共查询到20条相似文献，搜索用时 31 毫秒
1.
A. Le Gall M.A. Janssen A.G. Hayes C. Savage R.D. Lorenz R.L. Kirk S. Wall E.R. Stofan the Cassini Radar Team 《Icarus》2011,213(2):608-624
Large expanses of linear dunes cover Titan’s equatorial regions. As the Cassini mission continues, more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation (SAR, radiometry, scatterometry, altimetry) and with an increasing variety of observational geometries. In this paper, we report on Cassini’s radar instrument observations of the dune fields mapped through May 2009 and present our key findings in terms of Titan’s geology and climate. We estimate that dune fields cover ∼12.5% of Titan’s surface, which corresponds to an area of ∼10 million km2, roughly the area of the United States. If dune sand-sized particles are mainly composed of solid organics as suggested by VIMS observations (Cassini Visual and Infrared Mapping Spectrometer) and atmospheric modeling and supported by radiometry data, dune fields are the largest known organic reservoir on Titan. Dune regions are, with the exception of the polar lakes and seas, the least reflective and most emissive features on this moon. Interestingly, we also find a latitudinal dependence in the dune field microwave properties: up to a latitude of ∼11°, dune fields tend to become less emissive and brighter as one moves northward. Above ∼11° this trend is reversed. The microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion (relative to that of the dune), roughness and degree of sand cover. In agreement with radiometry and scatterometry observations, SAR images suggest that the fraction of interdunes increases northward up to a latitude of ∼14°. In general, scattering from the subsurface (volume scattering and surface scattering from buried interfaces) makes interdunal regions brighter than the dunes. The observed latitudinal trend may therefore also be partially caused by a gradual thinning of the interdunal sand cover or surrounding sand sheets to the north, thus allowing wave penetration in the underlying substrate. Altimetry measurements over dunes have highlighted a region located in the Fensal dune field (∼5° latitude) where the icy bedrock of Titan is likely exposed within smooth interdune areas. The hemispherical assymetry of dune field properties may point to a general reduction in the availability of sediments and/or an increase in the ground humidity toward the north, which could be related to Titan’s asymmetric seasonal polar insolation. Alternatively, it may indicate that either the wind pattern or the topography is less favorable for dune formation in Titan’s northern tropics. 相似文献
2.
All previous observations of seasonal change on Titan have been of physical phenomena such as clouds and haze. We present here the first observational evidence of chemical change in Titan's atmosphere. Images taken during 1999-2002 (late southern spring on Titan) with the W.M. Keck I 10-meter telescope at 8-13 μm show a significant accumulation of ethylene (C2H4) in the south polar stratosphere as well as north-south stratospheric temperature variation (colder at poles). Our observations restrict this newly discovered south polar ethylene accumulation to latitudes south of 60° S. The only other observations of the spatial distribution of C2H4 were those of Voyager I, which found a significant north polar accumulation in early northern spring. We see no build-up in the north, although the highest northern latitudes are obstructed from view in the current season. Our observations constrain any unobserved north polar accumulation of C2H4 to north of 50° N latitude. Comparison of the Voyager I results with our new results show seasonal chemical change has occurred in Titan's atmosphere. 相似文献
3.
An investigation of the Martian polar cap winds and their response to a variety of factors is carried out by a series of numerical experiments based on a zonally symmetric primitive equation model. These factors are the seasonal thermal forcing, mass exchange between polar caps and atmosphere, large-scale topography, and polar cap size. The thermal forcing sets up a circulation whose surface winds adjust to achieve angular momentum balance, with low-latitude easterlies and high-latitude westerlies. The maximum westerlies occur roughly where the horizontal temperature gradients are largest. This pattern changes when cap and atmosphere exchange mass. Corriolis forces acting on the net outflow or inflow produce easterlies at the surface during spring (outflow) and westerlies during winter (inflow). Topography appears to have a small effect, but cap size does play a role, the circulation intensity increasing with cap size. Peak surface winds occur when outflow or inflow is a maximum and are 20 m sec?1 during spring and 30 m sec?1 during winter for the northern hemisphere. The model results show that surface winds near the edge of a retreating polar cap are substantially enhanced, a result which is consistent with the Viking observations of local dust storm activity near the edge of the south polar cap during spring. The results also indicate that the surficial wind indicators near the south pole are formed during spring and those near the north pole during winter. The implication is that the high-latitude dune fields in the northern hemisphere are formed at a time when the terrain is being covered with frost. It is therefore suggested that the saltating particles are “snowflakes” which have formed by the mechanism proposed by Pollack etal. The model results for the winter simulation, which have formed by the mechanism transport by large-scale eddies, compare favorably with general circulation model (GCM) calculations. This suggests that the eddy transports may be less important than those associated with the net mass flow, and that 2-D climate modeling may be more succesful for Mars than Earth. 相似文献
4.
