Perchloric acid: A possible sink for stratospheric chlorine

The possibility that chlorine may deplete stratospheric chlorine has received considerable attention recently. The only termination steps considered up to now involve HCl formation by chlorine atom attack on hydrogen-bearing molecules. We propose that an important removal mechanism for chlorine in the stratosphere may be the formation of HClO₄ via the sequence of steps Cl + O₂ + O₃ → ClO₃ + O₂ ClO₃ + OH → HClO₄. In addition to being produced as often as HCl, HClO₄ may be more stable to radical attack and thus a more efficient sink than HCl for stratospheric chlorine.     

Prospects for asteroseismology

The observational basis for asteroseismology is being dramatically strengthened, through more than two years of data from the CoRoT satellite, the flood of data coming from the Kepler mission and, in the slightly longer term, from dedicated ground-based facilities. Our ability to utilize these data depends on further development of techniques for basic data analysis, as well as on an improved understanding of the relation between the observed frequencies and the underlying properties of the stars. Also, stellar modelling must be further developed, to match the increasing diagnostic potential of the data. Here we discuss some aspects of data interpretation and modelling, focusing on the important case of stars with solar-like oscillations.     

Astrophysical Radiation Dynamics: The Prospects for Scaling

The general principles of scaling are discussed, followed by a survey of the important dimensionless parameters of fluid dynamics including radiation and magnetic fields, and of non-LTE spectroscopy. The values of the parameters are reviewed for a variety of astronomical and laboratory environments. It is found that parameters involving transport coefficients – the fluid and magnetic Reynolds numbers – have enormous values for the astronomical problems that are not reached in the lab. The parameters that measure the importance of radiation are also scarcely reached in the lab. This also means that the lab environments are much closer to LTE than the majority of astronomical examples. Some of the astronomical environments are more magnetically dominated than anything in the lab. The conclusion is that a good astronomical environment for simulation in a given lab experiment can be found, but that the reverse is much more difficult. PACS NOS: 95.30.Jx, 95.30.Lz, 97.10.Ex, 97.10.Gz, 98.62.Mw The U.S. Government’s right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Stellar intensity interferometry: Prospects for sub-milliarcsecond optical imaging

Using kilometric arrays of air Cherenkov telescopes at short wavelengths, intensity interferometry may increase the spatial resolution achieved in optical astronomy by an order of magnitude, enabling images of rapidly rotating hot stars with structures in their circumstellar disks and winds, or mapping out patterns of nonradial pulsations across stellar surfaces. Intensity interferometry (once pioneered by Hanbury Brown and Twiss) connects telescopes only electronically, and is practically insensitive to atmospheric turbulence and optical imperfections, permitting observations over long baselines and through large airmasses, also at short optical wavelengths. The required large telescopes (～10 m) with very fast detectors (～ns) are becoming available as the arrays primarily erected to measure Cherenkov light emitted in air by particle cascades initiated by energetic gamma rays. Planned facilities (e.g., CTA, Cherenkov Telescope Array) envision many tens of telescopes distributed over a few square km. Digital signal handling enables very many baselines (from tens of meters to over a kilometer) to be simultaneously synthesized between many pairs of telescopes, while stars may be tracked across the sky with electronic time delays, in effect synthesizing an optical interferometer in software. Simulated observations indicate limiting magnitudes around m_V = 8, reaching angular resolutions ～30 μarcsec in the violet. The signal-to-noise ratio favors high-temperature sources and emission-line structures, and is independent of the optical passband, be it a single spectral line or the broad spectral continuum. Intensity interferometry directly provides the modulus (but not phase) of any spatial frequency component of the source image; for this reason a full image reconstruction requires phase retrieval techniques. This is feasible if sufficient coverage of the interferometric (u, v)-plane is available, as was verified through numerical simulations. Laboratory and field experiments are in progress; test telescopes have been erected, intensity interferometry has been achieved in the laboratory, and first full-scale tests of connecting large Cherenkov telescopes have been carried out. This paper reviews this interferometric method in view of the new possibilities offered by arrays of air Cherenkov telescopes, and outlines observational programs that should become realistic already in the rather near future.     

