Past and future of radio occultation studies of planetary atmospheres

Measurements of radio waves that have propagated through planetary atmospheres have provided exploratory results on atmospheric constituents, structure, dynamics, and ionization for Venus, Mars, Titan, Jupiter, Saturn, and Uranus. Highlights of past results are reviewed in order to define and illustrate the potential of occultation and related radio studies in future planetary missions.     

北京地区大气温度及重力波活动的季节变化

利用瑞利激光雷达观测数据,分析了北京地区35~70km高度范围内大气温度和重力波活动的季节变化.发现北京地区30~70km高度范围内的大气温度有明显的年周期变化:平流层顶最高温度出现在6,7月份,大约为270K;中间层70km高度最低温度也出现在6,7月份,大约为200K.以2014年10月14日晚数据为例,分析重力波势能密度,发现50km以下重力波势能存在耗散,而在50km以上重力波近乎无耗散地向上传播.通过对比35~50km高度范围内的平均势能密度,对北京地区重力波活动强弱的季节变化进行了研究.研究结果表明,北京上空重力波活动强度具有明显的年周期变化,冬季平均势能密度为18J·kg-1,夏季为8J·kg-1,且冬季重力波活动强度约为夏季的两倍.此外,还分析了春夏秋冬四个季节重力波势能密度随高度的变化.结果表明,不同季节和不同高度的重力波势能密度不同.      

损耗大气中惯性重力波的非线性相互作用方程及其初步讨论

根据弱相互作用理论,本文建立了损耗大气中极性重力波的非线性相互作用方程。这组方程在三个方面推广了前人的工作:考虑了波的空间传播;包含了粘滞产生的衰减;波谱可以是连续的。粘滞衰减率的大小与波的空间尺度以及传播方向有关。Coriolis力的引入使相互作用系数成为复量。根据这组方程,考察了惯性重力波的参量激发。结果表明:在共振条件满足时,主波存在一个阈值,阈值大小与次级波的损耗率成正比。当主波振幅大于这个阈值时,次级波将指数增长。在相互作用过程中,次级波的频率将发生变化,变化的大小与主波能量成正比。      

太阳风中的Alfven波耗散与电子和质子温度分布问题

根据动力论Alfven波耗散机制,详细计算了动力论Alfven波衰减的能量分配给质子和电子的比率.初步解释了高速太阳风中质子温度比电子温度高的事实,定性说明了r＜10R_s时电子快速冷却的原因.      

Planetary wave transience effects on the zonal mean flow: simulations with the COMMA-LIM model

A 3D circulation model has been used to investigate the role of the quasi two-day wave propagation, its dissipation and variability in the formation of the middle atmosphere zonal mean circulation. According to linear theory, the accelerations due to dissipation of steady planetary waves are relatively weak. This study shows that the transient quasi two-day wave is able to shift the summer jet towards the equator thereby attenuating the seasonal weakening of the westward jet.     

粘性耗散对重力波波包共振相互作用的影响

采用数值方法考察了粘性耗散对重力波波包共振相互作用的影响．结果表明,粘性耗散不仅衰减相互作用波的能量,而且强烈地影响波包之间局地能量的传递,使波包的形状发生进一步的改变．粘性耗散减缓相互作用过程的发展,阻止高波数的次级波与主波进行相互作用．在低热层高度上,耗散过程占支配地位,三波能量演变的特征发生了相当大的变化．大尺度主波通过与小尺度次级波的相互作用,更快地实现对大气的加热．      

内重力波传播的3维传输函数模式研究

在考虑背景风场及大气耗散的条件下,建立了3维内重力波传输函数数值模式.分析了300 km高度3维传输函数在频率波数域的特性,并以近地面单位脉冲点源为激发源,得到了内重力波在3维空间中的时空分布.讨论了不同时空尺度地面方波源激发的内重力波在电离层高度的能量分布特征.结果表明,(1)对内重力波而言,背景大气相当于一个带通滤波器,只有波动周期和波长分别在15～30 min和200～400 km之间的重力波扰动最容易上传到300km高度;(2)在背景风场的作用下水平面上以同心圆扩散的波阵面以及垂直方向上成漏斗状的波阵面发生了变形,并且逆风方向比顺风方向更有利于声重力波由对流层向电离层高度传播;(3)300km高度对时间尺度和空间尺度分别在20～30 min和150～250 km之间的地面方波源响应的总能量最强.      

A numerical model characterizing internal gravity wave propagation into the upper atmosphere

A new two-dimensional, time-dependent and fully nonlinear model is developed to numerically simulate plane wave motions for internal gravity waves in a non-isothermal and windy atmosphere that accounts for the dissipation due to eddy and molecular processes. The atmosphere has been treated as a well mixed total gas with a constant mean molecular weight. The effects of Rayleigh friction and Newtonian cooling are applied near the upper boundary of the model to simulate the radiation conditions as well as to act as a sponge layer; lateral boundaries are periodic over a horizontal wavelength to simulate a horizontally infinite domain. The thermal excitation to initiate upward propagating waves is spatially localized in the troposphere and is a Gaussian function of time. A time-splitting technique is applied to the finite difference equations that are derived from the Navier–Stokes equations. The time integration for these highly coupled equations is performed using an explicit second order Lax–Wendroff scheme and an implicit Newton–Raphson scheme. The wave solutions derived from the model are found to be broadly agreeable with those derived from a Wentzel–Kramers–Brillouin theory.     