A. Le Gall A.G. Hayes R. Ewing M.A. Janssen J. Radebaugh C. Savage P. Encrenaz 《Icarus》2012,217(1):231-242
Dune fields dominate ～13% of Titan’s surface and represent an important sink of carbon in the methane cycle. Herein, we discuss correlations in dune morphometry with altitude and latitude. These correlations, which have important implications in terms of geological processes and climate on Titan, are investigated through the microwave electromagnetic signatures of dune fields using Cassini radar and radiometry observations. The backscatter and emissivity from Titan’s dune terrains are primarily controlled by the amount of interdune area within the radar footprint and are also expected to vary with the degree of the interdunal sand cover. Using SAR-derived topography, we find that Titan’s main dune fields (Shangri-La, Fensal, Belet and Aztlan) tend to occupy the lowest elevation areas in Equatorial regions occurring at mean elevations between ～?400 and ～0 m (relative to the geoid). In elevated dune terrains, we show a definite trend towards a smaller dune to interdune ratio and possibly a thinner sand cover in the interdune areas. A similar correlation is observed with latitude, suggesting that the quantity of windblown sand in the dune fields tends to decrease as one moves farther north. The altitudinal trend among Titan’s sand seas is consistent with the idea that sediment source zones most probably occur in lowlands, which would reduce the sand supply toward elevated regions. The latitudinal preference could result from a gradual increase in dampness with latitude due to the asymmetric seasonal forcing associated with Titan’s current orbital configuration unless it is indicative of a latitudinal preference in the sand source distribution or wind transport capacity. 相似文献
5.
We present new in situ measurements of solar wind electron density as a function of heliolatitude. The data were obtained on Ulysses during its fast transit from south solar pole to north solar pole, at heliocentric distance about 1.5 AU, near the 1996 solar activity minimum. The density is measured accurately using the method of quasi-thermal noise spectroscopy with the Ulysses radio experiment, at a higher time resolution than the particle analysers on board. At low heliolatitudes (22° S to 21° N) the histogram of our data shows three main classes of flows with densities centered at 3.5, 7, and 12 cm-3, close to the values previously found by near-ecliptic space probes, in the region where fast coronal hole wind alternates with slower streamer belt wind. Poleward of 22° latitude where Ulysses encountered fast wind coming from coronal holes, the histogram of our data shows a single class of flow centered at 2.9 cm-3 with a roughly normal distribution. We find a density nearly independent of latitude, with the mean density from the south coronal hole 10% larger than that from the north, which may stem from a genuine north/south asymmetry and/or from the small decrease in solar activity during the time of the observations. We finally compare the data with some analytical models. 相似文献
6.
A spatiotemporal analysis of long-term measurements of the Sun’s magnetic field was carried out to study changes in its zonal structure and reversals of the polar fields in Cycles 21?–?24. A causal relationship between activity complexes, their remnant magnetic fields, and high-latitude magnetic fields has been demonstrated in the current cycle. The appearance of unipolar magnetic regions near the poles is largely determined by the decay of long-lived activity complexes. The nonuniform distribution of sunspot activity and its north–south asymmetry result in the asymmetry of remnant fields that are transported poleward due to meridional circulation. The asymmetry of high-latitude magnetic fields leads to an asynchrony of polar-field reversals in both hemispheres. The interaction of high-latitude unipolar magnetic regions with the polar fields affects the embedded coronal holes. The evolution of large-scale magnetic fields was also studied in a time–latitude aspect. It is shown that regular reversals of the Sun’s polar fields resulted from cyclic changes in high-latitude magnetic fields. A triple polarity reversal of the polar fields in Cycle 21 and short-term polarity alternations at the poles were interpreted taking into account the interaction of the remnant fields with the Sun’s polar fields. 相似文献
7.
