羊八井460GHz大气不透明度的测量

为了解羊八井观测站的亚毫米波观测条件,使用移动式亚毫米波望远镜(Portable Submillimeter Telescope,POST)测量了羊八井观测站460 GHz波段天顶方向大气不透明度.观测结果表明,羊八井观测站的460 GHz波段天顶方向大气不透明度的四分位数为1.25、1.42、1.63,观测时间段内大气不透明度τ_0≤1.0的时间比例约占3.4%.将此次观测结果与国际现有优秀亚毫米波台址比较,分析了影响亚毫米波不透明度的可能原因.     

Millimeter and submillimeter measurements of asteroid (2867) Steins during the Rosetta fly-by

The European Space Agency Rosetta Spacecraft passed within 803 km of the main belt asteroid (2867) Steins on 5 September 2008. The Rosetta Spacecraft carries a number of scientific instruments including a millimeter and submillimeter radiometer and spectrometer. The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190 GHz (1.6 mm) and 562 GHz (0.53 mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients. A 4096 channel CTS (chirp transform spectrometer) having 180 MHz total bandwidth and ∼44 kHz resolution is also connected to the submillimeter receiver. We present the continuum observations of asteroid (2867) Steins obtained during the fly-by with the MIRO instrument. Spectroscopic data were also collected during the fly-by using the MIRO spectrometer fixed-tuned to rotational lines of several molecules. Results of the spectroscopic investigation will be the topic of a separate publication.Comparative thermal models and radiative transfer calculations for Steins are presented. Emissivities of Steins were determined to be 0.6-0.7 and 0.85-0.9 at wavelengths of 0.53 and 1.6 mm, respectively. The thermal inertia of Steins was estimated to be in the range 450-850 J/(m² s^0.5 K). Assuming that the emissivity of Steins is determined by the Fresnel reflection coefficients of the surface material, the area-averaged dielectric constant of the surface material is in the range 4-20. These values are rock-like, and are unlike the powdered-regolith surface of the Moon.     

Microwave Millisecond Spike Emission in the Solar Flare of 1991 May 16

Properties, including the time duration, polarization, quasi-periodical oscillations and so on, of the microwave spike emissions observed at 2.5 GHz and 2.6 GHz during the solar flare of 1991 May 16 are analyzed statistically. The left-handed and right-handed circular polarizations of the spike emissions at 2.5, 2.6 and 3.1 GHz are reported in detail. At these 3 frequencies, most of the spikes are superposed on both the rising (and maximum) and the descending phase of the burst. It is noteworthy that spikes also appeared superposed on the small bursts that appeared after the main burst. The spike emission lasted 17 minutes. Polarization reversals on different timescales appearing in the spike emissions at 2.5 and 2.6 GHZ are described. Our statistical analysis indicates that the polarization reversals at 2.5 and 2.6 GHz differ in characters on average, the polarization reversal at 2.5 GHz is earlier than that of 2.6 GHz by about 1.5 minutes, and polarization reversal of the spike emission is more frequent at 2.5 GHZ.     

Jovian temperature and cloud variability during the 2009-2010 fade of the South Equatorial Belt

