A possible active galactic nucleus in M82?

MERLIN and VLA observations of the galaxy M82 have detected a jet-like feature emanating from one of the compact sources, 44.01+59.6. The proximity of this source to the dynamical centre of M82 led us to suspect that it could be a weak active nucleus rather than an SNR. We imaged this source using the EVN at 15 mas and although it shows a compact shell-like structure which could be consistent with an SNR, we note that the EVN image shows a bright region within the source which is elongated along the jet direction. There are distinct similarities between this source and the Sgr A complex at the centre of our own Galaxy which contains the compact AGN, Sgr A^*.     

VSOP/global VLBI observations of 2021+614: Detection of hotspot advance

We have analysed VSOP and global VLBI data at 5 GHz and VLBA + Effelsberg data at 15 GHz for 2021+614. By identifying the core as being between two bright hotspots, we show that 2021+614 is a compact symmetric object of size 40 pc. From comparison with earlier observations we deduce an apparent age of 375 yr for 2021+614. This provides additional support for the contention that compact symmetric radio objects associated with galaxies are in fact young radio sources and the possible precursors of the classical FR I or FR II radio galaxies.     

Observational support for radiative pumping of SiO masers in evolved stars

SiO maser emission in AGB stars is found to be systematically distributed in ring structures, displaying tangential linear polarization. Using the VLBA, we have performed observations of such SiO maser emission at 43 GHz in the circumstellar envelopes of several late-type stars. We simultaneously mapped the v=1 and v=2, J=1–0 transitions. Our new maps show, for most spots, a systematic spatial shift between both maser lines. We discuss the implication of these results on the possible pumping mechanisms, concluding that the existing data favour radiative pumping schemes.     

External atmospheric corrections in geodetic very-long-baseline interferometry

Three methods to correct for the atmospheric propagation delay in very-long-baseline interferometry (VLBI) measurements were investigated. In the analysis, the NASA R&D experiments from January 1993 to June 1995 were used. The methods were compared in correcting for the excess propagation delay due to water vapour, the “wet” delay, at one of the sites, the Onsala Space Observatory on the west coast of Sweden. The three methods were: (1) estimating the wet delay using the VLBI data themselves; (2) inferring the wet delay from water vapour radiometer (WVR) data, and (3) using independent estimates based on data from the global positioning system (GPS). Optimum elevation cutoff angles were 22^∘ and 26^∘ when using WVR and GPS data, respectively. The results were found to be similar in terms of reproducibility of the estimated baseline lengths. The shortest baselines tend to benefit from external measurements, whereas the lack of improvement in the longer baselines may be partly due to the large amount of data thrown away when removing observations at low elevation angles. Over a 2 week period of intensive measurements, the two methods using external data showed an overall improvement, for all baseline lengths, compared to the first method. This indicates that there are long-term systematic errors in the wet delay data estimated using WVR and GPS data. Received: 27 October 1998 / Accepted: 20 May 1999     

The application of a shift theorem analysis technique to multipoint measurements

A Fourier domain technique has been proposed previously which, in principle, quantifies the extent to which multipoint in-situ measurements can identify whether or not an observed structure is time stationary in its rest frame. Once a structure, sampled for example by four spacecraft, is shown to be quasi-stationary in its rest frame, the structure’s velocity vector can be determined with respect to the sampling spacecraft. We investigate the properties of this technique, which we will refer to as a stationarity test, by applying it to two point measurements of a simulated boundary layer. The boundary layer was evolved using a PIC (particle in cell) electromagnetic code. Initial and boundary conditions were chosen such, that two cases could be considered, i.e. a spacecraft pair moving through (1) a time stationary boundary structure and (2) a boundary structure which is evolving (expanding) in time. The code also introduces noise in the simulated data time series which is uncorrelated between the two spacecraft. We demonstrate that, provided that the time series is Hanning windowed, the test is effective in determining the relative velocity between the boundary layer and spacecraft and in determining the range of frequencies over which the data can be treated as time stationary or time evolving. This work presents a first step towards understanding the effectiveness of this technique, as required in order for it to be applied to multispacecraft data.     