Over an 18-year period, seasonal changes in the north-south asymmetry of polarization at high latitudes of Jupiter have been revealed by polarimetric observations in blue light. The average seasonal difference in the polarization degree between north and south is positive and equal to about 0.5%. There is some relationship between seasonal variations in the observed difference and the seasonal north-south asymmetry in solar radiation incident on Jupiter's atmosphere. There are two maxima on the observed seasonal curve, falling on the jovian spring and autumn and coinciding correspondingly with positive and negative maxima of the heliographic latitude of Jupiter. Two possible explanations are discussed: seasonal changes in insolation and/or time-dependent magnetospheric influence on the polar events. 相似文献
8.
We use data from the VIMS instrument on board the Cassini spacecraft to construct high sensitivity and high spatial-resolution maps of the locations of tropospheric clouds on Titan in the late northern winter season during which the Cassini prime mission took place. These observations show that, in this season, clouds on Titan are strongly hemispherically asymmetric. Mid-latitude clouds, in particular, occur only in the southern hemisphere and have not ever been observed in the north. Such an asymmetry is in general agreement with circulation models where sub-solar surface heating controls the locations of clouds and appears in conflict with models where perennial polar hazes prevent significant summertime polar heating from affecting the circulation. The southern mid-latitude clouds appear to be distributed uniformly in longitude, in contrast to some previous observations. Southern high-latitude clouds exhibit a significant concentration, however, between about 180° and 270°E longitude. A spatially and temporally uniform cloud always appears northward of ∼50°N latitude. This cloud appears unchanged over the course of the observations, consistent with the interpretation that it is caused by continuous ethane condensation as air subsides and radiatively cools through the tropopause. The location of this cloud likely provides a direct tracer of elements of north polar atmospheric circulation, potentially allowing continuous monitoring of circulation changes as Titan passes through equinox into north polar spring and summer. We show that a similar analysis of this dataset by Rodriguez et al. (2009) contains substantial errors and should not be used. 相似文献
9.
We present the solar wind plasma parameters obtained from the Ulysses spacecraft during its second pole-to-pole fast latitude scan near the 2001 solar maximum. We study the solar wind properties
from the electron density and core temperature measurements made by the radio receiver on Ulysses using the method of quasi-thermal noise spectroscopy. We analyze these parameters as functions of heliographic latitude and
distance. We present their histograms normalized to 1 AU and find a bimodal distribution for the electron core temperature.
The cooler population can be associated with the fast wind flow emanating from coronal holes present at various latitudes.
We discuss a slight north/south asymmetry found for the electron density. Finally, we compare the present results to those
obtained during the 1996 solar minimum and 1991 solar maximum. 相似文献
10.
Photospheric magnetic fields were studied using the Kitt Peak synoptic maps for 1976?–?2003. Only strong magnetic fields (B>100 G) of the equatorial region were taken into account. The north–south asymmetry of the magnetic fluxes was considered as well as the imbalance between positive and negative fluxes. The north–south asymmetry displays a regular alternation of the dominant hemisphere during the solar cycle: the northern hemisphere dominated in the ascending phase, the southern one in the descending phase during Solar Cycles 21?–?23. The sign of the imbalance did not change during the 11 years from one polar-field reversal to the next and always coincided with the sign of the Sun’s polar magnetic field in the northern hemisphere. The dominant sign of leading sunspots in one of the hemispheres determines the sign of the magnetic-flux imbalance. The sign of the north–south asymmetry of the magnetic fluxes and the sign of the imbalance of the positive and the negative fluxes are related to the quarter of the 22-year magnetic cycle where the magnetic configuration of the Sun remains constant (from the minimum where the sunspot sign changes according to Hale’s law to the magnetic-field reversal and from the reversal to the minimum). The sign of the north–south asymmetry for the time interval considered was determined by the phase of the 11-year cycle (before or after the reversal); the sign of the imbalance of the positive and the negative fluxes depends on both the phase of the 11-year cycle and on the parity of the solar cycle. The results obtained demonstrate the connection of the magnetic fields in active regions with the Sun’s polar magnetic field in the northern hemisphere. 相似文献
11.