Mid-infrared 7-20 μm imaging of Jupiter from ESO’s Very Large Telescope (VLT/VISIR) demonstrate that the increased albedo of Jupiter’s South Equatorial Belt (SEB) during the ‘fade’ (whitening) event of 2009-2010 was correlated with changes to atmospheric temperature and aerosol opacity. The opacity of the tropospheric condensation cloud deck at pressures less than 800 mbar increased by 80% between May 2008 and July 2010, making the SEB (7-17°S) as opaque in the thermal infrared as the adjacent equatorial zone. After the cessation of discrete convective activity within the SEB in May 2009, a cool band of high aerosol opacity (the SEB zone at 11-15°S) was observed separating the cloud-free northern and southern SEB components. The cooling of the SEBZ (with peak-to-peak contrasts of 1.0 ± 0.5 K), as well as the increased aerosol opacity at 4.8 and 8.6 μm, preceded the visible whitening of the belt by several months. A chain of five warm, cloud-free ‘brown barges’ (subsiding airmasses) were observed regularly in the SEB between June 2009 and June 2010, by which time they too had been obscured by the enhanced aerosol opacity of the SEB, although the underlying warm circulation was still present in July 2010. Upper tropospheric temperatures (150-300 mbar) remained largely unchanged during the fade, but the cool SEBZ formation was detected at deeper levels (p > 300 mbar) within the convectively-unstable region of the troposphere. The SEBZ formation caused the meridional temperature gradient of the SEB to decrease between 2008 and 2010, reducing the vertical thermal windshear on the zonal jets bounding the SEB. The southern SEB had fully faded by July 2010 and was characterised by short-wave undulations at 19-20°S. The northern SEB persisted as a narrow grey lane of cloud-free conditions throughout the fade process.The cool temperatures and enhanced aerosol opacity of the SEBZ after July 2009 are consistent with an upward flux of volatiles (e.g., ammonia-laden air) and enhanced condensation, obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. These changes occurred within cloud decks in the convective troposphere, and not in the radiatively-controlled upper troposphere. NH₃ ice coatings on aerosols at p < 800 mbar are plausible sources of the suppressed 4.8 and 8.6-μm emission, although differences in the spatial distribution of opacity at these two wavelengths suggest that enhanced attenuation by a deeper cloud (p > 800 mbar) also occurred during the fade. Revival of the dark SEB coloration in the coming months will ultimately require sublimation of these ices by subsidence and warming of volatile-depleted air.     

大气492 GHz亚毫米波不透明度的测量

使用移动式亚毫米波望远镜(POST)在位于青藏高原海拔3200米的紫金山天文台德令哈射电天文观测站址测量地球大气492GHz频率处天顶方向的不透明度(τ0)的结果．在1999—2000年冬季和2000—2001年冬季的两个观测季节内累计共进行了约870个小时的测量，取得了25842组τ0的有效数据．对数据的统计表明，观测季节内τ0值主要分布在1．5—3．0之间．观测时段内大气不透明度τ0≤1．0的时间比例约占3％．在给出实测资料的基础上，将所测量的亚毫米波不透明度与国际现有亚毫米波台址的不透明度进行了初步比较．     

In search of water vapor on Jupiter: Laboratory measurements of the microwave properties of water vapor under simulated jovian conditions

Detection and measurement of atmospheric water vapor in the deep jovian atmosphere using microwave radiometry has been discussed extensively by Janssen et al. (Janssen, M.A., Hofstadter, M.D., Gulkis, S., Ingersoll, A.P., Allison, M., Bolton, S.J., Levin, S.M., Kamp, L.W. [2005]. Icarus 173 (2), 447-453.) and de Pater et al. (de Pater, I., Deboer, D., Marley, M., Freedman, R., Young, R. [2005]. Icarus 173 (2), 425-447). The NASA Juno mission will include a six-channel microwave radiometer system (MWR) operating in the 1.3-50 cm wavelength range in order to retrieve water vapor abundances from the microwave signature of Jupiter (see, e.g., Matousek, S. [2005]. The Juno new frontiers mission. Tech. Rep. IAC-05-A3.2.A.04, California Institute of Technology). In order to accurately interpret data from such observations, nearly 2000 laboratory measurements of the microwave opacity of H₂O vapor in a H₂/He atmosphere have been conducted in the 5-21 cm wavelength range (1.4-6 GHz) at pressures from 30 mbars to 101 bars and at temperatures from 330 to 525 K. The mole fraction of H₂O (at maximum pressure) ranged from 0.19% to 3.6% with some additional measurements of pure H₂O. These results have enabled development of the first model for the opacity of gaseous H₂O in a H₂/He atmosphere under jovian conditions developed from actual laboratory data. The new model is based on a terrestrial model of Rosenkranz et al. (Rosenkranz, P.W. [1998]. Radio Science 33, 919-928), with substantial modifications to reflect the effects of jovian conditions. The new model for water vapor opacity dramatically outperforms previous models and will provide reliable results for temperatures from 300 to 525 K, at pressures up to 100 bars and at frequencies up to 6 GHz. These results will significantly reduce the uncertainties in the retrieval of jovian atmospheric water vapor abundances from the microwave radiometric measurements from the upcoming NASA Juno mission, as well as provide a clearer understanding of the role deep atmospheric water vapor may play in the decimeter-wavelength spectrum of Saturn.     

