Numerical MHD Simulation Of The Dynamics And Geoeffectiveness Of Some Interplanetary Compound Structures

The interplanetary (IP) space is a key node of the cause-effect chain for the space weather, as a pivot linking the Sun and the Earth. "Multiple magnetic cloud" (Multi-MC) and "shock overtaking MC" are two particular types of IP compound structures, and are both proved by observations of spacecraft and ground-based observatories to be two important IP sources of large geomagnetic storms. A comprehensive investigation of "Multi-MC" and "shock overtaking MC" is carried out by numerical MHD simulation. It is focused on dynamics and ensuing geoeffectiveness, closely combined with spacecraft observations. The main conclusions can be summarized as the following three points:1. The construction of a 2.5-dimensional magnetohydrodynamic code in the heliospheric meridional planeIn order to simulate the physical process of shock entering MC, a source code is constructed on basis of a compound scheme of shock-capturing methods. The numerical model can numerically solve the mathematical problem of interaction between a sharp discontinuity and a complicated smooth structure, guarantee the divergence-free condition of magnetic field in numerical computation. Thus it provides an indispensable premise and powerful platform for the research of physical problems.2. The numerical simulation of the "direct collision" and "oblique collision" for "shock overtaking MC" An incidental shock first catches up with a preceding MC, then penetrates through the MC, and finally merges with the MC-driven shock into a stronger compound shock. The MC body is highly compressed by the shock front along its normal. After the shock passage, the previously compressed MC body is gradually restored to an oblate morphology under the action of its inherent magnetic elasticity. The compression and rotation of the magnetic field serve as an efficient mechanism to cause a large geomagnetic storm. Moreover, when a shock penetrates an MC at the different depth, the resulting geomagnetic storm is also different. Regardless of the shock orientation, the shock penetration location regarding the maximum geoeffectiveness is right at MC core on the condition of very strong shock intensity. Quantified by the Dst index, the simulation results reveal that the geoeffectiveness of an individual MC is largely enhanced with 80% increment in maximum by an incidental shock. Furthermore, it is found that the oblique penetration of a shock through an MC leads to the MC deflection. The opposite deflections of MC body and shock aphelion in the MC-shock oblique collision occur simultaneously through the process of shock penetrating MC. The dependence of such deflection on the initial shock intensity and orientation is also explored. An appropriate angular difference between the initial eruption of an MC and an overtaking shock leads to the maximum deflection of the MC body. The larger the shock intensity is. the greater is the deflection angle. Therefore, the interaction of MCs with other disturbances could be a cause of deflected propagation of interplanetary coronal mass ejection (ICME).3. The numerical simulation of a Multi-MC formed by the "direct collision" of two MCsBoth slow preceding MC (MC1) and fast following MC (MC2) are initially launched along the heliospheric equator, one after another with different time interval. The coupling of two MCs involves the comprehensive interaction among the MC1 body. MC1- driven shock, MC2 body, and MC2-driven shock. 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. As the evolution proceeds, the MC1 body suffers from the larger and larger compression, and its original vulnerable magnetic elasticity becomes stiffer and staffer. So there exists a maximum compressibility of the Multi-MC when the accumulated elasticity can balance the external compression. This cutoff limit of compressibility mainly decides the maximally available geoeffectiveness of a Multi-MC, because the geoeffectiveness enhancement of MCs interacting is ascribed to the compression. The intense compression accompanying the southward magnetic field (B_s) event is responsible for the geoeffectiveness enhancement of "Multi-MC" and "shock overtaking MC". 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 the interplanetary Multi-MC.