Multiple Magnetic Clouds Research Articles

The first in situ observation of torsional Alfvén waves during the interaction of large-scale magnetic clouds

Abstract The large-scale magnetic cloud such as coronal mass ejections (CMEs) is the fundamental driver of the space weather. The interaction of the multiple-CMEs in interplanetary space affects their dynamic evolution and geo-effectiveness. The complex and merged multiple magnetic clouds appear as the in situ signature of the interacting CMEs. The Alfvén waves are speculated to be one of the major possible energy exchange/dissipation mechanism during the interaction. However, no such observational evidence has been found in the literature. The case studies of CME–CME collision events suggest that the magnetic and thermal energy of the CME is converted into the kinetic energy. Moreover, magnetic reconnection process is justified to be responsible for merging of multiple magnetic clouds. Here, we present unambiguous evidence of sunward torsional Alfvén waves in the interacting region after the super-elastic collision of multiple CMEs. The Walén relation is used to confirm the presence of Alfvén waves in the interacting region of multiple CMEs/magnetic clouds. We conclude that Alfvén waves and magnetic reconnection are the possible energy exchange/dissipation mechanisms during large-scale magnetic clouds collisions. This study has significant implications not only in CME-magnetosphere interactions but also in the interstellar medium where interactions of large-scale magnetic clouds are possible.

Solar and interplanetary triggers of the largest Dst variations of the solar cycle 23

We present the results of an investigation from the Sun to the Earth of the sequence of events that caused major Dst decreases (ΔDst≤–100nT during 1h) that occurred during 1996–2005. These events are expected to be better related to geomagnetic induced current (GIC) events than those events where any geomagnetic index is far from its quiet time value. At least one full halo CME with a speed on the plane of sky above 900km/s participates in every studied event. The seven events were triggered by interplanetary signatures, which arise as a consequence of interaction among different solar ejections. The interaction arises at different stages from the solar surface, between segments of a filament, to the interplanetary medium, appearing as ejecta or multiple-magnetic clouds (MultiMCs). In other cases, shock waves overtake or compress previous ICMEs and at other times the interaction also appears between magnetic clouds (MCs) and streams.

Three-dimensional MHD simulation of two coronal mass ejections' propagation and interaction using a successive magnetized plasma blobs model

A three-dimensional (3-D), time-dependent, numerical magnetohydrodynamic (MHD) model is used to investigate the evolution and interaction of two coronal mass ejections (CMEs) in the nonhomogeneous ambient solar wind. The background solar wind is constructed on the basis of the self-consistent source surface with observed line of sight of magnetic field and density from the source surface of 2.5 R(s) to Earth's orbit (215 R(s)) and beyond. The two successive CMEs occurring on 28 March 2001 and forming a multiple magnetic cloud in interplanetary space are chosen as a test case, in which they are simulated by means of a two high-density, high-velocity, and high-temperature magnetized plasma blobs model, and are successively ejected into the nonhomogeneous background solar wind medium along different initial launch directions. The dynamical propagation and interaction of the two CMEs between 2.5 and 220 R(s) are investigated. Our simulation results show that, although the two CMEs are separated by 10 h, the second CME is able to overtake the first one and cause compound interactions and an obvious acceleration of the shock. At the L1 point near Earth the two resultant magnetic clouds in our simulation are consistent with the observations by ACE. In this validation study we find that this 3-D MHD model, with the self-consistent source surface as the initial boundary condition and the magnetized plasma blob as the CME model, is able to reproduce and explain some of the general characters of the multiple magnetic clouds observed by satellite.

