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外文期刊>The Astrophysical journal
>A STUDY OF THE HELIOCENTRIC DEPENDENCE OF SHOCK STANDOFF DISTANCE AND GEOMETRY USING 2.5D MAGNETOHYDRODYNAMIC SIMULATIONS OF CORONAL MASS EJECTION DRIVEN SHOCKS
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A STUDY OF THE HELIOCENTRIC DEPENDENCE OF SHOCK STANDOFF DISTANCE AND GEOMETRY USING 2.5D MAGNETOHYDRODYNAMIC SIMULATIONS OF CORONAL MASS EJECTION DRIVEN SHOCKS
We perform four numerical magnetohydrodynamic simulations in 2.5 dimensions (2.5D) of fast coronal mass ejections (CMEs) and their associated shock fronts between 10?Rs and 300?Rs. We investigate the relative change in the shock standoff distance, Δ, as a fraction of the CME radial half-width, D OB (i.e., Δ/D OB). Previous hydrodynamic studies have related the shock standoff distance for Earth's magnetosphere to the density compression ratio (DR; ρ u/ρ d) measured across the bow shock. The DR coefficient, k dr, which is the proportionality constant between the relative standoff distance (Δ/D OB) and the compression ratio, was semi-empirically estimated as 1.1. For CMEs, we show that this value varies linearly as a function of heliocentric distance and changes significantly for different radii of curvature of the CME's leading edge. We find that a value of 0.8?± 0.1 is more appropriate for small heliocentric distances (30?Rs) which corresponds to the spherical geometry of a magnetosphere presented by Seiff. As the CME propagates its cross section becomes more oblate and the k dr value increases linearly with heliocentric distance, such that k dr?= 1.1 is most appropriate at a heliocentric distance of about 80?Rs. For terrestrial distances (215?Rs) we estimate k dr?= 1.8?± 0.3, which also indicates that the CME cross-sectional structure is generally more oblate than that of Earth's magnetosphere. These alterations to the proportionality coefficients may serve to improve investigations into the estimates of the magnetic field in the corona upstream of a CME as well as the aspect ratio of CMEs as measured in situ.
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机译：我们在2.5维（2.5D）的快速日冕质量抛射（CME）及其相关的激波锋在10？Rs和300？Rs之间执行了四个数值磁流体动力学模拟。我们以CME径向半宽D OB（即Δ/ D OB）的分数调查冲击支座距离Δ的相对变化。以前的水动力研究已经将地球磁层的冲击距离与在整个弓激波中测得的密度压缩比（DR;ρu /ρd）相关联。半经验性地将DR系数k dr（其为相对间隔距离（Δ/ D OB）与压缩比之间的比例常数）估计为1.1。对于CME，我们表明该值随日心距的变化呈线性变化，并且随着CME前缘的不同曲率半径而显着变化。我们发现，对于较小的日心中心距离（<30？Rs），0.8？±0.1的值更合适，这对应于Seiff提出的磁层的球形几何形状。随着CME的传播，其横截面变得更扁，并且k dr值随日心距线性增加，因此k dr？= 1.1最适合日心距约为80？Rs。对于地面距离（215？Rs），我们估计k dr？= 1.8？±0.3，这也表明CME的横截面结构通常比地球磁层的扁平。比例系数的这些更改可能有助于改进对CME上游电晕中磁场的估计以及就地测量的CME的纵横比的研究。
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