Photochemistry of the stratosphere of Venus: Implications for atmospheric evolution

The photochemistry of the stratosphere of Venus was modeled using an updated and expanded chemical scheme, combined with the results of recent observations and laboratory studies. We examined three models, with H₂ mixing ratio equal to 2 × 10^?5, 5 × 10^?7, and 1 × 10^?13, respectively. All models satisfactorily account for the observations of CO, O₂, O₂(¹Δ), and SO₂ in the stratosphere, but only the last one may be able to account for the diurnal behavior of mesospheric CO and the uv albedo. Oxygen, derived from CO₂ photolysis, is primarily consumed by CO₂ recombination and oxidation of SO₂ to H₂SO₄. Photolysis of HCl in the upper stratosphere provides a major source of odd hydrogen and free chlorine radicals, essential for the catalytic oxidation of CO. Oxidation of SO₂ by O occurs in the lower stratosphere. In the high-H₂ model (model A) the OO bond is broken mainly by S + O₂ and SO + HO₂. In the low-H₂ models additional reactions for breaking the OO bond must be invoked: NO + HO₂ in model B and ClCO + O₂ in model C. It is shown that lightning in the lower atmosphere could provide as much as 30 ppb of NO_x in the stratosphere. Our modeling reveals a number of intriguing similarities, previously unsuspected, between the chemistry of the stratosphere of Venus and that of the Earth. Photochemistry may have played a major role in the evolution of the atmosphere. The current atmosphere, as described by our preferred model, is characterized by an extreme deficiency of hydrogen species, having probably lost the equivalent of 10²–10³ times the present hydrogen content.     

Prospects for far infrared arrays

We report initial performance measurements of a 1/8 scale version of a 32×32 pixel array under development for SIRTF. This array demonstrates that we can reach the sensitivity limits set by the natural backgrounds in space while providing good imaging and photometric performance. Based on the achieved performance levels, we project the imaging capabilities of SIRTF in the far infrared to exceed by a factor of more than 10,000 those achieved by any preceding telescope.     

Prospects for predicting cycle 24

Although we have reliable data of solar polar fields only from the mid-1970s, it seems that the polar field at a minimum is well correlated with the next cycle, but the strength of the cycle is not correlated with the polar field produced at its end. We explain this by suggesting that the Babcock-Leighton mechanism of poloidal field generation from tilted active regions involves randomness, whereas the other aspects of the dynamo process are more ordered. To model actual cycles, we have to ‘correct’ our theoretical dynamo model by ‘feeding’ information about the polar field at the minima. Following this process, we find that our model fits the observed sunspot numbers of cycles 21–23 reasonably well and predicts that cycle 24 will be the weakest in a century.     

Some aspects of the stratospheric Cl-ClO-Cl cycle: Possible roles of ClO, ClNO3 and HOCl

Depending on such factors as (a) the probabilities of exciting the various vibrational states in ClO formed in the reaction of Cl with O₃, (b) the radiative lifetime of ClO^*, (c) ΔH_ƒ(ClO₃), and (d) the rate coeffic`ient of the relevant three-body reaction, the production of ClO₃ via the reaction ClO^* +O₂ +M→ClO₃ +M may be quite substantial in the stratosphere. The significance of this result lies in the subsequent elimination (from the stratosphere) of ClO₃ and its associated chlorine atom as HClO₄, in the manner recently suggested by Samonaitis and Heicklen. In the stratosphere, ClO₃ most probably photodissociates primarily into OClO and O. Upon photodissociation, OClO may also yield atomic oxygen. Thus the formation of ClO₃ from ClO^* and O₂, and the above-mentioned photodissociation steps constitute an interesting, indirect mechanism of O₂ dissociation into two odd oxygen species. Other aspects of ClO^* chemistry, applicable in stratospheric conditions, also deserve attention in view of Nicholl's recent interpretation of the Umkehr measurements by Brewer et al. The reactions of ClO with HO₂, and NO₂, possess the potential of significantly obstructing the completion of the C1-ClO-Cl cycle, at least in the region below 35 km. An accurate and critical study of the chemistry of oxyacids, higher oxides, and nitrates of chlorine in the stratospheric environment is needed. Obviously, this is only a partial list of the difficult problems associated with a proper understanding of stratospheric chlorine chemistry which appears to be far more complex than what is implied in the literature. (See also notes added in proof stage.)     

Prospects for spectroscopic reflected-light planet searches

High-resolution spectroscopic searches for the starlight reflected from close-in extrasolar giant planets have the capability of determining the optical albedo spectra and scattering properties of these objects. When combined with radial velocity measurements they also yield the true mass of the planet. To date, only two such planets have been targeted for reflected-light signals, yielding upper limits on the optical albedos of the planets. Here we examine the prospects for future searches of this kind. We present Monte Carlo estimates of prior probability distributions for the orbital velocity amplitudes and planet/star flux ratios of six bright stars known to harbour giant planets in orbits with periods of less than 5 d. Using these estimates, we assess the viability of these targets for future reflected-light searches using 4- and 8-m class telescopes.     

