Contribution of the ageing effect to the observed asymmetry of interplanetary magnetic clouds

Abstract

<jats:p><jats:italic>Context.</jats:italic> Large magnetic structures are launched away from the Sun during solar eruptions. They are observed as (interplanetary) coronal mass ejections (ICMEs or CMEs) with coronal and heliospheric imagers. A fraction of them are observed in situ as magnetic clouds (MCs). Fitting these structures properly with a model requires a better understanding of their evolution.</jats:p> <jats:p><jats:italic>Aims.</jats:italic> In situ measurements are made locally when the spacecraft trajectory crosses the magnetic configuration. These observations are taken for different elements of plasma and at different times, and are therefore biased by the expansion of the magnetic configuration. This ageing effect means that stronger magnetic fields are measured at the front than at the rear of MCs. This asymmetry is often present in MC data. However, the question is whether the observed asymmetry can be explained quantitatively from the expansion alone.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> Based on self-similar expansion, we derived a method for estimating the expansion rate from the observed plasma velocity. We next corrected the observed magnetic field and the spatial coordinate along the spacecraft trajectory for the ageing effect. This provided corrected data as in the case when the MC internal structure were observed at the same time.</jats:p> <jats:p><jats:italic>Results.</jats:italic> We apply the method to 90 best-observed MCs near Earth (1995–2012). The ageing effect is the main source of the observed magnetic asymmetry for only 28% of the MCs. After correcting for the ageing effect, the asymmetry is almost symmetrically distributed between MCs with a stronger magnetic field at the front and those at the rear of MCs.</jats:p> <jats:p><jats:italic>Conclusions.</jats:italic> The proposed method can efficiently remove the ageing bias within in situ data of MCs, and more generally, of ICMEs. This allows us to analyse the data with a spatial coordinate, such as in models or remote-sensing observations.</jats:p>

Exploring the radial evolution of Interplanetary Coronal Mass Ejections using EUHFORIA

“…From EUHFORIA simulations, we report values of ζ CME ranging between 1.09 and 0.63 at the different heliocentric distances considered (Subsection 4.5). Such numbers are comparable with typical observational values at 1 au, which suggests the asymmetries in the CME magnetic field profile are primarily due to actual cross-section asymmetries, although ageing may still provide a significant contribution leading to a mix of the two effects (Démoulin et al 2020). For the particular CME considered, and based on our simulations, we consider ageing may be most relevant for heliocentric distances closer than 0.5 au, where ζ CME was higher than 0.8, while at distances larger than 0.5 au the lower expansion rates (ζ CME < 0.8) suggest cross-sectional asymmetries as the dominant source of magnetic asymmetry.…”

“…Previous studies agree in considering the latter to be typically dominant at 1 au. However, the ageing effect has been found to be the main source of magnetic asymmetry for a significant minority of magnetic ejectas at 1 au, making the consideration and correction for its effects worthwhile, in particular, in the case of large events (Démoulin et al 2020). Furthermore, we note that the effect of ageing at different heliocentric distances than 1 au, particularly those closer to the Sun, cannot be discarded a priori when interpreting the results obtained from EUHFORIA simulations.…”

“…Démoulin et al 2008Démoulin et al , 2018. However, it should be stressed that more recent estimates considering magnetic ejectas with expansion rates ζ CME ∈ (0.8, 1.2) reported ageing effect as the main source of the observed magnetic asymmetry for more than one every four events (Démoulin et al 2020).…”

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

“…Indeed, we find that the velocity changes by less than 20% across the ME. In particular, we do not observe the consequence of such an expansion on the magnetic profile for ICMEs without a sheath, which implies the same on its extension with the magnetic flux conservation (see Démoulin et al 2020, for an analysis of expansion). Moreover, the evolution of these geometrical parameters are counteracted by a weaker magnetic field (due to magnetic flux conservation), so a less effective screening with time.…”

Categorization of Coronal Mass Ejection Driven Sheath Regions: Characteristics of STEREO Events

“…Inside the MEs of both types of sheaths, the magnetic field fluctuations stabilise to lower levels since MEs are low-beta structures.The abrupt transitions in the magnetic field strength, proton density, and dynamic pressure (see first, fourth, and fifth panel of Figure2) at the front of the MEs are also more pronounced for propagation sheaths. For both types of sheaths, the magnetic field strengths at the leading edges are higher compared to the rear, leading to asymmetric magnetic field profiles inside the MEs (seeDémoulin et al 2008) Démoulin et al (2020). suggested that these asymmetric magnetic field profiles can possibly occur due to a stronger compression on one side of the ME.…”