背景风场对中尺度受导重力波的影响

采用重力波的大气分层全波解法，计算得出了强逆风场作用下以及无风条件下中尺度重力波受导波模的色散曲线和衰减距离曲线，结果表明，强逆风场可以使得各中尺度受导波模衰减距离明显变长，并导致不同尺度重力波的衰减距离出现峰值分布，通过分析美国MillstoneHill台站和武汉电离层观象中的TID实测结果证实了这些风场支持的远距离传播波模的存在。     

中尺度电离层扰动的统计色散特性

利用Millstone Hill(288.5E,42.6N)地区的数字测高仪漂移模观测数据,统计分析了中尺度电离层行扰(TID)的色散特性.分析结果表明,该地区的TID在扰动频率-相速度平面的分布中存在若干明显的扰动出现率峰值.比较相应的理论研究结果发现,观测到的TID大都可以用不同的受导重力波传播模式加以解释: 频率较高的TID可以用地面和热层底部维持的各阶完全制导模解释,频率相对较低的TID可以用传统理论给出的由热层底部陡峭的温度梯度维持的L₁模和我们新近提出的由热层背景风场所维持的各阶导管模解释.      

强震前异常电场激发声重波对电离层的影响

震前地震孕育期地表异常增强的电场,通过大气电导率传输到电离层高度.该异常电场通过非稳态局部加热,可以在电离层高度激发声重波.基于该理论,利用一维时变中纬电离层物理模型,模拟了该扰动源对电离层电子密度的影响.结果表明,重力波引起的中性风速度扰动对电离层电子密度分布影响甚微,该机理无法解释震前电离层异常扰动现象.      

中高层大气行星波与惯性重力内波的非线性相互作用

在弱非线性理论基础上,将三维大气中行星波和惯性重力波从原始非弹性近似方程中分离出来,讨论了典型的2天行星波与惯性重力内波的非线性相互作用过程.从共振曲面和参量不稳定增长率来看,行星波倾向于与空间尺度较大的惯性重力波发生相互作用.利用潮汐波的等价重力波假设,讨论了2天行星波与半日潮及9.6h惯性重力波的相互作用,三波相互作用时能量守恒.非线性相互作用使2天波和潮汐波的波幅受到长期调制.      

重力波波包在三维非等温大气和水平风场中非线性传播的数值研究

采用全隐欧拉格式（FICE）对重力波波包在三维非等温大气、均匀和剪切风场中的非线性传播进行了数值模拟，给出了重力波波包三维非线性传播的全过程，分析了重力波的传播特性及背景温度场、风场对重力波传播的影响。结果表明：波包扰动速度振幅的增长比在WKB条件下振幅的增长要慢；波包非线性传播的路径、能量传输速度不同于WKB近似下的结果，非线性效应导致了重力波的传播特性的改变；温度场的非均匀性会改变重力波传播的路径和速度；剪切风场使扰动速度振幅的增加变得缓慢，使垂直波长减小。     

非等间距显式算法在重力波非线性传播数值模拟中的初步应用

采用3阶TVD型显式时间积分方法,建立了2维可压缩大气中重力波非线性传播的非等间距显式数值模式.对小振幅重力波传播过程的模拟结果表明,该模式能够很好地再现小振幅重力波的传播过程,并能保持能量守恒关系和各扰动量之间的相位关系,与线性重力波理论预测的结果吻合很好.对有限振幅重力波的模拟结果表明,与格点数相同的等间距网格中的模拟结果相比,采用非等间距网格能够以更高的分辨率模拟重力波的不稳定传播直到破碎的整个过程;在非等间距网格中的模拟结果与在加密一倍的等间距网格中的模拟结果相同,说明了用非等间距网格模拟重力波的饱和与破碎过程是正确的,并且采用非等间距网格能够大大减少计算量.      

重力波非线性传播过程中的饱和与破碎

采用水平方向的显式算法与垂直方向的隐式算法相结合的时间分裂法,建立了二维可压缩大气中重力波非线性传播的数值模式.用本模式对小振幅重力波传播过程的模拟结果与线性重力波理论预测的结果吻合很好,从而验证了本模式的正确性.我们用此模式模拟了有限振幅重力波在非线性传播过程中的饱和与破碎,结果表明,(1)翻转出现在饱和之前,但向破碎演化仍需要一段时间,由于非线性波-波和波-流相互作用使得非线性数值模拟的饱和高度(出现时间)高(早)于线性饱和理论预测的结果;(2)重力波在不稳定之前已经有能量向背景场中转移,破碎直接导致非线性波-波相互作用,造成能量向小尺度短波上转移;(3)背景风场的加速方向,形成射流的方向与重力波的水平传播方向一致,表明重力波与背景流的非线性相互作用加剧了背景风剪切和不稳定性的发展.      

非线性效应对重力波特性的影响

通过分析二维情况下重力波波包在等温大气中非线性传播的数值模拟结果,对非线性效应对重力波特性的影响做了定量分析。分析结果表明;重力波在非线性传播过程中,一方面由于非线性作用,随着波振幅的增加,垂直波长减小,同时平流成份增加;另一方面,尽管存在非线性效应,线性重力波的色散关系与偏振关系仍然适用。     

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.      

