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1.
We have examined the effects of vaporization from the nucleus of a comet and show that a latitude dependence of vaporization can, in some cases, explain asymmetries in cometary light curves. We also find that a non-uniform distribution of solar radiation over a comet can considerably shorten the vaporization lifetime compared to the results normally obtained by assuming that the nuclear surface is isothermal.Independent of any latitude effects, comets with CO2-dominated nuclei and with perihelion distances less than 0.5 AU have vaporization lifetimes less than or comparable to their dynamical ejection times. This may explain the observed deficit of comets with small perihelion distances. Similarly comets with CO2-dominated nuclei and perihelia near Jupiter's orbit have vaporization lifetimes that are shorter than the time for capture into short-period orbits. We suggest, therefore, that at least some new comets are composed in large part of CO2, while only H2O-dominated comets, with lower vaporization rates, can survive to be captured into short-period orbits. 相似文献
2.
The comet 67P/Churyumov-Gerasimenko is the current target of the mission Rosetta, initially planned to investigate comet 46P/Wirtanen. These two comets have similar orbits, except the distance to the Sun at perihelion, but different orbital histories and different masses. Thus, structures of the nuclei can be significantly different. The evolution of comet Wirtanen was simulated by several authors, while comet Churyumov-Gerasimenko became an object of high interest only recently and is not well investigated. In the present work we simulate the evolution of the nucleus, down to tens of meters below the surface, using an extended version of the model previously applied for comet Wirtanen [Kossacki et al., 1999. Comet 46P/Wirtanen: evolution of the subsurface layer. Icarus 142, 202-218.]. The model includes strengthening of the nucleus due to sintering of the ice grains. Simulations are performed for different latitudes, accounting for the evolution of the orbit and for changes of the nucleus orientation, as well as diurnal and seasonal changes of insolation. The calculated loss of water vapor from the comet is integrated over the nucleus surface and is compared with the observational data. We have found, that the sublimation through the dust mantle can be large enough to reproduce the profile of the total water production as a function of time from perihelion. The required dependence of thickness of the dust layer on latitude qualitatively matches present distribution of the absorbed solar flux. The non-gravitational acceleration in the comet motion together with the simulated sublimation flux are used in order to estimate the mass and the bulk density of the nucleus. 相似文献
3.
A new model of the sublimation of volatile ices from a cometary nucleus has been developed which includes the effects of diurnal heating and cooling, rotation period and pole orientation, and thermal properties of the ice and subsurface layers. The model also includes the contribution from coma opacity, scattering, and thermal emission, where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. The model is applied to the specific case of the 1986 apparition of Halley's comet. It is found that the generation of a cometary dust coma actually increases the total energy reaching the Halley nucleus. This results because of the significantly greater geometrical cross section of the coma as compared with the bare nucleus, and because the coma provides an essentially isotropic source of multiply scattered sunlight and thermal emission over the entire nucleus surface. For Halley, the calculated coma opacity is approximately 0.2 at 1 AU from the Sun, and 1.2 at perihelion (0.587 AU). At 1 AU this has little effect on dayside temperatures (maximum ≈200°K) but raises nightside temperatures (minimum ≈150°K) by about 40°K. At perihelion the higher opacity results in a nearly isothermal nucleus with only small diurnal and latitudinal temperature variations. The general surface temperature is 205°K with a maximum of 209°K at local noon on the equator. Some possible consequences of the results with respect to the generation of nongravitational forces, observed volatile production rates for comets, and cometary lifetimes against sublimation are discussed. 相似文献
4.
J. Klinger 《Icarus》1981,47(3):320-324
We consider spheres of water ice of about 1 km in radius moving on three different orbits with a common perihelion distance of 8 AU. As evaporation is negligible in these cases, we call them inactive ice bodies. The surface temperature has been numerically calculated for two extreme situations: (1) The spheres are composed of amorphous ice with a heat conduction to the interior presumed to be negligible. (2) The spheres are composed of compact hexagonal ice with a heat conduction coefficient known from laboratory experiments. Whereas in case 1 the temperature is an unambiguous function of heliocentric distance, in case 2 we observe a thermal “hysteresis” and the maximum temperature has a phase lag with respect to perihelion. The perihelion temperature depends on the eccentricity of the orbit. The case of active ice bodies is also discussed. We come to the conclusion that an ice body moving on the orbit of Tempel 2 must contain crystalline ice and the variations of the surface temperature must be smoothed out in an important way. In the case of Halley's orbit, we suppose that the center of the ice body still contains large amounts of amorphous ice. 相似文献
5.