Magnetohydrodynamic simulation of the interaction between two interplanetary magnetic clouds and its consequent geoeffectiveness: 2. Oblique collision

The numerical studies of the interplanetary coupling between multiple magnetic clouds (MCs) are continued by a 2.5‐dimensional ideal magnetohydrodynamic (MHD) model in the heliospheric meridional plane. The interplanetary direct collision (DC)/oblique collision (OC) between both MCs results from their same/different initial propagation orientations. Here the OC is explored in contrast to the results of the DC. Both the slow MC1 and fast MC2 are consequently injected from the different heliospheric latitudes to form a compound stream during the interplanetary propagation. The MC1 and MC2 undergo contrary deflections during the process of oblique collision. Their deflection angles of ∣δθ1∣ and ∣δθ2∣ continuously increase until both MC‐driven shock fronts are merged into a stronger compound one. The ∣δθ1∣, ∣δθ2∣, and total deflection angle Δθ (Δθ = ∣δθ1∣ + ∣δθ2∣) reach their corresponding maxima when the initial eruptions of both MCs are at an appropriate angular difference. Moreover, with the increase of MC2's initial speed, the OC becomes more intense, and the enhancement of δθ1 is much more sensitive to δθ2. The ∣δθ1∣ is generally far less than the ∣δθ2∣, and the unusual case of ∣δθ1∣ ≃ ∣δθ2∣ only occurs for an extremely violent OC. But because of the elasticity of the MC body to buffer the collision, this deflection would gradually approach an asymptotic degree. As a result, the opposite deflection between the two MCs, together with the inherent magnetic elasticity of each MC, could efficiently relieve the external compression for the OC in the interplanetary space. Such a deflection effect for the OC case is essentially absent for the DC case. Therefore, besides the magnetic elasticity, magnetic helicity, and reciprocal compression, the deflection due to the OC should be considered for the evolution and ensuing geoeffectiveness of interplanetary interaction among successive coronal mass ejections.

Magnetohydrodynamic simulation of the interaction between two interplanetary magnetic clouds and its consequent geoeffectiveness

Numerical studies of the interplanetary “multiple magnetic clouds (Multi‐MC)” are performed by a 2.5‐dimensional ideal magnetohydrodynamic (MHD) model in the heliospheric meridional plane. Both slow MC1 and fast MC2 are initially emerged along the heliospheric equator, one after another with different time intervals. The coupling of two MCs could be considered as the comprehensive interaction between two systems, each comprising of an MC body and its driven shock. The MC2‐driven shock and MC2 body are successively involved into interaction with MC1 body. The momentum is transferred from MC2 to MC1. After the passage of MC2‐driven shock front, magnetic field lines in MC1 medium previously compressed by MC2‐driven shock are prevented from being restored by the MC2 body pushing. MC1 body undergoes the most violent compression from the ambient solar wind ahead, continuous penetration of MC2‐driven shock through MC1 body, and persistent pushing of MC2 body at MC1 tail boundary. As the evolution proceeds, the MC1 body suffers from larger and larger compression, and its original vulnerable magnetic elasticity becomes stiffer and stiffer. So there exists a maximum compressibility of Multi‐MC when the accumulated elasticity can balance the external compression. This cutoff limit of compressibility mainly decides the maximally available geoeffectiveness of Multi‐MC because the geoeffectiveness enhancement of MCs interacting is ascribed to the compression. Particularly, the greatest geoeffectiveness is excited among all combinations of each MC helicity, if magnetic field lines in the interacting region of Multi‐MC are all southward. Multi‐MC completes its final evolutionary stage when the MC2‐driven shock is merged with MC1‐driven shock into a stronger compound shock. With respect to Multi‐MC geoeffectiveness, the evolution stage is a dominant factor, whereas the collision intensity is a subordinate one. The magnetic elasticity, magnetic helicity of each MC, and compression between each other are the key physical factors for the formation, propagation, evolution, and resulting geoeffectiveness of interplanetary Multi‐MC.

Numerical Simulation of the Interaction of Two Coronal Mass Ejections from Sun to Earth

