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1.
An extension of the seasonal climate model of R. D. Cess and J. Caldwell (1979, Icarus, 38, 349–357) to Saturn's upper troposphere is presented. The ring-modulated latitudinal dependence of the insolation, the ring thermal emission, the oblateness of the planet, the orbit eccentricity, and the latitudinal variation of the internal heat flux are taken into account. Calculations agree closely with the temperature—latitude profiles retrieved from Voyager IRIS measurements at atmospheric levels located above the 0.2-bar pressure level; they reproduce the observed large-scale hemispheric asymmetry which is then shown to result from the seasonally variable insolation. Aerosol absorption is found to be the dominant source of atmospheric solar heating in the troposphere and the model suggests an aerosol mean unit optical depth around the 0.25-bar level in the equatorial region and around the 0.35-bar level at other latitudes. The model fails to predict the retrieved temperature—latitude profiles below the 0.3-bar level. This discrepancy is attributed to the existence of clouds at these levels which are responsible for an additional far-infrared opacity not taken into account in the temperature retrieval. The cloud-top altitude would be about 0.3 bar except in the 20 to 40°N region where these clouds would be confined below the 0.6-bar level. The poor correlation between infrared measurements and visible images is discussed and a possible model of Saturn's cloud structure is proposed. 相似文献
2.
We present a new high-resolution map of thermal inertia derived from observations of planetary brightness temperature by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) obtained during the entire MGS primary mapping mission. Complete seasonal coverage provides a nearly global view of Mars, including the polar regions, at a spatial resolution of approximately 3 km. Our map of nighttime thermal-bolometer-based thermal inertia covers approximately 60% of the surface between 80° S and 80° N latitudes. We confirm the global pattern of high and low thermal inertia seen in lower resolution mapping efforts and provide greater detail concerning a third surface unit with intermediate values of both thermal inertia and albedo first identified by Mellon et al. 2000, Icarus 148, 437-455. Several smaller regional units with distinct characteristics are observed. Most notably, a unit of low thermal inertia () and low-to-intermediate albedo (0.09-0.22) dominates the region polewards of 65° S. We consider possible causes for these characteristics and conclude that a low-density mantle formed by desiccation of a previously ice-rich near-surface layer is the most likely explanation for the observed thermophysical properties. Global comparison of thermal inertia and elevation shows that high and low thermal inertia values can be found over a broad range of elevation, with only low values (30-) occurring at the highest elevations and the highest values occurring only at lower elevations. However, the lowest values () are found only at lower elevations, implying that the distribution of low thermal inertia material is not solely controlled by atmospheric pressure and the trapping of fines at high elevations. A new estimate of thermal inertia for the Viking and Pathfinder landing sites helps establish an important link between surface characteristics observed in situ and those derived from remote-sensing data. 相似文献
3.
We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C2H2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions. 相似文献
4.
Radiative control of surface temperature is a key characteristic of the martian environment and its low-density atmosphere. Here we show through meteorological modeling that surface temperature can be far from radiative equilibrium over numerous sloping terrains on Mars, where nighttime mesoscale katabatic winds impact the surface energy budget. Katabatic circulations induce both adiabatic atmospheric heating and enhancement of downward sensible heat flux, which then becomes comparable to radiative flux and acts to warm the ground. Through this mechanism, surface temperature can increase up to 20 K. One consequence is that warm signatures of surface temperature over slopes, observed through infrared spectrometry, cannot be systematically associated with contrasts of intrinsic soil thermal inertia. Apparent thermal inertia maps retrieved thus far possibly contain wind-induced structures. Another consequence is that surface temperature observations close to sloping terrains could allow the validation of model predictions for martian katabatic winds, provided contrasts in intrinsic thermal inertia can be ruled out. The thermal impact of winds is mostly discussed in this paper in the particular cases of Olympus Mons/Lycus Sulci and Terra Meridiani but is generally significant over any sloped terrains in low thermal inertia areas. It is even general enough to apply under daytime conditions, thereby providing a possible explanation for observed afternoon surface cooling, and to ice-covered terrains, thereby providing new insights on how winds could have shaped the present surface of Mars. 相似文献
5.
