Pc1 pearl waves with magnetosonic dispersion

Results of the analysis of 15 unusual Pc1 pearl wave events with inverse dispersion in comparison with the dispersion of well-known electromagnetic ion-cyclotron (EMIC) waves in the form of classic pearl pulsations are presented. Pulsations with the dynamical spectrum consisting of both falling tones only (first type) and events with structures, which start with the falling tones and then develop into rising tones (second type), have been discovered. The first type corresponds to the frequency dispersion of magnetosonic waves (R-waves), and the second type corresponds to the mixed frequency dispersion of R-waves and EMIC waves (L-waves). All events were observed during quiet geomagnetic periods. The duration of the events is about 20–30 min. For the interpretation of these phenomena, the cyclotron instability driven by energetic proton beams with relative mean velocity v₀ directed along the background magnetic field and corresponding to an energy ∼10–100 keV is considered. The interaction of such proton beams with waves having frequencies ω<ω_i (ω_i is the ion gyrofrequency) leads to the instability, which allows the fastest growth of electromagnetic oscillations with the dispersion of R-wave type. When the velocity of the proton beam decreases (v₀≈0), R-waves attenuate and L-waves (for the proton temperature T_⊥>T_∥) will be amplified. This instability is the reason for the generation of classic Pc1 pearl pulsations with the usual dispersion and allows explaining the transition of the dispersion from R- to L-waves.     

Satellite observations of Pc 1 pearl waves: The changing paradigm

Pc 1 pearls have been observed on the ground for about 70 years. During this time numerous publications have been written on the various properties of Pc 1 pearl waves, the related theory, and possible applications. Pc 1 waves with a clear pearl structure are only a fraction of all Pc 1 waves observed on ground, and this fraction depends on the latitude of observations, increasing from high to low latitudes. In fact, the spatial and temporal occurrence of Pc 1 pearls is closely connected with the location and development of the plasmapause. While it has been known roughly 40 years that Pc 1 waves are electromagnetic ion cyclotron waves generated by anisotropic, energetic ions in the near-equatorial magnetosphere, the formation of pearl structure is still largely in question. In situ observations of Pc 1 waves in the Earth's magnetosphere have been made since the 1970s by various satellites in different orbits. However, satellite observations of clear Pc 1 pearls are still rather few. Here we review a few crucial satellite-based observations of Pc 1 pearls, and evaluate their contribution to the understanding of pearl formation. We show that the long-held paradigm of the bouncing wave packet model is in serious contradiction with satellite observations and therefore outdated. Instead, observations support the idea that Pc 1 wave growth rate is successively modulated at the equator by long-period ULF waves.     

On the attenuation of sea waves by rain

Abstract

Some conflicting evidence on Reynolds' (1900) hypothesis that rain should attenuate any wave motion on the sea surface is discussed. It is concluded that rain drops ought to produce vortex rings in the sea which mix the water to a sufficient depth to affect most waves, as asserted by Reynolds. By introducing vertical and horizontal eddy viscosities to model the mixing process, an estimate is found for the rate of attenuation of wave energy by the rain. Consequently, the net effect on the wave field of attenuation by rain and generation by wind is calculated.     

利用激光雷达测量重力波三维结构

激光雷达观测得到的密度、温度等数据被广泛应用于大气重力波研究.瑞利激光雷达可以获取激光路径上的大气密度、温度数据.对于大气中的三维波动而言,单条路径上的观测参量能提取得到的波动信息有限.本文首先以单色重力波为例,分析了利用激光雷达直接观测三维波动结构的可行性.激光雷达垂直观测即可得到重力波的垂直波长,当激光雷达以一定的天顶角斜向测量时,所得到的波长包含了重力波的垂直波长以及水平波长信息.因此,利用激光雷达同时以三个方向（垂直、向南（天顶角30°）以及向西（天顶角30°））测量,可以提取得到重力波的垂直波长和水平波长.本文利用中国科学院国家空间科学中心研制的车载532 nm瑞利激光雷达的经向系统和纬向系统同时以不同的指向角观测大气重力波,对利用激光雷达获取三维波动结构的方法进行了分析研究.本文给出了北京地区激光雷达观测重力波的诸多案例,分析了30~60 km高度范围内北京地区大气重力波的垂直及水平波长信息.并以2017年11月7日观测的准单色重力波为例,结合再分析资料的风场数据,分析了该重力波的水平波长,垂直波长及传播方向等信息.