We present a three-dimensional compressible magnetohydrodynamics (MHD) model of the interaction of two coronal mass ejections (CMEs). Two identical CMEs are launched in the exact same direction into a preexisting solar wind, the second one 10 hr after the first one. Our global steady state coronal model possesses high-latitude coronal holes and a helmet streamer structure with a current sheet near the equator, reminiscent of near-solar minimum conditions. Within this model system, we drive the CMEs to erupt by the introduction of two three-dimensional magnetic flux ropes embedded in the helmet streamer. After an initial phase, when the trailing shock and the second CME propagate into the disturbed solar wind medium, they reach the edge of the first magnetic cloud, leading to complex magnetic interactions and a steep acceleration of the shock. Later, the trailing shock reaches the dense sheath of plasma associated with the leading shock, where it decelerates to a speed about 100 km s-1 larger than the speed of the leading shock. Eventually, the two shocks merge and a stronger, faster shock forms in association with a contact discontinuity between the old and new downstream regions. We find that the trailing shock always remains a fast-mode shock. A reverse shock is driven after the collision of the two magnetic clouds due to the difference in speed within the reconnection region. At Earth, the two magnetic clouds can still be distinguished, with a compressed and heated first cloud and a second overexpanded cloud. The transit time of this complex ejecta is reduced by about 6 hr compared to the case of the first CME without interaction. Our simulation is able to reproduce and explain some of the general features observed in satellite data for multiple magnetic clouds.

MHD simulation of the formation and propagation of multiple magnetic clouds in the heliosphere

A multiple-magnetic-cloud (Multi-MC) structure formed by the overtaking of two successive coronal mass ejections (CMEs) in the heliosphere is studied by using a 2.5-D MHD simulation. This simulation illustrates the process of the formation and propagation of two identical CMEs, which are ejected with speeds of 400 km s −1 and 600 km s −1 respectively and initially separated by 12 h. The results show that it takes ∼18 h for the fast cloud to catch up with the preceding slow one, then the two clouds form a Multi-MC structure that arrives at 1 AU three days later. The fast cloud is slowed down significantly because of the blocking by the preceding slow one. This implies that the travel time of a Multi-MC structure is dominated by the preceding slow cloud. Moreover, most primary observational characteristics of Multi-MC at 1 AU are well represented by the simulation. In addition, by combining observations, theoretical model and the simulation results, differences between Multi-MC and other types of in-situ observed double-flux-rope structure are addressed. A comparison of Multi-MC to coronal- mass-ejection cannibalization near Sun is also given.

Numeric simulations of in situ observations of interacting ejecta near 1 AU

Abstract We use a simple numerical model (Gonzalez-Esparza, J.A., Santillan, A., Ferrer, J. A numerical study of the interaction between two ejecta in the interplanetary medium: one and two dimensional hydrodynamic simulations, Ann. Geophys. 22, 3741–3749, 2004) to study the evolution of three events in the solar wind reported by Wang et al. (Wang, Y.M., Ye, P.Z., Wang, S. Multiple magnetic clouds: several examples during March–April 2001. J. Geophys. Res. 108, 1370, 2003, doi:10.1029/2003JA009850 ), where two interacting ejecta detected in situ by ACE near 1 AU were related to CMEs observed previously by SOHO-LASCO. The study is based on a 1-D hydrodynamic model using the ZEUS code (Stone, J.M., Norman, M. ZEUS 2-D: A radiation magnetohydrodynamics code for astrophysical flows in two dimensions, I, the hydrodynamics algorithms and tests, Astrophys. J. 80, 753, 1992). Although this model cannot address either the magnetic field dynamics or the complex geometrical effects intrinsic in the three-dimensional nature of the phenomena, it illuminates the transferring of momentum and evolution of interacting large-scale solar wind disturbances in those cases where there is no merging (magnetic reconnection) between the two ejecta. This model can reproduce, in some cases, characteristics of the events such as transit times and flow signatures as inferred from the two-point measurements from spacecraft.

A numerical study of the interaction between two ejecta in the interplanetary medium: one- and two-dimensional hydrodynamic simulations

