论文标题
ULF波动跨无碰撞冲击:2.5D局部混合模拟
ULF wave transmission across collisionless shocks: 2.5D local hybrid simulations
论文作者
论文摘要
我们通过分析11个2D局部混合模拟运行的输出,研究上游超低频率(ULF)波与无碰撞冲击的相互作用。我们的模拟冲击在4.29-7.42之间具有alfvénic马赫数,其$θ_{bn} $角度为15 $^\ circ $,30 $^\ circ $,45 $^\ circ $和50 $^\ circ $。 ULF波的预测会在所有这些上游发展。上游波的波长和幅度表现出对冲击的m $ _a $和$θ_{bn} $的复杂依赖。波长与两个参数呈正相关,对$θ_{bn} $的依赖性更强。 ULF波的幅度与反射光束速度和密度的乘积成正比,这也取决于m $ _a $和$θ_{bn} $。 ULF波与冲击的相互作用引起大规模(几十个上游离子惯性长度)的冲击波纹。冲击波纹的特性与ULF波性质有关,即THER波长和振幅。反过来,涟漪对跨冲击的ULF波传输产生了很大的影响,因为它们会改变局部冲击特性($θ_{bn} $,强度),因此相同的ULF波前局部相同的不同部分具有不同的特性。下游波动不像波前延伸,方向或波长相似于上游波。然而,某些特征在下游压缩波的傅立叶光谱中保守,这些光谱在大约对应于上游ULF波的波长的波长下凸起或变平。在横向下游光谱中,这些特征较弱。
We study the interaction of upstream ultra-low frequency (ULF) waves with collisionless shocks by analyzing the outputs of eleven 2D local hybrid simulation runs. Our simulated shocks have Alfvénic Mach numbers between 4.29-7.42 and their $θ_{BN}$ angles are 15$^\circ$, 30$^\circ$, 45$^\circ$ and 50$^\circ$. The ULF wave foreshocks develop upstream of all of them. The wavelength and the amplitude of the upstream waves exhibit a complex dependence on the shock's M$_A$ and $θ_{BN}$. The wavelength positively correlates with both parameters, with the dependence on $θ_{BN}$ being much stronger. The amplitude of the ULF waves is proportional to the product of the reflected beam velocity and density, which also depend on M$_A$ and $θ_{BN}$. The interaction of the ULF waves with the shock causes large-scale (several tens of upstream ion inertial lengths) shock rippling. The properties of the shock ripples are related to the ULF wave properties, namely thier wavelength and amplitude. In turn, the ripples have a large impact on the ULF wave transmission across the shock because they change local shock properties ($θ_{BN}$, strength), so that different sections of the same ULF wave front encounter shock with different characteristics. Downstream fluctuations do not resemble the upstream waves in terms the wavefront extension, orientation or their wavelength. However some features are conserved in the Fourier spectra of downstream compressive waves that present a bump or flattening at wavelengths approximately corresponding to those of the upstream ULF waves. In the transverse downstream spectra these features are weaker.