Abstract
Abstract. We present a comprehensive statistical analysis of mirror mode waves and the properties of their plasma surroundings in sheath regions driven by interplanetary coronal mass ejection (ICME). We have constructed a semi-automated method to identify mirror modes from the magnetic field data. We analyze 91 ICME sheath regions from January 1997 to April 2015 using data from the Wind spacecraft. The results imply that similarly to planetary magnetosheaths, mirror modes are also common structures in ICME sheaths. However, they occur almost exclusively as dip-like structures and in mirror stable plasma. We observe mirror modes throughout the sheath, from the bow shock to the ICME leading edge, but their amplitudes are largest closest to the shock. We also find that the shock strength (measured by Alfvén Mach number) is the most important parameter in controlling the occurrence of mirror modes. Our findings suggest that in ICME sheaths the dominant source of free energy for mirror mode generation is the shock compression. We also suggest that mirror modes that are found deeper in the sheath are remnants from earlier times of the sheath evolution, generated also in the vicinity of the shock. Keywords. Interplanetary physics (plasma waves and turbulence; solar wind plasma) – space plasma physics (waves and instabilities)
Highlights
Mirror mode (MM) waves generated by the mirror instability arise from the antiphase, low-frequency fluctuations of the magnetic field and plasma density when a sufficient temperature anisotropy is present in the plasma
We have identified mirror modes (MMs) from 91 interplanetary coronal mass ejection (ICME)-driven sheaths using a semi-automated method and investigated their occurrence and properties
We summarize the key findings of this study as follows:
Summary
Mirror mode (MM) waves generated by the mirror instability arise from the antiphase, low-frequency fluctuations of the magnetic field and plasma density when a sufficient temperature anisotropy is present in the plasma. MM waves are linearly polarized structures that appear as sharp increases and decreases in the magnetic field data (peaks and dips) They are frequently observed in heliospheric plasma, in particular in different sheath structures. The authors used a superposed epoch analysis to investigate how the occurrence of MMs depends on the ICME properties They discovered MM favoring conditions in the sheath regions of magnetic clouds (i.e., a subset of ICME featuring enhanced magnetic field, smooth rotation of the magnetic field direction and low plasma beta; e.g., Burlaga et al, 1981) and ICMEs with preceding shocks, whereas ICMEs without magnetic cloud structures and preceding shocks lacked these features.
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