The interaction of multiple Coronal Mass Ejections (CMEs) has been observed by LASCO coronagraphs and by near-Earth spacecraft, and it is thought to be an important cause of geo-effective storms, large Solar Energetic Particles events and intense Type II radio bursts. New and future missions such as STEREO, the LWS Sentinels, and the Solar Orbiter will provide additional observations of the interaction of multiple CMEs between the Sun and the Earth. We present the results of simulations of two and more CMEs interacting in the inner heliosphere performed with the Space Weather Modeling Framework (SWMF). Based on those simulations, we discuss the observational evidence of the interaction of multiple CMEs, both in situ and from coronagraphs. The clearest evidence of the interaction of the CMEs are the large temperature in the sheath, due to the shocks merging, and the brightness increase in coronagraphic images, associated with the interaction of the leading edges. The importance of having multiple satellites at different distances and angular positions from the Sun is also discussed.
Deep Dive into Observational evidence of CMEs interacting in the inner heliosphere as inferred from MHD simulations.
The interaction of multiple Coronal Mass Ejections (CMEs) has been observed by LASCO coronagraphs and by near-Earth spacecraft, and it is thought to be an important cause of geo-effective storms, large Solar Energetic Particles events and intense Type II radio bursts. New and future missions such as STEREO, the LWS Sentinels, and the Solar Orbiter will provide additional observations of the interaction of multiple CMEs between the Sun and the Earth. We present the results of simulations of two and more CMEs interacting in the inner heliosphere performed with the Space Weather Modeling Framework (SWMF). Based on those simulations, we discuss the observational evidence of the interaction of multiple CMEs, both in situ and from coronagraphs. The clearest evidence of the interaction of the CMEs are the large temperature in the sheath, due to the shocks merging, and the brightness increase in coronagraphic images, associated with the interaction of the leading edges. The importance of havin
Coronal Mass Ejections (CMEs) are the most extreme events occurring in our solar system, and their frequency highly depends on the phase of the solar cycle: from 6 a day near solar maximum to 0.5-0.8 a day near solar minimum (Gopalswamy, 2004). The typical propagation time of a CME from the Sun to the Earth is 2-3 days. Therefore, near solar maximum, there is a high probability that multiple CMEs will interact on their way to Earth. Ejecta resulting from the interaction of multiple CMEs have been reported and studied by Burlaga et al. (2002); Wang et al. (2003); Berdichevsky et al. (2003) and Farrugia & Berdichevsky (2004), among others. Numerical investigations of multiple CMEs propagating and interacting, including the simpler case of the interaction between a CME and a forward shockwave, have been pioneered by Vandas et al. (1997) and recently reported by Odstrčil et al. (2003); Gonzalez-Esparza et al. (2004); Schmidt & Cargill (2004); Lugaz et al. (2005Lugaz et al. ( , 2007) ) and Xiong et al. (2006).
Based on near-Earth in situ measurements only, the interaction region between the magnetic subclouds of a multiple-magnetic cloud event is among the only evidence that multiple CMEs interacted between the Sun and the Earth. This region is characterized by a lower magnetic field strength, a higher temperature, resulting in a larger plasma β (ratio of the thermal to the magnetic pressures) and it is associated with the reconnection between the two clouds (Wang et al., 2003). As noted by Burlaga et al. (2002), the speed profile of complex ejecta, although irregular, often shows variations of less than 100 km s -1 between the different ejecta. Thus, it can be hard to distinguish between an isolated CME and interacting CMEs, simply based on the speed profile of the events observed near-Earth.
The interaction of two CMEs near the Sun can sometimes be observed by the LASCO coronagraphs (e.g. Gopalswamy et al., 2001;Reiner et al., 2003). It can appear as CME “cannibalism” (Gopalswamy et al., 2001), where the faster ejection “swallows” the slower, preceding one. It can also appear as a brightness increase as the leading edge of the two CMEs interact, as is the case for the ejections from June 11, 1998 (see Figure 1). However, often, the only indication that multiple CMEs interacted on their way to Earth is when multiple Earth-directed ejections are observed by LASCO and a single structure (multiple-magnetic cloud events or complex ejecta) is observed at Earth.
It is the goal of this work to propose other evidence of CMEs interaction based on three-dimensional (3-D) magneto-hydrodynamic (MHD) simulations, relying both on existing (Wind, ACE, STEREO) and future missions (LWS Sentinels, Solar Orbiter). We briefly summarize the simulations used for this study in Section 2. In Section 3, we discuss in situ synthetic measurements at 1 AU, followed, in Section 4, by a presentation of the possible in situ observations closer to the Sun by future missions. In Section 5, we examine possible white-light observations of interacting CMEs by the STEREO Heliospheric Imagers. In Section 6, we conclude and discuss other possible observational evidence of interacting CMEs not included in the present work.
The two simulations on which this study is based have been published in Lugaz et al. (2005) and Lugaz et al. (2007). Both simulations are performed with a 3-D MHD code (BATS-R-US). In Lugaz et al. (2005) (therafter Simulation 1), two identical out-of-equilibrium Gibson-Low magnetic flux ropes (Gibson and Low, 1998) are added 10 hours after each other onto the solar surface into a solar wind characteristic of solar minimum (see also Manchester et al., 2004, for a description of the models). The interaction of those two ejections results in the passage of a multiple-magnetic cloud event at Earth. The two magnetic sub-clouds are preceded by a single shock wave, the result of the merging of the two shock waves driven by the ejections. In Lugaz et al. (2007) (thereafter Simulation 2), we investigate three homologous eruptions from NOAA active region 9236 in November 24, 2000. The three ejections were separated by 10 and 6.5 hours respectively and of equivalent velocities (1000-1200 km s -1 ). We use the solar wind model developed by Roussev et al. (2003) incorporating MDI magnetogram data and, which reproduces observations by Wind for the pre-eruption solar wind. We use out-of-equilibrium semi-circular magnetic flux ropes to initiate the three eruptions and are able to reproduce most of the LASCO and Wind observations. Simulation 2 was performed with the Space Weather Modeling Framework (for a description of the SWMF, see Tóth et al., 2005).
Here, we compare the results at 1 AU of Simulation 1 (solid line, thereafter referred as the interacting case) to the results of an identical but iso-lated CME (dash-dotted line from Manchester et al. (2004), thereafter referred as the isolated case), as seen in the left panel of Figure 2. The
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