Directional discontinuities (DDs) are common structures in the interplanetary space. Correctly determining the normal of a DD is important to understand the changes of phase fronts of magnetic fields adjacent to a discontinuity as well as helpful for the applications in many regimes of space physics research. In this work, we propose a new scheme to estimate the normal directions of DDs by finding the smallest standard deviation of normal magnetic fields derived from the cross product of magnetic fields on both sides of the discontinuity, based on the idea that the phase fronts of the adjacent magnetic fields are closely parallel to the DD plane. By comparing with the normal direction determined from Cluster multiple spacecraft, we show that our scheme can provide the same accuracy as that from the multispacecraft estimation. Moreover, our scheme gives a consistent result of normal estimations at different Cluster spacecraft. We notice that in some cases, the normal directions derived from the minimum variance analysis have large differences between those of multispacecraft method and our scheme implying significant influence of the kinetic effect of particles in the transition region of the discontinuity. A few events of STEREO B observations are further studied to show that our scheme can be applied to DDs in single‐spacecraft measurements with even small rotation of magnetic fields across the discontinuities. With the help of an accurate normal estimation, we can understand the variations of magnetic field phase fronts in the vicinity of discontinuities.

The compressed and turbulent sheath regions of interplanetary magnetic clouds (IMCs) provide a natural laboratory to study electromagnetic waves (EMWs) around the proton cyclotron frequency f cp. Based on the Morlet wavelet spectral analysis, the repeated filtering analysis and the minimum variance analysis of high-resolution magnetic field data from the STEREO spacecraft, 81 EMW events are identified in the sheath regions of six IMCs. These EMWs are all transverse, almost circularly polarized, and quasi-parallel propagating along the background magnetic field B 0. They can be left-handed (LH) or right-handed (RH) polarized in the spacecraft frame, where the occurrence rate of the LH-polarized EMWs is higher than that of RH-polarized ones, consistent with previous observations in the solar wind. Also, a comparative analysis of polarization sense of these EMWs has been made in the spacecraft and plasma frames. Our results show that more than half of EMW events suffer a polarization reversal from the spacecraft to plasma frames, which are deduced to propagate inward relative to the solar wind flow. Others are outward-propagating waves. In the plasma frame, the outward-propagating LH-EMWs and inward-propagating RH-EMWs have relatively higher occurrence rates than the inward-propagating LH-EMWs and outward-propagating RH-EMWs, respectively. Furthermore, in the plasma frame all the frequencies of LH-EMWs are below f cp , but the RH-EMW frequencies can exceed f cp. These results are helpful in understanding the physical properties of EMWs and their roles in the sheath regions of IMCs.

Electromagnetic waves (EMWs) near the proton cyclotron frequency f cp are transverse left-handed (LH) or right-handed (RH) polarized waves, and are ubiquitous in the solar wind. However, the characteristics of these waves in the sheath regions of interplanetary coronal mass ejections (ICMEs) are poorly understood. Through a comprehensive survey of Wind magnetic field and plasma data using dynamic spectra and repeated filtering analyses, 700 EMW events (7.1% of the analysis time) are identified in the 62 ICME sheath regions associated with quasi-perpendicular shocks involved with a low shock Mach number M f and low upstream β 1. In the ICME sheath regions, outward (inward)-propagating LH (RH) EMWs have relatively higher counts and longer duration than inward (outward)-propagating LH (RH) EMWs in the plasma frame, consistent with previous STEREO observations. The spatial distributions of the magnetic field, plasma, and frequency parameters of EMWs are also presented in both spacecraft and plasma frames, especially the proton (alpha) temperature anisotropy , α abundance N α /N p, and normalized differential alpha-proton speed V d/V A. After removing the Doppler shift, 81.1% (59%) of all outward (inward)-propagating LH EMWs have a frequency below (above) 0.5f cp, while 68.3% (64%) of all outward (inward)-propagating RH EMWs have a frequency smaller (greater) than 0.5f cp. Further investigations of local plasma parameters reveal that different excitation mechanisms for EMWs are in different subregions of the ICME sheath regions. These results are helpful in understanding the important role of EMWs in the solar wind–ICME coupling process with different sheath regions.