Non-simultaneous Forbush Decrease Events Observed at Middle Latitude Neutron Monitor Stations

다국어 초록 (Multilingual Abstract)

The Forbush Decrease(FD) events are recorded as the abrupt decrease of cosmic ray intensity at the ground neutron monitor (NM) stations. They are considered to be associated with the magnetic structures around the Earth. The associated magnetic structures are the magnetic clouds and interplanetary shocks originated from the coronal mass ejections. Oh et al. (2008) and Oh and Yi. (2009) reported that the FD events are observed globally simultaneous or non-simultaneous at different NM stations at the high latitude. They suggested that the simultaneity of FD event is determined by the strength of and propagation direction of of Climax, Irkutsk, and Jungfraujoch NM stations during the solar maximum period (1998-2002). Those NM stations have the cutoff rigidities of 3-6 GV and are located in the middle latitude. Previous studies before Oh et al. (2008) have mainly focused on the simultaneous FD events by globally distributed NM stations. Thus, the aim of this study is to investigate the statistical properties of the simultaneous and non-simultaneous FD events at the middle latitude and to compare these FD events at middle latitude with ones at the high latitude configured by Oh et al. [2008] and Oh and Yi (2009). Most FD　events(44 out of 54) are observed simultaneous in universal time(UT) at each NM station regardless of the longitudinal location. On the other hand, the other FD　events are not globally simultaneous but locally simultaneous in local time(LT). The stronger FD events tend to be simultaneous events, but the weaker FD events are non-simultaneous. There is a difference in cosmic ray intensity variation during the main phase due to the cutoff rigidity in comparison with the FD events observed at high latitude.

목차 (Table of Contents)

• 제1장 이론 1
• 1.1 우주선 1
• 1.2 우주선의 종류 3
• 1.2.1 은하우주선 (Galactic cosmic rays; GCR) 3
• 1.2.2 태양우주선 (Solar cosmic rays; SCR) 3
• 1.2.3 이상우주선 (Anomalous cosmic rays; ACR) 4
• 1.3 우주선 세기의 변화 6
• 1.3.1 지구내적 변화 (Terrestrial variations) 7
• 1.3.2 지구외적 변화 (Extra-terrestrial variations) 9
• 1.3.3 Forbush Decrease 현상 14
• 1.4 우주선과 지구 16
• 1.4.1 지구 자기권과 우주선 16
• 1.4.2 대기 연쇄 붕괴 (Atmospheric cascade) 18
• 1.4.3 Magnetospheric Propagation and Geomagnetic Cutoff 21
• 1.5 우주선의 검출 26
• 1.5.1 우주선의 상호작용 26
• 1.5.2 중성자 검출 27
• 1.5.3 Global Network of Neutron Monitors 30
• 1.6 고위도 우주선 중성자 검출기의 Forbush Decrease 현상 31
• 1.6.1 고위도 지방의 동시 FD 현상과 비동시 FD 현상 31
• 제2장 관측 자료 및 분석방법 34
• 2.1 관측 자료 34
• 2.2 Foubush Decrease event 선택 방법 36
• 2.2.1 Forbush Decrease 현상과 Ground Level Enhancement 36
• 2.2.2 Forbush Decrease 현상의 선택 기준 38
• 2.3 통계분석 46
• 2.3.1 Fisher의 직접확률법 (Fisher Exact Test) 46
• 2.3.2 독립표본 T 검정 (Student T-Test) 47
• 제3장 결과 48
• 3.1 중위도 우주선 중성자 검출기의 Forbush Decrease 현상 48
• 3.1.1 동시 FD 현상과 비동시 FD 현상 48
• 3.1.2 Forbush Decrease 현상의 통계적인 특징 58
• 3.1.2.1 Fisher의 직접확률법 (Fisher Exact Test) 58
• 3.1.2.2 독립표본 T검정 (Student T-Test) 59
• 3.2 고위도와 중위도 우주선 중성자 검출기의 FD 현상 비교 61
• 제4장 토의 및 결론 64
• 참고문헌 67
• Abstract 72