Statistical Study Of Abundances And Charge States Of Heavy Ions In ICMEs

In interplanetary coronal mass ejections（ICMEs）,the abundance of heavy ions reflects the height of source material.Due to the freeze-in effect,the charge distribution indicates the heating process for the initiation of coronal mass ejections（CMEs）in the lower corona.When ICMEs propagate to the Geo-space,the higher charge state heavy ions collide with the neutral atoms generating the soft X-rays.The abundance and flux of these heavy ions in ICMEs are important for the calibration of the magnetosphere imaging.Above all,it is important to study the signatures of heavy ions quantitively by statistics.In this work,we used a variety of statistical methods to investigate the characteristics of heavy ions in interplanetary coronal mass ejections and the relationships with velocity and the solar cycle.This statistical work studies the abundances and the charge states of the carbon,oxygen,and iron ions in 281 ICMEs measured at 1 AU by ACE spacecraft from 1998 to 2011.The Gaussian distribution test is applied,and the analysis of variance is used to quantify the similarity between two distributions of ionic charge states and abundances.The correlation analysis and grouping tests of the four factors possibly related to the heavy ion characteristics,such as the interplanetary magnetic field components,velocity,duration of ICMEs and solar activity level,are also carried out.The results show that the average charge states and abundance of heavy ions are highly related to velocity of ICMEs at 1AU:the O⁷⁺/O⁶⁺in ICMEs is positively correlated with the speed,which is contrary to that in the solar wind;the charge state of oxygen ions and He/O show statistical differences between the high-speed and the low-speed groups of ICMEs.In addition,the abundance and charge states of heavy ions are also significantly related to solar activity:the content of Mg ions in ICMEs is much higher than that in solar wind during the solar maximum years;the mean charge and the flux of oxygen ions show quasi-linear relations to the sunspot number（SN）during solar minimum,and show fluctuations during solar maximum;C,O,Fe ions have three types of charge state distribution and these types are highly related to solar cycle.This work suggests that the heating and acceleration of ICMEs and solar wind are different below the freeze-in height of oxygen.The abundance of Mg increasing with the solar activity indicates that CMEs are composed of more materials from Corona during solar maximum years.The three types of charge distribution of C,O,and Fe ions are related to solar minimum or solar maximum respectively.And the abundance of Mg,O,and Fe are positively related to the solar activity.These two results reveal that the solar magnetic field controls the composition and heating of heavy ions in CMEs.There may be a minimum-mode and a maximum-mode of the CME initiation.