Solar Origin of Bare Ion Anomalies in the Solar Wind and Interplanetary Coronal Mass Ejections

Abstract

Abstract Previous studies of the solar wind and interplanetary coronal mass ejections (ICMEs) have shown periods throughout solar cycle 23 when heliospheric measurements of ion composition appear anomalous. In these cases, C6+ and other bare ion densities, i.e., fully stripped ions, are unusually low, leading it to be classified as the Outlier solar wind. However, its origin and solar source(s) remain largely uncertain. In this work, we further characterize the Outlier wind to connect its heliospheric structure to its solar source to constrain the conditions of its formation. Through an analysis of the plasma and magnetic field properties of each occurrence between 1998 and 2011, we find that the Outlier plasma occurs in the slow solar wind or interplanetary mass ejections (∼460 km s−1), and comprises distinct, high density events lasting less than 10 hr. The number of events is correlated with the solar cycle, indicating the process leading to the depletion of bare ions is strongly governed by the magnetic field. Additionally, the events exhibit a bi- or unidirectional suprathermal electron strahl that is concurrent with changes in the magnetic field direction. Moreover, the Outlier wind’s composition, entropy, Alfvén speed, and proton temperature suggest a helmet streamer or active region origin. Together, the properties exhibited by the Outlier wind suggest a strong connection to the heliospheric current sheet and that the solar wind events are smaller scale versions of those seen in ICMEs, such as small magnetic flux ropes. However, more work is necessary to determine the source and creation process in the vicinity of the Sun.