Magnetic-reconnection exhausts in the sheath of magnetic clouds

Context. Reconnection exhausts are a common phenomenon in the solar wind. Many exhausts were observed between an interplanetary coronal mass ejection (ICME) and the ambient solar wind, or between two ICMEs, or within the interior of a single ICME. Observed exhausts are almost exclusively associated with ICMEs with low (often much lower than 1) proton beta. Aims. Often the sheath regions of ICMEs have a high level of plasma beta. Therefore we aim to find out whether the reconnection exhausts occur frequently in the sheath regions of ICMEs. Methods. We examined the plasma and magnetic-field data in the sheath of the magnetic cloud (i.e., ICME) observed on 18–20 October 1995, and identified six reconnection exhausts. Results. The six reconnection exhausts occured within regions of proton beta that was higher than unity. Five of them occurred on a high level (>2.2) of proton beta. Conclusions. Low proton beta is no exclusive condition for magnetic reconnection. Reconnection may occur frequently in the sheath of ICMEs when magnetic fields from different source regions, that is, from diffident orientations, are pushed together

The Relationship of Magnetic Twist and Plasma Motion in a Magnetic Cloud

Our recent investigations indicate that interplanetary magnetic clouds (MCs) have a high-twist core and a weak-twist outer shell. Utilizing the velocity-modified uniform-twist force-free flux rope model, we further investigate the relationship between the twist profile of magnetic field lines and the distribution of the plasma poloidal angular velocity inside an MC. The poloidal velocity in the MC is 11 km s−1. There are evidently positive correlations between the absolute value of the twist and the plasma poloidal angular velocity in peeled flux ropes or flux rope layers, although the correlation coefficients in flux rope layers are less than those in peeled flux ropes. This finding suggests that plasma flows are frozen-in magnetic field lines as we expected for interplanetary medium, of which the magnetic Reynolds number is large. Furthermore, based on this picture, we infer the axial velocity in the MC frame, which is less than 10 km s−1 and almost uniform in the cross section of the MC. Besides, it is inferred that the plasma flows velocity in the MC is much less than the local Alfvén speed

Comparison of counterstreaming suprathermal electron signatures of ICMEs with and without magnetic cloud: are all ICMEs flux ropes?

Context. Magnetic clouds (MCs), as in large-scale interplanetary magnetic flux ropes, are usually still connected to the Sun at both ends near 1 AU. Many researchers believe that all nonMC interplanetary coronal mass ejections (ICMEs) also have magnetic flux rope structures, which are inconspicuous because the observing spacecraft crosses the flanks of the rope structures. If so, the field lines of nonMC ICMEs should also usually be connected to the Sun at both ends. Aims. We want to know whether or not the field lines of most nonMC ICMEs are still connected to the Sun at both ends. Methods. This study examined the counterstreaming suprathermal electron (CSE) signatures of 272 ICMEs observed by the Advanced Composition Explorer (ACE) spacecraft from 1998 to 2008 and compared the CSE signatures of MCs and nonMC ICMEs. Results. Results show that only 10 of the 101 MC events (9.9% ) and 75 of the 171 nonMC events (43.9%) have no CSEs. Moreover, 21 of the nonMC ICMEs have high CSE percentages (more than 70%) and show relatively stable magnetic field components with slight rotations, which are in line with the expectations that the observing spacecraft passes through the flank of magnetic flux ropes. Therefore, the 21 events may be magnetic flux ropes but the ACE spacecraft passes through their flanks of magnetic flux ropes. Conclusions. Considering that most other nonMC events have disordered magnetic fields, we suggest that some nonMC ICMEs inherently have disordered magnetic fields, and therefore no magnetic flux rope structures

Magnetic-reconnection exhausts in the sheath of magnetic clouds

Context. Reconnection exhausts are a common phenomenon in the solar wind. Many exhausts were observed between an interplanetary coronal mass ejection (ICME) and the ambient solar wind, or between two ICMEs, or within the interior of a single ICME. Observed exhausts are almost exclusively associated with ICMEs with low (often much lower than 1) proton beta. Aims. Often the sheath regions of ICMEs have a high level of plasma beta. Therefore we aim to find out whether the reconnection exhausts occur frequently in the sheath regions of ICMEs. Methods. We examined the plasma and magnetic-field data in the sheath of the magnetic cloud (i.e., ICME) observed on 18–20 October 1995, and identified six reconnection exhausts. Results. The six reconnection exhausts occured within regions of proton beta that was higher than unity. Five of them occurred on a high level (>2.2) of proton beta. Conclusions. Low proton beta is no exclusive condition for magnetic reconnection. Reconnection may occur frequently in the sheath of ICMEs when magnetic fields from different source regions, that is, from diffident orientations, are pushed together