Partially ionized atmospheres of neutron stars with strong magnetic fields

We construct hydrogen atmosphere models for strongly magnetized neutron stars in thermodynamic equilibrium, taking into account partial ionization. The presence of bound states affects the equation of state, absorption coefficients, and polarizability tensor of a strongly magnetized plasma. Therefore the partial ionization influences the polarization vectors and opacities of normal electromagnetic waves, and thus the spectra of outgoing radiation. Here, we review a model suitable for the most typical neutron-star atmospheres and focus on the problems that remain to be solved for its extension to other atmospheric parameters.     

无碰撞激波上游低频波的激发及其对激波耗散机制的影响

由于很大一部分来自激波上游的粒子被激波面所反射,因此在准平行无碰撞激波的上游存在着等离子体束流．通过一维混合模拟方法,计算了束流密度较小(nb／n0=0．02)和较大(nb／n0=0．2)两种不同情形下的等离子体束流不稳定性．结果表明,在束流密度较小时,束流激发的主要是平行于背景磁场方向传播的右旋波,此波动只能对束流粒子产生影响,包括减速和加热．在束流密度较大时,束流可同时激发平行和反平行于背景磁场方向传播的右旋波,除能对束流粒子产生影响外,还可通过非共振作用加速和加热背景粒子．文中对准平行无碰撞激波耗散机制的影响也进行了讨论。     

Cylindrical and spherical dust-electron-acoustic shock waves due to dust charge fluctuation

Cylindrical and spherical dust-electron-acoustic (DEA) shock waves propagating in a dusty plasma (containing cold inertial electrons, hot Maxwellian electrons, stationary and streaming ions, and charge fluctuating stationary dust) are theoretically investigated by reductive perturbation method. It is shown that the effect of the dust charge fluctuation introduces some new features in the nonlinear propagation of the DEA waves, particularly the dust charge fluctuation provides a source of dissipation, and is responsible for the formation of the DEA shock structures. It is also found that the basic features of the DEA nonlinear structures are significantly modified by the non-planar (viz. cylindrical and spherical) geometry, and that the height of the cylindrical DEA shock structures are larger than that of the planar DEA shock structures, but smaller than that of the spherical ones. The implications of these results in laboratory dusty plasmas are briefly discussed.     

Lessons from laboratory experiments on reconnection

Magnetic reconnection has been studied in a laboratory experiment designed to model the basic two-dimensional neutral sheet configuration. However, the focus has been put on the inner region of the neutral sheet where the ions are effectively unmagnetized and MHD concepts are violated. In this parameter regime driven reconnection is governed by the fast dynamics of electrons. In true neutral sheets (B_z ⋍ 0) the current is carried by electrons. Thin current sheets (Δz ≳ c/ω_pe) rapidly form multiple X and 0 points due to the onset of the collisionless electron tearing mode. Magnetic energy is transported along the separator at the speed of whistler waves rather than Alfvén waves. Due to space charge separation the reconnection electric field E_y is, in general, not constant along the separator but localized near boundaries, nonuniformities in density and magnetic fields which limit the current I_y. This leads to localized particle acceleration, formation of anisotropic velocity distributions and instabilities. Reconnection and energization can be spatially separated which shows the importance of investigating both the global current system as well as critical local plasma properties. Experiments of current sheet disruptions are performed which demonstrate the processes of magnetic energy storage, transport, conversion and dissipation. Double layers and shock waves can be produced by current disruptions. The laboratory experiments show new dynamic features of reconnection processes not considered in MHD models yet relevant to narrow current sheets or the center of thick sheets.     

Analytical MHD model of three-dimensional time-dependent reconnection in a compressible plasma

We describe and analyse a model of three-dimensional time-varying reconnection in which the effect of surface waves is neglected. Reconnection is assumed to be caused by a localized decrease of the plasma conductivity inside the diffusion region. The localized dissipation gives rise to an electric field E^*, which determines the reconnection rate. As a result, the current sheet decays into a system of large-amplitude MHD waves, which propagate along the current sheet and thereby induce perturbations in the surrounding medium. Our model is applied to the case of reconnection at the magnetopause, in particular to the signatures referred to as FTEs (flux transfer events).     

Secular variations of OH nightglow emission and of the OH intensity-weighted temperature induced by gravity–wave forcing in the MLT region

Secular variations of OH airglow (8,3) band and of the intensity-weighted temperature induced by a gravity wave packet at three latitudes in the northern hemisphere in the Mesosphere/Lower Thermosphere region are investigated using a 2-D, time-dependent, fully nonlinear OH chemistry–dynamics model. A net-cycled average survives due to wave transience and dissipation. The integrated OH (8,3) volume emission rates show a 20 percent increase induced by the wave packet. The corresponding intensity-weighted temperature shows an initial increase up to 2 percent then a gradual decrease for the remainder of the simulation time by the same wave packet. These secular variations could be mistaken as long-period or short-period waves in the airglow observations. Therefore, care must be taken when analyzing the data from observations.     

