Dust optical properties: A comparison between cometary and interplanetary grains

A better understanding of cometary dust optical properties has been derived from extensive observations of comet Halley, complemented by other cometary observations at large phase angles and/or in the infrared. Also, further analysis of IRAS observations and improvements in inversion techniques for zodiacal light have led to some progress in our knowledge of interplanetary dust.

Synthetic curves for phase angle dependence of intensity and polarization are presented, together with typical albedo values. The results obtained for interplanetary dust are quite reminiscent of those found for comets. However, the heterogeneity of the interplanetary dust cloud is demonstrated by the radial dependence of its local polarization and albedo; these parameters are also found to vary with inclination of the dust grains' orbits with respect to the ecliptic. Such results suggest drastic alterations with temperature in the texture of cometary dust, and would favor an important asteroidal component in the zodiacal cloud.     

IRAS observations of the zodiacal background

The Infrared Astronomical Satellite (IRAS) sky survey measurements are used to constrain a model for the interplanetary dust emission. Efforts to date, although hampered somewhat by calibration uncertainties, indicate that the cloud is inclined to the ecliptic by approximately 1.7° with its line of ascending nodes at 70° ecliptic longitude. The data is well fit by a dust density distribution described in cloud symmetry plane coordinates by p = p_o (r_o/r)^1.1 exp [−4.2 (z/r)^1.2], where the dust has a gray emissivity and an equilibrium temperature at 1 AU of 280 K.     

The impact of IRAS results on the three-dimensional models (3D) of the global distribution of interplanetary dust

The three-dimensional (3D) distribution of micrometeoroids, representing the interplanetary dust cloud, has been investigated by light scattering analysis of the zodiacal light. Classical models imply, however, that there are homogeneous physical properties of the grains throughout the visible zodiacal cloud. IRAS results have questioned this assumption and suggested, e.g., a possible spatial decrease of albedo with solar distance, which would result in a more moderate decrease of number densities. The discussion of the conventional and corresponding modified models shows that the IRAS measurements propose a flatter interplanetary dust cloud than the visual models. This tendency is also supported by other IR-observations. A dynamical analysis reveals that this flattening of the dust cloud on account of the IRAS-data is achieved by a change in the major axis distribution of the particle orbits only, whereas in the other models the inclination distribution is also changed.     

IRAS observations of cometary dust

The large beam size of the Infrared Astronomy Satellite (IRAS) focal plane detector array is well suited to measuring the low level thermal emission from cometary dust. Eight comets discovered in 1983 and nine previously known periodic comets were observed by IRAS during its ten month active lifetime. Dust production rates are derived for a wide range of heliocentric distances. Grain properties are inferred from application of simple models to the long wavelength spectral energy distribution.     

Report on the Kiso cometary dust trail survey

Cometary dust trails were first observed by IRAS; they are widely known to be the origins of meteoric showers. A new window has been opened for the study of dust trails, using ground-based observations. We succeeded in obtaining direct images of the 22P/Kopff dust trail with the Kiso 1.05-m Schmidt telescope. Following this initial success, we have continued to perform a dust trail survey at Kiso. As a result of this survey, we have detected dust trails along the orbit of six periodic comets, between February 2002 and March 2004. The optical depth of these dust trails are 10⁻⁹ to 10⁻⁸, which is consistent with IRAS measurements. In this paper, we describe the observations and data reduction procedures, and report the brief result obtained between February 2002 and March 2004.     

On the evolution of dust in the solar vicinity.

The analysis of interplanetary dust shows that the majority of particles in out-of-ecliptic regions comes from comets and also that near solar dust, in the ecliptic regions, results most probably largely from comets. The intense radiation flux in the solar vicinity is expected to cause strong modifications in the material composition and surface structure of interplanetary dust particles and hence the analysis of near solar dust provides interesting insights into the evolution of meteoritic, especially cometary materials. Because of the lack of in-situ measurements our present knowledge concerning these processes derives from remote sensing, i.e. observations of the solar F-corona. In particular these are observations of albedo, polarization and colour temperature given in terms of average particle properties. For example the analysis of near infra-red F-corona data points to the existence of a strong component of irregularly structured silicate particles, most probably of cometary origin. The data may indicate a subsequent sublimation of different particles or different constituents of the particles. Here we compare particle properties derived from F-corona observations with model calculations of single particle properties and discuss perspectives of future analysis of cometary dust in the interplanetary cloud.     

