The interpretations of ultraviolet observations of comets

Summary Ultraviolet observations of comets from above the Earth's atmosphere have provided excellent new results and improved older ground based observations (OH) by an order of magnitude. Satellites are especially suitable because long integration times and observations during extended time intervals are possible.The existing cometary L observations have confirmed the relatively high overall gas production rates on the order of 10³⁰ molecule s^–1. The results strongly support the concept of an icy conglomerate solid cometary nucleus. Additional observations of hydroxyl and oxygen favor water to be one of the most abundant molecules in comets. The observations are in agreement with the predominent role of water in the evaporation process of the nuclear ices but are not proof in themselves.Water did not outnumber other consitutents by orders of magnitude in comets Bennett and Kohoutek. At least in these comets, carbon-containing molecules were possibly as numerous as water. Determination of the carbon scale length is necessary for a more quantitative statement.A hydrogen velocity of 7–9 km s^–1 was observed in comet Bennett as well as in comet Kohoutek for a variety of heliocentric distances and varying production rates. Determinations of the outflow velocity from L isophotes agree with line profile observations of L and H. Hydroxyl may constitute the main source for the hydrogen atoms with v _H - 8 km s^–1. The decay process, however, leading to this particular velocity is not yet known. Possibly a large portion of the OH radicals do not decay into hydrogen atoms or at least not into slow ones. If the high velocity component of 20 km s^–1 or more comprises a larger amount (up to 50%), most of the quoted hydrogen production rates must be revised upward.The intrinsic cometary brightness is only a very crude indicator of a comet's actual gas production rate as shown by comparison of comets Bennett and TSK. Comets can be successfully used as (extra ecliptic) space probes to measure interplanetary quantities, e.g., the curvature of the extended hydrogen clouds can be used for the absolute determination of the solar emission independent of instrumental calibration. Generally time dependent hydrogen density models must be used for the interpretation. The strength of the ultraviolet L emission favors its measurement as a standard procedure for the observation of comets (possibly together with OH (3090 Å)). These observations provide the most accurate results on the total cometary gas production rate and its variation with heliocentric distance.Dedicated to Professor L. Biermann in recognition of his inspiring guidance.On leave of absence from Max-Planck-Institut für Physik und Astrophysik, Munich.     

Synchronization of QAM Signals in the Presence of ISI

A data-aided carrier reconstruction method is described for quadrature amplitude modulation (QAM) data communications over narrow-band transmission channels. Previous analyses of this problem have assumed an "infinite" bandwidth of the communication link, so that negligible intersymbol interference (ISI) resulted. On the contrary, when ISI takes place, a different structure must be used which operates on sampled data. The performance of this structure is expressed analytically as a function of 1) the statistics of the message symbols, 2) the characteristics of the channel, and 3) the parameters of the tracking system. Excellent agreement has been found between the theory and the results of a computer simulation.     

Composition and structure of the atmosphere of Venus

Although in recent years much has been learned about the atmospheric composition and structure of Venus, there are many key questions which remain unanswered. The Pioneer Venus set of experiments is designed to provide information both individually and collectively to help understand and explain first of all the present state of the atmosphere (the composition and distribution in both the lower and upper parts, the state property profiles, the cloud compositions, the role of phase in the thermal structure, the planet's surface and interior composition, the high surface temperature, the stability of CO₂, the ionosphere — its chemistry and thermal structure, the existence of superrotation, the response of the upper atmosphere to changes in solar EUV and the solar wind) and secondly the origin and evolution of the atmosphere. This paper discusses these questions and the degree to which the Pioneer Venus instruments will respond to them.     

Low-cost simulator for basic radar signals

Most modern radar simulators are costly and need cumbersome operations while used. Besides, they have many features not suitable for preliminary level. Basic training of radio monitoring personnel can be carried out with a simple and inexpensive system capable of producing both staggered and frequency hopping pulse radar signals with radiation pattern modulation. For operator convenience the system is programmed with basic programming language     

A new spectrometric measurement of atmospheric 63 μm emission

Absolute zenith intensities of the atomic oxygen fine structure emission at 63 μm were measured above Kiruna, Sweden, on 9 December 1981 in the altitude regime of 85 km to 237 km. The measured data are compared with theoretical predictions for this emission. Both local thermodynamic equilibrium (LTE) and non-LTE conditions were assumed for the model intensity calculations. The importance of the 63 μm emission as a cooling mechanism of the thermosphere is briefly discussed.     

航空发动机2 级风扇的数值研究

运用ANSYS CFX 11.0和home-code程序对AI222发动机风扇流路气动参数进行了3维CFD(Computational Fluid Dynamics)计算;通过3维建模,实现了风扇主要积分特性和局部参数计算。通过与风扇进行大量试验结果的对比,验证了计算结果,并找出了产生差别的原因,得到了风扇总特性以及气流径向参数的计算值与试验值的吻合度。结果表明:通过运用3维建模计算方法简化了风扇设计过程,缩短了风扇从设计到试验的时间,降低了航空发动机部件的设计成本。     

Daytime O/N<Subscript>2</Subscript> Retrieval Algorithm for the Ionospheric Connection Explorer (ICON)

The NASA Ionospheric Connection Explorer Far-Ultraviolet spectrometer, ICON FUV, will measure altitude profiles of the daytime far-ultraviolet (FUV) OI 135.6 nm and N₂ Lyman-Birge-Hopfield (LBH) band emissions that are used to determine thermospheric density profiles and state parameters related to thermospheric composition; specifically the thermospheric column O/N₂ ratio (symbolized as \(\Sigma\)O/N₂). This paper describes the algorithm concept that has been adapted and updated from one previously applied with success to limb data from the Global Ultraviolet Imager (GUVI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission. We also describe the requirements that are imposed on the ICON FUV to measure \(\Sigma\)O/N₂ over any 500-km sample in daytime with a precision of better than 8.7%. We present results from orbit-simulation testing that demonstrates that the ICON FUV and our thermospheric composition retrieval algorithm can meet these requirements and provide the measurements necessary to address ICON science objectives.     

Critical Questions and Future Measurements — Collated Views of the Workshop Participants

At the ISSI Workshop 'The Origin and Composition of Cometary Material' a short questionnaire was devised by the 'Critical Measurements for the Future' Working Group and distributed to the attendees. The aim was to find out what they thought were the 'critical questions' and the key measurements needed to find answers. Results from the 15 respondents are collated and summarized. This revised version was published online in June 2006 with corrections to the Cover Date.