We identify mechanisms controlling the distribution of methane convection and large-scale circulation in a simplified, axisymmetric model atmosphere of Titan forced by gray radiation and moist (methane) convection. The large-scale overturning circulation, or Hadley cell, is global in latitudinal extent and provides fundamental control of precipitation and tropospheric winds. The precipitating, large-scale updraft regularly oscillates in latitude with seasons. The distance of greatest poleward excursion of the Hadley cell updraft is set by the mass of the convective layer of the atmosphere; convection efficiently communicates seasonal warming of the surface through the cold and dense lower atmosphere, increasing the heat capacity of the system. The presence of deep, precipitating convection introduces three effects relative to the case with no methane latent heating: (1) convection is narrowed and enhanced in the large-scale updraft of the Hadley cell; (2) the latitudinal amplitude of Hadley cell updraft oscillations is decreased; and (3) a time lag is introduced. These effects are observable in the location and timing of convective methane clouds in Titan’s atmosphere as a function of season. A comparison of simulations over a range of convective regimes with available observations suggest methane thermodynamic-dynamic feedback is important in the Titan climate. 相似文献
6.
The Galileo photopolarimeter–radiometer (PPR) made over 100 observations of Europa’s surface temperature. We have used these data to constrain a diurnal thermal model and, thus, map the thermal inertia and bolometric albedo over 20% of the surface. We find an increased thermal inertia at mid-latitudes that is widespread in longitude and does not appear to correlate with geology, albedo, or other observables. Our derived thermophysical properties can be used to predict volatile stability across the surface over the course of a day and in planning of infrared instruments on future missions. Furthermore, while observations in the thermal infrared can and have been used to find endogenic activity, no such activity was detected at Europa. We have calculated the detection limits of these PPR observations and find that 100 km2 hotspots with temperatures of 116–1200 K could exist undetected on the surface, depending on the location. 相似文献
7.
《Planetary and Space Science》2007,55(13):1990-2009
This study aims at interpreting the zonal and meridional wind in Titan's troposphere measured by the Huygens probe by means of a general circulation model. The numerical simulation elucidates the relative importance of the seasonal variation in the Hadley circulation and Saturn's gravitational tide in affecting the actual wind profile. The observed reversal of the zonal wind at two altitudes in the lower troposphere can be reproduced with this model only if the near-surface temperature profile is asymmetric about the equator and substantial seasonal redistribution of angular momentum by the variable Hadley circulation takes place. The meridional wind near the surface is mainly caused by the meridional pressure gradient and is thus a manifestation of the Hadley circulation. Southward meridional wind in the PBL (planetary boundary layer) is consistent with the near-surface temperature at the equator being lower than at mid southern latitudes. Even small changes in the radiative heating profile in the troposphere can substantially affect the mean zonal and meridional wind including their direction. Saturn's gravitational tide is rather weak at the Huygens site due to the proximity to the equator, and does not clearly manifest itself in the instantaneous vertical profile of wind. Nevertheless, the simulated descent trajectory is more consistent with the observation if the tide is present. Because of a different force balance in Titan's atmosphere from terrestrial conditions, PBL-specific wind systems like on Earth are unlikely to exist on Titan. 相似文献
8.
