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131.
B. Van der Holst S. Poedts E. Chané C. Jacobs G. Dubey D. Kimpe 《Space Science Reviews》2005,121(1-4):91-104
Simulations of coronal mass ejections (CMEs) evolving in the interplanetary (IP) space from the Sun up to 1 AU are performed
in the framework of ideal magnetohydrodynamics (MHD) by the means of a finite-volume, explicit solver. The aim is to quantify
the effect of the background solar wind and of the CME initiation parameters, such as the initial magnetic polarity, on the
evolution and on the geo-effectiveness of CMEs. First, three different solar wind models are reconstructed using the same
numerical grid and the same numerical scheme. Then, different CME initiation models are considered: Magnetic foot point shearing
and magnetic flux emergence. For the fast CME evolution studies, a very simple CME model is considered: A high-density and
high-pressure magnetized plasma blob is superposed on a background steady state solar wind model with an initial velocity
and launch direction. The simulations show that the initial magnetic polarity substantially affects the IP evolution of the
CMEs influencing the propagation velocity, the shape, the trajectory (and thus, the geo-effectiveness). 相似文献
132.
133.
134.
XIANG Changqing FENG Xueshang YAO Jiusheng 《空间科学学报》2006,26(1):8-13
A three-dimensional MHD simulation is conducted to study the steady solar wind in Carrington Rotation (CR) 1935 by using the three-dimensional numerical magnetohydrodynamic (MHD) model introduced by Feng et al. The numerical results demonstrate that the neutral current sheet has two peaks and two valleys, which is consistent with the result of PFSS model at Wilcox Solar Observatory (WSO). The obtained proton number density at 2.5 Rs is of the same order of magnitude as the result estimated from K-coronal brightness during the CRs 1733-1742 in 1983made by Wei et al. The radial velocity profile along heliocentric distance is consistent with that of low solar wind speed deduced by Sheeley and Wang et al. However, it is not able to reproduce the fast-speed flow in coronal holes and slow solar wind in streamers because of oversimplified energy equation adopted in our model. Future efforts must be made to remedy this deficiency. 相似文献
135.
A. Mangalam A. Prasad 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(2):738-748
We use our semi-analytic solution of the nonlinear force-free field equation to construct three-dimensional magnetic fields that are applicable to the solar corona and study their statistical properties for estimating the degree of braiding exhibited by these fields. We present a new formula for calculating the winding number and compare it with the formula for the crossing number. The comparison is shown for a toy model of two helices and for realistic cases of nonlinear force-free fields; conceptually the formulae are nearly the same but the resulting distributions calculated for a given topology can be different. We also calculate linkages, which are useful topological quantities that are independent measures of the contribution of magnetic braiding to the total free energy and relative helicity of the field. Finally, we derive new analytical bounds for the free energy and relative helicity for the field configurations in terms of the linking number. These bounds will be of utility in estimating the braided energy available for nano-flares or for eruptions. 相似文献
136.
Elena Provornikova Leon Ofman Tongjiang Wang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(2):645-654
EUV imaging observations from several space missions (SOHO/EIT, TRACE, and SDO/AIA) have revealed a presence of propagating intensity disturbances in solar coronal loops. These disturbances are typically interpreted as slow magnetoacoustic waves. However, recent spectroscopic observations with Hinode/EIS of active region loops revealed that the propagating intensity disturbances are associated with intermittent plasma upflows (or jets) at the footpoints which are presumably generated by magnetic reconnection. For this reason, whether these disturbances are waves or periodic flows is still being studied. This study is aimed at understanding the physical properties of observed disturbances by investigating the excitation of waves by hot plasma injections from below and the evolution of flows and wave propagation along the loop. We expand our previous studies based on isothermal 3D MHD models of an active region to a more realistic model that includes full energy equation accounting for the effects of radiative losses. Computations are initialized with an equilibrium state of a model active region using potential (dipole) magnetic field, gravitationally stratified density and temperature obtained from the polytropic equation of state. We model an impulsive injection of hot plasma into the steady plasma outflow along the loops of different temperatures, warm (~1 MK) and hot (~6 MK). The simulations show that hot jets launched at the coronal base excite slow magnetoacoustic waves that propagate to high altitudes along the loops, while the injected hot flows decelerate rapidly with heights. Our results support that propagating disturbances observed in EUV are mainly the wave features. We also find that the effect of radiative cooling on the damping of slow-mode waves in 1–6 MK coronal loops is small, in agreement with the previous conclusion based on 1D MHD models. 相似文献
137.