Habitat use and preference of adult pike (Esox lucius L.) in an anthropogenically impacted lowland river

To efficiently manage northern pike (Esox lucius), information is needed on its habitat use and preference. However, knowledge gaps still exist, especially on pike habitat use and preference in rivers characterised by artificial environments. We investigated the use of the main river, tributaries and side arms at the macro-scale, and the use and preference of riparian habitats by adult pike at the meso-scale in an anthropogenically impacted river basin. Adult pike were followed in winter and spring by radio telemetry. At the macro-scale pike intensively used the main river in winter and spring, but also frequented specific side arms in winter and specific tributaries in spring, which may indicate the importance of these habitats to adult pike. At the meso-scale, reedy semi-natural banks were used the most, irrespective of any assumption on habitat availability or use. The findings underline the value of protecting the least impacted, (semi)natural habitats for adult pike in an anthropogenically impacted river system. The large behavioural differences in habitat use between individuals at both habitat scales further underline the importance of habitat heterogeneity. The results also provide insight into the impact of riparian habitat restoration on adult pike and may be used to more efficiently manage pike rivers, e.g. by enhancing the lateral connectivity with river side arms or by reconstructing natural riparian habitats.     

对称模式下的CHAMP弯曲角掩星数据同化

简单介绍了无线电掩星技术探测行星大气的发展史,列举了该技术中存在的若干问题。从 Eyre提出的统计学的最优估计反演方法,比较了用相位、弯曲角和折射率作为同化因子时出现的问题和各自的优缺点。对弯曲角同化因子,以欧洲中期天气预报中心(ECMWF)资料为背景场,运用一维变分技术,进行CHAMP掩星观测资料变分同化反演,从而获得水汽和温度剖面。将反演获得的气象剖面与非同化的剖面作比较,并且采用相应的探空气球资料作为验证,可以看出变分同化技术比传统的标准反演技术反演误差小。证实掩星数据资料的一维变分同化技术可以改进目前的数值天气预报模式。     

GPS-LEO掩星探测地球大气的方法

近年来伴随导航定位技术与无线电掩星技术的结合，一种崭新的遥感地球大气的方法-Global Positioning System/Meteoroloby(GPS/NET)应运而生，该方法可以有效地探测大气温、压、湿等气象参数。本文在介绍利用GPS-LEO掩星技术探测地球大气的原理，以及反演大气参数的方法、步骤的基础上，给出了模拟掩星数据反演出的温度廓线、密度廓线及部分GPS/NET实验数据的初步反演结果。通过分析可以看出，利用掩星技术反演出的温度廓线与大气实际情况相符，并且与ECMWF、NCEP的结果吻合较好。在干空气假设条件下，近地面大气的反演结果与实际情况还存在一定误差，有待深入研究解决。     

Spectral survey of the Comet Halley atmosphere during an occultation of a star

On November 15, 1985 a star was occulted by Comet Halley. Seven consequent spectra of the star has been obtained with 5-minute exposition with the TV scanner at 6-meter telescope. Simultaneously seven spectra of the cometary atmosphere were obtained at the projected distance 40” from the star. A method is developed to calculate the optical thickness of the cometary atmosphere as a function of wave length and the nucleocentric distance. This function is given in the spectral range 3925 Å−4850 Å.     

COSMIC掩星资料同化对台风“天兔”预报影响的试验

台风数值预报的关键之一在于给定准确的模式初始场。本文使用气象、电离层和气候星座观测系统(COSMIC)的卫星资料,通过资料同化技术改善数值天气预报模式的初始场,进而评估数据同化技术对台风(或热带气旋)预报的改善效果。使用我国自主研发的数值天气预报模式,对1319号台风“天兔”进行模拟试验,对温度场、湿度场、涡度散度场及假相当位温场进行诊断分析。试验结果表明,与同化常规探空资料相比,COSMIC资料的同化可减小模式预报台风路径误差,台风附近降水落区更为集中。研究评估了掩星资料同化对模式的改进作用,并能为台风预报技术和防灾减灾提供参考。