     

江淮地区地震考古的新发现

对江淮地区近期发掘的四个考古遗址进行了地震考古研究,发现了丰富的构造变形现象,包括古地震事件。其中固镇小李家遗址的汉代砖室墓变形具有统一的方向性,显示受区域NE-SW向构造应力作用形成,与强烈地震的快速变形无关;合肥乱墩子遗址的多数竖穴土坑墓存在不规则变形,但M66的压扭变形可能受郯庐断裂右旋走滑活动牵引形成,也与地震力作用无关。在和县大城子遗址和马鞍山申东遗址发现古地震遗迹,主要为地震裂缝、文化层位错及褶曲。在和县大城子遗址通过文化层中木炭的AMS14C测年确定出西周中晚期的一次古地震事件（2 580±30)~(2 760±30） a BP;在马鞍山申东遗址通过文化层考古断代及地震裂缝与文化层的切割关系识别出一次古地震事件亦发生于西周中晚期。最后综合本次研究与前人对江淮地区西周时期的古地震研究成果,初步认为西周中晚期江淮地区处于中强地震活跃期。     

On resonant over-reflection of waves by jets

It is well known that internal or Rossby waves propagating across a jet can be amplified, a phenomenon usually referred to as over-reflection. In some cases, over-reflection can be infinitely strong – physically, this means that the reflected and transmitted waves can exist without an incident one, i.e. they are spontaneously emitted by the mean flow. In this article, it is shown that infinitely strong over-reflection (resonant over-reflection) occurs for gravity-wave scattering by ageostrophic jets in a rotating barotropic ocean and Rossby-wave scattering by a two-jet configuration on the quasigeostrophic beta-plane. It is further demonstrated that, generally, a resonantly over-reflected wave is always marginal to instability, i.e. either an increase or a decrease of its wavenumber transforms it into an unstable eigenmode localised near the jet.     

On the mean motion induced by transient inertio-gravity waves

Using standard two-scaling techniques we calculate the Eulerian- and Lagrangianmean flows induced at second order in amplitude by small-amplitude, transient, non-dissipative, vertically-propagating inertio-gravity waves on anf-plane. The example is an idealized one, but illustrates a number of important features that are typical of wave, mean-flow interaction in a rotating, stratified fluid. Physical discussion of the solution is given in Section 4 of the review byMcIntyre (1980), which appears elsewhere in this issue.     

Scattering of rossby waves by partial barriers

Abstract

The scattering of Rossby waves by partial barriers is studied. It is found that only a small fraction of the wave energy will penetrate the gap between South America and Antarctica if the wave period is less than a month.     

Generation of waves by advected pressure fluctuations

Abstract

Because of the dispersion of surface gravity waves, the rate at which advected pressure fluctuations generate wave energy is independent of the coherence time of the pressure fluctuations provided that this time is long compared with the period of the waves. Phillips' (1957) analysis is correct insofar as it concerns wave components exactly “resonant” with the advected pressure fluctuations, but it does not deal adequately with the “bandwidth” of the resonance.     

Natural self-cleaning of oil-polluted beaches by waves

Changes in the magnitude of oil pollution on five prototype study beaches on Israel's northern Mediterranean coast had been monitored at 2- to 4-weekly intervals from 21 November 1980 to 19 March 1981. Statistical analyses of these changes were employed to rank the beaches in order of decreasing self-cleaning capacity. Records of storm-induced wave data records for the winter of 1980–1981 were correlated to incidence of change in the magnitude of subsurface tarball pollution and the rate of emplaced fuel sample dissipation on the study beaches. Specific wave breaking processes, overtopping of offshore obstacles and wave refraction were singled out as accelerating or retarding wave energy transfer to the polluted beaches. Attribution of the occurrence of specific local littoral processes to observed change in beach pollution magnitude made it possible to assess potential oil spill damage. Elucidation of the causative processes provided theoretical corroboration for beach sensitivity ranking established earlier by statistical analysis.     

Resonance induced by Rossby Lee waves

Abstract

The mutual interaction of fields induced by spatially separated potential vorticity sources in a quasi-geostrophic barotropic flow is investigated using the weakly nonlinear approach. It is found that a powerful nonlinear response can be triggered by Rossby lee waves. This resonance phenomenon which dominates all other nonlinear corrections depends on certain global resonance conditions and on the change in the phase of the Rossby lee wave across the distance separating the sources. The response is particularly strong for topographic forcing possessing δ-function characterisitics.     

Sand wave formation by stationary waves

In this paper we develop a new hypothesis which relates the formation of sand waves in open flow to the effect of stationary (lee) waves on flow with smooth nonhomogeneous downstream depth or behind an obstacle. The lee wave moves upstream with the phase velocity equal to the absolute value of the flow velocity, hence the wave crest does not move. Results of our experiments show that sand waves on the flow bed appear only below the lee waves and characteristics of sand waves are determined by the wave properties. We investigated the nonlinear stationary waves for subcritical and supercritical Froude numbers. These results allow us to predict sand waves characteristics for a particular flow.     

Sunglitter radiance modulation by internal waves

Summary Quantitative sunglitter radiance modulations at the ocean surface due to internal waves were calculated applying a simple first order theory. The results have been compared with the image data of internal wave signatures northeast of Nantucket Shoals off the coast of Massachusetts, U.S.A., taken by the Earth Terrain Camera (ETC) during the Skylab mission. The maximum magnitudes of sunglitter radiance modulations predicted by the theory are comparable to the satellite image data. The extreme values of calculated modulations depend only weakly on the wavelength of the internal wave signature. This is consistent with the analysed ETC data. On the other hand, the shapes of simulated and measured image intensity transects across an internal wave packet are different. This may possibly be explained by the assumed idealized sinusoidal type of thermocline used in the calculations. The theory predicts that sunglint signatures of internal waves are advected relative to positions of extreme slope regions of the thermocline due to currents.