H. Yoshimura 《Astronomische Nachrichten》1994,315(5):371-390
We found an evidence that the luminosity of the Sun systematically decreased about 20 days before sunspot surface appearance by analysing time-lag correlation of time derivatives of running mean time profiles of the data of the Active Cavity Radiometer Irradiance Monitor (ACRIM) I experiment on board of Solar Maximum Mission (SMM) and of the data of the daily sunspot number. This indicates that sunspot flux tube cooling and heat transport blocking by the flux tubes start to take place in the interior of the solar convection zone well before the sunspot surface appearance. From this finding and our previous finding that the luminosity of the Sun systematically increased and the blocked heat appeared on the surface about 50 days after the sunspot surface appearance, a new view of sunspot formation and dynamics and a new view of the luminosity modulation emerged. (i) Sunspots of a solar cycle are formed from clusters of flux tubes which can be seen in the running mean time profile of the sunspot number as a peak with duration on the order of 100 to 200 days. (ii) Heat flow is blocked by the cluster of sunspot flux tubes inside the convection zone to decrease the luminosity about 20 days before the surface emergence of the sunspot cluster. (iii) The blocked heat appears on the surface about 50 days after the surface emergence of the cluster of sunspot flux tubes to heat up the surface. This appears as a thermal pulse in the running mean time profile of the ACRIM dat in between the peaks of the sunspot running mean time profile. This process of heating the surface makes the temperature gradient less steep and weakens the buoyancy of sunspot flux tubes below the surface. (vi) The radiative cooling of the surface layer by the excess heat release steepens the temperature gradient so that the buoyancy of the sub-surface magnetic flux tubes becomes stronger to cause the next surge of emergence of a cluster of sunspots and other magnetic activities, which creates a peak in the time profile of the sunspot number. We call this peak a magnetic pulse of the Sun and the coupled process of alternating pulsed appearance of heat and sunspots the magneto-thermal pulsation of the Sun. 相似文献
6.
A model of cometary activity is developed which integrates the feedback processes involving heat, gas, and dust transport, and dust mantle development. The model includes the effects of latitude, rotation, and spin axis orientation. Results are obtained for various grain size distributions, dust-to-ice ratios, and spin axis orientations. Attention is focused on the development, change of structure and distribution of dust mantles and their mutual interaction with ice surface temperature and gas and dust production. In this model the dust mantle controls the mechanism of gas transport not onlu by its effect on the temperature but, more importantly, by its own dynamic stability. Results suggest that an initially homogeneous short-period comet with a “cosmic” dust-to-water ice ratio, typical orbit, rotation rate, and grain size distribution would develop at most only a thin (<1 mm) cyclic mantle at all points on the nucleus. Such a fully developed temporary mantle would exist throughout the diurnal cycle only beyond ～4AU. Thus, cyclic behavior would be expected for such an idealized comet, at least for most of its lifetime. Long-term irreversible mantle development on comets with typical rotation rates was not found except regionally on Encke and also on objects with perihelia ?1.5 AU. Even in these cases, free silicate exists, after a few cycles, only as relatively rare large grains and agglomerates with radii ～1 cm scattered over exposed ice. Full mantle development would require hundreds to thousands of cycles. In the case of an initially homogeneous comet Encke, this slow incipient mantle development is shown to be the direct result of its peculiar axial orientation. High obliquity appears required for long-term mantle development for typical rotation rates and perihelia ?1.5 AU. Heat conduction into the nucleus for an incompletely mantled or bald comet has been found to be very important in maintaining relatively higher ice surface temperatures, and hence fluxes, during those portions of the diurnal and orbital cycles which would otherwise be cooler. It is also shown to be at least one cause of post perihelion brightness asymmetries, especially in lower obliquity comets. Maximum heliocentric distances at which 1-μm dust, sand, pebbles, cobbles, and boulders can be permanently ejected from