Pre-storm behaviour of NmF2 and TEC (GPS) over equatorial and low latitude stations in the Indian sector

The pre-storm behavior of NmF₂ and TEC over an equatorial station, Trivandrum (8.47°N, 76.91°E, dip 0.6°S) and a low latitude station, Waltair (17.7°N,83.3°E, dip 20°N) has been studied for a total of 18 strong geomagnetic storms with DST ? −100 nT. The simultaneous measurements of GPS-TEC and NmF₂ over Trivandrum and Waltair during the period 2000–2005 have been considered for the present study. It is found that there is a substantial increase in NmF₂ and TEC before the onset of the storm over Waltair, while the increase is not present at Trivandrum. The origin of pre-storm enhancements in electron density still remains unresolved owing to several conditions in their potential sources and occurrence mechanisms. In the present study an attempt is made to identify the possible mechanisms responsible for such enhancements in electron density of the F-region.     

竖直平板自然对流强化换热的实验

研究各种强化换热结构对竖直平板自然对流换热的强化作用.通过比较圆柱扰流、圆锥扰流、三棱柱扰流、三棱锥扰流、四棱柱扰流、四棱锥扰流和直角三角翼纵向涡发生器LVG(Longitudinal vortex gener-ator)等对自然对流换热的强化效果,发现:单种换热结构中直角三角翼纵向涡发生器的强化换热效果最好,组合结构中直角三角翼叉排与圆柱扰流的混合排列换热效果最好,其能将竖直平板自然对流换热效果提高24.%.      

Ground and satellite-based observations of ionospheric plasma bubbles and blobs at 5.65° latitude in the Brazilian sector

This investigation uses simultaneous observations from all-sky imager system and an ionosonde collocated at Araguatins (5.65° S, 48.07° W and dip-latitude of 4.17° S), a near-equatorial region in Brazil. These simultaneous observations were used to investigate the occurrence of plasma bubbles and blobs in the field of the imaging system and their association with atypical range Spread-F signature in ionograms. Also, in-situ observation of plasma density from Swarm satellites were used to support the ground-based observations. Using a few cases, a methodology will be established to identify in the plasma blobs (atypical ESF) in the ionograms when there is the simultaneous observation of plasma bubbles and blobs in the field of view of the ionosonde. For this purpose, simultaneous sequence of OI 630.0 nm nightglow images and ionograms are presented for different case studies; 1. when there is the absence of a plasma bubble or blob, 2. when there is only the occurrence of plasma bubbles and 3. when there is the occurrence of plasma bubbles and blobs, in order to compare traces in the ionogram in all these case studies. With these we can cover all kinds of signatures in the ionograms corresponding to no irregularities, plasma bubbles only and plasma bubbles-blobs. These OI 630.0 nm nightglow and ionograms recorded simultaneously make it possible to establish a novel methodology to recognize in ionograms cases when there is the occurrence of Spread-F signature associated with bubble-blob in the FOV of the ionosonde.     

Mini-TED跨声速气动特性及其增升机理

通过求解二维可压Navier-Stokes方程,研究了NACA0012翼型加装微型后缘增升装置（mini-TED）后的跨声速流场特性,与Gurney flap （GF）对比分析了几何参数对mini-TED后方涡系及翼型气动特性的影响.将mini-TED的几何细节参数定义为弦向长度和有效高度,两者方向正交.在相同迎角下仅改变mini-TED的弦向长度,后缘涡系结构虽发生变化,但翼型气动力几乎没有影响;反之仅改变有效高度则后缘涡系和翼型气动力系数同时发生明显改变,且与同等高度下的GF气动系数相近.结果表明：有效高度是影响翼型气动特性的决定因素.有效高度改变了mini-TED后涡系的发生范围,而相对于整个翼型绕流,后缘涡系的大小是影响翼型流场最重要的因素,而涡系的微观结构和形态的改变影响相对很小.加装mini-TED后上表面激波位置后移、下表面激波强度削弱,从而翼型表面压力分布特性发生了改变.随有效高度增大,mini-TED诱导的涡系发生区域随之增大,引流作用增强,翼型升力系数、阻力系数和低头力矩系数提高,同时相同迎角下翼型的升阻比明显提高.     

Solar cycle effect on temporal and spatial variation of topside ion density measured by SROSS C2 and ROCSAT 1 over the Indian longitude sector

