共查询到20条相似文献，搜索用时 31 毫秒
1.
《Planetary and Space Science》2007,55(12):1741-1756
The dynamics of Venus’ mesosphere (70–110 km) is characterized by the superposition of two different wind regimes: (1) Venus’ retrograde superrotation; (2) a sub-solar to anti-solar (SS–AS) flow pattern, driven by solar EUV heating on the sunlit hemisphere. Here, we report on new ground-based velocity measurements in the lower part of the mesosphere. We took advantage of two essentially symmetric Venus elongations in 2001 and 2002 to perform high-resolution Doppler spectroscopy (R=120,000) in 12C16O2 visible lines of the 5ν3 band and in a few solar Fraunhofer lines near 8700 Å. These measurements, mapped over several points on Venus’ illuminated hemisphere, probe the region of cloud tops. More precisely, the solar Fraunhofer lines sample levels a few kilometers below the UV features (i.e. near ∼67 km), while the CO2 lines probe an altitude higher by about 7 km. The wind field over Venus’ disk is retrieved with an rms uncertainty of 15–25 m s−1 on individual measurements. Kinematical fit to a one- or two-component circulation model indicates the dominance of the zonal retrograde flow with a mean equatorial velocity of ∼75 m s−1, exhibiting very strong day-to-day variations (±65 m s−1). Results are very consistent for the two kinds of lines, suggesting a negligible vertical wind shear over 67–74 km. The SS–AS flow is not detected in single-day observations, but combining the results from all data suggests that this component may invade the lower mesosphere with a ∼40 m s−1 velocity. 相似文献
2.
《New Astronomy》2014
Ultraviolet spectra of FK Comae and V1794 Cygni observed with the Hubble Space Telescope Cosmic Origins Spectrograph (HST COS) and the International Ultraviolet Explorer (IUE) satellites were analyzed for the period 1981–2011. Temporal variations of line fluxes of the O I 1306 Å, C II 1336 Å, C IV 1550 Å, He II 1640 Å and Mg II k & h 2800 Å, produced in the transition regions and chromospheres of these stars, imply variations in density and temperature changes in the line emitting regions as a result of the rapid rotation and magnetic fields responsible for stellar activity. Results are consistent with the models of Ramsey et al. (1981), Oliveira and Foing (1999), and Korhonen et al. (2000). 相似文献
3.
An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ～10 m s?1 weaker than cloud level winds in the cores of eastward jets and ～5 m s?1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2 × 10?5 m2 s?3) and smaller in westward jet regions (1.6 × 10?5 m2 s?3) than the global mean value (4.1 × 10?5 m2 s?3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level. 相似文献
4.
《New Astronomy》2017
We report the results of spectral and photometric observations of SU UMa. Available spectra from International Ultraviolet Explorer (IUE) and CCD photometry of five nights were used. Some profiles revealing the variations of some spectral lines at different times are presented. There is variation with time for photometric observations and the brightness of SU UMa is changed from 12.9 mag. to 15.6 mag in 5 nights with clear display of burst. The ultraviolet luminosity for emitting region is in the range of (6.9*1030 erg s−1). The mass accretion rate is in the range of (9.8*10−13 M⊙yr−1). The line flux modulations at different times can be explained in terms of the mass transfer instability model (Bath 1973). 相似文献
5.
《New Astronomy》2014
Among evolved massive stars likely in transition to the Wolf–Rayet phase, IRC + 10420 is probably one of the most enigmatic. It belongs to the category of yellow hypergiants and it is characterized by quite high mass loss episodes. Even though IRC + 10420 benefited of many observations in several wavelength domains, it has never been a target for an X-ray observatory. We report here on the very first dedicated observation of IRC + 10420 in X-rays, using the XMM-Newton satellite. Even though the target is not detected, we derive X-ray flux upper limits of the order of 1–3 × 10−14 erg cm−2 s−1 (between 0.3 and 10.0 keV), and we discuss the case of IRC + 10420 in the framework of emission models likely to be adequate for such an object. Using the Optical/UV Monitor on board XMM-Newton, we present the very first upper limits of the flux density of IRC + 10420 in the UV domain (between 1800 and 2250 Å and between 2050 and 2450 Å). Finally, we also report on the detection in this field of 10 X-ray and 7 UV point sources, and we briefly discuss their properties and potential counterparts at longer wavelengths. 相似文献
6.