Broadband submillimeter measurements of the full Moon center brightness temperature and application to a lunar eclipse

We report on observations of the full Moon brightness temperature covering the frequency range of 300-950 GHz, and also on observations of the lunar eclipse of July 16, 2000, though only covering the frequency range of 165-365 GHz due to poor atmospheric transmission at higher frequencies. All observations were performed from the summit of Mauna Kea (HI) using a Fourier Transform Spectrometer mounted on the Caltech Submillimeter Observatory and supplemented by measurements of the atmospheric opacity using a 183 GHz Water Vapor Monitor. The telescope was pointed to the center of the lunar disk (with a footprint of ∼45-15 km on the Moon at 300 through 900 GHz). In order to obtain the correct values of the Moon brightness temperatures at all frequencies we carefully corrected for the atmospheric absorption, which varies across the submillimeter domain. This correction is fully described. The measured pre-eclipse brightness temperature is around 337 K in the 165-365 GHz range. This temperature slightly increases with frequency to reach ∼353 K at 950 GHz, according to previous broader band data. The magnitude of the temperature drop observed during the eclipse at 265 GHz (central frequency of the band covered) was about ∼70 K, in very good agreement with previous millimeter-wave measurements of other lunar eclipses. We detected, in addition, a clear frequency trend in the temperature drop that has been compared to a thermal and microwave emission model of the lunar regolith, with the result of a good match of the relative flux drop at different frequencies between model and measurements.     

Reconciling the centimeter- and millimeter-wavelength ammonia absorption spectra under jovian conditions: Extensive millimeter-wavelength measurements and a consistent model

Over 1000 laboratory measurements of the 2-4 mm-wavelength opacity of ammonia have been made under simulated jovian atmospheric conditions using a high-precision laboratory system developed at Georgia Tech. These laboratory measurements of the opacity of ammonia were made of various gas mixtures of hydrogen (∼77.5-85.5%), helium (∼12.5-13.5%), and ammonia (1-10%) at pressures between 1 and 3 bars and temperatures between 200 and 300 K. Laboratory measurements were also made of the opacity of pure ammonia at pressures between 0.05 and 1 bar and temperatures between 200 and 300 K. Using these millimeter-wavelength measurements and close to 2000 cm-wavelength measurements made by Hanley et al. (2009), a new consistent model has been developed to accurately characterize the absorption spectra of ammonia in a hydrogen/helium atmosphere in the 1 mm to 30 cm wavelength range. This model can be used in the 1-30 cm wavelength range at pressures up to 20 bars and temperatures from 200 to 500 K and in the 1 mm to 1 cm wavelength range at pressures up to 3 bars and temperatures from 200 to 300 K. These measurements and the accompanying model will enable better interpretation of the centimeter- and millimeter-wavelength emission spectra of the jovian planets.     

Comparison and Analysis of the Light Variations of 3C 273, 3C 279 and 3C 345

The blazar 3C 345 is quite similar to the γ-ray blazar 3C 279 in respect of redshift, spectral energy distribution from the radio to the X-ray wave band, and so on. However, it was not detected by EGRET. We compared the differences and similarities between 3C 345 and the γ-ray sources 3C 279 and 3C 273 in respect of variation amplitudes (in the 8 GHz, 22 GHz, 37 GHz and B band), the smallest variation time scales in the optical and the time lags between different bands to search possible reason for the lack of the γ-ray radiation in 3C 345. From our analyses it is found that the variation amplitudes in the radio band for 3C 345 and 3C 279 are quite similar; the variation amplitudes in the optical wave band gradually decrease in the order 3C 279, 3C 345 and 3C 273, and variations on the order of days are found in these three sources. It is also found that the time lag in 3C 345 is much longer than that in 3C 279 and is approximately the same as that in 3C 273. Based on the similarity of other observational properties between 3C 345 and 3C 273, such as the presence of the big blue bump and their comparable infrared luminosities, it is proposed that the spectral energy distribution and the luminosity in the γ-ray band for 3C 345 are similar to those for 3C 273. It is indicated by our simple calculations that if 3C 273 has a similar redshift to that of 3C 345, then even at the burst state, EGRET could not detect any radiations from 3C 273. This might be the reason why 3C 345 had never been detected by EGRET.     

