A Forbush decrease (FD) is a depression of cosmic ray (CR) intensity observed by ground-based neutron monitors (NMs). The CR intensity is thought to be modulated by the heliospheric magnetic structures including the interplanetary coronal mass ejection (ICME) surrounding the Earth. The different magnitude of the decreasing in intensity at each NM was explained only by the geomagnetic cutoff rigidity of the NM station. However, sometimes NMs of almost the same cutoff rigidity in northern and southern hemispheres observe the asymmetric intensity depression magnitudes of FD events. Thus, in this study we intend to see the effects on CR intensity modulation of FD event recorded at different NMs due to different ICME propagation directions as an additional parameter in the model explaining the CR modulation. Fortunately, since 2006 the coronagraphs of twin spacecraft of the STEREO mission allow us to infer the propagation direction of ICME associated with the FD event in 3-dimension with respect to the Earth. We suggest the hypothesis that the asymmetric CR modulations of FD events are determined by the propagation directions of the associated ICMEs.

Abstract
1. Introduction
2. Data
3. Results
4. Summary and Discussions
Acknowledgments
References

• Jongil Jung(Astronomy, Space Science and Geology, Chungnam National University)
• Suyeon Oh(Department of Earth Science Education, Chonnam National University) Corresponding Author
• Yu Yi(Astronomy, Space Science and Geology, Chungnam National University)
• Jongdae Sohn(Korean Astronomy and Space Science Institute)