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.