Robert E. Samuelson 《Planetary and Space Science》2003,51(2):127-145
Heating occurs in Titan's stratosphere from the absorption of incident solar radiation by methane and aerosols. About 10% of the incident sunlight reaches Titan's surface and causes heating there. Thermal radiation redistributes heat within the atmosphere and cools to space. The resulting vertical temperature profile is stable against convection and a state of radiative equilibrium is established. Equating theoretical and observed temperature profiles enables an empirical determination of the vertical distribution of thermal opacity. A uniformly mixed aerosol is responsible for most of the opacity in the stratosphere, whereas collision-induced absorption of gases is the main contributor in the troposphere. Occasional clouds are observed in the troposphere in spite of the large degrees of methane supersaturation found there. Photochemistry converts CH4 and N2 into more complex hydrocarbons and nitriles in the stratosphere and above. Thin ice clouds of trace organics are formed in the winter and early spring polar regions of the lower stratosphere. Precipitating ice particles serve as condensation sites for supersaturated methane vapor in the troposphere below, resulting in lowered methane degrees of supersaturation in the polar regions. Latitudinal variations of stratospheric temperature are seasonal, and lag instantaneous response to solar irradiation by about one season for two reasons: (1) an actual instantaneous thermal response to a latitudinal distribution of absorbing gases, themselves out of phase with the sun by about one season, and (2) a sluggish dynamical response of the stratosphere to the latitudinal transport of angular momentum, induced by radiative heating and cooling. Mean vertical abundances of stratospheric organics and aerosols are determined primarily by atmospheric chemistry and condensation, whereas latitudinal distributions are more influenced by meridional circulations. In addition to preferential scavenging by precipitating ice particles from above, the polar depletion of supersaturated methane results from periodic scavenging by short-lived tropospheric clouds, coupled with the steady poleward march of the continuously drying atmosphere due to meridional transport. 相似文献
9.
Spectra taken by Cassini’s Composite Infrared Spectrometer (CIRS) between 10 and 600 cm−1 (17-1000 μm) of surface thermal emission of Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus have been used to derive the thermal inertia and bolometric Bond albedo values. Only an upper limit for the bolometric Bond albedo of Iapetus’ dark leading side could be determined due to the insensitivity of the thermal model to albedo when albedos are very low. The thermal inertia in this region however is better constrained. The CIRS coverage of Enceladus is extensive enough that the latitudinal variation in these values from 60°S to 70°N has been determined in 10° wide bins. The bolometric Bond albedos determined here are consistent with literature values which show the surface of the saturnian icy moons to be covered in ice contaminated to varying degrees. The thermal inertia of the moons is shown to be in the range 9-, approximately 2-6 times lower than that of the Galilean satellites, implying a less well consolidated and more porous surface. The thermal inertias of Iapetus and Phoebe are somewhat higher, suggesting that the very low thermal inertias of satellites from Rhea inwards may be related to their probable coating of E-ring material. Latitudinal variations on the surface of Enceladus show that the bolometric Bond albedo and thermal inertia increase towards the active plume source at the south pole. 相似文献
10.
The energy balance at the surface of an airless planetary body is strongly influenced by the bolometric Bond albedo and the surface thermal inertia. Both of these values may be calculated through the application of a thermal model to measured surface temperatures. The accuracy of either, though, increases if the value of the other is better constrained. In this study, we used the improved global bolometric Bond albedo map of Iapetus derived from Cassini VIMS and ISS and Voyager ISS data in conjunction with Cassini CIRS temperature data to reevaluate surface thermal inertia across Iapetus. Results showed the thermal inertia of the dark terrain varies between 11 and 14.8 J m−2 K−1 s−1/2 while the light material varies between 15 and 25 J m−2 K−1 s−1/2. Using an approximation to the thermal properties of the dark overburden derived from our thermal inertia results, we can implement our thermal model to provide estimates on the dark material thickness, which was found to lie between 7 cm and 16 cm. In order to develop an accurate global thermal model, a weighted function that approximates the surface thermal inertia across Iapetus was developed and verified via our measurements. The global bolometric Bond albedo map, surface thermal inertia map, and the thermal model are then used to synthesize global temperature maps that may be used to study the stability of volatiles. 相似文献
11.