《New Astronomy》2007,12(6):435-440
A detailed analysis of emission lines of carbon-like silicon reveals that some ratios of n = 3 → 2 line intensities are sensitive to the electron density, ne. The ratio between two groups of 3d → 2p transition lines of 55.246 Å and 55.346 Å provides a good diagnostic of ne because of the combined characteristic of sensitivity to electron density and relative insensitivity to temperature. From this ratio, a lower limit of the electron density of 0.6 × 108 cm−3 was set for Procyon, which is consistent with the values constrained by C V and Si X emission lines. Significant discrepancies were found between theoretical predictions and observations for the 3s → 2p lines relative to 3d → 2p lines in Procyon, recently measured using the Chandra high-resolution transmission grating instrument. The difference of more than a factor of 3, cannot be explained by uncertainties of atomic data. Ness and co-workers also suggested that the effect of opacity appeared not to be a major factor for the discrepancy. For the 3s → 2p line at 61.611 Å, present work indicates that the large discrepancy may be from the contamination of a S VIII line at 61.645 Å. For lines at 61.702 and 61.846 Å, we suggest that the discrepancies may be attributed to contaminations by currently yet-unknown spectral lines. 相似文献
7.
《Astroparticle Physics》2009,30(6):366-372
We present results of a search for relativistic magnetic monopoles with the Baikal neutrino telescope NT200, using data taken between April 1998 and February 2003. No monopole candidates have been found. We set an upper limit 4.6 × 10−17 cm−2 s−1 sr−1 for the flux of monopoles with βm = 1. This is a factor of 20 below the Chudakov–Parker bound which is inferred from the very existence of large-scale galactic magnetic fields. 相似文献
8.
《Planetary and Space Science》2007,55(11):1494-1501
In this work, we calculate the neutral Na production rates on the Moon and Mercury, as due to the impacts of meteoroids having an impact probability on the surface that can influence the daily observations of the exosphere: the meteoroids radius range considered for the Moon and Mercury are 10−8–0.15 and 10−8–0.10 m, respectively. We also estimate the mass of meteoroids that has impacted the surfaces of the Moon and Mercury in the last 3.8 Gy (after the end of the Late Heavy Bombardment).The results of our model are that (i) the Na production rates are ∼(3–4.9)×104 and ∼(1.8–2.3)×106 atoms cm−2 s−1, for Moon and Mercury, respectively, and (ii) in the last 3.8 Gy, the mass of meteoroids that has impacted the whole surface of the Moon and Mercury has been 8.86×1018 and 2.66×1019 g, respectively. 相似文献
9.
We describe the science motivation and development of a pair production telescope for medium-energy (∼5–200 MeV) gamma-ray polarimetry. Our instrument concept, the Advanced Energetic Pair Telescope (AdEPT), takes advantage of the Three-Dimensional Track Imager, a low-density gaseous time projection chamber, to achieve angular resolution within a factor of two of the pair production kinematics limit (∼0.6° at 70 MeV), continuum sensitivity comparable with the Fermi-LAT front detector (<3 × 10−6 MeV cm−2 s−1 at 70 MeV), and minimum detectable polarization less than 10% for a 10 mCrab source in 106 s. 相似文献
10.
To ascertain the importance of sputtering by solar wind ions on the formation of a sodium exosphere around Mercury and the Moon, we have irradiated with 4 keV He ions, the Na bearing tectosilicates: albite, labradorite, and anorthoclase, as well as adsorbed Na layers deposited on albite and on olivine (a neosilicate that does not contain Na). Sodium at the surface and near surface (<40 Å) was quantified with X-ray photoelectron spectroscopy before and after each irradiation to determine the depletion cross section. We measured a cross section for sputtering of Na adsorbed on mineral surfaces, σs ≈ 1 × 10?15 cm2 atom?1. In addition, mass spectrometric analyses of the sputtered flux show that a large fraction of the Na is sputtered as ions rather than as neutral atoms. These results have strong implications for modeling the sodium population within the mercurian and the lunar exospheres. 相似文献
11.
《New Astronomy》2007,12(2):104-110
The double-station observation of a meteor is a kind of method for measuring the physical parameters of the meteor effectively. Four special groups of photographs about meteoric trains taken at two stations during Leonids 2001 have been collected. One representative group of them has been measured and analyzed in detail. The results reveal that this train has a screw-like structure and looks to have many spoke beams. The train moves forward in the form of left-hand screw. The horizontal drift velocity northward is 29.6 m s−1 and the vertical drift velocity upward is 12.3 m s−1. In addition, the material of the meteoric train has an outward expanding velocity up to about 79 m s−1. The possible explanation is that the persistent meteoric train is disturbed by the high altitude wind. 相似文献
12.