重力波波包在真实大气中传播特性的数值研究

采用二维全隐欧拉（FICE）格式，对重力波波包在真实大气中的非线性传播和演变过程进行了数值模拟，模拟结果表明，在中层大气下部激发的向上传播的重力波波包在传播到中层顶之前，波相关能量沿着射线路径传播，非线性效应和背景温场对波能量传播路径的影响很小，当波包传播到低热层大气后，波包饱和波相关能量几乎完全沿着水平方向传播，垂直方向的能量传播受到抑制，这与在无耗大气下，WKB近似条件下的线性重力波理论的预言相差很大，深入的分析表明抑制重力波波包向上传输能量的关键因素是大气分子粘性的垂直非均匀性。非线性和背景温场的影响不足以完全抑制波能量的向上传播，此外，在波包的整个传播过程中，由于非线性，背景温度和背景耗散的共同作用，重力波波包的垂直波长随时间明显减小，这些结果说明大气的分子粘性特别是分子粘性的垂直不均匀性对重力波波包在中、高层大气的非线性传播过程起着重要作用。     

从射电天文观测数据反演电离层行扰的相速

本文结合ＴＩＤｓ的形成机制及其测量技术，对它的传播特性作了有益于测量的讨论，指出：（１）ＴＩＤｓ是由两个正交波组成；（２）测量电子总含量所表征的ＴＩＤｓ与测量电子浓度所表征者同样有效，据此导出了从测量数据反演其相速的公式，并对波幅的抑制效应作了讨论。     

损耗大气中随机重力波场的传输方程

从包含分子粘性的非线性相互作用方程出发, 采用随机相位近似, 推导了损耗大气中重力波场的非线性传输方程。由于分子损耗的引入, 谱演变的速率发生了改变, 在某些情况下, 还会得到与无耗条件下的传输规律相反的结论。损耗大气中重力波场的传输方程是研究中高层大气重力波能量收支平衡的出发点。      

大气重力波与电子密度扰动的耦合

本文从相互耦合的大气流体方程和双流体等离子体方程出发, 导出了赤道F区大气重力波和电子密度扰动的耦合色散关系, 据此对两者的共振相互作用作了进一步的理论分析。结果表明, 大气重力波可以通过共振耦合将部分能量转换给带电粒子, 为赤道扩展F提供初始电子密度扰动;在这过程中, 等离子体不稳定性对共振条件和共振耦合有着重要的影响。      

Multi-instrument observations of large-scale atmospheric gravity waves/traveling ionospheric disturbances associated with enhanced auroral activity over Svalbard

This study reports on observations of large-scale atmospheric gravity waves/traveling ionospheric disturbances (AGWs/TIDs) using Global Positioning System (GPS) total electron content (TEC) and Fabry–Perot Interferometer’s (FPI’s) intensity of oxygen red line emission at 630?nm measurements over Svalbard on the night of 6 January 2014. TEC large-scale TIDs have primary periods ranging between 29 and 65?min and propagate at a mean horizontal velocity of $～$749–761?m/s with azimuth of $～$345–347$°$ (which corresponds to poleward propagation direction). On the other hand, FPI large-scale AGWs have larger periods of $～$42–142?min. These large-scale AGWs/TIDs were linked to enhanced auroral activity identified from co-located all-sky camera and IMAGE magnetometers. Similar periods, speed and poleward propagation were found for the all-sky camera ($～$60–97?min and $～$823?m/s) and the IMAGE magnetometers ($～$32–53?min and $～$708?m/s) observations. Joule heating or/and particle precipitation as a result of auroral energy injection were identified as likely generation mechanisms for these disturbances.     

Investigation of the characteristics of wavelike oscillations of post-sunset equatorial ionospheric irregularity by decomposing fluctuating TEC

Post-sunset ionospheric irregularities are common features of the equatorial ionosphere that affect radio communication and navigation systems; their triggering physical mechanism is not yet fully understood. Atmospheric gravity wave is considered as a seeding mechanism for the occurrence of ionospheric irregularities (Abdu et al., 2009). To understand the effects of atmospheric waves, characteristics of wavelike oscillation from ionospheric total electron content (TEC) fluctuation that can be obtained from superposition of different oscillation modes have been investigated. Decomposing fluctuating TEC into different oscillation modes and investigating oscillation characteristics of each component is also important to get insight about the characteristics of individual atmospheric waves that may cause TEC fluctuation. In this paper we have investigated characteristics of components of fluctuating TEC obtained from SCINDA GPS receiver installed at Bahir Dar, (geographic coordinate, 11.5°N, 37.6° E, and dip latitude of 2.5°N) Ethiopia during April 2012. First Empirical Mode Decomposition (EMD) has been applied to decompose TEC fluctuation into different oscillation modes that are known as Intrinsic Mode Function (IMF). Hilbert-Huang Transform (HHT) and Continuous Wavelet Transform (CWT) have been applied to investigate the characteristics of wave-like oscillations. Applying EMD on fluctuating vTEC corresponding to a GPS satellite, five components are found. Results from HHT and CWT have shown excellent agreement. In addition, it is found out that the median periods of oscillation of those five components are 9, 17, 47, 78, and 118 min. Of these periods, 17 and 47 min respectively are oscillation periods of components of TEC fluctuation with occurrence frequency of 92% and 91% that may be interpreted as the manifestation of two frequently occurring components of atmospheric gravity waves that are likely generated by the motion of solar terminator.     

The ionospheric effect of atmospheric gravity waves excited prior to strong earthquake

We have computed perturbations in the nighttime mid-latitude F2 region ionosphere that could be produced by internal atmospheric gravity waves generated before strong earthquakes through ionospheric Joule heating due to the seismogenic electric field of short duration. There is a strong anisotropy of the atmospheric gravity wave effect with respect to the imminent earthquake epicentre, the electron density changes being maximum poleward and equatorward of the epicentre and being minimum eastward and westward of it. It should be noted that the duration of the electron density perturbation in the F2 region ionosphere is much longer than the duration of the primary precursor of an earthquake – the enhancement of the vertical electric field at the Earth’s surface, which initiates the atmospheric gravity wave generation. This fact is important from the practical point of view of predicting catastrophic earthquakes.     

Lower ionosphere response to external forcing: A brief review

There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail.