Variability of foF2 in the African equatorial ionosphere

This paper presents the impact of diurnal, seasonal and solar activity effects on the variability of ionospheric foF2 in the African equatorial latitude. Three African ionospheric stations; Dakar (14.8°N, 17.4°W, dip: 11.4°N), Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N) and Djibouti (11.5°N, 42.8°E, dip: 7.2°N) were considered for the investigation. The overall aim is to provide African inputs that will be of assistance at improving existing forecasting models. The diurnal analysis revealed that the ionospheric critical frequency (foF2) is more susceptible to variability during the night-time than the day-time, with two peaks in the range; 18–38% during post-sunset hours and 35–55% during post-midnight hours. The seasonal and solar activity analyses showed a post-sunset September Equinox maximum and June Solstice maximum of foF2 variability in all the stations for all seasons. At all the stations, foF2 variability was high for low solar activity year. Overall, we concluded that equatorial foF2 variability increases with decreasing solar activity during night-time.     

Behavior of the equivalent slab thickness over three European stations

The total electron content (TEC) derived from the global positioning system (GPS) and the F2-layer peak electron density obtained from Digisonde data have been used to study the diurnal, seasonal and solar activity variations of the ionospheric equivalent slab thickness (τ) over three European stations located at Pruhonice (50.0°N, 15.0°E), Ebro (40.8°N, 0.5°E) and El Arenosillo (37.1°N, 353.3°E). The diurnal variation of the τ is characterized by daytime values lower than nighttime ones for all seasons at low solar activity while daytime values larger than nighttime characterizes the diurnal variation for summer at high solar activity. A double peak is noticeable at dusk and at dawn, better expressed for winter at low solar activity. The seasonal variations of τ depend on local time and solar activity, the daytime values of τ increases from winter to summer whereas nighttime values of τ show the opposite. The effect of the solar activity on τ depends on local time and season, there being very sensitive for winter nighttime values of τ. The results of this study are compared with those presented by other authors.     

Comparative study of foF2 measurements with IRI-2007 model predictions during extended solar minimum

The unusually deep and extended solar minimum of cycle 23/24 made it very difficult to predict the solar indices 1 or 2 years into the future. Most of the predictions were proven wrong by the actual observed indices. IRI gets its solar, magnetic, and ionospheric indices from an indices file that is updated twice a year. In recent years, due to the unusual solar minimum, predictions had to be corrected downward with every new indices update. In this paper we analyse how much the uncertainties in the predictability of solar activity indices affect the IRI outcome and how the IRI values calculated with predicted and observed indices compared to the actual measurements. Monthly median values of F2 layer critical frequency (foF2) derived from the ionosonde measurements at the mid-latitude ionospheric station Juliusruh were compared with the International Reference Ionosphere (IRI-2007) model predictions. The analysis found that IRI provides reliable results that compare well with actual measurements, when the definite (observed and adjusted) indices of solar activity are used, while IRI values based on earlier predictions of these indices noticeably overestimated the measurements during the solar minimum. One of the principal objectives of this paper is to direct attention of IRI users to update their solar activity indices files regularly. Use of an older index file can lead to serious IRI overestimations of F-region electron density during the recent extended solar minimum.     

The variability and IRI2007-predictability of hmF2 over South Africa

This paper presents an investigation into the variability and predictability of the maximum height of the ionospheric F2 layer, hmF2 over the South African region. Data from three South African stations, namely Madimbo (22.4°S, 26.5°E, dip angle: −61.47°), Grahamstown (33.3°S, 26.5°E, dip angle: −64.08°) and Louisvale (28.5°S, 21.2°E, dip angle: −65.44°) were used in this study. The results indicate that hmF2 shows a larger variability around midnight than during the daytime for all seasons. Monthly median hmF2 values were used in all cases and were compared with predictions from the IRI-2007 model, using the URSI (Union Radio-Scientifique Internationale) coefficient option. The analysis covers the diurnal and seasonal hourly hmF2 values for the selected months and time sectors e.g. January, July, April and October for 2003 and 2005. The time ranges between (03h00–23h00 UT; LT = UT + 2h) representing the local sunrise, midday, sunset and midnight hours. The time covers sunrise, midday, sunrise, and midnight hours (03–06h00 UT, 07–11h00 UT, sunrise 16–18h00 UT and 22–23h00 UT; LT = UT + 2h). The dependence of the results on solar activity levels was also investigated. The IRI-2007 predictions follow fairly well the diurnal and seasonal variation patterns of the observed hmF2 values at all the stations. However, the IRI-2007 model overestimates and underestimates the hmF2 value during different months for all the solar activity periods.     