M.R. Sanad 《New Astronomy》2010,15(8):688-694
We present ultraviolet spectra of the AE Aqr binary system taken by the International Ultraviolet Explorer (IUE) during the period from 1978 to 1993, to accomplish a large scale study of what happens to the ultraviolet fluxes of different emission lines during different orbital phases. Five profiles of AE Aqr binary system show variations of line fluxes at different orbital phases are presented. We concentrated on studying N V emission line at 1240 Å, O I emission line at 1306 Å, He II emission line at 1640 Å, N III emission line at 1749 Å and Si III emission line at 1892 Å, produced in the line emitting gas (Eracleous et al., 1994, Eracleous and Horne, 1996), by calculating the line fluxes of these spectral lines. Our results show that there are spectral variations of line fluxes for the aforementioned emission lines at different times, similar to the light curves found for AE Aqr binary system by using ASCA, XMM – Newton and Chandra X-ray observations (Mauche, 2006). We attributed these spectral variations to the variations of the mass-transfer rate (Ikhsanov et al., 2004) and to the collisions between the compact blobs and the fluffy blobs, where the collisions are supersonic, shocking the gas, causing heating, then expansion (Eracleous et al., 1994, Eracleous and Horne, 1996, Horne, 2009). 相似文献
13.
《New Astronomy》2007,12(6):507-521
The dynamics of the dwarf-spheroidal (dSph) galaxies in the gravitational field of the Galaxy is investigated with particular reference to their susceptibility to tidal break-up. Based on the observed paucity of the dSphs at small Galactocentric distances, we put forward the hypothesis that subsequent to the formation of the Milky Way and its satellites, those dSphs that had orbits with small perigalacticons were tidally disrupted, leaving behind a population that now has a relatively larger value of its average perigalacticon to apogalacticon ratio and consequently a larger value of its r.m.s. transverse to radial velocities ratio compared to their values at the time of formation of the dSphs. We analyze the implications of this hypothesis for the phase space distribution of the dSphs and that of the dark matter (DM) halo of the Galaxy within the context of a self-consistent model in which the functional form of the phase space distribution of DM particles follows the King model, i.e. the ‘lowered isothermal’ distribution and the potential of the Galaxy is determined self-consistently by including the gravitational cross-coupling between visible matter and DM particles. This analysis, coupled with virial arguments, yields an estimate of ≳270 km s−1 for the circular velocity of any test object at galactocentric distances of ∼100 kpc, the typical distances of the dSphs. The corresponding self-consistent values of the relevant DM halo model parameters, namely, the local (i.e., the solar neighbourhood) values of the DM density and velocity dispersion in the King model and its truncation radius, are estimated to be ∼0.3 GeV cm−3, >350 km s−1 and ≳150 kpc, respectively. Similar self-consistent studies with other possible forms of the DM distribution function will be useful in assessing the robustness of our estimates of the Galaxy’s DM halo parameters. 相似文献
14.
In this work we investigated changes of the water emission from a model comet of the size and orbital elements of Comet P/2008 R1 (Garradd). We performed simulations for model cometary nuclei of different compositions and two different orientations in space. Our simulations indicate, that the emission of water decreases from one orbital period to another one, but in some cases slowly. When the rotation axis of the nucleus lies in the orbital plane the seasonal maximum of water production during the first two orbital periods can be as high as about 1026 mol s?1, but decreases by two orders of magnitude during only 50 orbital periods. The highest rate of water production after many orbital periods is expected when the rotation axis is perpendicular to the orbital plane – the seasonal maximum of water production can be about 5 × 1025 mol s?1 during the first two orbital periods after activation of the comet and no more than 0.8 × 1025 mol s?1 500 orbital periods later. The upper estimate for the production of water derived from observations of P/2008 R1 (Garradd) by Jewitt et al. (Jewitt, D., Yang, B., Haghighipour, N. [2009]. Astron. J. 137, 4313–4321) is 5 × 1025. 相似文献
15.