the subsolar point by H2O (CO2) are (in AU): 6.9 (16.8), 5.2 (11.5), 1.8 (3.0), 0.21 (0.34) and 0.07 (0.11), respectively. A detailed anatomy of temperature, gas and dust fluxes vs latitude and longitude for a homogeneous rotating comet with fixed axis is given for comparison with future observations. Most H2O flux histories deduced from brightness data are found to be in reasonable agreement with the model, allowing for uncertainty in radius and albedo. A clear exception is Encke. It is shown that the large discrepancy between Encke's observed and model predicted fluxes, based on radar cross section, can be used to evaluate the extent of exposed ice (<10%). The model is then used to place an active area so as to explain a reported sharp drop in flux on approach to the Sun at 0.78 AU. An active area or areas, <10% of the comet's surface, centered near 65°N latitude appears indicated. Although cyclic mantles are generally indicated for the set of parameters we used, our results show that a global mantle only 1 to 3 cm thick (depending on the orbit) consisting of a full range of grain sizes can cause irresversible evolution to a noncometary body. We investigated the long-term evolution of such a postulated initially thinly mantled cometary object. It was found that after the first few passes and until the end of its dynamic lifetime the object averaged <3 × 10?12 g cm?1 sec?1 H2O flux. Therefore, if cometary objects evolve into Apollo asteroids, ice should always be accessible within 10 m of the surface despite numerous close perihelion passages. The possible impact of factors not included in the model, such as initial inhomogeneities, coma scattering of radiation, and global redistribution of ejected silicate around the nucleus, are discussed. 相似文献
7.
Paul R. Weissman 《Icarus》1983,55(3):448-454
D. J. Michels, N. R. Sheeley, Jr., R. A. Howard, and M. J. Koomen (Science215, 1097–1102, 1982) observed a comet which appears to have impacted the Sun. Z. Sekanina (Astron. J..87, 1059–1072, 1982) showed that the comet, 1979XI, was probably a member of the Kreutz group of sungrazing comets. The sungrazers typically have perihelia of 1.2–1.9 solar radii but Sekanina found q = 0.35 R⊙ for 1979XI. It is interesting to speculate how the perihelion may have been reduced to this small value. The change in perihelion can not be explained by planetary, stellar, or nongravitational perturbations. Tidal splitting of the nucleus on a previous perihelion passage is also ruled out, through a random splitting event near aphelion of the comet's orbit is a remote possibility. The most plausible explanation is collision with another body, most likely a comet, at large heliocentric distance. However, the expected probability of such an event is exceedingly small. Another aspect of the problem is whether the nucleus of 1979XI sublimated completely before impacting the Sun. Assuming a water ice nucleus, it is shown that a surface layer of only 5–15 m thickness would be sublimated prior to impact. Although it is likely that the nucleus tidally disrupted after crossing the solar Roche limit, the ultimate destruction of the nucleus probably resulted from the shock of hitting the denser regions of the solar atmosphere, just above the photosphere. 相似文献
8.
One of the goals of comet research is the determination of the chemical composition of the nucleus because it provides us
with the clues about the composition of the nebula in which comet nuclei formed.
It is well accepted that photo-chemical reactions must be considered to establish the abundances of mother molecules in the
coma as they are released from the comet nucleus or from distributed dust sources in the coma. However, the mixing ratios
of mother molecules in the coma changes with heliocentric distance. To obtain the abundances in the nucleus relative to those
in the coma, we must turn our attention to the release rates of mother molecules from the nucleus as a function of heliocentric
distance. For this purpose, we assume three sources for the coma gas: the surface of the nucleus (releasing mostly water vapor),
the dust in the coma (the distributed source of several species released from dust particles), and the interior of the porous
nucleus (the source of many species more volatile than water). The species diffusing from the interior of the nucleus are
released by heat transported into the interior. Thus, the ratio of volatiles relative to water in the coma is a function of
the heliocentric distance and provides important information about the chemical composition and structure of the nucleus.
