Radio observations of Jupiter-family comets

Radio observations from decimetric to submillimetric wavelengths are now a basic tool for the investigation of comets. Spectroscopic observations allow us: (i) to monitor the gas production rate of the comets, by directly observing the water molecule, or by observing secondary products (e.g., the OH radical) or minor species (e.g., HCN); (ii) to investigate the chemical composition of comets; (iii) to probe the physical conditions of cometary atmospheres: kinetic temperature and expansion velocity. Continuum observations probe large-size dust particles and (for the largest objects) cometary nuclei.Comets are classified from their orbital characteristics into two separate classes: (i) nearly isotropic, mainly long-period comets and (ii) ecliptic, short-period comets, the so-called Jupiter-family comets (JFCs). These two classes apparently come from two different reservoirs, respectively, the Oort cloud and the trans-Neptunian scattered disc. Due to their different history and—possibly—their different origin, they may have different chemical and physical properties that are worth being investigated.The present article reviews the contribution of radio observations to our knowledge of the JFCs. The difficulty of such a study is the commonly low gas and dust productions of these comets. Long-period, nearly isotropic comets from the Oort cloud are better known from Earth-based observations. On the other hand, JFCs are more easily accessed by space missions. However, unique opportunities to observe JFCs are offered when these objects come by chance close to the Earth (like 73P/Schwassmann-Wachmann 3 in 2006), or when they exhibit unexpected outbursts (as did 17P/Holmes in 2007).About a dozen JFCs were successfully observed by radio techniques up to now. Four to ten molecules were detected in five of them. No obvious evidence for different properties between JFCs and other families of comets is found, as far as radio observations are concerned.     

A Special Radio Spectral Fine Structure Used for Plasma Diagnostics in Coronal Magnetic Traps

A specific combination of spectral fine structures in meter –  decimeter dynamic spectra of solar radio burst emission is reported in observations carried out at the Astrophysical Institute Potsdam. We describe and interpret the occurrence of zebra patterns in fast drifting (type III burst-like) envelopes of absorbed continuum emission. A possible mechanism of the origin of such an involved spectral pattern is put forward, leading to a necessarily multinonequlibrium component coronal plasma. The suggested mechanism is based on the fact that during the passage of a fast electron beam through the corona the loss cone instability (which is caused by electrons captured in a magnetic trap generating the continuum) is quenched. As result, a fast drift burst appears in absorption, and the zebra pattern becomes visible on the low background emission. This zebra pattern is generated by a group of electrons with a nonequilibrium distribution over transverse velocities. In the absence of the beam the pattern is invisible against the background of the stronger continuum. It is shown that the mechanism is sensitive to the distribution parameters of the different electron ensembles. Therefore the effect in dynamic radio spectra is comparatively rare but its proper existence underlines that the simultaneous presence of different ensembles of electrons in the flaring corona can be quite a frequent situation. This can explain some problems in deconvolving X-ray photon spectra to electron energy spectra.     

Influence of Radio Wave Propagation on the Properties of Solar Microwave Bursts

Scintillation of radio signals passing through the solar corona is considered. An expression describing the dynamic spectrum of these scintillations on the basis of multibeam propagation of radio waves is derived. Properties of the analytically calculated spectrum are shown to coincide with zebra-structure properties of solar radio bursts. It is determined that the time profile of the scintillations caused by multibeam propagation may appear as impulses of emission or absorption or may have a sawtooth form. It is concluded that assuming specific emission source features is not the only way to explain the zebra structure, since the effect of multibeam propagation of radio waves through the solar corona and interplanetary space yields a simple explanation of the phenomenon discussed.     

2130MHz太阳射电望远镜中频放大器的调试

本文介绍云南天文台２１３０ＭＨｚ太阳射电望远镜中频放大器的结构原理，技术指标及测试结果。     

银道面HII区复合体G18．2－0．3的射电及红外发射

本文作者用上海天文台的２５米天线和１２．２ＧＨｚ的接收系统，对银道面附近的一个ＨＩＩ区复合体Ｇ１８．２—０．３进行了射电连续谱观测．采用沿源运动轨迹，对观测曲线进行多子源模型拟合的方法，将该复合体中的６个子源从延展的ＨＩＩ区辐射中分离了出来，并导出了每个子源在１２．２ＧＨｚ频率上的流量和角大小．结合外台站在２６９５ＭＨｚ，４７５０ＭＨｚ和１０．５５ＧＨｚ上的流量还得到了这些子源的辐射频谱．分析表明它们具有热辐射性质．对具有射电复合线（Ｈ７６α）资料和角尺度的４个子源导出了它们的电子密度在（１—３）×１０２ｃｍ－３间．此外通过对Ｇ１８．２—０．３复合体子源ＩＲＡＳ资料的分析和模型拟合还得到了每个子源周围冷尘埃的等效黑体温度．其中４个子源并具有ＩＲＡＳ－ＬＲＳ资料．分析它们得到了与该ＨＩＩ区成协尘埃的性质．根据５个子源的ＩＲＡＳ流量、频带以及色改正因子又求出了这些子源在１—５００μｍ间的总红外光度在２×１０４—１０５Ｌ间．它表明与Ｇ１８．２—０．３中各子源成协的新形成星是一些大质量的ＯＢ型星，它们在同一个延展ＨＩＩ区中如此密集证明了大质量星“致密—堆积”形成模式的合理性．     

