Cosmic ray scattering in compressible turbulence

We study the scattering of low-energy cosmic rays (CRs) in a turbulent, compressive magnetohydrodynamic (MHD) fluid. We show that compressible MHD modes – fast or slow waves with wavelengths smaller than CR mean free paths induce cyclotron instability in CRs. The instability feeds the new small-scale Alfvénic wave component with wavevectors mostly along magnetic field, which is not a part of the MHD turbulence cascade. This new component gives feedback on the instability through decreasing the CR mean free path. We show that the ambient turbulence fully suppresses the instability at large scales, while wave steepening constrains the amplitude of the waves at small scales. We provide the energy spectrum of the plane-parallel Alfvénic component and calculate mean free paths of CRs as a function of their energy. We find that for the typical parameters of turbulence in the interstellar medium and in the intercluster medium the new Alfvénic component provides the scattering of the low-energy CRs that exceeds the direct resonance scattering by MHD modes. This solves the problem of insufficient scattering of low-energy CRs in the turbulent interstellar or intracluster medium that was reported in the literature.     

The interaction of cosmic rays and magnetized plasma

A large flux of cosmic rays streaming through a magnetized plasma creates cavities of low plasma density and low magnetic field. The magnetic field focuses the cosmic ray trajectories into the cavities with the possible formation of filaments or beams of high-energy cosmic rays.     

Analysis of the effect of a mean velocity field on a mean field dynamo

We study semi-analytically and in a consistent manner the generation of a mean velocity field     by helical magnetohydrodynamical (MHD) turbulence, and the effect that this field can have on a mean field dynamo. Assuming a prescribed, maximally helical small-scale velocity field, we show that large-scale flows can be generated in MHD turbulent flows via small-scale Lorentz force. These flows back-react on the mean electromotive force of a mean field dynamo through new terms, leaving the original α and β terms explicitly unmodified. Cross-helicity plays the key role in interconnecting all the effects. In the minimal τ closure that we chose to work with, the effects are stronger for large relaxation times.     

长江下游越冬期冷暖长期变化与大型环流低频振动

本文运用上海112年月平均温度资料和1月、10月的月平均海平面气压场110年资料,分析了以上海资料为代表的长江下游地区越冬季节温度由偏冷转向偏暖的长期变化及其有关联的大型环流系统的低频振动。发现近百年中前冬11月、12月和后冬2月这三个月的冷暖长期变化相似,特点是前50年左右偏冷占优势;后50年左右偏暖占优势,相应地10月大型环流中亚洲大陆冷高压的优势是前50年偏强偏东;后50年偏弱偏西,北太平洋高压的变化趋势则是前50年偏弱;后50年偏强。1月气温的长期变化与上述11月、12月和2月的冷暖变化不同,对应的大型环流低频振动也不同。     

非局部对流理论及其在太阳模型中的应用(英文)

在Li&Yang (2 0 0 1 )所给出的局部对流理论的基础上 ,我们进一步采用梯度型方案给出了非局部对流理论 ,并将它用于太阳模型中。这一理论考虑了恒星对流区内的非局部效应 ,它得到了一个与原来用混合长理论或局部理论给出的结果有所区别的对流区 ,扩散效应很明显。但是 ,目前我们的理论还不能处理时间相关的对流以及对流超射等问题。这些问题将在后续工作中加以考虑。当把这一理论应用于太阳模型中时 ,我们发现它对标准太阳模型的改正非常微小。我们讨论了这一现象 ,并对其加以解释     

A scheme for resampling, filtering, and subsampling unevenly spaced laser Doppler anemometer data

Laser Doppler Anemometry (LDA) has proved a powerful tool for quantifying fluid turbulence and is increasingly being applied in fields such as fluvial sedimentology and geomorphology. When operated in the burst-signal processing mode, high-frequency velocity fluctuations are measured at irregular time internals. In many situations, there is a need to transform these data to obtain evenly spaced velocity values but at a lower frequency. However, clear guidelines for this type of data processing are lacking. Three steps are necessary in order to transform the original files into evenly spaced data: (1) resampling at the average sampling rate, (2) low-pass filtering with half-power frequency adjusted to the final sampling frequency, and (3) decimating at the desired frequency. The decision taken at each step will affect the resulting signal and may cause, if not assessed carefully, severe problems in the signal such as aliasing errors. This paper examines each stage of data processing and details the advantages and drawbacks of different techniques in relation to the effects on turbulence statistics (variance, instantaneous shear stress, etc.). A standard method and specific guidelines are finally proposed. An erratum to this article is available at .     

Wave Flow Simulations Over Arctic Leads

We investigate the flow over Arctic leads using a mesoscale numerical model, typical of both summer and winter, under idealised conditions. We find that Arctic leads may be the source of standing atmospheric internal gravity waves during both seasons. The summertime wave may be compared with the wave generated by a small ridge, though with the phase reversed. The mechanism for exciting the wave is found to be the internal boundary layer developing due to horizontal variations in surface temperature and roughness length. During the more exploratory wintertime simulations, with substantial temperature difference between the lead and the ice surface, we find that secondary circulations and intermittent wave-breaking may occur. The effects of the lead appear far downstream.