Causes and Consequences of Magnetic Complexity Changes within Interplanetary Coronal Mass Ejections: A Statistical Study

Abstract

We present the first statistical analysis of complexity changes affecting the magnetic structure of interplanetary coronal mass ejections (ICMEs), with the aim of answering the questions: How frequently do ICMEs undergo magnetic complexity changes during propagation? What are the causes of such changes? Do the in situ properties of ICMEs differ depending on whether they exhibit complexity changes? We consider multispacecraft observations of 31 ICMEs by MESSENGER, Venus Express, ACE, and STEREO between 2008 and 2014 while radially aligned. By analyzing their magnetic properties at the inner and outer spacecraft, we identify complexity changes that manifest as fundamental alterations or significant reorientations of the ICME. Plasma and suprathermal electron data at 1 au, and simulations of the solar wind enable us to reconstruct the propagation scenario for each event, and to identify critical factors controlling their evolution. Results show that ∼65% of ICMEs change their complexity between Mercury and 1 au and that interaction with multiple large-scale solar wind structures is the driver of these changes. Furthermore, 71% of ICMEs observed at large radial (>0.4 au) but small longitudinal (<15°) separations exhibit complexity changes, indicating that propagation over large distances strongly affects ICMEs. Results also suggest that ICMEs may be magnetically coherent over angular scales of at least 15°, supporting earlier theoretical and observational estimates. This work presents statistical evidence that magnetic complexity changes are consequences of ICME interactions with large-scale solar wind structures, rather than intrinsic to ICME evolution, and that such changes are only partly identifiable from in situ measurements at 1 au.

Multi-spacecraft Observations of the Evolution of Interplanetary Coronal Mass Ejections between 0.3 and 2.2 au: Conjunctions with the Juno Spacecraft

“…Four of the 10 events (40%) display at least one significant change in flux rope orientation with increasing heliocentric distance at which it was observed. This is consistent with the 38% of flux rope events (5/13) found to have significant changes in orientation between MESSENGER and 1 au by Scolini et al (2021b); however, this result must be taken with care due to the small sample size of events in both studies. We leave the causes behind such changes in orientation as an open question to be investigated by future studies.…”

“…As ICMEs propagate, many physical processes can affect their evolution (see Manchester et al 2017), including deflections that significantly alter their course (Gosling et al 1987) and rotations or changes to their inclination (e.g., Nieves-Chinchilla et al 2013). Such changes in orientation or reorientations of the magnetic structure can be considered as increases to the magnetic complexity of an ICME (e.g., Scolini et al 2021b) and have mostly been attributed to their interaction with other solar wind transients (Winslow et al 2016(Winslow et al , 2021a(Winslow et al , 2021b.…”

“…We note that although the fitting was conducted in RTN coordinates at each spacecraft, the difference in angular separation between spacecraft is small and thus we assume the R-T plane as equivalent between observations when compared to the large uncertainties of the LFF fitting model. As in Scolini et al (2021b), we define a significant change in flux rope orientation as greater than ±45°in θ 0 or ±60°in f 0 . This is in line with the large uncertainties in flux rope orientation produced by the LFF model: for such fits, Lepping et al (2003) estimated errors of ±13°and ±30°in θ and f, respectively, whereas Al-Haddad et al (2013 previously found that differing fitting models may vary in their estimate of the flux rope axis orientation by ±45°in θ or ±60°in f. Significant changes in orientation are highlighted in bold in Table 2.…”

“…The multispacecraft events identified in Salman et al (2020) were utilized by Lugaz et al (2020) to investigate the relationship between the global and local expansion of ICMEs; such measures of local expansion and comparison to global expansion rate for individual ICMEs had not previously been possible, and it was found that the two measures of expansion have little relation to each other. Scolini et al (2021b) also used the same multi-spacecraft catalog to investigate how the magnetic complexity of ICMEs changes as they propagate, and the causes behind such changes; ∼65% of ICMEs were found to change their complexity between MESSENGER and 1 au, with interactions between multiple large-scale solar wind structures the main driver of these changes.…”

On the utility of flux rope models for CME magnetic structure below 30Rs

“…Since magnetic field hodograms are increasingly being used in the analysis of in-situ ICME observations (e.g. Nieves-Chinchilla et al, 2019;Scolini et al, 2022;Davies et al, 2021), including recent PSP events (e.g. Nieves-Chinchilla et al, 2020;Winslow et al, 2021), we constructed hodogram visualizations of both the MHD simulation data and the in-situ flux rope model fits to further evaluate the quality of the flux rope model reconstructions.…”