紫金山天文台13：7米射电望远镜天线表面精度的首次射电全息测量

１９９２年１月，在紫金山天文台１３．７米毫米波望远镜上进行了用射电全息术的相位恢复方法测量天线表面精度的尝试。利用望远镜的２２ＧＨｚ系统，用强水脉泽源ＯＲＩＯＮ－ＫＬ作为信号源测量了天线的聚焦和偏焦方向图，采用Ｍｉｓｅｌｌ算法获得了天线口面上的相位分布，由此得到的天线表面相对于理想抛物面偏差的均方根值与１９８９年用经典的经纬仪带尺方法测量的结果基本相符。     

云南天文台四波段太阳射电望远镜群天线伺服系统数控化自动化工程

本文详细介绍了云南天文台四波段射电望远镜天线群伺服控制系统的改造和更新技术。采用TP—801A型单板机和步进电机数控系统代替了原来的机械系统,从而实现了数控化和自动化。     

Global sounding of sporadic E layers by the GPS/MET radio occultation experiment

The GPS radio occultation technique is sensitive for layered structures with horizontal scales of around 100 km and with vertical scales of a few 100 m or more at the Earth's limb. These structures cause strong fluctuations of the GPS L1 and L2 phase paths which have been measured by a GPS receiver onboard of Microlab-1 satellite in 730 km orbit during the GPS/Meteorology experiment (GPS/MET of UCAR, Boulder). By means of GPS/MET radio occultation data, profiles of electron density fluctuations are derived for the mesosphere/lower thermosphere region with a height resolution of around 1 km. Data analysis of 1900 radio occultation events in June/July 1995, 1540 events in October 1995, and 2690 events in February 1997 confirms seasonal dependence of sporadic E layers. The meridian slices of average sporadic E activity show a dominance of plasma irregularities in the summer hemisphere. The irregularities mainly occur at heights 90–110 km. Auroral and equatorial sporadic E, electron density depletions, and multiple ionization layers are also present in the high resolution GPS/MET data. The multiple layers often have a distance of around 5–10 km in height, and appear up to a height of 140 km (upper height limit for 50 Hz sampling rate of GPS receiver). For February and June, the GPS/MET observations are compared to ground-based observations of the Asia/Australia ionosonde chain.     

Plasma and the universe: large scale dynamics,filamentation, and radiation

One of the earliest predictions about the morphology of the universe is that it be filamentary (Alfvén, 1950). This prediction followed from the fact that volumewise, the universe is 99.999% matter in the plasma state. When the plasma is energetic, it is generally inhomogeneous with constituent parts in motion. Plasmas in relative motion are coupled by the currents they drive in each other and nonequilibrium plasma often consists of current-conducting filaments.In the laboratory and in the Solar System, filamentary and cellular morphology is a well-known property of plasma. As the properties of the plasma state of matter is believed not to change beyond the range of our space probes, plasma at astrophysical dimensions must also be filamentary.During the 1980s a series of unexpected observations showed filamentary structure on the Galactic, intergalactic, and supergalactic scale. By this time, the analytical intractibility of complex filamentary geometries, intense self-fields, nonlinearities, and explicit time dependence had fostered the development of fully three-dimensional, fully electromagnetic, particle-in-cell simulations of plasmas having the dimensions of galaxies or systems of galaxies. It had been realized that the importance of applying electromagnetism and plasma physics to the problem of radiogalaxy and galaxy formation derived from the fact that the universe is largely aplasma universe. In plasma, electromagnetic forces exceed gravitational forces by a factor of 10³⁶, and electromagnetism is 10⁷ times stronger than gravity even in neutral hydrogen regions, where the degree of ionization is a miniscule 10^–4.The observational evidence for galactic-dimensioned Birkeland currents is given based on the direct comparison of the synchrotron radiation properties of simulated currents to those of extra-galactic sources including quasars and double radio galaxies.     

High resolution radio recombination line observations

Summary Over the last decade sensitive observations of radio recombination line emission using high angular resolution synthesis telescopes have become available. As a result it has now become possible to image the physical parameters deduced from radio recombination lines across individual sources. In the case of HII regions this work has resulted in detailed images of radial velocities, electron temperatures and the abundance of singly ionized helium (Y⁺). Direct observational evidence has been found for pressure broadening and non-LTE effects. Dramatic variations have been found in the ratio of He⁺ to H⁺, from as low as a few percent (the galactic centre) to as high 34% in one region of W3. Detailed images have been obtained of the partially ionized medium (CII and H regions) close to HII regions. Observations of recombination lines at very low frequencies have revealed the existence of very low density ionized gas in all directions in our galaxy. Higher resolution observations have led to a partial understanding of this medium. The first complete velocity field of the ionized gas in the centre of our galaxy has been obtained. Very recently the first images were made of extragalactic radio recombination lines, offering the possibility to study the kinematics of the ionized gas in the central few hundred parsecs of external galaxies.