We investigated the diurnal, seasonal and latitudinal variations of ion density Ni over the Indian low and equatorial topside ionosphere within 17.5°S to 17.5°N magnetic latitudes by combining the data from SROSS C2 and ROCSAT 1 for the 9 year period from 1995 to 2003 during solar cycle 23. The diurnal maximum density is found in the local noon or in the afternoon hours and the minimum occurs in the pre sunrise hours. The density is higher during the equinoxes as compared to that in the June and December solstice. The local time spread of the daytime maximum ion density increases with increase in solar activity. A north south asymmetry with higher ion density over northern hemisphere in the June solstice and over southern hemisphere in December solstice has been observed in moderate and high solar activity years. The crest to crest distance increases with increase in solar flux. Ion density bears a nonlinear relationship with F_10.7 cm solar flux and EUV flux in general. The density increases linearly with solar flux up to ∼150 sfu (1 sfu = 10⁻²²Wm⁻²Hz⁻¹) and EUV flux up to ∼50 units (10⁹ photons cm⁻² s⁻¹). But beyond this the density saturates. Inverse saturation and linear relationship have been observed in some season or latitude also. Inter-comparison of the three solar activity indices F_10.7 cm flux, EUV flux and F_10.7P (= (F_10.7 + F_10.7A)/2, where F_10.7A is the 81 day running average value of F_10.7) shows that the ion density correlates better with F_10.7P and F_10.7 cm fluxes. The annual average daytime total ion density from 1995 to 2003 follows a hysteresis loop as the solar cycle reverses. The ion density at 500 km over the Indian longitude sector as obtained by the international reference ionosphere is in general lower than the measured densities during moderate and high solar activity years. In low solar activity years the model densities are equal or higher than measured densities. The IRI EIA peaks are symmetric (±10°) in equinox while densities are higher at 10°N in June solstice and at 10°S in the December solstice. The model density follows F_10.7 linearly up to about F_10.7 > ∼150 sfu and then saturates.     

Early morning enhancement in ionospheric electron density during intense magnetic storms

This study reports on ionospheric disturbances that occurred in the early morning hours in the South American–Atlantic sector during a few intense/super storm events. The events were observed at latitudes close to the southern crest of the equatorial ionization anomaly (EIA) as an unusual intensification of the F region electron density peak at local times when the EIA is not usually developed. All the events were observed at pre dawn-morning hours, under conditions of northward interplanetary geomagnetic field. Large scale traveling ionospheric disturbances that are launched during highly disturbed conditions and/or equatorward surges in the thermospheric meridional winds seem to be the most probable causes of the observed disturbances.     

On the uncertainties in the measurement of absolute (true) TEC over Indian equatorial and low latitude sectors

The Indian sector encompasses the equatorial and low latitude regions where the ionosphere is highly dynamic and is characterized by the equatorial ionization anomaly (EIA) resulting in large latitudinal electron density gradients causing errors and uncertainties in the estimation of range delays in satellite based navigation systems. The diurnal and seasonal variations of standard deviations in the TEC data measured during the low sunspot period 2004–2005 at 10 different Indian stations located from equator to the anomaly crest region and beyond are examined and presented. The day-to-day variability in TEC is found to be lowest at the equatorial station and increases with latitude up to the crest region of EIA and decreases beyond.     

面向导航增强的低轨星座设计与应用

随着传统产业升级和新兴技术的发展,社会生产和生活对分米级、厘米级的实时精准定位需求日益凸显。低轨卫星轨道高度低、信号强度大、短时间内几何构型变化快,因此应用低轨卫星开展导航增强服务成为研究热点。低轨卫星的增强服务性能依赖于星座的快速组网和设计,低轨卫星星座构型、轨道高度、轨道倾角等是影响其覆盖性能和增强性能的关键因素。全面分析了低轨星座设计的关键要素,包括轨道高度、轨道倾角和单星覆盖性、地面人口密度、空间环境等,在此基础上设计了单构型和复合构型低轨导航增强星座,并进一步分析低轨星座的覆盖性能。结果显示:复合低轨导航增强星座可以实现对全球的连续覆盖,同时满足极地高密度覆盖和低纬度的连续覆盖需求,对北斗导航系统的增强效果明显。     

Responses of the African equatorial ionization anomaly (EIA) to some selected intense geomagnetic storms during the maximum phase of solar cycle 24

This study investigates the morphology of the GPS TEC responses in the African Equatorial Ionization Anomaly (EIA) region to intense geomagnetic storms during the ascending and maximum phases of solar cycle 24 (2012–2014). Specifically, eight intense geomagnetic storms with Dst ≤ ?100 nT were considered in this investigation using TEC data obtained from 13 GNSS receivers in the East African region within 36–42°E geographic longitude; 29°N–10°S geographic latitude; ± 20°N magnetic latitude. The storm-time behavior of TEC shows clear positive and negative phases relative to the non-storm (median) behavior, with amplitudes being dependent on the time of sudden commencement of the storm and location. When a storm starts in the morning period, total electron content increases for all stations while a decrease in total electron content is manifested for a storm that had its sudden commencement in the afternoon period. The TEC and the EIA crest during the main phase of the storm is significantly impacted by the geomagnetic storm, which experiences an increase in the intensity of TEC while the location and spread of the crest usually manifest a poleward expansion.     

颗粒传热增强对微型脉冲推力器点火过程的影响

为了分析点火药产物中炽热颗粒对固体脉冲推力器点火过程的影响,采用了颗粒对壁面的冲击热增强模型来模拟炽热颗粒对推进剂表面的传热增强效应,进行了微型脉冲推力器点火过程的数值分析。结果表明,颗粒热增强效应显著影响脉冲推力器的点火启动过程,颗粒的热增强效应显著缩短了点火延迟时间,并使点火启动过程的压强增大,点火过程对颗粒含量较少的点火药更敏感;设计微型脉冲推力器点火器时要严格匹配产物颗粒含量,并严格控制点火药量范围。