The large-scale magnetic structure of interplanetary coronal mass ejections (ICMEs) has been shown to cause decreases in the galactic cosmic ray (GCR) flux measured in situ by spacecraft, known as Forbush decreases (Fds). We use measurements of the GCR count rate obtained by MESSENGER during its orbital phase around Mercury to identify such Fds related to the passage of ICMEs and characterize their structure. Of the 42 ICMEs with corresponding high-quality GCR data, 79% are associated with a Fd. Thus a total of 33 ICME-related Fds were identified, 24 of which (73%) have a two-step structure. We use a superposed epoch analysis to build an average Fd profile at MESSENGER and find that despite the variability of individual events, a two-step structure is produced and is directly linked with the magnetic boundaries of the ICME. By using results from previous studies at Earth and Mars, we also address whether two-step Fds are more commonly observed closer to the Sun; we found that although likely, this is not conclusive when comparing to the wide range of results of previous studies conducted at Earth. Finally, we find that the percentage decrease in GCR flux of the Fd is greater at MESSENGER on average than at Earth and Mars, decreasing with increasing heliocentric distance. The relationship between the percentage decrease and maximum hourly decrease is also in agreement with previous studies, and follows trends relating to the expansion of ICMEs as they propagate through the heliosphere.
Characteristics and energy dependence of recurrent galactic cosmic-ray flux depressions and of a Forbush decrease with LISA Pathfinder
在内日球层观测到的银河宇宙线（GCR）能谱受太阳活动、太阳极性以及太阳和行星际起源结构的调制。LISA Pathfinder（LPF）上的高计数率粒子探测器（PD）用于子系统诊断，用于测量银河宇宙线和太阳高能粒子的积分通量，积分通量大于70 MeV n $^{-1}$，计数率为6500 s $^{-1}$。以15 s的采样时间收集PD数据。本文报告了2016年8月2日GCR通量周期性低气压和福布什下降的特征和能量依赖性。行星际飞行任务的能力，携带PD仪器性能的目的，在监测行星际日冕物质抛射（ICME）的通道进行了讨论。
