A statistical study of the average iron charge state distributions inside magnetic clouds for solar cycle 23

Magnetic clouds (MCs) are the interplanetary counterparts of coronal magnetic flux ropes. They can provide valuable information regarding flux rope characteristics at their eruption stage in the corona, which is unable to be explored in situ at present. In this paper, we make a comprehensive survey of the average iron charge-state ($⟨Q⟩Fe$) distributions inside 96 MCs for solar cycle 23 using Advanced Composition Explorer (ACE) data. Since the $⟨Q⟩Fe$ in the solar wind are typically around 9+ to 11+, the Fe charge state is defined as being high when the $⟨Q⟩Fe$ is larger than 12+, which implies the existence of a considerable amount of Fe ions with high charge states (e.g., ≥16+). The statistical results show that the $⟨Q⟩Fe$ distributions of 92 (∼96%) MCs can be classified into four groups with different characteristics. In group A (11 MCs), the $⟨Q⟩Fe$ shows a bi-modal distribution with both peaks being higher than 12+. Group B (4 MCs) presents a unimodal distribution of $⟨Q⟩Fe$, with its peak being higher than 12+. In groups C (29 MCs) and D (48 MCs), the $⟨Q⟩Fe$ remains higher and lower than 12+ throughout ACE’s passage through the MC, respectively. Possible explanations of these distributions are discussed.