Effect of atmospheric dust loading on martian albedo measurements

This work is devoted to the analysis of the variation of albedo measured by orbiting instruments with atmospheric opacity on Mars. The study has been conduced by analysing Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data from martian regions with different surface albedo.In support of these data, synthetic spectra with different surface albedo and atmospheric opacities have been computed, so that a comparison has been performed. The synthetic spectra have been retrieved by using two different grain sizes for suspended dust (0.5 and 1.2 μm), allowing a comparison between the two models and the observations.Using the DCI, a parameter describing the quantity of dust deposited on the surface, the effectiveness of the single scattering approximation has been tested for low atmospheric opacity by analysing the quality of the linear fit up to different atmospheric opacity.For more opaque conditions two kinds of fits have been applied to the data, linear and second-order degree polynomial. In this case, we found that the polynomial fit better describes the observations.The analysis of these data made it possible to notice a peculiar trend, already reported by Christensen (1988), of the albedo over Syrtis Major after the occurrence of dust storms, but, differently from that work, now the study of DCI together with atmospheric opacity and albedo allowed us to robustly confirm the hypothesis made by Christensen.Finally, the comparison between observations and synthetic spectra computed with models with different particles grain sizes indicates that dust particles of 0.5 μm diameter are the most effective to change the aerosol atmospheric opacity on Mars.     

Characterization of the atmosphere above a site for millimeter wave astronomy

The Sardinia Radio Telescope (SRT) is a challeging scientific project managed by the National Institute for Astrophysics (INAF), it is being developed at 30 km North of the city of Cagliari, Italy. The goal of the SRT project is to build a general purpose, fully steerable, 64 m diameter radio telescope, capable of operating with high efficiency in the centimeter and millimeter frequency range (0.3–100 GHz). In portions of this frequency range, especially towards the high end, astronomical observations can be heavily deteriorated by non-optimal atmospheric conditions, especially by water vapor content. The water molecule permanent electric dipole in fact, leads to pressure broadened rotational transitions around the 22.23 GHz spectral line. Furthermore, water vapor’s continuum absorption and emission may influence higher frequency observations too. To a lower degree, cloud liquid black body radiation can also affect centimeter and millimeter observations. In addition to this, inhomogeneities in water vapor distributions can cause signal phase errors which introduce a great amount of uncertainty to VLBI mode observations. The Astronomical Observatory of Cagliari (OA-CA) has obtained historical timeseries of radiosonde profiles conducted at the airport of Cagliari. Through the radiosonde measurements and an appropriate radiative transfer model, we have performed a statistical analysis of the SRT site’s atmosphere which accounts for atmospheric opacity at different frequencies, integrated water vapor (IWV), integrated liquid water (ILW) and cloud cover distributions during the year. This will help to investigate in which period of the year astronomical observations at different frequencies should be performed preferably. The results show that, at the SRT site, K-band astronomical observations are possible all year round, the median opacity at 22.23 GHz is 0.10 Np in the winter (Dec-Jan-Feb) and 0.16 Np in the summer (Jun-Jul-Aug). Integrated water vapor during winter months ranges, on average, between 7 and 15 mm. Cloud cover is usually not present for more than 36% of the time during the year. The atmospheric opacity study indicates that observations at higher frequencies (50–100 GHz) may be performed usefully: the median opacity at 100 GHz is usually below or equal to 0.2 Np in the period that ranges from January to April.     

Thermal structure and composition of Jupiter’s Great Red Spot from high-resolution thermal imaging

