Determination of the Kelvin-Helmholtz Wave Parameters on the Magnetopause in Single-spacecraft Observations

Kelvin-Helmholtz (K-H) waves are formed from the triggeringof the K-H instability on the magnetopause, which is a candidatemechanism for solar wind entry into the magnetosphere, especially undernorthward interplanetary magnetic field conditions. In this study, aK-H wave event was identified from the observation of probe Bof the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission on 15 May 2008. A new method to determinethe wave parameters of the K-H waves in single-spacecraft observationsis proposed. The dominant wave period is determined by three kinds ofspectrograms for three key parameters, namely the ion density, the iontemperature, and the z component of magnetic field. The phasevelocity is estimated by calculating the center-of-mass velocity of thedetected K-H vortex region. This approximation is validated bycomparison with other alternative methods. The method to determine thewave parameters is a first step to further study K-H wave properties and their relationship with interplanetaryconditions.     

大功率无线电波加热低电离层

等离子体对大功率电波的欧姆耗散会使电子温度升高,进而导致电子密度和其他等离子体参数改变,实现电离层的地面人工变态.本文基于大功率无线电波与低电离层相互作用的自洽模型,分析了不同入射条件下电离层参数的变化,主要结论如下:电离层D区是电波的主要吸收区,并且其吸收强度随入射频率的升高而降低,当入射频率为6 MHz（有效入射功率为200 MW）时电子温度的最大增幅约为520 K,电子密度最大增幅为7300 cm^-3左右;电子温度达到饱和所需时间小于电子密度的饱和时间,前者具有μs量级,后者具有ms量级;停止加热后,电子温度和密度迅速恢复到初始状态,恢复时间均小于各自的饱和时间,但量级相当;入射功率越高,电子温度和密度的增幅越大,并且饱和时间也越长,在相同入射条件下,夜晚的饱和时间要大于白天.     

Connection between winter mesopause region temperatures and diurnal LF reflection height variations measured at Collm

Scale height, H, estimates are calculated from the decrease/increase of ionospheric virtual reflection heights of low-frequency (LF) radio waves at oblique incidence in suitably defined morning intervals around sunrise during winter months. The day-to-day variations of H qualitatively agree with daily mean temperature variations around 90 km from meteor radar measurements. Since mesospheric long-period temperature variations are generally accepted to be the signature of atmospheric planetary waves, this shows that LF reflection height measurements can be used for monitoring the dynamics of the upper middle atmosphere. The long-term variations of monthly mean H estimates have also been analysed. There is no significant trend, which is in agreement with other measurements of mesopause region temperature trends.     

Observations of HF propagation on a path aligned along the mid-latitude trough

Observations of the direction of arrival and time of flight of HF signals propagating on a 1400 km path oriented along the mid-latitude trough are presented. At night, the signal commonly arrives from directions offset from the great circle bearing by up to 80° and these events have been categorised into five main types. Statistics indicating how often these categories of propagation were observed in the period August 2006 to September 2007 are presented. The physical mechanisms which result in the off great circle propagation are also discussed.     

双向电脉冲激励压电超声换能器实验研究

本文对双向电脉冲激励压电陶瓷超声换能器进行实验研究。对两种类型的换能器进行了实验,进而利用迭加原理对换能器所产生的脉冲超声波波形进行了讨论。结果证明,双向电脉冲是激励压电超声换能器比较理想的电脉冲波形,可以得到幅值大、脉冲持续时间短的超声波,有广泛的应用前景。     

Ionospheric and geomagnetic Pc5 oscillations as observed by the ionosonde and magnetometer at Sodankylä

The study is based on the data of the rapid-run ionosonde at the Sodankylä Geophysical Observatory at auroral latitude (L?=?5.25) which routinely performs one-minute sounding since 2007. This dataset allows a unique opportunity for investigating possible effects of ultra-low frequency (ULF, 1–7?mHz) waves in the auroral ionosphere. Suitable observations were made during moderately disturbed geomagnetic conditions typically at recovery of the geomagnetic storms caused by solar wind high-speed streams, in the daytime between 9 and 16 MLT. The ionospheric oscillations corresponding to Pc5 geomagnetic pulsations were found in variations of the virtual height of the F layer and the power of ionosonde reflections from E and F layers. The later are most probably caused by modulation of electron precipitation, which is also manifested in weak (about 0.01–0.06?dB) variations of cosmic noise absorption. The most important and novel result is that the pulsations of power of reflection from E and F layers typically has a spectral maximum at nearly half the periodicity of the Pc5 geomagnetic pulsations, whereas such spectral peak is negligible in the geomagnetic pulsations.     

同步的快速捕获

时分多址(TDMA)扩频通信系统的同步捕获是一个很重要的问题,如何实现快速同步则更重要。采用声表面波器件,时间延时门和一致检测窗能十分满意地解决同步的快速捕获,精度达正负半个码元。本文还讨论了两种基本的快速同步捕获的方法,並给出了试验结果。     

近壁区Reynolds应力输运方程的理论模型

在湍流边界层的近壁区,采用对称共振三波的理论模型描述相干结构,根据理论模型对Reynolds应力输运方程逐项进行计算和分析,结果与直接数值模拟符合很好.这不仅在理论上有益于对湍流物理机制的了解,而且展现了一种可能性,即根据相干结构的理论知识来改造湍流模型,使之具有更清晰的物理内涵以提高近壁区的预估精度.