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.      

Seismogenic ionospheric anomalies associated with the strong Indonesian earthquake occurred on 11 April 2012 (M = 8.5)

Ionospheric perturbations in possible association with a major earthquake (EQ) (M?=?8.5) which occurred in India-Oceania region are investigated by monitoring subionospheric propagation of VLF signals transmitted from the NWC transmitter (F?=?19.8?kHz), Australia to a receiving station at Varanasi (geographic lat. 25.3°N, long 82.99°E), India. The EQ occurred on 11 April 2012 at 08:38:35?h UT (magnitude?≈?8.5, depth?=?10?km, and lat.?=?2.3°N, long.?=?93.0°E). A significant increase of few days before the EQ has been observed by using the VLF nighttime amplitude fluctuation method (fixed frequency transmitter signal). The analysis of total electron contents (TEC) derived from the global positioning system (GPS) at three different stations namely, Hyderabad (latitude 17.38°N, longitude 78.48°E), Singapore (latitude 1.37°N, longitude 103.84°E) and Port Blair (latitude 11.62°N, longitude 92.72°E) due to this EQ has also been presented. Significant perturbation in TEC data (enhancements and depletion) is noted before and after the main shock of the EQ. The possible mechanisms behind these perturbations due to EQ have also been discussed.     

Science objectives of the magnetic field experiment onboard Aditya-L1 spacecraft

The Aditya-L1 is first Indian solar mission scheduled to be placed in a halo orbit around the first Lagrangian point (L1) of Sun-Earth system in the year 2018–19. The approved scientific payloads onboard Aditya-L1 spacecraft includes a Fluxgate Digital Magnetometer (FGM) to measure the local magnetic field which is necessary to supplement the outcome of other scientific experiments onboard. The in-situ vector magnetic field data at L1 is essential for better understanding of the data provided by the particle and plasma analysis experiments, onboard Aditya-L1 mission. Also, the dynamics of Coronal Mass Ejections (CMEs) can be better understood with the help of in-situ magnetic field data at the L1 point region. This data will also serve as crucial input for the short lead-time space weather forecasting models.The proposed FGM is a dual range magnetic sensor on a 6?m long boom mounted on the Sun viewing panel deck and configured to deploy along the negative roll direction of the spacecraft. Two sets of sensors (tri-axial each) are proposed to be mounted, one at the tip of boom (6?m from the spacecraft) and other, midway (3?m from the spacecraft). The main science objective of this experiment is to measure the magnitude and nature of the interplanetary magnetic field (IMF) locally and to study the disturbed magnetic conditions and extreme solar events by detecting the CME from Sun as a transient event. The proposed secondary science objectives are to study the impact of interplanetary structures and shock solar wind interaction on geo-space environment and to detect low frequency plasma waves emanating from the solar corona at L1 point. This will provide a better understanding on how the Sun affects interplanetary space.In this paper, we shall give the main scientific objectives of the magnetic field experiment and brief technical details of the FGM onboard Aditya-1 spacecraft.     

Seasonal,annual, and interannual variability in MLT quasi-two-day waves over the low-latitude region Kolhapur (16.8°N; 74.2°E)

This study presents the quasi-two-day wave (Q2DW) characteristics of the mesosphere and lower thermosphere (MLT) region obtained by taking hourly mean values of horizontal wind velocities for 4? years (August 2013–July 2017) through continuous measurements using a medium-frequency (MF) radar (operating frequency – 1.98?MHz) located at the low-latitude Indian station Kolhapur (16.8°N; 74.2°E). The MF radar located at Kolhapur was upgraded in 2013, and these results of Q2DW have been reported for the first time after upgrading. The present study investigated variability in seasonal, annual, interannual, and solar indices of Q2DWs traveling in zonal (EW) and meridional (NS) components in the MLT region. The Q2DW activity is observed to be stronger during austral summer (January–February) (EW?=?～5?m/s and NS?=?～8–10?m/s) than during boreal summer (June–July) (EW = ～5 m/s and NS = ～6–8?m/s). The Q2DW amplitudes are larger in the meridional component than in the zonal one. A strong semiannual oscillation (SAO) has been observed in Q2DWs, with peak during January–February and June–July. In addition, small enhancement is seen in meridional Q2DW in October (～5–6?m/s). It is observed that the entire spectrum (40–60?h) measured between 86 and 94?km contributes to the SAO amplitudes during January–February and June–July, whereas the waves measured between 42?h and 52?h contribute to enhancement in October similar to that reported elsewhere. In general, the Q2DW amplitude shows large interannual variability. The easterlies developed in the global circulation model in Northern hemisphere during May intensify up to around summer solstice. Q2DW activity peaks during westerly shear zone and intensifies with time at a lower thermospheric altitude (above 90?km). Small positive correlations (r?=?0.2 for sunspot number and r?=?0.1 for 10.7?cm solar flux) have been observed between Q2DW amplitudes and solar activity.     

