Characteristics and evolution of sheath and leading edge structures of interplanetary coronal mass ejections in the inner heliosphere based on Helios and Parker Solar Probe observations

Abstract

Context. We investigated the plasma and magnetic field characteristics of the upstream regions of interplanetary coronal mass ejections (ICMEs) and their evolution as function of distance to the Sun in the inner heliosphere. Results are related both to the development of interplanetary shocks, sheath regions, and compressed solar wind plasma ahead of the magnetic ejecta (ME). Aims. From a sample of 45 ICMEs observed by Helios 1/2 and the Parker Solar Probe, we aim to identify four main density structures; namely shock, sheath, leading edge, and ME itself. We compared characteristic parameters (proton particle density, plasma-beta, temperature, magnetic field strength, proton bulk speed, and duration) to the upstream solar wind in order to investigate the interrelation between the different density structures. Methods. For the statistical investigation, we used plasma and magnetic field measurements from 40 well-observed Helios 1/2 events from 1974-1981. Helios data cover the distance range from 0.3-1 au. For comparison, we added a sample of five ICMEs observed with the Parker Solar Probe from 2019-2021 over the distance range of 0.32-0.75 au. Results. It is found that the sheath structure consists of compressed plasma as a consequence of the turbulent solar wind material following the shock and lies ahead of a region of compressed ambient solar wind. The region of compressed solar wind plasma is typically found directly in front of the magnetic driver and seems to match the bright leading edge commonly observed in remote sensing observations of CMEs. From the statistically derived density evolution over distance, we find the CME sheath becomes denser than the ambient solar wind at about 0.06 au. From 0.09-0.28 au, the sheath structure density starts to dominate over the density within the ME. The ME density seems to fall below the ambient solar wind density over 0.45-1.18 au. Besides the well-known expansion of the ME, the sheath size shows a weak positive correlation with distance, while the leading edge seems not to expand with distance from the Sun. We further find a moderate anti-correlation between sheath density and local solar wind plasma speed upstream of the ICME shock. An empirical relation is derived connecting the ambient solar wind speed with sheath and leading edge density. We provide constraints to these results in this paper. Conclusions. The average starting distance for actual sheath formation could be as close as 0.06 au. The early strong ME expansion quickly ceases with distance from the Sun and might lead to a dominance in the sheath density between 0.09 and 0.28 au. The leading edge can be understood as a separate structure of compressed ambient solar wind directly ahead of the ME and is likely the bright leading edge of CMEs often seen in coronagraph images. The results allow for better interpretation of ICME evolution and possibly the observed mass increase due to enlargement of the sheath material. The empirical relation between sheath and leading edge density and ambient solar wind speed...

On the Mesoscale Structure of Coronal Mass Ejections at Mercury’s Orbit: BepiColombo and Parker Solar Probe Observations

“…In fact, the CME analyzed in this work might still have been experiencing significant expansion by the time it was detected at Parker, since the sheath appears overall less dense than the following ejecta, in agreement with Salman et al (2021), who found that expansion sheaths tend to display lower densities, on average. Furthermore, Temmer & Bothmer (2022) estimated that the sheath density tends to overcome the ejecta density in the heliocentric distance range of 0.09-0.28 au, after which expansion of the driver generally weakens. We also note that Giacalone et al (2023) came to the same conclusion (i.e., that the 2022 February 15 CME was overexpanding by the time it impacted Parker) by observing the intensity of energetic particle increase behind the shock, possibly resulting from ions filling an expanding volume associated with the propagation of a blast wave.…”

Refined Modeling of Geoeffective Fast Halo CMEs During Solar Cycle 24

“…different solar cycles) and different versions of the models (Falkenberg et al., 2010). In addition, while the CME geometry derivation has been largely investigated (e.g., Mays et al., 2015), the estimation of the CME density and its evolution from remote observations has not been done comprehensively until recently (Temmer & Bothmer, 2022; Temmer et al., 2021).…”

“…On average, CMEs consist of several distinct parts, which we are able to distinguish well in white‐light coronagraph images, with the most pronounced structures referred to as flux ropes and shock‐sheaths (Vourlidas et al., 2013). The same structures can be identified from in‐situ measurements, where also additional sub‐structures are detected, but it is not fully understood how these relate to the observed white‐light structures (Kilpua et al., 2020; Temmer & Bothmer, 2022). Globally, the flux rope and shock‐sheath halo CME parts can be approximated by two simple geometrical shapes: a cone shape coming from the original ice‐cream cone model, representing the flux rope, and an ellipsoid representing the shock‐sheath (Xie et al., 2004).…”

“…The same structures can be identified from in-situ measurements, where also additional sub-structures are detected, but it is not fully understood how these relate to the observed white-light structures (Kilpua et al, 2020;Temmer & Bothmer, 2022). Globally, the flux rope and shock-sheath halo CME parts can be approximated by two simple geometrical shapes: a cone shape coming from the original ice-cream cone model, representing the flux rope, and an ellipsoid representing the shock-sheath (Xie et al, 2004).…”

A Survey of Coronal Mass Ejections Measured In Situ by Parker Solar Probe during 2018–2022

“…From a linear approximation, Salman et al (2020b) found the sheath formation to start at around 0.24 au. In a recent study, from a statistically derived density evolution over r using Helios and PSP observations, Temmer & Bothmer (2022) theorized the sheath to start forming even before that, at ∼0.06 au. However, we observe that only two out of the eight ICMEs in our list within 0.44 au have sheaths and none before 0.38 au.…”