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Strahldichtemodulation des Sonnenglitzerns durch interne Wellen

Zusammenfassung Quantitative Strahldichtemodulationen des Sonnenglitzerns an der Meeresoberfl?che durch interne Wellen werden unter Verwendung einer einfachen ersten Ordnungstheorie berechnet. Die Ergebnisse werden mit Bilddaten von internen Wellensignaturen nord?stlich von Nantucket Shoals vor der Küste von Massachusetts, U.S.A., verglichen, die w?hrend der Skylab-Mission von der Erdbeobachtungskamera (ETC) aufgenommen wurden. Die Werte der maximalen Strahldichtemodulation des Sonnenglitzerns, die von der Theorie vorhergesagt werden, sind mit den Satellitenbilddaten vergleichbar. Die Extremwerte der berechneten Modulationen h?ngen nur schwach von der Wellenl?nge der internen Wellensignaturen ab. Dieses stimmt mit den analysierten ETC Daten überein. Auf der anderen Seite sind die Kurvenverl?ufe zwischen den simulierten und gemessenen Bildintensit?tsprofilen entlang eines internen Wellenpaketes unterschiedlich. Dieses kann m?glicherweise durch den angenommenen idealisierten sinusf?rmigen Typ der Thermokline erkl?rt werden, der für die Berechnungen verwendet wurde. Die Theorie macht die Vorhersage, da? Signaturen des Sonnenglitzerns von internen Wellen durch Str?mungen advektiert werden relativ zu den Positionen der extremen Neigungsregionen der Thermokline.

     

Kinematic dynamo induced by helical waves

We investigate numerically the kinematic dynamo induced by the superposition of two helical waves in a periodic box as a simplified model to understand the dynamo action in astronomical bodies. The effects of magnetic Reynolds number, wavenumber and wave frequency on the dynamo action are studied. It is found that this helical-wave dynamo is a slow dynamo. There exists an optimal wavenumber for the dynamo growth rate. A lower wave frequency facilitates the dynamo action and the oscillations of magnetic energy emerge at some particular wave frequencies.     

Peculiarities of energy transport in the atmosphere by acoustic gravity waves

The dependence of energy transport by acoustic gravity waves (AGWs) on their spectral properties is studied. On the basis of the analysis of expressions for group velocities and energy fluxes of AGWs, it is shown that there exist separate frequencies and wavelengths at which the energy transport in space is most efficient. Comparison of the obtained results with the data of observations on board the Dynamic Explorer 2 satellite shows that, in the upper atmosphere of the Earth’s polar regions, AGWs with spectral parameters corresponding to the maximum of energy transport predominate.     

Scattering of waves by subterranean structures via the boundary element method

The boundary element method (BEM) is used to study the two-dimensional wave field generated when buried structures of arbitrary shape (i.e. inclusions) in an elastic medium are illuminated (or insonified) by dynamic line sources. Both steady-state responses and time-domain transients are presented. The problem is formulated in the frequency domain by means of appropriate Green's functions. The evaluation of the singular integrals is achieved (and to the best of the writers' knowledge, for the first time in the technical literature) in analytical form, which results in improvements in computational efficiency and accuracy. Closed-form solutions for regular geometries are then used to validate the method. The interaction of two cavities, the formation of shadow zones by inclusions and the complexity of the scattered field from bodies with irregular shapes are used as examples to demonstrate the versatility of the method. The responses computed in the time domain were invariably found to be causal, even for non-convex domains, which belies a recent assertion by some researchers that the application of boundary element methods to concave domains is associated with non-causal effects.     

On the development of baroclinic waves influenced by friction and heating

Summary The influence of surface skin friction and a specific type of heating on the stability of baroclinic waves in a two-level, quasi-geostrophic model is investigated. It is found that the effect of friction alone changes the neutral stability curve in such a way that a broader band of wavelengths are unstable for a given value of the vertical windshear. The neutral stability curve is independent of the intensity of friction in this case. The effect of heating is to make all waves longer than a certain critical wave length unstable, but the amplification rate is very small for large values of the wavelength. The combined effect of friction and heating will in general tend to stabilize the waves. The amplification rate is investigated in all cases.Numerical integrations of the linearized equations show that the flow in the stable case will reach a steady state characterized by vanishing dissipation and heat transport, but with a certain wave length dependent ratio between the kinetic energy of the vertical shear flow and the vertical mean flow. It is shown that this ratio can be predicted from the steady state solutions in the adiabatic, frictionless case. A comparison is made between the predictions of the energetics of the waves in the model and observational studies.Research supported by the Section on Atmospheric Sciences, National Science Foundation, GP-2561.Contribution No. 99 from the Department of Meteorology and Oceanography, University of Michigan.