Dunes in the Martian north polar erg show two dominant orientations. When seen at frost cap minimum, dunes north of 80°N record east winds, dunes south of 80°N record west winds. Many of the transverse dunes are considered to be reversing dunes. Dunes in two fields may have reversed at least once during the lifetime of the Viking Orbiters. Poor agreement exists among published predictive models of north polar winds and the interpretations derived from the major published map of the north polar dunes. We propose that the average polar winds are: (1) strong, off-pole northwest winds in fall; (2) moderate west winds in winter; (3) latitude-dependent weak to strong off-pole northeast winds in spring; and (4) weak west winds in summer. Viking images of near-polar clouds confirm much of the hypothesis. Images discussed in other studies can be given alternative interpretations that support this hypothesis also. Over millenia, the combination of reversing west and east winds could produce the binodal distributions of dune orientations observed at the north pole. 相似文献
12.
Yehouda Enzel Rivka Amit Uri Dayan Onn Crouvi Ron Kahana Baruch Ziv David Sharon 《Global and Planetary Change》2008,60(3-4):165-192
Modern-day synoptic-scale eastern Mediterranean climatology provides a useful context to synthesize the diverse late Pleistocene (60–12 ka) paleohydrologic and paleoenvironmental indicators of past climatic conditions in the Levant and the deserts to its south and east. We first critically evaluate, extract, and summarize paleoenvironmental and paleohydrologic records. Then, we propose a framework of eastern Mediterranean atmospheric circulation features interacting with the morphology and location of the southeast Mediterranean coast. Together they strongly control the spatial distribution of rainfall and wind pattern. This cyclone–physiography interaction enforces the observed rainfall patterns by hampering rainfall generation south and southeast of the latitude of the north Sinai coast, currently at 31°15′.The proposed framework explains the much-increased rains in Lebanon and northern Israel and Jordan as deduced from pollen, rise and maintenance of Lake Lisan, and speleothem formation in areas currently arid and semiarid. The proposed framework also accounts for the southward and eastward transition into semiarid, arid, and hyperarid deserts as expressed in thick loess accumulation at the deserts' margins, dune migration from west to east in the Sinai and the western Negev, and the formation of hyperarid (< 80 mm yr− 1) gypsic–salic soils in the southern Negev and Sinai. Our climatic synthesis explains the hyperarid condition in the southern Negev, located only 200–250 km south of the much-increased rains in the north, probably reflecting a steeper rainfall gradient than the present-day gradient from the wetter Levant into its bordering southern and eastern deserts.At present, the rainiest winter seasons in Lebanon and northern and central Israel are associated with more frequent (+ 20%), deeper Cyprus Lows traversing the eastern Mediterranean at approximately the latitude of southern Turkey. Even these wettest years in northern Israel do not yield above average annual rainfall amounts in the hyperarid southern Negev. This region is mainly influenced by the Active Red Sea Troughs that produce only localized rains. The eastern Mediterranean Cyprus Lows also produce more dust storms and transport higher amounts of suspended dust to the loess area than any other atmospheric pattern. Concurrent rainfall and dust are essential to the late Pleistocene formation of the elongated thick loess zone along the desert northern margin. Even with existing dust storms, the lack of rain and very sparse vegetation account for the absence of late Pleistocene loess sequences from the southern Negev and the formation of hyperarid soils.When the north Sinai coast shifted 30–70 km northwest due to last glacial global sea level lowering, the newly exposed coastal areas supplied the sand and dust to these active eastern Mediterranean cyclones. This enforced the latitude of the northern boundary of the loess zone to be directly due east of the LGM shoreline. This shift of coast to the northwest inhibited rainfall in the southern Levant deserts and maintained their hyperaridity. Concurrently, frequent deep eastern Mediterranean Cyprus Lows were funneled along the northern Mediterranean increasing (probably doubling) the rains in central and northern Israel, Lebanon, southwestern Syria and northern Jordan. These storms and rains formed lakes, forests, and speleothems only a short distance north of the deserts in the southern Levant. 相似文献
13.
