20 Years of ACE Data: How Superposed Epoch Analyses Reveal Generic Features in Interplanetary CME Profiles

Abstract

Interplanetary coronal mass ejections (ICMEs) are magnetic structures propagating from the Sun's corona to the interplanetary medium. With over 20 years of observations at the L1 libration point, ACE offers hundreds of ICMEs detected at different times during several solar cycles and with different features such as the propagation speed. We investigate a revisited catalog of more than 400 ICMEs using the superposed epoch method on the mean, median, and the most probable values of the distribution of magnetic and plasma parameters. We also investigate the effects of the speed of ICMEs relative to the solar wind, the solar cycle, and the existence of a magnetic cloud on the generic ICME profile. We find that fast-propagating ICMEs (relatively to the solar wind in front) still show signs of compression at 1 au, as seen by the compressed sheath and the asymmetric profile of the magnetic field. While the solar cycle evolution does not impact the generic features of ICMEs, there are more extreme events during the active part of the cycle, widening the distributions of all parameters. Finally, we find that ICMEs with or without a detected magnetic cloud show similar profiles, which confirms the hypothesis that ICMEs with no detected magnetic clouds are crossed further away from the flux rope core. Such a study provides a generic understanding of processes that shape the overall features of ICMEs in the solar wind and can be extended with future missions at different locations in the solar system. Plain Language Summary Interplanetary coronal mass ejections (ICMEs) are magnetized clouds of solar material expelled from the Sun's atmosphere into the interplanetary medium. In the present article, we use plasma and magnetic data from the NASA ACE mission positioned at the Lagrange 1 point and perform statistical studies over the 400 ICMEs detected over the last 20 years. Such an approach, called a superposed epoch analysis, provides a temporal description of the generic features of ICMEs. In particular, we find that ICMEs that propagate faster than their surrounding solar wind still show compressed regions of plasmas and stronger magnetic field. While we find that the 11-year solar cycle does not affect the generic properties of ICMEs, we observe more extreme events during the active phase. Finally, we also find that a subset of ICMEs, called magnetic clouds and often associated with a stronger and more coherent magnetic field, have the same properties as other ICMEs. This confirms that most ICMEs may be magnetic clouds but which detection is limited by the spacecraft crossing. Such a study, based on long-term heliospheric missions, allows us to probe physical processes that occur during the propagation of ICMEs, which can help better predict space weather and its consequences.

Statistical Analysis of Magnetic Field Fluctuations in Coronal Mass Ejection-Driven Sheath Regions

“…Sheath events listed by Kilpua et al (2019) for the years 1997-2018 with some additional events listed by Regnault et al (2020) have been analyzed in this study. OMNI data were used to identify sheaths in the list given by Kilpua et al (2019); a few sheaths with Wind data gaps were excluded, leaving a total of 79 sheaths analyzed in the present study (Supplementary Table S1).…”

Magnetic Structure and Propagation of Two Interacting CMEs From the Sun to Saturn

“…The properties of interplanetary CMEs (or ICMEs; e.g., Kilpua et al, 2017) have been studied mainly around 1 AU (e.g., Cane & Richardson, 2003;Jian et al, 2006Jian et al, , 2018Nieves-Chinchilla et al, 2019;Owens, 2018;Regnault et al, 2020;I. G. Richardson & Cane, 2010), that is where continuous in-situ measurements of the solar wind have been available for several decades and up to this day.…”

The two-step Forbush decrease: a tale of two substructures modulating galactic cosmic rays within coronal mass ejections

“…The superposed epoch analysis (or Chree analysis, Chree 1913) consists in normalizing a dataset, com-monly a time series of different events on a similar timescale, and obtaining the time series' behavior of a statistically relevant parameter for each normalized time bins. In the context of ICMEs, previous studies (Lepping et al 2003;Rodriguez et al 2016;Masías-Meza et al 2016;Janvier et al 2019;Regnault et al 2020) typically take well-defined times for the normalization: the time of the discontinuity, the start time of the ME or MC, and the end time of the ME or MC. In the following, the normalization is similar to the studies of Masías-Meza et al (2016); Regnault et al (2020): the unit time for the normalized time series is taken as the length of the sheath (if it exists), and the ME is normalized to three times this unit time.…”

“…In the context of ICMEs, previous studies (Lepping et al 2003;Rodriguez et al 2016;Masías-Meza et al 2016;Janvier et al 2019;Regnault et al 2020) typically take well-defined times for the normalization: the time of the discontinuity, the start time of the ME or MC, and the end time of the ME or MC. In the following, the normalization is similar to the studies of Masías-Meza et al (2016); Regnault et al (2020): the unit time for the normalized time series is taken as the length of the sheath (if it exists), and the ME is normalized to three times this unit time. This choice is motivated by the previously cited studies, which observed that sheaths have on average a duration three times less that of the ME.…”

“…We also used the Richardson and Cane ICME list 2 that we revised, as reported in (Regnault et al 2020). This revision includes checking/redefining the frontiers of ICMEs and only taking into account ICMEs with clear signatures.…”