The geometry of the zodiacal dust bands

A program is in progress to use the best available IRAS data to determine the orbital parameters of the ensemble of particles which make up the zodiacal dust bands. Refinements on the early results of Gautier and Hauser include the use of a more complete set of data and improved treatment of ring profile effects due to dispersions in orbital parameters. Results to date include an analysis of the sensitivity of the observable parameters of the dust bands to changes in the orbital parameters of the particle ensemble.     

Solar orbiter, a high-resolution mission to the sun and inner heliosphere

The scientific rationale of the Solar Orbiter is to provide, at high spatial (35 km pixel size) and temporal resolution, observations of the solar atmosphere and unexplored inner heliosphere. Novel observations will be made in the almost heliosynchronous segments of the orbits at heliocentric distances near 45 R and out of the ecliptic plane at the highest heliographic latitudes of 30° – 38°. The Solar Orbiter will achieve its wide-ranging aims with a suite of sophisticated instruments through an innovative design of the orbit. The first near-Sun interplanetary measurements together with concurrent remote observations of the Sun will permit us to determine and understand, through correlative studies, the characteristics of the solar wind and energetic particles in close linkage with the plasma and radiation conditions in their source regions on the Sun. Over extended periods the Solar Orbiter will deliver the first images of the polar regions and the side of the Sun invisible from the Earth.     

地面观测研究彗星尘埃的一种新方法

本文根据尘埃吸收系数Q_v的不同模型(由实验和观测建立的模型)求解了不同性质的尘埃的辐射平衡方程.得到了性质不同,颗粒大小不同,日心距不同的尘埃的温度.如果我们把邻近两个波长的观测到的强度比值所确定的色温度当作尘埃的真实温度,那么我们就能够通过近红外的地面观测来研究尘埃的性质(而不要远红外的空间观测).      

Recent infrared observations of comets with UKIRT and IRAS

A preliminary analysis of infrared observations of comets P/Crommelin and P/Tempel 1 is presented. Comet P/Crommelin was observed from UKIRT over the range 1–20 micron, using standard filters. From the shape of the thermal emission spectrum, the temperature of the dust grains is estimated (T = 314 ± ₃₃⁴⁴K) and also the dust production rate (1.3 × 10⁵gs^?1). Comet P/Tempel 1 was observed with the Infrared Astronomical Satellite (IRAS). The emission is found to be considerably extended and there is also evidence for temperature variation of the dust grains as indicated by the 12 to 25 micron flux ratio.     

Infrared emission and optical scattering by the high galactic latitude dust cloud L1642

We present a comparison of optical surface brightness and extinction data with IRAS infrared maps at 60 and 100 μm for the high galactic latitude cloud L1642. We have derived the optical albedo of the grains and the ratio of 100 μm and optical opacities. The albedo does not depend strongly on wavelength; on the average the albedo is 0.5 between 3450 and 5500 Å. L1642 is detected also at 12 and 25 μm, where the morphology of the brightness distribution supports an explanation in terms of non-equilibrium emission from small grains and/or large aromatic molecules.     

Interstellar dust models

Interstellar dust models, previously constrained only from the extinction curve, have been radically changed with the arrival of IRAS observations of the dust infrared emission. An important component of interstellar dust is likely to be made of small particles that show a fluctuating temperature upon impinging single photons and which can produce large near and mid infrared excesses ubiquitously observed in the Galaxy and external galaxies. The analysis of COBE data should soon improve our understanding of dust infrared emissivity and particularly for big grains in the submillimeter domain. We will discuss the key observations (spectral features, broad-band colors, correlations with gas tracers…) which put the best constraints on any dust models and show that the next generation of IR/submm satellites (ISO, SIRTF…) should improve our knowledge of interstellar dust composition and the dust redistribution of the stellar energy inside galaxies.     

Out-of-ecliptic distribution of interplanetary dust derived from near earth flux

The out-of-ecliptic distribution of interplanetary dust, i.e. its number density, mainly has been subject to optical or infrared remote sensing techniques. As the population in interplanetary space is made up of orbiting particles which will cross the ecliptic plane, determination of their orbital properties there gives a possibility also to derive their out-of-ecliptic distribution. Determination of orbital elements is provided by advanced detectors capable of measuring the vector of impact velocity. In a simple model, which applies for advanced detectors in near earth orbit, the feasibility of the method to determine the out-of-ecliptic spatial distribution of dust has been tested.     