High spatial resolution images of Mars were acquired with the Advanced Electro-Optical System (AEOS) 3.63-meter telescope at the Maui Space Surveillance System (MSSS) during both the 2001 and 2003 Mars apparitions. Comparisons are made of the surface albedo patterns obtained from these AEOS images to the surface albedo maps constructed from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data taken during the same time periods. These comparisons demonstrate that the images provide albedo information in a limited area surrounding the sub-Earth point that is consistent with the TES-derived albedo field. Additionally, it is shown that by employing adaptive optics (AO), the typical ground-based observing season of Mars can be extended. This is the only known published AO data set of Mars with temporal coverage over an entire apparition. Changes in the surface albedo affect the local ground temperature, which impacts the depth of the planetary boundary layer (PBL) above the surface. Since it is the state of the PBL that controls surface/atmospheric interaction, albedo variations have the power to alter the amount of dust that is lifted. A one-dimensional radiative/convective version of the NASA Ames Mars General Circulation Model is used to demonstrate that the measured albedo variations can alter the daytime ground temperatures by as much as 5 K, which in turn alters the structure of the planetary boundary layer (PBL). Therefore, albedo changes are thermodynamically important, and the ability to characterize them, should orbital observations become unavailable, is a valuable capability. 相似文献
12.
We have used a 2-D microphysics model to study the effects of atmospheric motions on the albedo of Titan's thick haze layer. We compare our results to the observed variations of Titan's brightness with season and latitude. We use two wind fields; the first is a simple pole-to-pole Hadley cell that reverses twice a year. The second is based on the results of a preliminary Titan GCM. Seasonally varying wind fields, with horizontal velocities of about 1 cm sec-1 at optical depth unity, are capable of producing the observed change in geometric albedo of about 10% over the Titan year. Neither of the two wind fields can adequately reproduce the latitudinal distribution of reflectivity seen by Voyager. At visible wavelengths, where only haze opacity is important, upwelling produces darkening by increasing the particle size at optical depth unity. This is due to the suspension of larger particles as well as the lateral removal of smaller particles from the top of the atmosphere. At UV wavelengths and at 0.89 micrometers the albedo is determined by the competing effects of the gas the haze material. Gas is bright in the UV and dark at 0.89 micrometers. Haze transport at high altitudes controls the UV albedo and transport at low altitude controls the 0.89 micrometers albedo. Comparisons between the hemispheric contrast at UV, visible, and IR wavelengths can be diagnostic of the vertical structure of the wind field on Titan. 相似文献
13.
Tetsuya Tokano 《Icarus》2003,164(1):50-78
In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface-atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution is largely consistent with the modeled global water cycle, so a large fraction of hydrogen is likely to exist as water, at low and mid latitudes in the form of adsorbed water. Under the present climate the water content in the shallow subsurface becomes higher in the northern hemisphere than in the southern hemisphere as a result of global water cycle, regardless of the initial water distribution in the soil or adsorptive capacity. The higher annual maximum soil temperature in the south, stronger net northward transport of atmospheric water vapor, and the emission of vapor from the northern residual polar cap in northern summer contribute to this hemispheric asymmetry. The generally higher adsorptive capacity of clay minerals in the northern plains may further increase this bias. The longitudinal inhomogeneity is caused by several factors, such as thermal inertia, adsorptive capacity, and atmospheric surface pressure. The water abundance is locally high in low thermal inertia regions (e.g., Arabia Terra) and at deep places where the surface pressure is high (e.g., Hellas); it is low in soil with a low adsorptive capacity (e.g., Tharsis) and high thermal inertia regions (e.g., Solis Planum). Most of the soil humidity near the surface at low and mid latitudes may originate from the atmosphere. The model implies that the upper soil layer should be largely ice-free because otherwise an excessive sublimation and vapor emission into the atmosphere in warm seasons would violate the observational constraints. Moreover, the more uniform latitudinal variation of the observed hydrogen abundance near the surface compared to that of deeper layers is indicative of the presence of adsorbed water instead of ground ice because the adsorbed water content does not as steeply depend on latitude as the ground ice stability. Concerning the regolith mineralogy, montmorillonite can much better account for the observed water cycle than palagonite. While the presence of permanent ground ice appears likely in the polar region below a thin layer, large seasonal cycle of phase change between pore ice and adsorbed water may be possible. Regolith adsorption/desorption is neither negligible nor crucial for the seasonal atmospheric water cycle, but the surface-atmosphere coupling is a major prerequisite for the long-term evolution of subsurface water distribution. 相似文献
14.
Images of Mars in the visible to near-infrared acquired from 1996 to 2005 using the Hubble Space Telescope WFPC2 have been used to model the martian surface photometric function at 502, 673, 953, and 1042 nm. These data range in spatial resolution from 12 to 70 km/pixel at the sub-Earth point, and in phase angle coverage from 0.34° to 40.5°. The WFPC2 images have been calibrated to radiance factor or I/F and projected to a cylindrical map for coregistration and comparison to similarly mapped spacecraft data sets of albedo, topography, thermal inertia, composition, and geology. We modeled the observed I/F as a function of phase angle using Minnaert, Lambert, lunar-Lambert, and Hapke photometric functions for numerous regions of interest binned into albedo units defined by Viking and TES albedo maps, and thermal-inertia units defined by TES thermal-inertia maps. Visibly opaque water-ice clouds and data acquired under high dust opacity conditions were excluded from the analysis. Our modeling suggests that under average to low atmospheric dust opacity conditions and over this range of phase angles, the photometric properties of the martian surface at 502, 673, 953, and 1042 nm are best modeled by lunar-Lambert functions with parameters derived for three surface units defined by low, moderate, and high TES bolometric albedos. 相似文献
15.
Damon P. Samonelli 《Icarus》1983,54(3):524-538
Voyager 1 IRIS observations of Amalthea, although initially indicating an unusually high temperature, now give a temperature of only 164 ± 5°K, a value consistent with the Earth-based measurement by G. H. Rieke [Icarus25, 333–334 (1975)] of 155 ± 15°K. We numerically modeled the temperature profile in the satellite's surface layer as a function of location and time of day, assuming a triaxial ellipsoid shape and thermal properties similar to those of the lunar soil. The major heat source is direct insolation, but temperatures are increased slightly by thermal radiation from Jupiter (?9°K), by sunlight reflected from the planet (?5°K), and by charged particle bombardment (?2°K). Maximum calculated temperatures reach 166°K, and we estimate that the temperature that Voyager would have measured under these circumstances is ≈160°K, in agreement with the observed temperature. Possible sources of error in the model are discussed in detail, including satellite shape effects, unusually low emissivity, uncommonly rough surface, abnormal thermal intertia, variability of the charged particle flux, and Joule heating. The IRIS observation strongly suggests that (i) the Amalthean surface has an emissivity near unity; (ii) the charged particle flux on the satellite at the time of observation was no more than 20 times larger than the flux indicated by Pioneer observations; and (iii) Joule heating of the satellite is insignificant (a conclusion also supported by rough calculations). The IRIS observation cannot, however, put any useful limits on the thermal inertia of the Amalthean surface layer. 相似文献
16.
Current methods for deriving thermal inertia from spacecraft observations of planetary brightness temperature generally assume that surface properties are uniform for any given observation or co-located set of observations. As a result of this assumption and the nonlinear relationship between temperature and thermal inertia, sub-pixel horizontal heterogeneity may yield different apparent thermal inertia at different times of day or seasons. We examine the effects of horizontal heterogeneity on Mars by modeling the thermal behavior of various idealized mixed surfaces containing differing proportions of either dust, sand, duricrust, and rock or slope facets at different angles and azimuths. Latitudinal effects on mixed-surface thermal behavior are also investigated. We find large (several 100 J m−2 K−1 s−1/2) diurnal and seasonal variations in apparent thermal inertia even for small (∼10%) admixtures of materials with moderately contrasting thermal properties or slope angles. Together with similar results for layered surfaces [Mellon, M.T., Putzig, N.E., 2007. Lunar Planet. Sci. XXXVIII. Abstract 2184], this work shows that the effects of heterogeneity on the thermal behavior of the martian surface are substantial and may be expected to result in large variations in apparent thermal inertia as derived from spacecraft instruments. While our results caution against the over-interpretation of thermal inertia taken from median or average maps or derived from single temperature measurements, they also suggest the possibility of using a suite of apparent thermal inertia values derived from single observations over a range of times of day and seasons to constrain the heterogeneity of the martian surface. 相似文献
17.