High-performance altimeter Doppler processing for measuring sea level height under varying sea state conditions

This paper describes an innovative method for processing nadir altimeter data acquired in Synthetic Aperture Radar (SAR) mode, enhancing the system performances over open ocean. Similarly to the current SAR data processing scheme, the so-called LR-RMC (Low Resolution with Range Migration Correction) method, originally designed by Phalippou and Demeester (2011), includes Doppler beam forming, Doppler shift correction and range correction. In LR-RMC, however, an alternative and less complex averaging (stacking) operation is used so that all the Doppler beams produced in a radar cycle (4 bursts of 64 beams for the open-burst Sentinel-3-mode altimeter) are incoherently combined to form a multi-beam echo. In that manner, contrarily to the narrow-band SAR technique, the LR-RMC processing enlarges the effective footprint to average out the effects of surface waves and particularly those from small sub-mesoscale structures (<1 km) that are known to impact SAR-mode performances. On the other hand, the number of averaged beams is as high as in current SAR-mode processing, thus providing a noise reduction at least equally good. The LR-RMC method has the added benefit of reducing the incoherent integration time with respect to the SAR-mode processing (50 ms compared to 2.5 s) limiting possible surface movement effects. By processing one year of Sentinel-3A SRAL SAR-mode data using the LR-RMC method, it is shown that the swell impact on the SAR altimeter performances is totally removed and that an improvement of 10–50% is obtained in the measurement noise of the sea surface height and significant wave height with respect to SAR mode. Additionally, observational capabilities over the middle scales are enhanced potentially allowing the ocean mesoscale features to be retrieved and observations assimilated more usefully in ocean models.     

Solitary and periodic waves in magneto–rotating plasmas: Effect of the Kappa–Cairns distributed electrons

The nonlinear propagation of ion–acoustic (IA) waves in a magneto–rotating plasma is studied by considering the Kappa-Cairns electron distribution. Employing the perturbation scheme, Korteweg–de Vries equation is derived. It is seen that both positive and negative potential solitons can be supported in the present plasma model. The numerical results reveal that the Kappa-Cairns distributed electrons modify features of the electrostatic waves significantly. The effects of non–thermal parameters, plasma rotation frequency, ion temperature, and the wave propagation angle on electrostatic solitary wave structures are also discussed here. It is found that the plasma parameters considerably influence the propagation of IA waves in rotating plasmas. Furthermore, using the bifurcation theory of planar dynamical systems to the K-dV equation, we have presented the existence of solitary and periodic traveling waves. Our study may be helpful to understand the behavior of ion–acoustic wave in the rotating plasma.     

Wake structure and similar behavior of wake profiles downstream of a plunging airfoil

Very limited attention has already been paid to the velocity behavior in the wake region in unsteady aerodynamic problems.A series of tests has been performed on a flapping airfoil in a subsonic wind tunnel to study the wake structure for different sets of mean angle of attack,plunging amplitude and reduced frequency.In this study,the velocity profiles in the wake for various oscillation parameters have been measured using a wide shoulder rake,especially designed for the present experiments.The airfoil under consideration was a critical section of a 660 kW wind turbine.The results show that for a flapping airfoil the wake structure can be of drag producing type,thrust producing or neutral,depending on the mean angle of attack,oscillation amplitude and reduced frequency.In a thrust producing wake,a high-momentum high-velocity jet flow is formed in the core region of the wake instead of the conventional low-momentum flow.As a result,the drag force normally experienced by the body due to the momentum deficit would be replaced by a thrust force.According to the results,the momentum loss in the wake decreases as the reduced frequency increases.The thrust producing wake pattern for the flapping airfoil has been observed for suffi ciently low angles of attack in the absence of the viscous effects.This phenomenon has also been observed for either high oscillation amplitudes or high reduced frequencies.According to the results,for different reduced frequencies and plunging amplitudes,such that the product of them be a constant,the velocity profiles exhibit similar behavior and coalesce on each other.This simi larity parameter works excellently at small angles of attack.However,at near stall boundaries,the similarity is not as evident as before.     

基于压缩感知的金属加筋板兰姆波健康监测技术

针对大型复杂板结构安全评价需要，发展了一种基于压缩感知的金属加筋板结构兰姆波健康监测技术。利用压缩感知技术从稀疏阵列得到的少量检测数据中恢复出加筋板结构中兰姆波的频散特性，并提出了一种将余弦相似度和皮尔森相似度相结合的稀疏阵列兰姆波复合成像方法，以实现加筋板结构中大范围缺陷检测和成像。实验结果表明，压缩感知技术可以恢复出金属加筋板中兰姆波的频率-波数关系，提出的兰姆波复合成像方法能够实现金属加筋板中单缺陷和多缺陷的检测及定位。研究工作为复杂板结构损伤检测提供了一种可行的技术方案。     

Longitudinal MLT wind structure at higher mid-latitudes as seen by meteor radars at central and Eastern Europe (13°E/49°E)

The mid-latitude mesosphere and lower thermosphere (MLT) wind speeds measured by two SKiYMET meteor radars (MRs) at Collm (51°N, 13°E) and Kazan (56°N, 49°E) during 2016–2017 were analyzed to study longitudinal wind structures. The differences between monthly mean prevailing wind speeds and tidal amplitudes were compared with the corresponding differences obtained from TIMED/TIDI satellite winds and gradient wind speeds from the AURA/MLS instrument. It is shown that the MR wind difference between the two sites is statistically significant. The difference of the horizontal prevailing winds can be explained by a superposition of the background zonal flow, which is different at the two latitudes, with stationary planetary waves of different origin. Non-migrating tides contribute significantly to the difference of the semidiurnal tidal winds between the two sites.