni) from the Defense Meteorological Satellite Program (DMSP) spacecraft at 0930 and 2130 LT have been analyzed to investigate the yearly variations of global plasma densities in the topside ionosphere at magnetic latitudes from 60°S to 60°N. Results indicate that there are strong yearly variations in the DMSP Ni at 840 km. The annual components of longitude-averaged Ni dominate at most latitudes with maxima around the June solstices in the Northern Hemisphere and the December solstice in the Southern Hemisphere. In contrast, seasonal anomaly (maxima Ni around the December solstice) exists in the northern equatorial zone. Moreover, the differences in Ni at the two solstices are not symmetrical about the magnetic equator, being generally higher in the Southern Hemisphere than in the Northern Hemisphere. Conjugate-averaged Ni is substantially greater at the December solstice than at the June solstice. This annual asymmetry is modulated by solar activity effect and has latitudinal and longitudinal structures. The longitude effects of the annual asymmetry depend on local time, being stronger in the evening sector than in the morning sector. The solstice differences and annual asymmetry are more marked with increasing solar activity. The annual asymmetry appears not only in the rising phase of the solar cycle but also in the declining phase. Thus the solar condition differences between the two solstices do not account for the Ni asymmetry. The concentration of neutral oxygen [O], provided from the NRLMSIS model, shows a similar pattern of annual and hemispheric asymmetries. Moreover, effects of the HWM model neutral winds are also constituent with the change patterns of Ni. Therefore, considering the principal processes in the topside ionosphere, the changes of [O] and the rates of thermospheric winds should contribute to the annual asymmetry in Ni at 840-km altitude.