Structure and fluctuations of a slow ICME sheath observed at 0.5 au by the Parker Solar Probe

Abstract

Context. Sheath regions ahead of interplanetary coronal mass ejections (ICMEs) are compressed and turbulent global heliospheric structures. Their global and fine-scale structure are outstanding research problems, and only a few studies have been conducted on this topic closer to the Sun than 1 au. Comprehensive knowledge of the sheath structure and embedded fluctuations and of their evolution in interplanetary space is important for understanding their geoeffectiveness, their role in accelerating charged particles to high energies, the interaction of ICMEs with the ambient wind, and the transport of energy between boundaries. Aims. Our key aims are to investigate in detail the overall structure, as well as nature (stochastic, chaotic, or periodic) and origin, of magnetic fluctuations within a sheath ahead of a slow ICME in the inner heliosphere. Methods. We used magnetic field and plasma observations from the Parker Solar Probe (PSP) during a sheath region observed at ∼0.5 au on March 15, 2019, ahead of a slow and slowly expanding streamer blow-out CME bracketed between a slower and faster stream. To examine the magnetohydrodynamic-scale turbulent properties, we present an analysis of the fluctuation amplitudes, magnetic compressibility of fluctuations, partial variance of increments (PVI), normalised cross helicity (σc), and normalised residual energy (σr). We also conducted a Jensen-Shannon permutation entropy and complexity analysis. Results. The investigated sheath consisted of slower and faster flows that were separated by a brief (∼15 min) change in the magnetic sector bounded by current sheet crossings and a velocity shear zone. The fluctuation amplitudes and frequency of high PVI values were larger and higher throughout the sheath than in the upstream wind and had dominantly negative σr and strongly positive σc. The velocity shear region marked a strong increase in temperature and specific entropy, and the following faster flow had large local patches of positive σr as well as larger fluctuation amplitudes and higher PVI values, in particular at smaller timescales. Fluctuations in the preceding wind and in the sheath were found to be stochastic. However, sheath fluctuations showed lower entropy and higher complexity, with entropy showing a reducing and complexity an increasing trend with increasing time lag. Conclusions. The two-part sheath structure was likely a result of a warp in the heliospheric current sheet (HCS) that was swept up and compressed into the sheath. The driving ejecta accelerated and heated the wind at the back of the sheath, which then interacted with the slower wind ahead of the HCS warp. This also caused some distinct differences in fluctuation properties across the sheath. Sheaths of slow ICMEs originating as streamer blow-outs can thus have complex structure where fluctuation properties are not just downstream shock properties, but are generated within the sheath. At short timescales, fluctuations feature fully developed and imbalanced MHD turbulence, while at longer scales, fluctuations are increasingly dominated by intermittent coherent and ordered structures.

The S-Web Origin of Composition Enhancement in the Slow-to-moderate Speed Solar Wind

“…Since we aim to use PVI to identify the sharp magnetic boundaries of coherent flux tubes and/or small flux-rope plasmoids, we choose a temporal scale of ℓ PVI = 2.13 minutes and an averaging window of 24 hr (10 times the magnetic field's mean correlation timescale above). Again, one makes a determination of the significance of any given PVI peak via thresholding, where some authors have used PVI > 2 (Pecora et al 2021), >2.4 (Greco et al 2008), >3 (Kilpua et al 2022), or even larger thresholds of >4-6 (e.g., Servidio et al 2011;Zhou et al 2019). Here, we use PVI > 3.0 during each of our CR 2002 solar wind intervals for ease of comparison between the HS and PS PVI statistics.…”

A Coronal Mass Ejection Impacting Parker Solar Probe at 14 Solar Radii

“…Despite WISPR observations, the CME in situ counterparts did not reach PSP (Howard et al 2019;Hess et al 2020;Liewer et al 2020;Wood et al 2020;Braga & Vourlidas 2021;Howard et al 2022). Conversely, PSP obtained in situ measurements for some events, but mostly far from the solar corona (Lario et al 2020;Nieves-Chinchilla et al 2020;Kilpua et al 2022;Pal et al 2022;Ledvina et al 2023). Some in situ signatures of a CME leg observed at 14 R e were reported by McComas et al (2023), but they were not simultaneous with the remote sensing observations.…”

The Faraday Effect Tracker of Coronal and Heliospheric Structures (FETCH) instrument

“…For example, MHD waves facilitate the transfer of energy between spatial and temporal scale sizes as shown in turbulence studies (Viall and Borovsky, 2020). The waves have a significant role in discontinuities and shocks (Kilpua et al, 2022); they can drive particle heating and acceleration (Suzuki and Inutsuka, 2005), and their presence constrains the characteristics of the plasma that can support them (Hollweg et al, 1982;.…”