N. Mateshvili D. Fussen F. Vanhellemont C. Bingen E. Dekemper N. Loodts C. Tetard 《Planetary and Space Science》2009,57(8-9):1022-1031
The SPICAM instrument onboard Mars Express has successfully performed two Martian years (MY 27 and MY28) of observations. Water ice cloud optical depths spatial and temporal distribution was retrieved from nadir measurements in the wavelength range 300–320 nm. During the northern spring the cloud hazes complex distribution was monitored. The clouds in the southern hemisphere formed a zonal belt in the latitude range 30–60°S. The edge of the retreating north polar hood merged with the northern tropical clouds in the range 250–350°E. The development of the aphelion cloud belt (ACB) started with the weak hazes formation (cloud optical thickness 0.1–0.3) in the equatorial region. At the end of the northern spring, the ACB cloud optical thickness reached already values of 0.3–1. The ACB decay in the end of the northern summer was accompanied with a presence of clouds in the north mid-latitudes. The expanded north polar hood merged with the north mid-latitude clouds in the eastern hemisphere. The interannual comparison indicates a decrease in cloud activity immediately after a strong dust storm in southern summer of MY28. The strong dust storms of the MY28 may also be a reason of the observed north polar hood edge shifting northward by 5°. 相似文献
14.
The Mariner 9 Ultraviolet Spectrometer has observed the 2550 Å ozone spectral absorption feature on Mars. This absorption was previously detected in the south polar region by Mariner 7 in 1969. Mariner 9 did not observe ozone at any time in the equatorial region, nor at the south polar cap during its summer season. However, ozone was found in the north polar region beginning at a latitude of 45°N and extending northward. Ozone later appeared in the southern hemisphere southward of 50°S as the Mars autumnal equinox approached. The presence of ozone on Mars seems to be coupled to the water vapor content of its atmosphere. 相似文献
15.
Abstract– We examine Martian northern high‐latitude and polar impact craters (NPICs) to better understand the north polar materials and polar processes. We examine topographic characteristics for 346 NPICs and compare them to global fit data (e.g., Garvin et al. 2003 ; Boyce and Garbeil 2007 ) as well as to a small set (N = 92) of southern high‐latitude and polar impact craters (SPICs). We find that the NPIC population above 57° N is significantly shallower than the global crater population. This suggests that the NPICs (1) were initially shallow due to target properties of polar geologic units; (2) were once deeper, but have been infilled due to polar processes; or (3) a combination of both. Indeed, many of the NPICs exhibit considerable noncentral peak interior topographic features (IFTs), which may be indicative of infilling processes. The NPIC IFTs also appear to display trends in their preferential orientation within the crater cavity; some SPICs display similar interior features, but do not show a clear preference in their orientation within the crater cavity. In addition, the NPIC population displays cavity wall slope trends that seem to indicate steepening of slopes with increasing crater diameter in comparison to the global slope trend ( Garvin et al. 2003 ). These trends suggest that the NPICs are unique in their geometry when compared to the global data set as well as with the SPICs further indicating that the north polar region may exhibit target properties and polar processes not seen in the south polar region or elsewhere on Mars. 相似文献
16.
Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic maps from Kitt Peak are used to analyze the polar coronal holes of solar activity cycles 22 and 23 (from 1990 to end of 2003). In the beginning of the declining phase of solar cycles 22 and 23, the north polar coronal holes (PCHs) appear about one year earlier than the ones in the south polar region. The solar wind velocity and the solar wind ionic charge composition exhibit a characteristic dependence on the solar wind source position within a PCH. From the center toward the boundary of a young PCH, the solar wind velocity decreases, coinciding with a shift of the ionic charge composition toward higher charge states. However, for an old PCH, the ionic charge composition does not show any obvious change, although the latitude evolution of the velocity is similar to that of a young PCH. 相似文献
17.
O. Prakash S. Umapathy A. Shanmugaraju P. Pappa kalaivani Bojan Vršnak 《Solar physics》2010,266(1):135-147
Solar coronal holes (CHs) are large regions of the corona magnetically open to interplanetary space. The nearly rigid north?–?south CH boundaries (CHBs) of equatorward extensions of polar CHs are maintained while the underlying photospheric fields rotate differentially, so interchange magnetic reconnection is presumed to be occurring continually at the CHBs. The time and size scales of the required reconnection events at CHBs have not been established from previous observations with soft X-ray images. We use TRACE 195 Å observations on 9 December 2000 of a long-lived equatorial extension of the negative-polarity north polar CH to look for changes of ??5 arcsec to >?20 arcsec at the western CHB. Brightenings and dimmings are observed on both short (≈?5 minutes) and long (≈?7 hours) time scales, but the CHB maintains its quasi-rigid location. The transient CHB changes do not appear associated with either magnetic field enhancements or the changes in those field enhancements observed in magnetograms from the Michelson Doppler Imager (MDI) on SOHO. In seven hours of TRACE observations we find no examples of the energetic jets similar to those observed to occur in magnetic reconnection in polar plumes. The lack of dramatic changes in the diffuse CHB implies that gradual magnetic reconnection occurs high in the corona with large (??10°) loops and/or weak coronal fields. We compare our results with recent observations of active regions at CHBs. We also discuss how the magnetic polarity symmetry surrounding quasi-rigid CHs implies an asymmetry in the interchange reconnection process and a possible asymmetry in the solar wind composition from the eastern and western CHB source regions. 相似文献
18.