C/SiC-SiO_(2)复合材料的制备及性能研究北大核心CSCD

采用溶胶凝胶和前驱体浸渍裂解混合工艺,制得了不同SiO_(2)/SiC比例的C/SiC-SiO_(2)复合材料,研究了SiO_(2)添加量对复合材料微观结构、力学性能的影响。结果表明:当添加SiO_(2)的质量分数约25%时,材料的拉伸强度和压缩强度与C/SiC材料性能相当;而当添加SiO_(2)的质量分数超过25%时,材料的强度与模量均随SiO_(2)含量的增加呈降低趋势。此外,SiO_(2)含量约25%的C/SiC-SiO_(2)复合材料的浸渍相成本较C/SiC材料降低约24%左右,这为C/SiC复合材料的快速低成本制备提供了新的技术支撑。     

Cu58MnCo钎焊1Cr11Ni2W2MoV马氏体不锈钢接头组织与性能

采用Cu58MnCo钎焊连接1Cr11Ni2W2MoV,研究了工作气氛、钎焊温度和保温时间对接头组织和性能的影响。结果表明：钎焊接头主要包括α''+[γ（Fe）,γ（Mn）]相,α（Co）+δ相和[Cu,γ（Mn）]+α（Mn）相;钎焊温度升高或保温时间延长,α''+[γ（Fe）,γ（Mn）]相和[Cu,γ（Mn）]+α（Mn）相增多,α（Co）+δ相含量减少,接头拉剪强度降低;工作气氛为70 Pa时所获接头性能优于真空度1×10^－2下所得的接头性能。     

2D C/SiC复合材料阻尼性能研究

测试了CVI工艺制备的2D C/S iC复合材料的阻尼性能,讨论了其阻尼机制,分析了涂层、测试频率和热处理等因素对其阻尼峰的影响。结果表明,2D C/S iC复合材料的阻尼峰值可达1.86×10-2,其阻尼机制主要为微裂纹扩展、增强纤维阻尼及界面阻尼;S iC涂层可大幅度降低2D C/S iC复合材料的阻尼性能,消除阻尼特性曲线上的阻尼峰;2D C/S iC复合材料阻尼峰随加载频率的升高而降低;高温热处理使2D C/S iC复合材料阻尼峰降低。     

Al 量对 Ni-Al-Hf-Cr-W 系合金显微组织结构的影响

研究了Ni-20Hf-8Cr-8W-（4～6）Al系列合金的显微组织以及Al含量对共晶组织的影响,结果表明：此系列合金中,随着Al含量的微小变化,会析出不同的共晶组织;当Al含量大于5.5wt%时,出现（β-NiAl+Ni7Hf2）或（γ′+β-NiAl+Ni7Hf2）共晶：当Al量小于5.5wt%时,会出现（γ′+Ni7Hf2）、（γ+γ′+Ni7Hf2）或（γ+Ni7Hf2）共晶组织。并且得出（Ni-20Hf-8Cr-8W-5.3Al）wt%的合金具有（γ′+Ni7Hf2）共晶,适宜选做为以Ni7Hf2增强的共晶复合材料的合金成份。     

指挥调度ISDN数字用户接口设计

提出指挥调度系统数字用户接口的实现方案,详细介绍了2B D接口的软硬件设计方法,为开发新型、全数字、大容量的指挥调度系统做了充分的技术准备。     

二维波动板流动显示方法研究

介绍了在水洞中进行的二维波动板周围流场的流动显示实验。通过氢气泡、粒子示踪法和粒子成像速度仪三种方法定量和定性地显示了二维波动板周围的流场结构。揭示了特征参数C／U与流场特性的关系,探讨了鱼类波状摆动对流动分离的抑制机理。同时还对三种不同的流动显示技术进行了探讨。