Meteor fluxes and visual magnitudes from EISCAT radar event rates: a comparison with cross-section based magnitude estimates and optical data

Incoherent scatter radars (ISR) are versatile instruments for continuous monitoring of ionisation processes in the Earths atmosphere. EISCAT, The European Incoherent Scatter facility has proven effective also in meteor studies. The time resolution of the radar can be reduced to a few milliseconds, sufficient to resolve the passage of individual meteors through the narrow ISR beam. Methods for group and phase velocity determination of the meteoroids and the discrepancy between the results related to the target behaviour are presented. The radar cross sections of echoes associated with moving meteoroids (–meteor head echoes) are very small and increase with decreasing wavelength. The parent meteoroids are found to have visual magnitudes far below the detection limit of most optical observations. The equivalent visual magnitude limit of the smallest objects observed by EISCAT in the current experiments has been estimated by two different methods, both from the cross-section measurements and from the measured event rates. Both methods give a limit value of +10 for the smallest objects while the upper limit is +4. The lower limit of the visual magnitude for the collocated optical measurement system is +4. Thus the two detection systems observe two different meteor size ranges, with the radar almost reaching micrometeorite population. Meteor fluxes estimated from the event rates and the radar system parameters agree well with previous extrapolated values for this size range.     

SCIPION, a new flexible ionospheric sounder in Senegal

SCIPION is a new state of the art digital sounder that has been developed by France Telecom-CNET for ionospheric monitoring and research. Extensive data processing using DSP technology has resulted in a low power, low cost and full featured system for both vertical and oblique soundings. A SCIPION system is in the process of being installed in Dakar, Senegal, to study HF propagation in the sub-equatorial ionosphere. However, preliminary results have still been obtained during experiments wit a prototype system. In this paper, the system is described and some illustrative examples of its capabilities are shown.     

Centaurus A – NGC 5128

Summary. At a distance of 3.4 Mpc, NGC 5128 (Centaurus A) is by far the nearest active radio galaxy. It is often considered to be the prototype Fanaroff-Riley Class I ‘low-luminosity’ radio galaxy, and as such it plays an important role in our understanding of a major class of active galaxies. Its proximity has spawned numerous detailed investigations of its properties, yielding unrivalled but still incomplete knowledge of its structure and dynamics. The massive elliptical host galaxy is moderately triaxial and contains a thin, strongly warped disk rich in dust, atomic and molecular gas and luminous young stars. Its globular cluster ensemble has a bimodal distribution of metallicities. Deep optical images reveal faint major axis extensions as well as a system of filaments and shells. These and other characteristics are generally regarded as strong evidence that NGC 5128 has experienced a major merging events at least once in its past. The galaxy has a very compact, subparsec nucleus exhibiting noticeable intensity variations at radio and X-ray wavelengths, probably powered by accretion events. The central object may be a black hole of moderate mass. Towards the nucleus, rich absorption spectra of atomic hydrogen and various molecular species suggest the presence of significant amounts of material falling into the nucleus, presumably ‘feeding the monster’. Emanating from the nucleus are linear radio/X-ray jets, becoming subrelativistic at a few parsec from the nucleus. At about 5 kpc from the nucleus, the jets expand into plumes. Huge radio lobes extend beyond the plumes out to to 250 kpc. A compact circumnuclear disk with a central cavity surrounds the nucleus. Its plane, although at an angle to the minor axis of the galaxy, is perpendicular to the inner jets. The jet-collimating mechanism, probably connected to the circumnuclear disk, appears to precess on timescales of order a few times 10 years. This review summarizes the present state of knowledge of NGC 5128 and its associated radio source Centaurus A. Underlying physical processes are outside its scope: they are briefly referred to, but not discussed. Received 30 December 1997     

Observations of the Sunyaev–Zel''dovich effect at high angular resolution towards the galaxy clusters A665, A2163 and CL0016+16

We report on the first observation of the Sunyaev–Zel'dovich (SZ) effect, a distortion of the Cosmic Microwave Background radiation (CMB) by hot electrons in clusters of galaxies, with the Diabolo experiment at the IRAM 30 m telescope. Diabolo is a dual-channel 0.1 K bolometer photometer dedicated to the observation of CMB anisotropies at 2.1 and 1.2 mm. A significant brightness decrement in the 2.1 mm channel is detected in the direction of three clusters (Abell 665, Abell 2163 and CL0016+16). With a 30 arcsec beam and 3 arcmin beamthrow, this is the highest angular resolution observation to date of the SZ effect. Interleaving integrations on targets and on nearby blank fields have been performed in order to check and correct for systematic effects. Gas masses can be directly inferred from these observations.     

