A Special Kind of Interplanetary Coronal Mass Ejection

Abstract It is generally believed that coronal mass ejections (CMEs) have magnetic flux rope structures because of their helical shapes. However, only about 30%–40% of interplanetary CMEs (ICMEs) have a local magnetic flux rope structure. The usual explanations are that the spacecraft only crossed the flank of the ropes and failed to detect the complete magnetic flux rope structure or that some processes destroyed these magnetic flux rope structures. Several studies suggest that some ICMEs inherently possess disordered magnetic fields and consequently exhibit no magnetic flux-rope structures. We introduce a special kind of ICME which has a low magnetic field magnitude and stable magnetic field direction, relatively fast expansion speed, and lower proton temperature and density. All three of the measured magnetic field components are relatively stable. We want to know whether these ICMEs also have magnetic flux rope structures or not. We identified 20 special ICMEs and analyzed their evolution based on their observed characteristics. We took a special ICME as an example, which had an apparent rope configuration at 1 au but evolved to a special ICME at 5.4 au, to illustrate that this kind of ICME could come from magnetic clouds (MCs) whose rope structure had been being stretched due to expansion. We inferred that the missing obvious flux rope structure may be due to the expansion of MCs, not the flank crossing effect. However, more than 50% of the events were associated with the dominant x-component of the magnetic field, which indicates a leg crossing. Therefore, the detection of part of these special ICMEs may also be the result of the leg-crossing effect.