Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
复制标题
宇宙结构冲击下非线性粒子加速的效率
DOI：
10.1086/426855
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发表时间：
2004
期刊：
影响因子：
--
通讯作者：
T. W. Jones
中科院分区：
文献类型：
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作者：
Hyesung Kang;T. W. Jones
通过对一维准平行平面激波中扩散激波加速度(DSA)的数值模拟，计算了宇宙线(CR)修正的天体物理激波在较宽激波马赫数和激波速度范围内的演化。模拟包括种子CRS的热泄漏注入，以及预先存在的上游CR种群。假设为类玻姆扩散。我们模拟了类似于宇宙结构薄饼周围预期的激波，以及由上游气体温度T0=104K-107.6 K和激波马赫数MS=2.4133的流动驱动的其他吸积激波。我们发现，当注入粒子加速到中等相对论能量(p/mc≳1)时，CR修正激波演化到时间渐近状态，并且当结果以特征扩散长度和扩散时间表示时，具有相同马赫数但具有不同激波速度的两个激波定性地演化为相似的。我们通过计算激波时产生的总CR能量与初始参照系中通过激波的总动能的比率来确定和比较模拟冲击中CR加速的“效率”。对于这些模型，这一效率比的时间渐近值主要由激波马赫数控制，正如前面提到的CR激波的相似性所预期的那样。在存在预先存在的CR源的情况下，激波演化过程类似于与单独的热泄漏CR源相比，在更高的热注入速率下的激波演化。这一附加贡献对模拟的高马赫数激波的激波后或CR特性影响很小或没有影响。模拟的高马赫数激波都朝着效率约50%的方向发展，与上游CR压力无关。另一方面，上游冲击压力增加了中等强度冲击(ms~少数)的总冲击能量，因为它是冲击冲击波压力的重要组成部分。这些激波已被证明在宇宙结构形成过程中主导着耗散，因此这种增强的效率可能会显著增加它们作为宇宙射线源的潜在重要性。
We have calculated the evolution of cosmic-ray (CR) modified astrophysical shocks for a wide range of shock Mach numbers and shock speeds through numerical simulations of diffusive shock acceleration (DSA) in one-dimensional quasi-parallel plane shocks. The simulations include thermal leakage injection of seed CRs, as well as preexisting, upstream CR populations. Bohm-like diffusion is assumed. We model shocks similar to those expected around cosmic structure pancakes, as well as other accretion shocks driven by flows with upstream gas temperatures in the range T0 = 104-107.6 K and shock Mach numbers spanning Ms = 2.4-133. We show that CR-modified shocks evolve to time-asymptotic states by the time injected particles are accelerated to moderately relativistic energies (p/mc ≳ 1), and that two shocks with the same Mach number, but with different shock speeds, evolve qualitatively similarly when the results are presented in terms of a characteristic diffusion length and diffusion time. We determine and compare the "efficiencies" of CR acceleration in our simulated shocks by calculating the ratio of the total CR energy generated at the shock to the total kinetic energy that would pass through the shock over time in its initial frame of reference. For these models the time-asymptotic value for this efficiency ratio is controlled mainly by shock Mach number, as expected from the aforementioned similarity in CR shocks. In the presence of a preexisting CR population, shock evolution proceeds similarly to that for higher thermal injection rates compared to thermal leakage CR sources alone. This added contribution has little or no impact on the postshock or CR properties of the high Mach number shocks simulated. The modeled high Mach number shocks all evolve toward efficiencies ~50%, regardless of the upstream CR pressure. On the other hand, the upstream CR pressure increases the overall CR energy in moderate strength shocks (Ms ~ a few), since it is a significant fraction of the shock ram pressure. These shocks have been shown to dominate dissipation during cosmic structure formation, so such enhanced efficiency could significantly increase their potential importance as sources of cosmic rays.