极点热层密度变化特征及其受太阳风扇形结构调制

本文基于2002年至2010年的GRACE卫星的观测密度统计分析南北极点的热层大气密度的世界时（即磁地方时）变化.研究发现：在9—11月份地球处于行星际磁场为背向太阳的扇区内（背向扇区）时，南极点热层密度在约17∶00 UT（13∶30 MLT）达到最大值，比日平均值高约22%；而在6—8月份，当地球处于行星际磁场为面向太阳的扇区内（面向扇区）时，北极点热层密度在06∶00 UT（12∶30 MLT）达到最大值，比日平均值高约13%.南极点的磁纬是-74°，其在15∶30 UT处于磁地方时正午，恰与极尖区位置重合.北极点在5∶30 UT处于磁地方时正午，此时北极点与极尖区位置最靠近.因此，极点热层大气密度的磁地方时变化可能是其周期性靠近极尖区的结果.南北极点热层密度的磁地方时变化分别在背向和面向扇区内更明显，这可能与行星际磁场B_y分量对南北半球密度的不同影响有关.统计结果还表明，极点热层大气密度的磁地方时变化在冬季半球内不明显.这可能是由于在冬季半球，沉降于极尖区的粒子相比夏季半球少、沉降高度低，因而能量沉降所引起的热层上部的密度增强较小.

Variation of the polar thermospheric mass density and its association with the sector structure of the solar wind

A statistical analysis of the polar neutral densities from 2002 to 2010 observed by the GRACE satellite shows that the densities at both the South and North Poles had a UT (Magnetic Local Time, MLT) variation. In September-November, the neutral density at the South Pole in an away solar wind sector maximizes at about 17:00 UT (13:30 MLT), which is about 22% above the daily average. In June-August, the neutral density at the North Pole in the toward sector maximizes at about 06:00 UT (12:30 MLT), which is about 13% above the daily average. The magnetic latitude of the South Pole is about -74°, and the pole reaches the magnetic local noon at about 15:30 UT. The North Pole reaches the magnetic local noon at about 5:30 UT. This is the time when the North Pole is close to the cusp region. Therefore, the MLT variation of the polar thermospheric density could be caused by the periodic wavy between the cusp and poles. The MLT variations of the thermospheric density at the South and North poles are more obvious in the away and toward sectors. This may be caused by the different effects of the IMF B_y on the thermospheric densities in the Southern and Northern Hemispheres. The statistical results also show that the MLT variation of the polar neutral density is not obvious in the winter hemisphere. This could be associated with the fact that the cusp ion precipitating energy flux in the summer hemisphere is larger than that in the winter hemisphere, and the average particle energy is lower in the summer hemisphere. Consequently, there is stronger cusp density enhancement in the summer hemisphere.