计算机在无线通讯系统中的抗干扰措施

针对计算机系统的主要干扰源，结合笔者在实际工作中的体会就计算机在无线通讯系统中的抗干扰问题进行了讨论，并给出了相应的各种抗干扰措施。     

Jovian decametric events during the period of the impacts of SL-9

Strong decametric narrow-band radio bursts, produced by impacts of fragments D, E, G, K, N, S and W of Comet Shoemaker-Levy 1993e (SL-9) on the Jupiter, have been detected during our observations from July 17 to 22, 1994 at the Xin-Xiang temporary Jovian Decametric Station of Beijing Astronomical Observatory. Plots of relative signal intensity of these bursts at 10 frequencies between 24.0 and 28.5MHz are presented. The major features of events are sumarized and discussed.The project is supported by the National Natural Science Foundation of China.Correspondent of this paper. Dr. Han is leaving from Beijing for the Astronomical Center of Zielona Gora, Pedagogical University, ul. Lubuska 2, 65 265 Zielona Gora, Poland to be a post-doctor. Contact with him by email HJL@bao01.bao.ac.cn or by airmail to Poland.     

全球定位系统(GPS)技术的最新进展 第六讲 GPS气象学

介绍了地基GPS气象学及空基GPS气象学的概念、基本原理、方法以及它们在数值天气预报和气候变化研究中的应用。     

空间大地测量的最新进展(三)

4 卫星雷达测高的最新进展4.1 从GEOS-3到SEASAT-1卫星雷达测高是从卫星上安置的雷达测高仪垂直向海面发射电脉冲,并被海面垂直反射至卫星.由电脉冲在测高仪和海面之间往返的时间,即可推算卫星对于瞬时海面的高度ha.由于电波束发散成为圆锥状,ha实际上是以εha(ε是波束圆锥的开角)为半径的一个圆形波迹范围内的平均高度.若ε=1°,ha=500-1000km,则此半径约为10km.在上节所述的激光测高中,地面光迹的直径为70m.因此,激光测高的分辨率远远好于雷达测高.为了把雷达测高结果换算为卫星至大地水准面的距离,需要加入一系列改正.测高卫星的位置可由该星的精密轨道求得,由此可算出该卫星至所选定的平均地球椭球面的距离h(即大地高),于是大地水准面高N=h-ha.     

上海天文台GPS掩星技术研究现状

简要地介绍了GPS/LEO卫星无线电掩星技术的基本原理、意义以及国际上的最近动态 ,列举了几个主要的LEO卫星计划 ;重点分析了GPS掩星探测地球大气技术中需要注意的若干问题 ,并推出可能采取的改进方案和相应的技术路线。上海天文台在GPS/LEO空基气象学中已经成功地提出 :通过通约LEO/GPS轨道方法 ,实现掩星地面观测点控制的新思路。作为一个应用实例 ,针对东中国海洋上空大气状况对中国东部经济发达的地区特别是上海地区的天气有着重要影响的背景 ,提出利用通约LEO/GPS卫星可对该地区大气实施监控。为了充分利用GPS/LEO掩星观测资源 ,上海天文台开展了振幅反演的计算方法研究 ;提出了在振幅反演中存在着无线电信号几何衰减和物理衰减两种不同的衰减机制 ,并考虑了它们的数学模型以及对反演结果的影响。简要地叙述了掩星质量因子的定义和计算方法。作为国内天文地球动力学研究中心 ,上海天文台将与国内外一些合作单位考虑建立GPS/LEO掩星技术观测处理的软件系统。作为空基GPS气象学的推广 ,还考虑进行山基和飞机载掩星观测实验 ,论证用这些方法监测局部大气剖面的可行性以及它们在国民经济和科学研究上的作用     

山基GPS掩星观测实验及其反演原理

分别在湖北省药姑山和九宫山的山头上开展GPS掩星观测实验,成功获取山基掩星观测数据,对掩星事件进行了分析和统计.给出利用山基掩星观测数据反演大气折射指数剖面和电波弯曲角的原理和算法.利用山基GPS掩星观测模拟数据,对反演方法进行试算和检验,结果表明反演方法准确可行.将该反演方法应用于观测数据的反演,获得了观测点高度以下的大气折射率剖面,以及电波弯曲角.实验结果和原理研究表明,山基掩星观测技术是一种潜在的低层大气环境监测新技术.