Global and local expansion of magnetic clouds in the inner heliosphere

Abstract

Context. Observations of magnetic clouds (MCs) are consistent with the presence of flux ropes detected in the solar wind (SW) a few days after their expulsion from the Sun as coronal mass ejections (CMEs). Aims. Both the in situ observations of plasma velocity profiles and the increase of their size with solar distance show that MCs are typically expanding structures. The aim of this work is to derive the expansion properties of MCs in the inner heliosphere from 0.3 to 1 AU. Methods. We analyze MCs observed by the two Helios spacecraft using in situ magnetic field and velocity measurements. We split the sample in two subsets: those MCs with a velocity profile that is significantly perturbed from the expected linear profile and those that are not. From the slope of the in situ measured bulk velocity along the Sun-Earth direction, we compute an expansion speed with respect to the cloud center for each of the analyzed MCs. Results. We analyze how the expansion speed depends on the MC size, the translation velocity, and the heliocentric distance, finding that all MCs in the subset of non-perturbed MCs expand with almost the same non-dimensional expansion rate (ζ). We find departures from this general rule for ζ only for perturbed MCs, and we interpret the departures as the consequence of a local and strong SW perturbation by SW fast streams, affecting the MC even inside its interior, in addition to the direct interaction region between the SW and the MC. We also compute the dependence of the mean total SW pressure on the solar distance and we confirm that the decrease of the total SW pressure with distance is the main origin of the observed MC expansion rate. We found that ζ was 0.91 ± 0.23 for non-perturbed MCs while ζ was 0.48 ± 0.79 for perturbed MCs, the larger spread in the last ones being due to the influence of the solar wind local environment conditions on the expansion.

Investigating the observational signatures of magnetic cloud substructure

“…Calculations of the average magnetic field strength over the duration of each cloud reveal that it is comparable (∼17 nT) in all but one of these events (∼32 nT), and the values obtained are within the range expected for a typical magnetic cloud [Lepping et al, 2003]. We find that z is smaller than the mean value obtained for the nonperturbed (not overtaken) magnetic clouds analyzed by Démoulin et al [2008] at 1 AU (z = 0.8 ± 0.2), and those analyzed by Gulisano et al [2010] between 0.3 and 1 AU (z = 0.9 ± 0.2). This is consistent with the presence of an overtaking solar wind stream behind these magnetic clouds, as the lowest values of z are found for overtaken clouds (see Table 1).…”

“…Nearly all of the clouds in this study are followed by fast solar wind streams. We have found that the nondimensional expansion rates of these MCs are low, with values of z < 0.8, indicating that these magnetic clouds are perturbed [Gulisano et al, 2010]. As a fast stream overtakes a magnetic cloud, it is expected to compress the flux rope and so decrease its expansion rate, as shown in MHD simulations [Xiong et al, 2006] and in in situ observations [Gulisano et al, 2010].…”

“…However, the difference, DV x , between the leading and trailing edges is not a direct indicator of the expansion rate of a plasma element, since, in particular, DV x depends on the size of the magnetic cloud so larger magnetic clouds have typically larger DV x values. A better measure of the expansion rate of the magnetic cloud is given by the nondimensional expansion rate, z, defined from theoretical considerations by Démoulin et al [2008] and from data analysis by Gulisano et al [2010],…”

Dynamical evolution of a magnetic cloud from the Sun to 5.4 AU

“…4 panel 3) smoothly and monotonically increases beyond this time, with a strong discontinuity of B y a bit later, on March 26, 13:00 UT. At the center of the flux rope we expect a global extreme of F y (Dasso et al 2006Gulisano et al 2010), in particular, as it was observed when this same cloud was located at 1 AU (panel 3 of Fig. 3).…”

“…However, observations of the MC field configuration are approximately consistent with this magnetic configuration at different heliodistances, ranging from 0.3 to 5 AU (e.g., Bothmer & Schwenn 1998;Leitner et al 2007), so that we expect a small anisotropy on the expansion along different cloud directions (i.e., l ≈ m ≈ n). From observations of different MCs at significantly different heliodistances (Wang et al 2005b;Leitner et al 2007) and from observations of the velocity profile slope from single satellite observations (Démoulin et al 2008;Gulisano et al 2010), it has been found that ζ ≈ 0.8. Since ζ is a combination of l and n, which depends on γ for each cloud, a systematic difference on l and n would be detected in a set of MCs with variable γ angle.…”