Abstract. We studied the heliospheric evolution in one and two dimensions of the interaction between two ejecta-like disturbances beyond the critical point: a faster ejecta 2 overtaking a previously launched slower ejecta 1. The study is based on a hydrodynamic model using the ZEUS-3-D code. This model can be applied to those cases where the interaction occurs far away from the Sun and there is no merging (magnetic reconnection) between the two ejecta. The simulation shows that when the faster ejecta 2 overtakes ejecta 1 there is an interchange of momentum between the two ejecta, where the leading ejecta 1 accelerates and the tracking ejecta 2 decelerates. Both ejecta tend to arrive at 1AU having similar speeds, but with the front of ejecta 1 propagating faster than the front of ejecta 2. The momentum is transferred from ejecta 2 to ejecta 1 when the shock initially driven by ejecta 2 passes through ejecta 1. Eventually the two shock waves driven by the two ejecta merge together into a single stronger shock. The 2-D simulation shows that the evolution of the interaction can be very complex and there are very different signatures of the same event at different viewing angles; however, the transferring of momentum between the two ejecta follows the same physical mechanism described above. These results are in qualitative agreement with in-situ plasma observations of "multiple magnetic clouds" detected at 1AU.

An Interplanetary Origin of Great Geomagnetic Storms: Multiple Magnetic Clouds

AbstractAn event of multiple magnetic cloud (Multi‐MC) in interplanetary space on March 31, 2001 caused the largest geomagnetic storm with Dst=–387nT during the 23th solar maximum (2000~2001). By analyzing the data from the ACE spacecraft, we describe the characteristics of magnetic fields and plasma within this Multi‐MC at 1AU. We also identify its solar sources, and coronal mass ejections (CMEs) by utilizing the observations from the Solar and Heliospheric Observatory (SOHO) and the GOES satellite. In this instance, the Multi‐MC became more effective with the extraordinary enhanced southward magnetic fields, and largely extended the duration of the Dst storm. The observations and the theoretical analysis reveal that the strength of the magnetic fields including the southward component increased several times due to the compression between the sub‐clouds in the Multi‐MC, and therefore intensified the corresponding effectiveness. Thus, we suggest that the compression between the sub‐clouds of Multi‐MC should be a mechanism in causing great geomagnetic storms. In addition, we find that the CMEs, which formed the Multi‐MC, seemed not to originate from the same solar region.

Multiple magnetic clouds: Several examples during March–April 2001

Multiple magnetic cloud (Multi‐MC), which is formed by the overtaking of successive coronal mass ejections (CMEs), is a kind of complex structure in interplanetary space. Multi‐MC is worthy of notice due to its special properties and potential geoeffectiveness. Using the data from the ACE spacecraft, we identify the three cases of Multi‐MC in the period from March to April 2001. Some observational signatures of Multi‐MC are concluded: (1) Multi‐MC only consists of several magnetic clouds and interacting regions between them; (2) each subcloud in Multi‐MC is primarily satisfied with the criteria of isolated magnetic cloud, except that the proton temperature is not as low as that in typical magnetic cloud due to the compression between the subclouds; (3) the speed of solar wind at the rear part of the front subcloud does not continuously decrease, rather increases because of the overtaking of the following subcloud; (4) inside the interacting region between the subclouds, the magnetic field becomes less regular and its strength decreases obviously, and (5) β value increases to a high level in the interacting region. We find out that two of three Multi‐MCs are associated with the great geomagnetic storms (Dst ≤ −200 nT), which indicate a close relationship between the Multi‐MCs and some intense geomagnetic storms. The observational results imply that the Multi‐MC is possibly another type of the interplanetary origin of the large geomagnetic storm, though not all of them have geoeffectiveness. Based on the observations from Solar and Heliospheric Observatory (SOHO) and GOES, the solar sources (CMEs) of these Multi‐MCs are identified. We suggest that such successive halo CMEs are not required to be originated from a single solar region. Furthermore, the relationship between Multi‐MC and complex ejecta is analyzed, and some similarities and differences between them are discussed.

Multiple magnetic clouds in interplanetary space

An interplanetary magnetic cloud (MC) is usually considered the byproduct of a coronal mass ejection (CME). Due to the frequent occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one MC catches up with another, should be a relatively common phenomenon. A simple flux rope model is used to get the primary magnetic field features of multi-MCs. Results indicate that the magnetic field configuration of multi-MCs mainly depends on the magnetic field characteristics of each member of multi-MCs. It may be entirely different in another situation. Moreover, we fit the data from the Wind spacecraft by using this model. Comparing the model with the observations, we verify the existence of multi-MCs, and propose some suggestions for further work.