The interaction between the geomagnetic and interplanetary magnetic fields is studied through its effects upon the intensities of solar electrons reaching the polar caps during times of strongly anisotropic electron fluxes in the magnetosheath. During the particle event of 18 November 1968, electrons of solar origin were observed outside the magnetopause with detectors aboard OGO-5. This is the only case on record for which high resolution directional flux observations are available for determining in detail the electron angular distribution, and thus the electron density in the magnetosheath. Correlative studies of these satellite observations and concurrent measurements by riometers and ionospheric forward scatter systems in both polar regions have revealed that the initial stage of the associated Polar Cap Absorption event is attributable to the prompt arrival of solar electrons. The electron flux precipitating into the south polar region was equal to or larger than the mean directional flux in interplanetary space, whereas over the north pole it was equal to or less than the backscattered flux. This evidence of a north-south asymmetry in the solar electron flux at a time when the interplanetary magnetic field vector was nearly parallel with the ecliptic plane supports an open magnetospheric model. The ratio of particle intensities in the High Polar Latitude and Low Polar Latitude regions in the southern hemisphere is consistent with that determined at times when the interplanetary electron fluxes were isotropic. The analysis indicates that an anisotropic electron flux may be isotropized at the magnetopause before propagating into the polar regions. 相似文献
19.
C. T. Russell 《Solar physics》1975,42(1):259-269
While at present we are able to deduce from ground records only qualitative properties of the solar wind, in the future quantitative deductions may be possible, in a statistical sense, from an examination of polar cap magnetograms together with records of geomagnetic activity. The qualitative inferences that are possible now indicate several important features of the behavior of the solar wind over the last 100 years. First, there appear to be significant long term changes in either the solar wind velocity, the magnetic field strength, the variability of the field or some combination of all three. Second, a heliographic latitude dependence of these parameters exists, whose amplitude depends on sunspot number. Third, with the exception of the most recent solar cycle, there is little north-south asymmetry in these solar parameters. Finally, there is a double sunspot cycle modulation of geomagnetic activity, the most likely cause of which is a modulation of the interplanetary magnetic polarity with latitude, and which in turn implies the presence of a solar polar magnetic dipole. The amplitude of this modulation has undergone significant changes since 1868, being large then and at the present, but effectively disappearing from 1908 to 1948. 相似文献
20.
Peter M. Woiceshyn 《Icarus》1974,22(3):325-344
The Mariner 9 S-band radio occultation measurements, which were taken over half a Martian year, were examined for seasonal variations in atmospheric pressures and temperatures. Seasonally related atmospheric pressure oscillations on a global scale were discovered when the pressures were compared on equi-potential levels. There was a global increase in pressure of about 13% between northern winter and spring seasons, and a global decrease in pressure of nearly 14% between northern spring and summer seasons. The maximum global pressure occurred during the northern spring season approximately one Martian month prior to aphelion. These pressure oscillations were correlated with the seasonal growth and decay, and the total area of the polar caps.Temperatures in the mid-latitude regions near the subsolar points were highest during the northern winter season when Mars was closest to the sun. In addition, high latitudinal temperature gradients (up to 2°K per degree latitude) were found. This has important atmospheric dynamical implications, especially for the growth of baroclinic waves.Occultation observations also indicated that the average elevation of the southern hemisphere was nearly 4km higher than the northern hemisphere when referenced to an equipotential level. The occultation measurements showed that the atmospheric pressures near the surface in the southern hemisphere were 33 to 43% lower than the atmospheric pressures near the surface in the northern hemisphere. In addition to other parameters, the asymmetry in the density of the Martian atmosphere and the hemispheric altitude differences are important in understanding the seasonal dynamic processes that exist in the polar cap regions and in the Martian atmosphere generally. 相似文献