Abstract
In this paper we perform a detailed analysis of interplanetary flux ropes observed between June~15--17, 2012 at Venus and subsequently at Earth's Lagrange L1 point, while the observation points were separated by about 0.28~AU in radial distance and $5^{\circ}$ in heliographic longitude. The flux ropes were associated with coronal mass ejections (CMEs) that erupted from the Sun on June~12--14, 2012 \textbf{(SOL2012-06-12, SOL2012-06-13, and SOL2012-06-14)}. We examine the CME--CME interactions by using \emph{in-situ} observations from the almost radially aligned spacecraft at Venus and L1, as well as by using heliospheric modelling and imagery. The June~14 CME reached the June~13 CME near the orbit of Venus and significant interaction occurred before they both reached Earth. The shock driven by the June~14 CME propagated through the June~13 CME and the two CMEs coalesced, creating the signatures of one large, coherent flux rope at L1. We discuss the origin of the strong interplanetary magnetic fields related to this sequence of events, the complexity of interpreting solar wind observations in the case of multiple interacting CMEs, and the coherence of the identified flux ropes at different observation points.
Highlights
We investigate the interactions, magnetic field structure, and coherence of the interplanetary counterparts of a series of coronal mass ejections (CMEs) that erupted from the Sun between June 12 and 14, 2012
We have analyzed the interplanetary counterparts of three CMEs that erupted from the Sun between June 12 and 14, 2012 using multipoint measurements from Venus Express (VEX) at 0.7 AU, and from Wind and Advanced Composition Explorer (ACE) at 1 AU
The shock driven by CME3 (S3) and the following FR3 arrived at Venus when VEX was back in the magnetosheath on the nightside; the sheath and the front part of FR3 were compressed in the Venusian magnetosheath
Summary
Coronal mass ejections (CMEs; e.g., Webb and Howard, 2012) are the key drivers of space weather storms at Earth (e.g., Gosling et al, 1991; Webb et al, 2000; Huttunen et al, 2002; Richardson and Cane, 2012; Kilpua et al, 2017b) and related hazards for many modern technologies and infrastructures in orbit and on the ground (e.g., Schrijver et al, 2015; Eastwood et al, 2017). Lugaz et al (2018) have recently reported clear differences in the magnetic field components of ICME flux ropes at observation points only ∼ 0.01 AU apart These studies imply that flux ropes embedded in CMEs may not be coherent structures on a global-scale, or that significant temporal evolution can occur over relatively short radial distances in the heliosphere (see discussion in Owens et al, 2017). Observations at Earth’s Lagrange L1 point (at 1 AU) show a weak ejecta (June 12 CME) followed by a coherent and strong flux rope structure This flux rope had the highest magnetic field magnitudes (about 40 nT) measured in the near-Earth solar wind during Solar Cycle 24. The paper is organized as follows: in section 2 we describe the data sets used in this study; in section 3 we present remotesensing observations of the Sun, the solar corona, and the inner heliosphere, together with in-situ observations at Venus and Earth; this section includes results from a global heliospheric simulation of the CMEs’ propagation; in section 4 we present insitu flux rope reconstructions; and in section 5 we discuss and summarize our results
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