Multispacecraft Observation of Unidirectional and Bidirectional Alfvén Waves within Large-scale Magnetic Clouds

Abstract

Recent years have seen growing evidence of the existence of Alfvén waves within interplanetary magnetic flux ropes, which are believed to be an important aspect of dynamics connecting the Sun and the heliosphere. Previous studies, due to localized observation by single spacecraft, focused on sunward or antisunward Alfvén waves propagating along with magnetic field lines. In this Letter, for the first time, we use multispacecraft observations to verify and analyze two large-scale magnetic clouds (MCs), when the spacecraft had quite different spatial separations. What surprises us is that not only unidirectional but bidirectional Alfvén waves exist in the large-scale MC, which is rooted to the Sun. We speculate that unidirectional Alfvén waves within an MC are generated by distortions produced within a preexisting flux rope, and bidirectional Alfvén waves are emitted from the center of reconnection and then travel outward along with two loop legs of an MC.

First Analysis of In Situ Observation of Surface Alfvén Waves in an ICME Flux Rope

“…These modes are appealing because they carry significant energy from the subphotospheric regions to the corona, and provide energy for coronal heating (Van Doorsselaere et al 2008). There is a high possibility of identifying AWs in interplanetary space when a magnetic flux rope erupts, no matter the mode in which they are present (Wang et al 2019), However, sometimes they are hardly distinguishable from a flux rope configuration (Higginson & Lynch 2018), and therefore their interrelationships are more complex than reported in several studies.…”