南大西洋异常区高能质子通量南北向差异

利用2012年8月NOAA N15，N16，N17，N18卫星中能质子和电子探测器（MEPED）>6.9MeV质子能档的数据，研究了800km高度处南大西洋异常区（SAA）质子分布的南北方向差异.数据显示，质子计数率沿经线随纬度的变化近似满足高斯分布，且向南运动的质子多于向北运动的质子.MEPED具有0°和90°两个探测方向，可对质子投掷角分布进行估算，从而消除在探测南向和北向质子时探测器方向角偏差所造成的影响.结果表明，在经度40°W、纬度13°S至23°S位置上，仍然存在南向质子比北向质子多20%~30%的现象.其原因可能与地磁异常使南向质子镜像点更低，更多质子散射进入大气层沉降有关.分析还发现，南大西洋异常区质子通量存在地方时变化.日侧比夜侧强约20%，这可能是由于地磁场日变化引起的. 

The Southward and Northward Difference of Proton Flux in South Atlantic Anomaly

This paper discusses the southward and northward proton distributions at 800km height of the South Atlantic Anomaly (SAA) according to the data detected by the >6.9MeV channel of the Medium Energy Proton and Electron Detector (MEPED) of NOAA N15, N16, N17 and N18 satellites. The data show that the counting rate of protons satisfies approximately the Gauss distribution with latitudes along a longitude while the number of southward protons is larger than the northward protons. MEPED has two heads on 0° and 90° directions. It can be used to evaluate the pitch-angle distribution of protons, then to eliminate the direction errors of the detectors between detecting the southward and the northward protons. The result shows that the southward protons are 20%~30% more than the northward protons in the latitudes of 13°S about 23°S along 40°W longitude. The phenomenon may be related to the lower mirror position in the southern hemisphere than in the northern hemisphere due to the geomagnetic anomaly. It causes more southward protons precipitating in SAA. Otherwise, the proton flux in SAA presents a local time dependency. The flux in the dayside is about 20% more than that in the nightside. It may be caused by the daily variation of the geomagnetic filed.