Multi-scale structure and composition of ICME prominence material from the Solar Wind Analyser suite - Solar Orbiter
Multi-scale structure and composition of icme prominence material from the solar wind analyser suite
(Solar Orbiter nugget #4 by Ryan M. Dewey1, Susan T. Lepri1, Stefano Livi1,2, Christopher J. Owen3, Philippe Louarn4, Raffaella D’Amicis5, and the Solar Orbiter/SWA team)
Measurements of heavy ion composition in the solar wind can be leveraged to both identify interplanetary coronal mass ejections (ICMEs) as well as extract information about the origin, release, and structure of these events. Observed remotely, coronal mass ejections at the Sun usually exhibit a characteristic three-part structure: a bright leading edge of highly ionized plasma, a low-density magnetically dominated cavity, and an embedded dense cold core associated with a solar prominence [1,2]. The coronal processes that create this spatial structure impart specific signatures in the plasma composition related to their varying thermal and density structure. The extreme energy release and heating of coronal plasma associated with eruption ionize the local plasma to elevated charge states (e.g., Fe16+) compared to the typical solar wind (Fe8–12+) [e.g., 3-5]. Conversely, prominence plasma originates from deeper in the solar atmosphere where cooler temperatures and denser material result in low-charge material (e.g., Fe2+) [6,7]. When observed in situ in the heliosphere, ICMEs often possess a magnetic cloud structure led by a density pileup and are associated with changes in plasma composition compared to the surrounding solar wind [e.g., 8,9]. However, while high-charge-state material is commonly observed within ICMEs , low-charge-state material is only rarely measured. A survey spanning over a decade of observations near Earth found only ~4% of ICMEs contained this low-charge prominence plasma .
Reconciling the differences in structure inferred from remote CME and in situ ICME measurements remains an observational and theoretical challenge. The low-charge-state material is expected to survive the eruption and be ejected into interplanetary space in or about the ICME [12,13]. However, remote observations of the prominence eruptions suggest that this material fills only a small fraction of the ICME  or that it fragments  such that the chance of observing this plasma in situ in the heliosphere is low. Furthermore, evolution of the ICME and its contents may contribute to differences in the remote and in situ observation. Magnetic reconnection between an ICME and the surrounding solar wind, for example, may dilute the low-charge material by changing magnetic topology to allow different plasma populations to mix [e.g., 16]. The Solar Orbiter mission  delivers new insight into the occurrence and characteristics of prominence plasma within ICMEs by providing dedicated plasma composition measurements at unprecedented resolution in the inner heliosphere.
The Solar Wind Analyser instrument suite  on Solar Orbiter recorded a short interval of prominence plasma during a series of ICMEs that passed over the spacecraft during late October to early November 2021. The suite includes three separate sensors: the Proton-Alpha System (PAS) that measures characteristics of the abundant proton and alpha ions; the Heavy Ion Sensor (HIS) that measures plasma composition via characteristics of minor heavy ion species; and the Electron Analyser System (EAS) that measures characteristics of solar wind electron populations. Measurements from the three sensors and the magnetometer  over a 7.5-day period are presented in Figure 1 and show the passage of two ICMEs with sharply contrasting features. The second event displays more typical in situ signatures, including a low-density flux rope with elevated carbon, oxygen, and iron charge states. However, the first ICME contains a short interval dominated by dense low-charge plasma (orange interval).
Figure 1. Solar Orbiter time series of the late October to early November 2021 ICMEs. PAS proton (a) density, (b) speed, and (c) temperature. HIS (d) carbon charge state distribution, (e) oxygen charge state distribution, (f) iron charge state distribution, (g) O7+/O6+ ratio (red) and C6+/C5+ ratio (blue), (h) Fe/O abundance ratio using Fe6-20+ and O5-8+. EAS (i) electron strahl pitch angle distribution. MAG (j) field strength (black) and vector components (RTN in red, green, and blue, respectively). Vertical lines indicate specific features of the ICMEs.
Despite the short duration of this prominence interval compared to the duration of the ICMEs, the SWA suite highlights the detail of its structure over a range of spatiotemporal scales in Figure 2. Bracketed by orange lines, the high-density low-charge material lasts for only 36 minutes and is embedded within a large-scale magnetic rotation. Within the prominence interval, distinct changes in the electron strahl pitch angle measured by EAS divide the interval into fourteen segments ranging from <30 seconds to several minutes in duration. These segments, bounded by the vertical black lines, are associated with small-scale rotations in the magnetic field and changes in the plasma characteristics. The fast time resolution of PAS and HIS, at 4 and 30 seconds respectively, capture fine structures in the bulk properties and plasma composition. Each of the low-charge species (H+, He+, He2+, C+, O+, and Fe1-4+) share similar large-scale spatiotemporal structure (enhancements at both the beginning and end of the interval) but exhibit individual behavior about the small-scale magnetic field structures. Highly ionized species (C6+, O6+, Fe8-12+, and Fe14-20+) are persistent throughout the interval but are anti-correlated with the low-charge material, showing depletions during both low-charge enhancements. The multi-scale structure evident within the plasma and magnetic field, the anticorrelation between low- and high-charge species abundance, and the small amount of low-charge plasma outside the main interval suggest gradual loss of the low-charge material from, and entry of the high-charge plasma into, the magnetic structures. Mixing of these populations more likely occurred during transit rather than during eruption.
Figure 2. Detailed structure of the prominence interval. PAS (a-b) speed, (c) temperature, (d) H+ density, (e) He+(black) and He2+ (red) densities. HIS count rates for: (f) C+ (black) and C6+ (red); (g) O+ (black) and O6+ (red); and (h) Fe1-4+ (black), Fe8-12+ (blue), Fe14-20+ (red). EAS (i) electron strahl pitch angle distribution. MAG (j-l) magnetic field RTN components, with faint gray indicating ±|B|.
Observations of interplanetary prominence material are rare and present unique opportunities to study ICME formation, eruption, internal structure, and evolution. These in situ Solar Orbiter plasma measurements from SWA provide significant improvements over earlier data sets – in particular, the composition observations are unparalleled in their temporal and species resolution. HIS operates over an order of magnitude faster than previous similar instruments, allowing it to record the fine structure of the prominence interval. Previously flown instruments would have recorded the entire interval as a single measurement . Taken together, the small, inhomogeneous spatiotemporal structure of the prominence plasma, low-charge plasma confinement and leakage, and high-charge plasma entry are consistent with fragmentation of the prominence plasma near the Sun. Additional events captured by the Solar Wind Analyser suite on Solar Orbiter in future will reveal the frequency of this process.
These observations, alongside additional compositional analysis and comparison with spacecraft measurements of these ICMEs near Earth, are being prepared for publication.
1University of Michigan, Ann Arbor, MI, USA
2Southwest Research Institute, San Antonio, TX, USA
3Mullard Space Science Laboratory, University College London, Holmbury St. Mary, UK
4Laboratoire de Physique des Plasmas, Ecole Polytechnique, Palaiseau, France
5INAF-Istituto di Astrofisica e Planetologia Spaziali, Roma, Italy
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