Our goal is to determine the abundance ratios of various mother molecules relative to water from many remote-sensing observations
of the coma as a function of heliocentric distance. Comet Hale-Bopp is ideal for this purpose since it has been observed using
instruments in many different wavelength regions over large ranges of heliocentric distances. The ratios of release rates
of species into the coma are than modeled assuming various chemical compositions of the spinning nucleus as it moves from
large heliocentric distance through perihelion. Since the heat flow into the nucleus will be different after perihelion from
that before perihelion, we can also expect different gas release rates after perihelion compared to those observed before
perihelion. Since not all the data are available yet, we report on progress of these calculations.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
R. Schulz 《Solar System Research》2009,43(4):343-352
One of the two planetary cornerstone missions of the European Space Agency is the Rosetta mission to comet 67P/Churyumov-Gerasimenko. Rosetta is a rendezvous mission with a comet nucleus, which combines an Orbiter with a Lander. It will monitor the evolution of the comet nucleus and the coma as a function of increasing and decreasing solar flux input along the comet’s pre- and post-perihelion orbit. Different instrumentations will be used in parallel, from multi-wavelength spectrometry to in-situ measurements of coma and nucleus composition and physical properties. Rosetta will go in orbit around the nucleus of its target comet 67P/Churyumov-Gerasimenko, when it is still far from the Sun and accompany the comet along its way to perihelion and beyond. In addition the Rosetta Lander Philae will land on the nucleus surface, before the comet is too active to permit such a landing (i.e. at around r = 3 AU) and examine the surface and subsurface composition of the comet nucleus as well as its physical properties. 相似文献
10.
11.
The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright
comet over a large range of heliocentric distances rh. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion.
CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate
reached 1031 s−1 at perihelion. Gas production curves obtained for various species show several behaviours with rh.
Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus
and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other
phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H2CO or HNC, they permit to constrain their origin in the coma.
We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet
moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet
Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large rh) or free sublimation (typical example: H2O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a
model with crystallization of amorphous ice close to the nuclear surface.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
Physical evolution of Jupiter family (JF) comets is considered as a simultaneous process of erosion and fading. Dynamical effects are limited to discrete changes of the perihelion distance, that result in changes of the evaporation rate. Assuming that the JF comet population is in a steady state, a distribution function of this population in the two dimensional phase space consisting of radius and active fraction of the nucleus surface is found as the solution of a set of kinetic equations, each one of them for a different perihelion distance. With use of the distribution function some statistical properties of the comet population, like the total number of comets in the considered region of the phase space, the number of objects that evaporate or get dormant per unit time, etc., are obtained. The cumulative distribution function with respect to the absolute brightness is calculated and compared with the observed one as a check on the considered models. 相似文献
13.
《Icarus》1987,72(3):535-554
An analytical model has been developed to simulate the chemical differentiation of a homogeneous, initially unmantled cometary nucleus composed of water ice, putative unclathrated CO2 ice, and silicate dust in specified proportions. Selective sublimation of any free CO2 ice present in a new comet should produce a surface layer of water ice and dust overlying the undifferentiated core. This surface layer modifies the temperature of buried CO2 ice and restricts the outflow of gaseous CO2. On each orbit, water sublimation closer to perihelion temporarily reduces the thickness of the water ice and dust layer and liberates dust. Most of the dust is blown off the nucleus, but a small amount of residual dust remains on the surface (cf. H. L. F. Houpis, W. H. Ip, and D. A. Mendis, 1986, Astrophys. J., in press). Our model includes the effects of nucleus rotation, arbitrary orientation of the rotation axis, latitude, heat conduction into the interior of the nucleus, restriction of CO2 gas outflow by the water ice and dust layer, and the use of thermal conductivities for both amorphous and crystalline water ice as appropriate, featuresthat were not included in the Houpis et al. model. The model also accounts for the erosion of the water ice surface, which Houpis et al. appear to have accounted for and which is an important effect. Specifically, we investigate the effects of varying the permeability of the surface water ice layer, the mass fraction of CO2, the orbit and the latitude, using the orbital parameters of Comets Halley and Tempel 2. It is found that CO2 gas production should exceed H2O gas production beyond ∼3 AU, and at 1 AU CO2 gas production should be between 20 to 25% of H2O gas production. The depth of CO2 ice and the variation in the depth of CO2 ice throughout an orbit are affected significantly by the perihelion of the orbit. The effects due to water ice permeability are significant but much less than expected on the basis of flow area. Latitude and CO2 concentration produce relatively small effects. Under all conditions considered here, CO2 ice should always be found within ∼1 m from the surface of comet nuclei if it is present as a free species to begin with. This result is probably generally valid for unmantled portions of most comets and qualitatively simulates the behavior of an abundant, highly volatile component in an H2O/silicate matrix. Comparison of these and similar results with observations could yield information regarding the permeability and chemical composition of cometary material and suggest sampling strategies to minimize fractionation effects. The method is applicable to other nonwater ices. 相似文献
14.