Thermal-IR imaging from space-borne and ground-based observatories was used to investigate the temperature, composition and aerosol structure of Jupiter’s Great Red Spot (GRS) and its temporal variability between 1995 and 2008. An elliptical warm core, extending over 8° of longitude and 3° of latitude, was observed within the cold anticyclonic vortex at 21°S. The warm airmass is co-located with the deepest red coloration of the GRS interior. The maximum contrast between the core and the coldest regions of the GRS was 3.0-3.5 K in the north-south direction at 400 mbar atmospheric pressure, although the warmer temperatures are present throughout the 150-500 mbar range. The resulting thermal gradients cause counter-rotating flow in the GRS center to decay with altitude into the lower stratosphere. The elliptical warm airmass was too small to be observed in IRTF imaging prior to 2006, but was present throughout the 2006-2008 period in VLT, Subaru and Gemini imaging.Spatially-resolved maps of mid-IR tropospheric aerosol opacity revealed a well-defined lane of depleted aerosols around the GRS periphery, and a correlation with visibly-dark jovian clouds and bright 4.8-μm emission. Ammonia showed a similar but broader ring of depletion encircling the GRS. This narrow lane of subsidence keeps red aerosols physically separate from white aerosols external to the GRS. The visibility of the 4.8-μm bright periphery varies with the mid-IR aerosol opacity of the upper troposphere. Compositional maps of ammonia, phosphine and para-H₂ within the GRS interior all exhibit north-south asymmetries, with evidence for higher concentrations north of the warm central core and the strongest depletions in a symmetric arc near the southern periphery. Small-scale enhancements in temperature, NH₃ and aerosol opacity associated with localized convection are observed within the generally-warm and aerosol-free South Equatorial Belt (SEB) northwest of the GRS. The extent of 4.8-μm emission from the SEB varied as a part of the 2007 ‘global upheaval,’ though changes during this period were restricted to pressures greater than 500 mbar. Finally, a region of enhanced temperatures extended southwest of the GRS during the survey, restricted to the 100-400 mbar range and with no counterpart in visible imaging or compositional mapping. The warm airmass was perturbed by frequent encounters with the cold airmass of Oval BA, but no internal thermal or compositional effects were noted in either vortex during the close encounters.     

Evidence that Synchrotron Emission from Nonthermal Electrons Produces the Increasing Submillimeter Spectral Component in Solar Flares

We investigate the origin of the increasing spectra observed at submillimeter wavelengths detected in the flare on 2 November 2003 starting at 17:17 UT. This flare, classified as an X8.3 and 2B event, was simultaneously detected by RHESSI and the Solar Submillimeter Telescope (SST) at 212 and 405 GHz. Comparison of the time profiles at various wavelengths shows that the submillimeter emission resembles that of the high-energy X rays observed by RHESSI whereas the microwaves observed by the Owens Valley Solar Array (OVSA) resemble that of ∼50 keV X rays. Moreover, the centroid position of the submillimeter radiation is seen to originate within the same flaring loops of the ultraviolet and X-ray sources. Nevertheless, the submillimeter spectra are distinct from the usual microwave spectra, appearing to be a distinct spectral component with peak frequency in the THz range. Three possibilities to explain this increasing radio spectra are discussed: (1) gyrosynchrotron radiation from accelerated electrons, (2) bremsstrahlung from thermal electrons, and (3) gyrosynchrotron emission from the positrons produced by pion or radioactive decay after nuclear interactions. The latter possibility is ruled out on the grounds that to explain the submillimeter observations requires 3000 to 2×10⁵ more positrons than what is inferred from X-ray and γ-ray observations. It is possible to model the emission as thermal; however, such sources would produce too much flux in the ultraviolet and soft X-ray wavelengths. Nevertheless we are able to explain both spectral components at microwave and submillimeter wavelengths by gyrosynchrotron emission from the same population of accelerated electrons that emit hard X rays and γ rays. We find that the same 5×10³⁵ electrons inferred from RHESSI observations are responsible for the compact submillimeter source (0.5 arcsec in radius) in a region of 4500 G low in the atmosphere, and for the traditional microwave spectral component by a more extended source (50 arcsec) in a 480 G magnetic field located higher up in the loops. The extreme values in magnetic field and source size required to account for the submillimeter emission can be relaxed if anisotropy and transport of the electrons are taken into account.     

The Periodicity Analysis of Radio Flux Variability of PKS 1510-089 with the Period04 Method

We have collected the 37 GHz radio data of PKS 1510-089 from 1990 to 2005, and obtained its long-term light curve after making data processing. From the light curve we can find that the activity in PKS 1510-089 is very strong. In this paper, the Period04 method is used for the first time to analyze the light periodicity of PKS 1510-089. The result indicates that its radio emission at 37 GHz has the periods of (1.87±0.13) yr and (0.87±0.07) yr. This result is consistent with the results obtained by Xie et al. in 2004, 2005, 2008, and Wu et al. in 2005 with other methods of data analysis.     