Tearing,coalescence and fragmentation processes in solar flare current sheet and drifting pulsating structures

The paper presents a summary of results from two different simulations which study the tearing, coalescence and fragmentation of current sheets, the associated production of energetic electrons and of plasma waves from these electrons which could explain drifting pulsation structures observed at radio wavelengths. Using a 2.5-D particle-in-cell (PIC) model of the current sheet it is shown that due to the tearing mode instability the current sheet tears into plasmoids and these plasmoids later on coalesce into larger ones. During these processes electrons are accelerated and they produce observable electromagnetic waves. Furthermore, the 3-D PIC model with two current sheets extended in the electric current direction shows their fast fragmentation associated with the exponential dissipation of the free magnetic field energy. An example of the drifting pulsating structure which is considered to be a radio signature of the above mentioned processes in solar flares is shown.     

On the heating mechanism of magnetic flux loops in the solar atmosphere

Hα filtergrams and magnetograms indicate that bright features (such as plages and granulation boundaries) correspond to areas of strong vertical magnetic fields and dark features (such as fibrils and filaments) are associated with strong horizontal magnetic field. It was suggested by /1/ that there is an excess dissipation of waves, available for heating, in regions of vertical magnetic fields. With this suggestion in mind, we have investigated the physical heating mechanism due to ponderomotive forces exerted by turbulent waves along curved magnetic flux loops. Results show that the temperature difference (ΔT) between the inside and outside of the flux loop can be classified into three parts; ΔT = ΔT₁ + ΔT₂ + ΔT₃; in which ΔT₁ and ΔT₃ represent the heating or cooling effect from the ponderomotive force, and ΔT₂ is the heating effect due to conversion of turbulent energy from the localized plasma. The specific physical mechanism (i.e., the ponderomotive forces exerted by turbulent waves), is used to illustrate solar atmospheric heating via an example leading to the formulation of plages.     

MHD surface waves – a source of global magnetospheric modes (resonances)?

Various mechanisms have been proposed for explanation of the global magnetospheric modes whose frequency peaks are in the frequency range 1–4 mHz. Recent papers claim: basic characteristics of the 1–4 mHz activity events observed on ground give evidences for an existence of MHD surface mode excited on the Earth magnetopause. The discrete frequencies of such MHD surface wave modes suggest an emergence of standing wave structures along the magnetic field lines lying on the magnetopause. Such discrete frequencies of MHD surface waves on magnetopause however, are not stable, at all. Contrariwise, MHD surface wave modes supported by the two plasma boundaries – the magnetopause and the plasmapause, are in accordance with existing experimental facts: discrete set of almost stable frequencies, field amplitude peaks and positions, energy dissipation, and field distribution from high to low latitudes. Mechanisms of the global magnetospheric mode resonance are pointed out as well as tools for their identification and discrimination.     

Structure of the atmospheres of Mars and Venus below 100 kilometers

The present state of knowledge of the structure of the atmospheres of Mars and Venus below 100 km is reviewed. Both atmospheres have been characterized to a remarkable degree, considering their remoteness from Earth. Mars' atmosphere is more variable, and less well defined. The variability is seasonal, diurnal, and latitude dependent, and also is strongly affected by atmospheric dust. Venus' atmosphere is less variable and better defined, as a result of numerous spacecraft missions. Mars' atmosphere is generally statically stable from the surface to 100 km, and its temperatures are controlled by radiative transfer moderated by vertical flow. Venus' lower atmosphere is stratified, with stable layers predominating. Its temperatures also are controlled by radiation and vertical motion. Thermal tidal motions occur in both atmospheres at about the same pressure levels, with larger amplitudes at Venus, consistent with the larger solar input at Venus.     

Comparative neutral composition instrumentation and new results

Composition and gas density measurement at all altitudes in the atmospheres of earth and other planets are made by mass spectrometers. Because of the impartiality and large dynamic range they are particularly favored for exploratory missions. Measurements of trace constituents, inert gases and height profiles, especially below clouds, are objectives where mass spectrometry is most useful. Significant advances have been made in recent years in development of light weight automated instruments. Experiments conducted in rarified atmospheres have been more successful or results were less controversial than in attempts to analyze high pressure atmospheres. Gas sampling and conditioning techniques are highly specific because of measurement environments and engineering constraints on the mission, and are usually the most critical elements in the experiment. Chemical sample enrichment and scrubbing for noble gas enhancement are additional sample conditioning techniques now employed. Dynamic range of more than 10⁸ is achievable. Reliable measurements of complex organic or chemically active trace constituents with mixing ratios of less than 10^?9 still require significant instrument development particularly where weight, power and sampling time are severely restricted. Adaptation of familiar and proven laboratory techniques for flight instruments is usually not straightforward and practical.     

Comparative analysis of chemical composition and optical properties of gaseous and disperse phases of the Earth-group planetary atmospheres

The models of chemical composition and structure of the Earth-type planetary atmospheres are offered. The optical properties of gaseous and disperse phases of the atmospheres are investigated.