Numerical studies have been performed to interpret the observed "shock overtaking magnetic cloud (MC)" event by a 2.5 dimensional magnetohydrodynamic (MHD) model in heliospheric meridional plane. Results of an individual MC simulation show that the MC travels with a constant bulk flow speed. The MC is injected with very strong inherent magnetic field over that in the ambient flow and expands rapidly in size initially. Consequently, the diameter of MC increases in an asymptotic speed while its angular width contracts gradually. Meanwhile, simulations of MC-shock interaction are also presented, in which both a typical MC and a strong fast shock emerge from the inner boundary and propagate along heliospheric equator, separated by an appropriate interval. The results show that the shock firstly catches up with the preceding MC, then penetrates through the MC, and finally merges with the MC-driven shock into a stronger compound shock. The morphologies of shock front in interplanetary space and MC body behave as a central concave and a smooth arc respectively. The compression and rotation of magnetic field serve as an efficient mechanism to cause a large geomagnetic storm. The MC is highly compressed by the the overtaking shock. Contrarily, the transport time of incidental shock influenced by the MC depends on the interval between their commencements. Maximum geoeffectiveness results from that when the shock enters the core of preceding MC, which is also substantiated to some extent by a corresponding simplified analytic model. Quantified by $Dst$ index, the specific result gives that the geoeffectiveness of an individual MC is largely enhanced with 80% increment in maximum by an incidental shock.
Deep Dive into Magnetohydrodynamic Simulation of the Interaction between Interplanetary Strong Shock and Magnetic Cloud and its Consequent Geoeffectiveness.
Numerical studies have been performed to interpret the observed “shock overtaking magnetic cloud (MC)” event by a 2.5 dimensional magnetohydrodynamic (MHD) model in heliospheric meridional plane. Results of an individual MC simulation show that the MC travels with a constant bulk flow speed. The MC is injected with very strong inherent magnetic field over that in the ambient flow and expands rapidly in size initially. Consequently, the diameter of MC increases in an asymptotic speed while its angular width contracts gradually. Meanwhile, simulations of MC-shock interaction are also presented, in which both a typical MC and a strong fast shock emerge from the inner boundary and propagate along heliospheric equator, separated by an appropriate interval. The results show that the shock firstly catches up with the preceding MC, then penetrates through the MC, and finally merges with the MC-driven shock into a stronger compound shock. The morphologies of shock front in interplanetary space
Coronal mass ejection (CME) is one of the most frequently eruptive phenomena in solar atmosphere, which causes significant changes in coronal structure accompanied by observable mass outflow. A great deal of CME observation data has been accumulated by spacecraft OSO-7, Skylab, P78-1, SMM, ISEE3, Helios, Yohkoh, SOHO, Ulysses, Wind, ACE et al. over the past 30 years. A typical CME is launched into interplanetary (IP) space with magnetic flux of 10 23 maxwell and plasma mass of 10 16 g [Gosling, 1990;Webb et al., 1994]. The "solar flare myth" that CMEs have no fundamental association (in terms of cause and effect) with flares [Gosling, 1993;Gosling and Hundhausen, 1995] is quite controversial [e.g., Svestka, 1995;Dryer, 1996]. It is more favorable of the equal importance of CME and flare concerning the source of IP transient disturbances and non-recurrent geomagnetic storms [Dryer, 1996]. Statistical research shows that nearly half of all CMEs form magnetic clouds (MCs) in IP space [Klein and Burlaga, 1982;Cane et al., 1997].
MC is very concerned in space community, because its regular magnetic field with large southward magnetic component always leads to geomagnetic storm. The characteristics of MCs, as defined by Burlaga et al. [1981], are enhanced magnetic field, smooth rotation of the magnetic field, low proton temperature, and a low ratio of proton thermal to magnetic pressure β p . Many studies modeled an MC by an ideal local cylinder with a force-free field [e.g., Goldstein, 1983;Burlaga, 1988;Farrugia et al., 1993;Kumar and Rust, 1996;Osherovich and Burlaga, 1997], though in real situation an MC should probably be a curved loop-like structure with its feet connecting to the solar surface [Larson et al., 1997]. Numerical simulations have been carried out to investigate the behavior of isolated D R A F T July 17, 2021, 5:13pm D R A F T X -5
loop-like MCs with various magnetic field strengths, axis orientations and speeds, based on the flux rope model [e.g., Vandas et al., 1995Vandas et al., , 1996aVandas et al., , b, c, 1997a, b;, b;Vandas and Odstrcil, 2000;Vandas et al., 2002;Groth et al., 2000;Odstrcil et al., 2002;Schmidt and Cargill, 2003;Vandas, 2003;Manchester et al., 2004a, b]. A great consistency was found between the in-situ observations, theoretical analyses and numerical simulations.
Recent studies have focused on the existence of more complex structure, with less defined characteristics and a possible association with interactions among CMEs, shocks, MCs, and corotating regions, such as complex ejecta [Burlaga et al., 2002], multiple MCs [Wang et al., 2002[Wang et al., , 2003a]], shock-penetrated MCs [Wang et al., 2003b;Berdichevsky et al., 2005],
and so on. Most of the different physical phenomena, which are likely to occur during the propagation of a following faster CME overtaking a preceding slower CME, have been studied by both 2.5-dimensional (2.5D) and 3-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations: the interaction of a shock wave with an MC [Vandas et al., 1997a;Odstrcil et al., 2003], the interaction of two MCs [Odstrcil et al., 2003;Gonzalez-Esparza et al., 2004;Lugaz et al., 2005;Wang et al., 2005], and the acceleration of electrons associated with the shock-cloud interaction [Vandas and Odstrcil, 2004].
The establishment of space weather forecasting system is ongoing as urgently needed by human civilization. Numerical MHD model may play a critical role in it [Dryer, 1998].
IP medium is a pivotal node in cause-effect chains of solar-terrestrial transporting events.
The correlation between Dst index and various IP parameters have been comprehensively studied [e.g., Burton et al., 1975;Vassiliadis et al., 1999] and applied in related numerical simulations [e.g., Vandas, 2003]. Moreover, some observation-data-driven numerical models have already been applied in the real time “fearless forecasting”: (1) HAF
(Hakamada-Akasofu-Fry) model based on kinetics [Fry et al., 2001[Fry et al., , 2005;;Intriligator et al., 2005;McKenna-Lawlor et al., 2005];
(2) STOA (Shock Time of Arrival) based on classical self-similarity blast wave theory [Smart and Shea, 1985];
(3) ISPM (Interplanetary Shock Propagation Model) based on 2.5D MHD simulation [Smith and Dryer, 1990]; (4) an ensemble of above three models [Dryer et al., 2001[Dryer et al., , 2004;;McKenna-Lawlor et al., 2002;Fry et al., 2003Fry et al., , 2004]]. events [Wang et al., 2003b];
(2) shock ahead of MC after completely penetrating it, such as March 20-21 2003 event [Berdichevsky et al., 2005]. Ruling out the possibility of weak shock dissipation in low β MC plasma, the MC-shock compound at 1 AU changes from category 1 to 2, as their eruption interval decreases at solar corona. MC-shock interaction is also an IP cause of large geomagnetic storms [Wang et al., 2003b, c]. Obviously MC with a penetrating shock at various stages may result in different geoeffectiveness.
In this paper, studies are presented to
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