GPS/LEO掩星技术中超折射效应的修正

在GPS掩星探测地球大气技术中，Abel积分变换要求大气折射指数n是折射半径a的单值函数，当大气折射率的垂直梯度达到小于一个极限值dN／dr≈-0．16N-unit m^-1上述单值性不成立，称其为超折射，此时Abel积分变换不再适用,如果还是在形式上应用经典的Abel变换，在反演结果中就会产生负大气折射率偏差．描述了低对流层中超折射现象的物理特性和数学表示；在广义Abel积分变换的基础上，讨论了超折射层内和超折射层下的大气剖面反演算法；选择了一个简单的采样间隔内等大气折射率垂直梯度假设，对英国高分辨率无线电探空观测资料进行模拟计算，验证了负超折射与大气折射率偏差的关系，并提出的广义Abel变换的合理性.     

Letter to the Editor: The collisional broadening of carbon radio recombination lines

The density matrix equations of highly excited carbon are analytically solved as functions of line number and temperature. Allowing for the series of the impact approximation theory and quadrupole interaction potential, the line widths are found with the theoretical error less than 0.06 in the range of temperatures T _e = 25 − 100 K. To determine the medium temperature and density by using the experimental values of line widths, the Doppler, radiational and impact broadening are calculated for the lines with the principal quantum numbers n > 300. This revised version was published online in July 2006 with corrections to the Cover Date.     

Some statistical features of solar radio SVC in the rising branch of cycle 22

For the rising branch of Cycle 22, 1987 January—1989 November, we plot the daily total flux S at each of the five wavelengths 2.0, 3.4, 6.0, 10.7 and 21.2 cm against the apparent sunspot area of the dominant sunspot A_y and find that, for several large active areas, the locus is located below the average regression line, and the increase in the radio flux is insignificant compared to the increase in the sunspot area.     

GPS无线电掩星数据处理系统

GPS无线电掩星技术能提供高精度、高分辨率、全球覆盖的地球电离层和中性层大气剖面。它具有全天候、低费用、无系统长期漂移等优点。从1995年4月至1997年3月,首次GPS／MET试验的成功显示了GPS掩星技术对监测全球大气能够发挥重要的作用,从而成为当前空间探测技术的研究热点之一。该文主要叙述了无线电掩星技术的数据处理系统的有关情况,并介绍了美国UCAR的掩星数据处理系统CDAAC的概况,可作为我国开展无线电掩星计划有关工作的借鉴。     

An ultraluminous X-ray microquasar in NGC 5408?

We studied the radio source associated with the ultraluminous X-ray source in NGC 5408  ( L _X≈ 10⁴⁰ erg s⁻¹)  . The radio spectrum is steep (index  ≈−1  ), consistent with optically thin synchrotron emission, not with flat-spectrum core emission. Its flux density (≈0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same in the March 2000 and December 2004 observations, suggesting steady emission rather than a transient outburst. However, it is orders of magnitude higher than expected from steady jets in stellar-mass microquasar. Based on its radio flux and spectral index, we suggest that the radio source is either an unusually bright supernova remnant, or, more likely, a radio lobe powered by a jet from the black hole (BH). Moreover, there is speculative evidence that the source is marginally resolved with a radius ∼30 pc. A faint H  ii region of similar size appears to coincide with the radio and X-ray sources, but its ionization mechanism remains unclear. Using a self-similar solution for the expansion of a jet-powered electron–positron plasma bubble, in the minimum-energy approximation, we show that the observed flux and (speculative) size are consistent with an average jet power  ≈ 7 × 10³⁸ erg s⁻¹∼ 0.1 L _X∼ 0.1 L _Edd  , an age ≈10⁵ yr, a current velocity of expansion ≈80 km s⁻¹. We briefly discuss the importance of this source as a key to understand the balance between luminosity and jet power in accreting BHs.     

The 21-cm forest

We examine the prospects for studying the pre-reionization intergalactic medium (IGM) through the so-called 21-cm forest in spectra of bright high-redshift radio sources. We first compute the evolution of the mean optical depth τ for models that include X-ray heating of the IGM gas, Wouthuysen–Field coupling, and reionization. Under most circumstances, the spin temperature T _S grows large well before reionization begins in earnest; this occurs so long as the X-ray luminosity of high-redshift starbursts (per unit star formation rate) is comparable to that in nearby galaxies. As a result,  τ≲ 10⁻³  throughout most of reionization, and background sources must sit well beyond the reionization surface in order to experience absorption that is measurable by square-kilometre class telescopes. H  ii regions produce relatively large 'transmission gaps' and may therefore still be observable during the early stages of reionization. Absorption from sheets and filaments in the cosmic web fades once T _S becomes large and should be rare during reionization. Minihaloes can produce strong (albeit narrow) absorption features. Measuring their abundance would yield useful limits on the strength of feedback processes in the IGM as well as their effect on reionization.     

Spectral variations and long-period intensity variations of auroral kilometric radiation from INTERBALL-2 satellite measurements

A statistical analysis of the auroral kilometric radiation (AKR) measurements in the POLRAD experiment on the INTERBALL-2 satellite has revealed a dependences of the size and location of the AKR generation region on geomagnetic activity: the generation region rises upward and expands with increasing magnetic disturbances. Based on our two-year measurements, we found seasonal AKR intensity variations: the AKR maximum and minimum are observed in winter and summer, respectively. The seasonal variations and the dependence of the spectrum on geomagnetic activity are assumed to have a common physical nature—the background-plasma density variations in the region of the AKR source attributable to plasma flows from the ionosphere into the magnetosphere.     

Ska Correlator Advances

When the SKA was proposed, a major technical obstacle to its feasibility was the cost of the correlator. Significant advances made in correlator design since then are described. These advances have made SKA correlator possible within reasonable cost constrains. At the same time performance issues with the proposed FX architecture have been addressed.     

The relationship between class I and class II methanol masers

The Australia Telescope National Facility Mopra millimetre telescope has been used to search for 95.1-GHz class I methanol masers towards 62 6.6-GHz class II methanol masers. A total of 26 95.1-GHz masers were detected, 18 of these being new discoveries. Combining the results of this search with observations reported in the literature, a near complete sample of 66 6.6-GHz class II methanol masers has been searched in the 95.1-GHz transition, with detections towards 38 per cent (25 detections; not all of the sources studied in this paper qualify for the complete sample, and some of the sources in the sample were not observed in the present observations).
There is no evidence of an anticorrelation between either the velocity range, or peak flux density of the class I and II transitions, contrary to suggestions from previous studies. The majority of class I methanol maser sources have a velocity range that partly overlaps with the class II maser transitions. The presence of a class I methanol maser associated with a class II maser source is not correlated with the presence (or absence) of main-line OH or water masers. Investigations of the properties of the infrared emission associated with the maser sources shows no significant difference between those class II methanol masers with an associated class I maser and those without. This may be consistent with the hypothesis that the objects responsible for driving class I methanol masers are generally not those that produce main-line OH, water or class II methanol masers.