火星尘埃与探测

火星表面尘埃与太阳辐射、热辐射的相互作用直接影响火星大气的结构、热平衡和动力学过程,并会产生改变火星表面反照率和火星地貌的长期效应.火星尘埃环境还对登陆于火星表面的着陆器能源系统和光学载荷等系统构成影响.为此需开展火星大气尘埃的直接就位探测.在介绍了火星的尘埃特性与主要探测方法基础上,提出了采用微质量计技术开展火星表面尘埃就位探测的综合探测器方案.探测器包含3种传感器.尘埃累积传感器通过设置其敏感晶体表面朝上,可以探测火星表面尘埃的沉积质量与速率;荷电尘埃传感器通过加置不同极性的偏置电压,可以探测荷正电尘埃和荷负电尘埃的累积特性;磁尘传感器通过在敏感晶体后加设小型永久磁铁,可以探测磁性尘埃的累积特性.传感器感测质量范围为10^-11~10^-4g.火星尘埃综合探测器可应用于未来的火星着陆探测计划.     

A comparison of the dust properties in recent periodic comets

Measurements of the thermal emission from the cometary dust coma can be used to derive the rate of dust production from the nucleus as well as the size distribution of absorbing grains. More than ten short-period comets have now been observed in the infrared over a wide range in heliocentric distance. Dust production rates are derived for these comets based on theoretical models of the thermal emission from small absorbing grains and calculations of dust grain velocities. The mean size and albedo of the dust grains is similar in these comets, with the exception of Comet Crommelin, which seems to have had larger, darker grains.     

Comet-solar wind interactions: A dusty point of view

Anticipating the new results from the space missions to Comet Halley and Comet Giacobini-Zinner, we make a brief review of recent theoretical and observational studies of dust-plasma environment. In order to relate different disciplines in cometary research in the context of comet-solar wind interaction, two separate issues: (a) surface processes and (b) plasma processes are considered to indicate how various kinds of observations of cometary dust comas and tails may be used to infer the conditions of solar wind - comet interaction and the corresponding plasma processes in the cometary ionospheres and ion tails (and vice-versa). In particular, it is suggested that the narrow sunward-pointing dust streamers emitted from the cometary nuclei could be related to the electrostatic transport of sub-micron dust over the nuclear surfaces at large heliocentric distances; and the striae sometimes observed in cometary dust tails at smaller heliocentric distances could be the consequence of electrostatic fragmentation of fluffy dust particles in the ion tails.     

Observations of high velocity gas in the local interstellar medium

Red-shifted high velocity gas components (HVC) with velocities between 40 to 120 kms⁻¹ have been detected in UV spectra towards several stars observed with IUE. The gas producing the components is detected across several degrees in the sky but it is patchy in nature. The physical properties of the material responsible for the HVC appear quite similar to those derived from direct observations of supernova remnants. Recent additional observations of the gas kinematics have been made through high resolution spectra of the NaD lines. Also, IRAS maps of infrared dust emission are presented for one region of sky of particular interest. In this report we review the observational material and discuss possibilities for the nature and origin of the HVC gas.     

地球磁鞘中磁场起伏特性的分布

本文利用行星际起伏通过激波后的变化的MHD模型, 具体讨论了地球磁鞘中磁场起伏特性在黄道面内的分布.主要结果是:(1)行星际磁场起伏的强度和各向异性在磁鞘中被显著放大;(2)行星际磁场基本位于黄道面内时, 磁鞘中磁场起伏特性(强度、相对起伏和各向异性等)呈现明显的晨一昏不对称性, 早晨侧(准平行激波)显著地高于黄昏侧(准垂直激波);(3)行星际磁场方向对磁场起伏特性在磁鞘中的分布有强烈的控制作用, 早晨侧响应灵敏, 黄昏侧反响不大.相对地讲, 黄昏侧的磁活动较之早晨侧稳定;(4)行星际磁场转南的增强将导致磁鞘中磁场起伏的最大区域自黄道面低纬向北极高纬移动, 南-北不对称性磁活动随之加强, 最强大致出现在磁场与黄道面相交成大约45°时, 而晨-昏不对称性的强弱程度则发生相反变化;(5)行星际磁场的相对起伏增加, 晨-昏不对称性反随之减弱.磁鞘中磁场起伏分布的特性与卫星观测大体符合, 是磁顶、边界层某些晨-管不对称性出现的可能起因之一.      

The derivation of Halley parameters from observations

A set of nominal model parameters for P/Halley is derived from its light curve and spectra. In those cases where Halley observations are not sufficient, the average value derived from a large set of other comets has been used, or data from comet Bennett, Halley's best analogue has been taken. The derived parameters include nucleus mass, size, density, albedo, rotation period, axial inclination, and surface temperature, the composition of the parent molecules, the total gas and dust production rates, distributions for the dust size and bulk density as well as various other parameters.