Physical evolution of Jupiter family (JF) comets is considered as a simultaneous process of erosion and fading. Dynamical effects are limited to discrete changes of the perihelion distance, that result in changes of the evaporation rate. Assuming that the JF comet population is in a steady state, a distribution function of this population in the two dimensional phase space consisting of radius and active fraction of the nucleus surface is found as the solution of a set of kinetic equations, each one of them for a different perihelion distance. With use of the distribution function some statistical properties of the comet population, like the total number of comets in the considered region of the phase space, the number of objects that evaporate or get dormant per unit time, etc., are obtained. The cumulative distribution function with respect to the absolute brightness is calculated and compared with the observed one as a check on the considered models. 相似文献
15.
Cosmic abundance, vapor pressure, and molecular weight considerations restrict the likely gas candidates for Pluto's atmosphere to Ne, N2, CO, O2, and Ar, in addition to the already detected CH4. The vapor pressures and cosmic abundances of these gases indicate that all except Ne should be saturated in Pluto's atmosphere. The vapor pressure of Ne is so high that the existence of solid or liquid Ne on Pluto's surface is very unlikely; cosmic abundance arguments imply that Ne cannot attain saturation in Pluto's atmosphere. At both perihelion, N2 should dominate the saturated gases. CO2 should have the next highest mixing ratio, followed by O2 and Ar. CH4 should have the smallest mixing ratio. Because vapor pressures of these gases vary with temperature at diverse rates, the bulk and constituent mixing ratios of Pluto's atmosphere should vary with season. Between perihelion and aphelion, the column abundance of CH4 may change by a factor of 260 while that of N2 changes by only a factor of 52. The potential seasonal variation of Pluto's atmosphere was investigated by considering the behavior of these gases when individually mixed with CH4. The effects of diurnal and latitudinal variation of insolation and eclipses on the atmosphere also were investigated. Seasonal effects are shown to dominate. It was shown that the atmospheric bulk may not be a minimum near aphelion but rather at intermediate distances from the Sun during summer/winter inadequate ice deposits may allow the atmosphere to collapse by freezing out over winter latitudes. If the atmosphere does not collapse, its weight is sufficient to keep it distributed uniformly around Pluto's surface. In this case, the atmosphere tends to regulate the surface temperature to a seasonally dependent value which is uniform over the globe.Finally, the likely global circulation regimes for each model atmosphere as a function of temperature were investigated and it was concluded that if CH4, O2, or CO dominates the atmosphere, Pluto will exhibit cyclic variations between an axially symmetric circulation system at perihelion and a baroclinic wave regime at aphelion. However, if N2 dominates, as is likely, the wave regime should hold continuously. If the atmosphere collapses to a thin halo during summer/winter seasons, only a weak, symmetric circulation should occur. 相似文献
16.
Shin Yabushita 《Earth, Moon, and Planets》1996,73(3):215-220
Earlier, a study has been made of the transport mechanism of volatile molecules such as N2 and CO through cometary nuclei as they are heated by radioactive elements. Coupled equations of heat and gas transport in the presence of gas sublimation and recondensation, as well as a heat source, were numerically solved. And it was shown that supervolatiles such as N2 and CO are transported through the pores of the nucleus, and consequently the volatile molecules become more abundant near the surface than deep inside the nucleus. Here, the process is investigated for a wider range of paramaters such as porosity and nuclear radius. It is shown that provided the central temperature attains the sublimation point of the super-volatiles, they are transported toward the surface regardless of the values of the parameters. 相似文献
17.