A thermal regime of the troposphere of Venus is mainly determined by the greenhouse effect. A closeness of the real temperature gradient to the adiabatic one indicates that turbulent heat fluxes are also essential. Additional problems arise as only about 11% of the solar radiation absorbed by the planet reaches the surface, and most of it is taken up in the clouds at altitudes of 60–70 km. The present study summarizes experimental data on atmospheric parameters related to turbulence and estimates turbulent fluxes and turbulence characteristics. These data confirm the author's hypothesis of an anomalous downward turbulent heat flux in the free atmosphere. A normal upward turbulent heat flux exists in the planetary boundary layer. 相似文献
18.
Terry Z. Martin 《Icarus》1981,45(2):427-446
A Mars average data set (MADS) has been constructed from thermal and albedo measurements of the Viking Infrared Thermal Mapper; by merging information from all longitudes; and, ensuring reasonably complete longitudinal sampling, a representation of mean Mars behavior is obtained. Brightness temperatures at 7, 9, 11, 15, and 20 μm and albedo information in the band 0.3–3.0 μm have been binned using 2° latitude strips, 24 times of day, 3 emission angle intervals, and 23 nonoverlapping Ls periods covering 1.43 Mars years starting at Ls = 84°. The MADS is ideally suited to parametric study of latitudinal, diurnal, angular, and seasonal dependences. Data are presented for surface thermal and albedo behavior in clear and dusty atmospheric conditions; the thermal response of the atmospheric temperature to a major dust storm is found to be consistent with Mariner 9 data from the 1971 storm. Examples of use of the MADS, which is available through the Mars Consortium, indicate how averaged data reveal specific surface and atmospheric phenomena. 相似文献
19.
Scot C.R. Rafkin 《Planetary and Space Science》2012,60(1):147-154
A review of non-local, deep transport mechanisms in the atmosphere of Earth provides a good foundation for examining whether similar mechanisms are operating in the atmospheres of Mars and Titan. On Earth, deep convective clouds in the tropics constitute the upward branch of the Hadley Cell and provide a conduit through which energy, moisture, momentum, aerosols, and chemical species are moved from the boundary layer to the upper troposphere and lower stratosphere. This transport produces mid-tropospheric minima in quantities such as water vapor and moist static energy and maxima where the clouds detrain. Analogs to this terrestrial transport are found in the strong and deep thermal circulations associated with topography on Mars and with Mars dust storms. Observations of elevated dust layers on Mars further support the notion that non-local deep transport is an important mechanism in the atmosphere of Mars. On Titan, the presence of deep convective clouds almost assures that non-local, deep transport is occurring and these clouds may play a role in global cycling of energy, momentum, and methane. Based on the potential importance of non-local deep transport in Earth's atmosphere and supported by evidence for such transport in the atmospheres of Mars and Titan, greater attention to this mechanism in extraterrestrial atmospheres is warranted. 相似文献
20.
Masaru Yamamoto 《Icarus》2011,211(2):993-1006
Heat and material transport processes caused by convective adjustment and mixing are important in modeling of Venus’ atmosphere. In the present study, microscale atmospheric simulations near the venusian surface were conducted using a Weather Research and Forecasting model to elucidate the thermal and material transport processes of convective adjustment and mixing. When convective adjustment occurs, the heat and passive tracer are rapidly mixed into the upper stable layer with convective penetration. The convective adjustment produces large eddy diffusions of heat and passive tracer, which may explain the large eddy diffusions estimated in the radiative-convective equilibrium model.For values of surface heat flux Q greater than a threshold (=0.064 K m s−1 in the present study), the convectively mixed layer with high eddy diffusion coefficients grows with time. In contrast, the mixed layer decays with time for Q values smaller than the threshold. The thermal structure near the surface is controlled not only by extremely long-term radiative processes, but also by microscale dynamics with time scales of several hours. A mixed layer with high eddy diffusion coefficients may be maintained or grow with time if the surface heat flux is high in the volcanic hotspot and adjacent areas. 相似文献