Diffuse Component Spectra of Solar Active Regions at Submillimeter Wavelengths

Solar maps at 212 and 405 GHz obtained by the Solar Submillimetric Telescope (SST) show regions of enhanced brightness temperature, which coincide with the location of active regions. A statistical study of the radio emission from these active regions was performed for the first time at such high frequencies during 23 days on June and July 2002, when the atmospheric opacity was low. The brightest regions on the maps were chosen for this study, where the brightness excess observed varies from 3 to 20% above quiet Sun levels (i.e., 200–1000 K) at both wavelengths. Sizes of the regions of enhanced emission calculated at half the maximum value were estimated to be between 2′ and 7′. These sizes agree with observed sizes of active regions at other wavelengths such as Hα and ultraviolet. An important result is that the flux density spectra of all sources increase toward submillimeter frequencies, yielding flux density spectral index with an average value of 2.0. The flux density of the active region sources were complemented with that from maps at 17 and 34 GHz from the Nobeyama Radio Heliograph. The resulting spectra at all four frequencies were fit considering the flux density to be due to thermal bremsstrahlung from the active region. In the calculations, the source radius was assumed to be the mean of the measured values at 212 and 405 K. The effective temperatures of the radio emitting source, assumed homogeneous, obtained from this fit were 0.6–2.9 × 10⁴ K, for source diameters of 2′–7′.     

A model of Titan's aerosols based on measurements made inside the atmosphere

The descent imager/spectral radiometer (DISR) instrument aboard the Huygens probe into the atmosphere of Titan measured the brightness of sunlight using a complement of spectrometers, photometers, and cameras that covered the spectral range from 350 to 1600 nm, looked both upward and downward, and made measurements at altitudes from 150 km to the surface. Measurements from the upward-looking visible and infrared spectrometers are described in Tomasko et al. [2008a. Measurements of methane absorption by the descent imager/spectral radiometer (DISR) during its descent through Titan's atmosphere. Planet. Space Sci., this volume]. Here, we very briefly review the measurements by the violet photometers, the downward-looking visible and infrared spectrometers, and the upward-looking solar aureole (SA) camera. Taken together, the DISR measurements constrain the vertical distribution and wavelength dependence of opacity, single-scattering albedo, and phase function of the aerosols in Titan's atmosphere.Comparison of the inferred aerosol properties with computations of scattering from fractal aggregate particles indicates the size and shape of the aerosols. We find that the aggregates require monomers of radius 0.05 μm or smaller and that the number of monomers in the loose aggregates is roughly 3000 above 60 km. The single-scattering albedo of the aerosols above 140 km altitude is similar to that predicted for some tholins measured in laboratory experiments, although we find that the single-scattering albedo of the aerosols increases with depth into the atmosphere between 140 and 80 km altitude, possibly due to condensation of other gases on the haze particles. The number density of aerosols is about 5/cm³ at 80 km altitude, and decreases with a scale height of 65 km to higher altitudes. The aerosol opacity above 80 km varies as the wavelength to the −2.34 power between 350 and 1600 nm.Between 80 and 30 km the cumulative aerosol opacity increases linearly with increasing depth in the atmosphere. The total aerosol opacity in this altitude range varies as the wavelength to the −1.41 power. The single-scattering phase function of the aerosols in this region is also consistent with the fractal particles found above 60 km.In the lower 30 km of the atmosphere, the wavelength dependence of the aerosol opacity varies as the wavelength to the −0.97 power, much less than at higher altitudes. This suggests that the aerosols here grow to still larger sizes, possibly by incorporation of methane into the aerosols. Here the cumulative opacity also increases linearly with depth, but at some wavelengths the rate is slightly different than above 30 km altitude.For purely fractal particles in the lowest few km, the intensity looking upward opposite to the azimuth of the sun decreases with increasing zenith angle faster than the observations in red light if the single-scattering albedo is assumed constant with altitude at these low altitudes. This discrepancy can be decreased if the single-scattering albedo decreases with altitude in this region. A possible explanation is that the brightest aerosols near 30 km altitude contain significant amounts of methane, and that the decreasing albedo at lower altitudes may reflect the evaporation of some of the methane as the aerosols fall into dryer layers of the atmosphere. An alternative explanation is that there may be spherical particles in the bottom few kilometers of the atmosphere.     