E. Van Hemelrijck 《Earth, Moon, and Planets》1983,29(1):83-93
This paper describes variations in the insolation on Mercury resulting from fluctuations of the orbital eccentricity (0.11≤e≤0.24) of the planet. Equations for the instantaneous and the daily insolation are briefly discussed and several numerical examples are given illustrating the sensitivity of the solar radiation to changes ine. Special attention is paid to the behavior of the solar radiation distribution curves near sunrise and sunset which at the warm pole of Mercury (longitudes ±90°) occur as the planet goes through perihelion. It has been found that for eccentricities larger than about 0.194 there exists two permanent thermal bulges on opposite sides of the Mercurian surface that alternately point to the Sun at every perihelion passage. The critical value ofe past which the Sun shortly sets after perihelion is near 0.213. 相似文献
18.
E. Hadamcik A. K. Sen A. C. Levasseur‐Regourd S. Roy Choudhury J. Lasue R. Gupta R. Botet 《Meteoritics & planetary science》2014,49(1):36-44
Comet C/2009 P1 (Garradd) was observed by imaging polarimetry for nearly 5 months from October 2011 to March 2012, over an intermediate phase angle range (28°–35°). Two months before perihelion and one month after, dust particles seem to be ejected all around the optocenter and jets extend to distances greater than 40,000 km. An increase of activity is noticed in intensity and polarization after perihelion. Two months before perihelion and one month after, the dust emission seems to be all around the optocenter. Two and three months after perihelion the jets are mainly toward the solar direction with an extension of more than 20,000 km projected on the sky. The values of the aperture polarization are comparable to those of other comets. On the polarization maps in October 2011 and January 2012 the higher polarization zones extend in large regions perpendicularly to the solar direction where jets are also observed. In February and March 2012, the polarization in the jets is larger in the solar direction than in the surrounding coma. By its activity visible on intensity images and polarization maps at large distances from the nucleus, comet Garradd probably belongs to the high‐Pmax class of comets. 相似文献
19.
Arguments are presented to suggest that surface layers of the nuclei of periodic comets consist of crystallized rather than
amorphous water ice and thermal modelling of such nuclei is presented. The rate of sublimation of water from a rotating nucleus
is found to be greater than that from a uniformly heated nucleus. When the model is applied to P/Halley, the sublimation rate
at perihelion is found to be 8.1 × 1029 mol s−1 for a nucleus rotating with a period of 50 hours and 7.6 × 1029 for a uniformly heated nucleus on the premise that the effective radius of the nucleus is 2.5 km. The total sublimation of
water per revolution is 5.38 × 1036 molecules for P/Halley and 3.91 × 1036 molecules per P/Crommelin. The result so obtained is discussed in relation to the observational data. 相似文献
20.
A one-dimensional simulation of pure water-ice cometary nuclei is presented, and the effect of the nucleus as a heat reservoir is considered. The phase transition from amorphous to crystalline ice is studied for two cases: (1) where the released latent heat goes entirely into heating adjacent layers and (2) where the released latent heat goes entirely into sublimation. For a Halley-like orbit it was found that for case 1 the phase boundary penetrates about 15 m on the first orbit and does not advance until sublimation brings the surface to some 10 m from the phase boundary. For case 2 the phase boundary penetrates about 1 m below the surface and remains at this depth as the surface sublimates. For an orbit like that of Schwassmann-Wachmann 1 the phase boundary penetrates about 50 m initially for case 1 and about 1 m for case 2. There is no further transformation until the entire comet is heated slowly to near the transition temperature, after which the entire nucleus is converted to crystalline ice. For an Encke-type orbit case 1 gives a nearly continuous transition of the entire nucleus to crystalline ice, while for case 2 the initial penetration is about 8 m and remains at this depth relative to the surface as sublimation decreases the cometary radius. Thus the entire comet is converted to crystalline ice just before it is completely dissipated. 相似文献