We present an analysis of archival X-ray observations of the Type IIL supernova SN 1979C. We find that its X-ray luminosity is remarkably constant at (6.5 ± 0.1) × 1038 erg s?1 over a period of 12 years between 1995 and 2007. The high and steady luminosity is considered as possible evidence for a stellar-mass (～5–10 M⊙) black hole accreting material from either a supernova fallback disk or from a binary companion, or possibly from emission from a central pulsar wind nebula. We find that the bright and steady X-ray light curve is not consistent with either a model for a supernova powered by magnetic braking of a rapidly rotating magnetar, or a model where the blast wave is expanding into a dense circumstellar wind. 相似文献
16.
A radial anisotropy in the flux of cosmic rays in heliosphere was theoretically predicted by Parker and others within the framework of the diffusion–convection mechanism. The solar wind is responsible for sweeping out the galactic cosmic rays, creating a radial density gradient within the heliosphere. This gradient coupled with the interplanetary magnetic field induces a flow of charged particles perpendicular to the ecliptic plane which was measured and correctly explained by Swinson, and is hereafter referred as ‘Swinson flow’. The large area GRAPES-3 tracking muon telescope offers a powerful probe to measure the Swinson flow and the underlying radial density gradient of the galactic cosmic rays at a relatively high rigidity of ∼100 GV. The GRAPES-3 data collected over a period of six years (2000–2005) were analyzed and the amplitude of the Swinson flow was estimated to be (0.0644 ± 0.0008)% of cosmic ray flux which was an ∼80σ effect. The phase of the maximum flow was at a sidereal time of (17.70 ± 0.05) h which was 18 min earlier than the expected value of 18 h. This small 18 min phase difference had a significance of ∼6σ indicating the inherent precision of the GRAPES-3 measurement. The radial density gradient of the galactic cosmic rays at a median rigidity of 77 GV was found to be 0.65% AU−1. 相似文献
17.
A heliopause spectrum at 122 AU from the Sun is presented for galactic electrons over an energy range from 1 MeV to 50 GeV that can be considered the lowest possible local interstellar spectrum (LIS). The focus of this work is on the spectral shape of the LIS below ∼1.0 GeV. The study is done by using a comprehensive numerical model for solar modulation in comparison with Voyager 1 observations at ∼112 AU from the Sun and PAMELA data at Earth. Below ∼1.0 GeV, this LIS exhibits a power law with E−(1.55 ± 0.05), where E is the kinetic energy of these electrons. However, reproducing the PAMELA electron spectrum averaged for 2009, requires a LIS with a different power law of the form E−(3.15 ± 0.05) above ∼5 GeV. Combining the two power laws with a smooth transition from low to high energies yields a LIS over the full energy range that is relevant and applicable to the modulation of cosmic ray electrons in the heliosphere. The break occurs between ∼800 MeV and ∼2 GeV as a characteristic feature of this LIS. The power-law form below ∼1 GeV produces a challenge to the origin of these low energy galactic electrons. On the other hand, the results of this study can be used as a gauge for astrophysical modeling of the local interstellar spectrum for electrons. 相似文献
18.
《Planetary and Space Science》2007,55(14):2063-2070
Ribbons are regularly spaced, between 2 and 6 km, troughs that exist on venusian tesserae, which are mainly located in, and characterize to, venusian crustal plateaus. Independent of the geological or temporal relations with other features, regularly and similarly spaced ribbons on several tesserae strongly suggest a thermal control on the thickness of the deformed layer. This can be used to constraint the heat flow at the time of ribbon formation, which holds important implications for the viability of the hypotheses that address the origin and evolution of crustal plateaus. For a brittle–ductile transition ∼1–3 km deep (as proposed from ribbon spacing), realistic strain rates, and a present-day surface temperature of 740 K, the implied heat flow is very high, 130–780 mW m−2. If Venus has experienced higher surface temperatures due to climate forcing by massive volcanism, then the heat flow could be greatly reduced. For surface temperatures of 850–900 K the heat flow is 190–560, 60–230 and 20–130 mW m−2 for brittle–ductile transition depths of 1, 2 and 3 km, respectively. Heat flow values around 80–100 mW m−2 are reasonable for venusian hotspots, based on terrestrial analogs, but hardly consistent with coldspot settings. High surface temperatures are also required to maintain the crustal solidus deeper than a few kilometers during the formation of ribbon terrains. For the obtained heat flows, a solidus deeper than ∼30 km (the likely mean value for the crustal thickness) is difficult to achieve. This suggests that a substantial proportion of the crust beneath crustal plateaus was emplaced subsequently to the time when ribbon terrains were formed. Alternatively, at that time a magma reservoir inside the crust could have existed. 相似文献
19.