138.
《中国航空学报》2016,(6):1553-1562
This paper deals with the numerical solution of inviscid compressible flows. The threedimensional Euler equations describing the mentioned problem are presented and solved numerically with the finite volume method. The evaluation of the numerical flux at the interfaces is performed by using the Toro Vazquez-Harten Lax Leer(TV-HLL) scheme. An essential feature of the proposed scheme is to associate two systems of differential equations, called the advection system and the pressure system. It can be implemented with a very simple manner in the standard finite volume Euler and Navier–Stokes codes as extremely simple task. The scheme is applied to some test problems covering a wide spectrum of Mach numbers, including hypersonic, low speed flow and three-dimensional aerodynamics applications. 相似文献
139.
The properties of magnetohydrodynamic waves and instabilities of laboratory and space plasmas are determined by the overall
magnetic confinement geometry and by the detailed distributions of the density, pressure, magnetic field, and background velocity
of the plasma. Consequently, measurement of the spectrum of MHD waves (MHD spectroscopy) gives direct information on the internal
state of the plasma, provided a theoretical model is available to solve the forward as well as the inverse spectral problems.
This terminology entails a program, viz. to improve the accuracy of our knowledge of plasmas, both in the laboratory and in
space. Here, helioseismology (which could be considered as one of the forms of MHD spectroscopy) may serve as a luminous example.
The required study of magnetohydrodynamic waves and instabilities of both laboratory and space plasmas has been conducted
for many years starting from the assumption of static equilibrium. Recently, there is a outburst of interest for plasma states
where this assumption is violated. In fusion research, this interest is due to the importance of neutral beam heating and
pumped divertor action for the extraction of heat and exhaust needed in future tokamak reactors. Both result in rotation of
the plasma with speeds that do not permit the assumption of static equilibrium anymore. In astrophysics, observations in the
full range of electromagnetic radiation has revealed the primary importance of plasma flows in such diverse situations as
coronal flux tubes, stellar winds, rotating accretion disks, and jets emitted from radio galaxies. These flows have speeds
which substantially influence the background stationary equilibrium state, if such a state exists at all. Consequently, it
is important to study both the stationary states of magnetized plasmas with flow and the waves and instabilities they exhibit.
We will present new results along these lines, extending from the discovery of gaps in the continuous spectrum and low-frequency
Alfvén waves driven by rotation to the nonlinear flow patterns that occur when the background speed traverses the full range
from sub-slow to super-fast.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
140.
K.M. Schure J. Vink A. Achterberg R. Keppens 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Observations show that the magnetic field in young supernova remnants (SNRs) is significantly stronger than can be expected from the compression of the circumstellar medium (CSM) by a factor of four expected for strong blast waves. Additionally, the polarization is mainly radial, which is also contrary to expectation from compression of the CSM magnetic field. Cosmic rays (CRs) may help to explain these two observed features. They can increase the compression ratio to factors well over those of regular strong shocks by adding a relativistic plasma component to the pressure, and by draining the shock of energy when CRs escape from the region. The higher compression ratio will also allow for the contact discontinuity, which is subject to the Rayleigh–Taylor (R–T) instability, to reach much further out to the forward shock. This could create a preferred radial polarization of the magnetic field. With an Adaptive Mesh Refinement MHD code (AMRVAC), we simulate the evolution of SNRs with three different configurations of the initial CSM magnetic field, and look at two different equations of state in order to look at the possible influence of a CR plasma component. The spectrum of CRs can be simulated using test particles, of which we also show some preliminary results that agree well with available analytical solutions. 相似文献