Sounding of Titan’s atmosphere at submillimeter wavelengths from an orbiting spacecraft

An investigation of the capabilities and science goals of a submillimeter-wave heterodyne sounder onboard a Titan orbiter is presented. Based on a model of Titan’s submillimeter spectrum, and including realistic instrumental performances, we show that passive limb observations of Titan’s submillimeter radiation would bring novel and unique information on the dynamical and chemical state of Titan’s atmosphere, particularly in the so far poorly probed 500-900 km region. The 300-360, 540-660 and 1080-1280 GHz spectral ranges appear especially promising, and could be explored with an instrument equipped with a tunable local oscillator system. Vertical temperature profiles can be determined up to ∼1200 km using rotational lines of CH₄, CO, and HCN. Winds can be measured over the 200-1200 km altitude range with an accuracy of 3-5 m/s from Doppler shift measurements of any strong optically thin line. Numerous molecular species are accessible, including H₂O, NH₃, CH₃C₂H, CH₂NH, and several nitriles (HC₃N, HC₅N, CH₃CN, and C₂H₃CN). Many of them are expected to be detectable in a large fraction of the atmosphere and in some cases at all levels, providing an observational link between stratospheric and thermospheric chemistry. Isotopic variants of some of these species can also be measured, providing new measurements of H, C, N, and O isotopic ratios. Mapping of the thermal, wind, and composition fields, best achieved from a polar orbit and with an articulated antenna, would provide a new view of the couplings between chemistry and dynamics over an extended altitude range of Titan’s atmosphere. Additional science goals at Saturn and Enceladus are briefly discussed.     

Extension of atmospheric dust loading to high altitudes during the 2001 Mars dust storm: MGS TES limb observations

Mars Global Surveyor (MGS) visible (solarband bolometer) and thermal infrared (IR) spectral limb observations from the Thermal Emission Spectrometer (TES) support quantitative profile retrievals for dust opacity and particle sizes during the 2001 global dust event on Mars. The current analysis considers the behavior of dust lifted to altitudes above 30 km during the course of this storm; in terms of dust vertical mixing, particle sizes, and global distribution. TES global maps of visible (solarband) limb brightness at 60 km altitude indicate a global-scale, seasonally evolving (over 190-240° solar longitudes, L_S) longitudinal corridor of vertically extended dust loading (which may be associated with a retrograde propagating, wavenumber 1 Rossby wave). Spherical radiative transfer analysis of selected limb profiles for TES visible and thermal IR radiances provide quantitative vertical profiles of dust opacity, indicating regional conditions of altitude-increasing dust mixing ratios. Observed infrared spectral dependences and visible-to-infrared opacity ratios of dust scattering over 30-60 km altitudes indicate particle sizes characteristic of lower altitudes (cross-section weighted effective radius, ), during conditions of significant dust transport to these altitudes. Conditions of reduced dust loading at 30-60 km altitudes present smaller dust particle sizes . These observations suggest rapid meridional transport at 30-80 km altitudes, with substantial longitudinal variation, of dust lifted to these altitudes over southern hemisphere atmospheric regions characterized by extraordinary (m/s) vertical advection velocities. By L_S=230° dust loading above 50 km altitudes decreased markedly at southern latitudes, with a high altitude (60-80 km) haze of fine (likely) water ice particles appearing over 10°S-40°N latitudes.     

Orbital effects on Mercury’s escaping sodium exosphere

We present results from coronagraphic imaging of Mercury’s sodium tail over a 7° field of view. Several sets of observations made at the McDonald Observatory since May 2007 show a tail of neutral sodium atoms stretching more than 1000 Mercury radii (R_m) in length, or a full degree of sky. However, no tail was observed extending beyond 120 R_m during the January 2008 MESSENGER fly-by period, or during a similar orbital phase of Mercury in July 2008. Large changes in Mercury’s heliocentric radial velocity cause Doppler shifts about the Fraunhofer absorption features; the resultant change in solar flux and radiation pressure is the primary cause of the observed variation in tail brightness. Smaller fluctuations in brightness may exist due to changing source rates at the surface, but we have no explicit evidence for such changes in this data set. The effects of radiation pressure on Mercury’s escaping atmosphere are investigated using seven observations spanning different orbital phases. Total escape rates of atmospheric sodium are estimated to be between 5 and 13 × 10²³ atoms/s and show a correlation to radiation pressure. Candidate sources of Mercury’s sodium exosphere include desorption by UV sunlight, thermal desorption, solar wind channeled along Mercury’s magnetic field lines, and micro-meteor impacts. Wide-angle observations of the full extent of Mercury’s sodium tail offer opportunities to enhance our understanding of the time histories of these source rates.