We present results of our study of the rheologies and ages of lava flows in the Elysium Mons region of Mars. Previous studies have shown that the geometric dimensions of lava flows reflect rheological properties such as yield strength, effusion rate and viscosity. In this study the rheological properties of lava flows in the Elysium Mons region were determined and compared to the rheologies of the Ascraeus Mons lava flows. We also derived new crater size-frequency distribution measurements (CSFDs) for the Elysium lava flows to identify possible changes in the rheological properties with time. In addition, possible changes in the rheological properties with the distance from the caldera of Elysium Mons were analyzed.In total, 35 lava flows on and around Elysium Mons were mapped, and divided into three groups, lava flows on the flanks of Elysium Mons, in the plains between the three volcanoes Elysium Mons, Hecates and Albor Tholus and lava flows south of Albor Tholus. The rheological properties of 32 of these flows could be determined. Based on our morphometric measurements of each individual lava flow, estimates for the yield strengths, effusion rates, viscosities, and eruption duration of the studied lava flows were made. The yield strengths of the investigated lava flows range from ～3.8 × 102 Pa to ～1.5 × 104 Pa, with an average of ～3.0 × 103 Pa. These yield strengths are in good agreement with estimates for terrestrial basaltic lava flows. The effusion rates are on average ～747 m3 s?1, ranging from ～99 to 4450 m3 s?1. The viscosities are on average ～4.1 × 106 Pa s, with a range of 1.2 × 105 Pa s to 3.1 × 107 Pa s. The eruption durations of the flows were calculated to be between 6 and 183 days, with an average of ～51 days. The determined rheological properties are generally very similar to those of other volcanic regions on Mars, such as on Ascraeus Mons in the Tharsis region. Calculated yield strengths and viscosities point to a basaltic/andesitic composition of the lava flows, similar to basaltic or andesitic a’a lava flows on Earth.Absolute model ages of all 35 lava flows on Elysium Mons were derived from crater size-frequency distribution measurements (CSFD). The derived model ages show a wide variation from about 632 Ma to 3460 Ma. Crater size-frequency distribution measurements of the Elysium Mons caldera show an age of ～1640 Ma, which is consistent with the resurfacing age of Werner (2009). Significant changes of the rheologies with time could not be observed. Similarly, we did not observe systematic changes in ages with increasing distances of lava flows from the Elysium Mons caldera. 相似文献
20.
The Venus Express (VEX) mission has been in orbit to Venus for more than 4 years now. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument onboard VEX observes Venus in two channels (visible and infrared) obtaining spectra and multi-wavelength images of the planet that can be used to sample the atmosphere at different altitudes. Day-side images in the ultraviolet range (380 nm) are used to study the dynamics of the upper cloud at 66–72 km while night-side images in the near infrared (1.74 μm) map the opacity of the lower cloud deck at 44–48 km. Here we present a long-term analysis of the global atmospheric dynamics at these levels using a large selection of orbits from the VIRTIS-M dataset covering 860 Earth days that extends our previous work (Sánchez-Lavega, A. et al. [2008]. Geophys. Res. Lett. 35, L13204) and allows studying the variability of the global circulation at the two altitude levels. The atmospheric superrotation is evident with equatorial to mid-latitudes westward velocities of 100 and 60 m s?1 in the upper and lower cloud layers. These zonal velocities are almost constant in latitude from the equator to 50°S. From 50°S to 90°S the zonal winds at both cloud layers decrease steadily to zero at the pole. Individual cloud tracked winds have errors of 3–10 m s?1 with a mean of 5 m s?1 and the standard deviations for a given latitude of our zonal and meridional winds are 9 m s?1. The zonal winds in the upper cloud change with the local time in a way that can be interpreted in terms of a solar tide. The zonal winds in the lower cloud are stable at mid-latitudes to the tropics and present variability at subpolar latitudes apparently linked to the activity of the South polar vortex. While the upper cloud presents a net meridional motion consistent with the upper branch of a Hadley cell with peak velocity v = 10 m s?1 at 50°S, the lower cloud meridional motions are less organized with some cloud features moving with intense northwards and southwards motions up to v = ±15 m s?1 but, on average, with almost null global meridional motions at all latitudes. We also examine the long-term behavior of the winds at these two vertical layers by comparing our extended wind tracked data with results from previous missions. 相似文献