Prospects for solar flare X-ray polarimetry

The history of solar flare X-ray polarimetry is reviewed and it is shown that as yet, there is no experimental evidence for such polarization. The present experimental limits are at the level of a few percent but these results may be biased by a large thermal component at low energies which may decrease the apparent polarization. To avoid this difficulty it will be necessary to make observations at higher energies where thermal emission is less important.The theoretical estimates of the polarization expected in the solar flare are also reviewed. The best present theoretical estimates are in the range of a few percent and are consistent with the present experimental limits.In this paper we discuss a new satellite instrument that has sufficient sensitivity at high energies to detect the polarization that is predicted by the present theories. The instrument sensitivity for a moderate (M class) event approaches polarization levels of 1% in each of 7 energy bins spanning the 10 to 100 keV range for integration times as short as 10 s. Comparable results can be obtained for an X class flare in 1 s.Presidential Young Investigator.     

The atmospheric temperatures over Olympus Mons on Mars: An atmospheric hot ring

We study the thermal fields over Olympus Mons separating seasons (northern spring and summer against southern spring and summer) and local time observations (day side versus night side). Temperature vertical profiles retrieved from Planetary Fourier Spectrometer on board Mars Express (PFS-MEX) data have been used. In many orbits (running north to south along a meridian) passing over the top of the volcano there is evidence of a hot air on top of the volcano, of two enhancement of the air temperature both north and south of it and in between a collar of air that is colder than nearby at low altitudes, and warmer than nearby at high altitudes. Mapping together many orbits passing over or near the volcano we find that the hot air has the tendency to form an hot ring around it. This hot structure occurs mostly between LT = 10.00 and 15.00 and during the northern summer. Distance of the hot structure from the top of the volcano is about 600 km (10° of latitude). The hot atmospheric region is 300-420 km (5-7°) wide. Hot ring temperature contrasts of about 40 K occur at 2 km above the surface and decrease to 20 K at 5 km and to 10 K at 10 km. The atmospheric circulation over an area of 40° × 40° (latitudes and longitudes) is affected by the topography and the presence of Olympus Mons (−133°W, 18°N). We discuss also the thermal stability of the atmosphere over the selected area using the potential temperatures. The temperature field over the top of the volcano shows unstable atmosphere within 10 km from the surface. Finally, we interpret the hot temperatures around volcano as an adiabatic compression of down-welling branch coming from over the top of volcano.Different air temperature profiles are observed in the same seasons during the night, or in different seasons. In northern spring-summer during the night the isothermal contours do not show the presence of the volcano until we reach close to the surface very much, where a thermal inversion is observed. The surface temperature shows higher values (by 10 K) in correspondence of the scarp (an abrupt altimetry variation of roughly 5 km) on south (6°N) and north (30°N) sides of volcano. During the southern spring-summer, on the contrary the isothermal curves run parallel to the surface even on top the volcano, just like the GCM have predicted.     

Titan's atmospheric engine: an overview

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.     

Connecting atmospheric science and atmospheric models for aerocapture at Titan and the outer planets

Many atmospheric measurement systems, such as the sounding instruments on Voyager, gather atmospheric information in the form of temperature versus pressure level. In these terms, there is considerable consistency among the mean atmospheric profiles of the outer planets Jupiter through Neptune, including Titan. On a given planet or on Titan, the range of variability of temperature versus pressure level due to seasonal, latitudinal, and diurnal variations is also not large. However, many engineering needs for atmospheric models relate not to temperature versus pressure level but atmospheric density versus geometric altitude. This need is especially true for design and analysis of aerocapture systems. Drag force available for aerocapture is directly proportional to atmospheric density. Available aerocapture “corridor width” (allowable range of atmospheric entry angle) also depends on height rate of change of atmospheric density, as characterized by density scale height. Characteristics of hydrostatics and the gas law equation mean that relatively small systematic differences in temperature versus pressure profiles can integrate at high altitudes to very large differences in density versus altitude profiles. Thus, a given periapsis density required to accomplish successful aerocapture can occur at substantially different altitudes (∼150-300 km) on the various outer planets, and significantly different density scale heights (∼20-50 km) can occur at these periapsis altitudes. This paper will illustrate these effects and discuss implications for improvements in atmospheric measurements to yield significant impact on design of aerocapture systems for future missions to Titan and the outer planets. Relatively small-scale atmospheric perturbations, such as gravity waves, tides, and other atmospheric variations can also have significant effect on design details for aerocapture guidance and control systems. This paper will discuss benefits that would result from improved understanding of Titan and outer planetary atmospheric perturbation characteristics. Details of recent engineering-level atmospheric models for Titan and Neptune will be presented, and effects of present and future levels of atmospheric uncertainty and variability characteristics will be examined.     

Nucleosynthesis in nova explosions: Prospects for its observation with focusing telescopes

Nova explosions are caused by the explosive burning of hydrogen in the envelope of accreting white dwarfs. During the thermonuclear runaway some radioactive isotopes are synthesized, which emit γ -rays when they decay. The γ -ray signatures of a nova explosion still remain undetected, because even the best instruments like SPI onboard INTEGRAL are not sensitive enough for the dim and broad lines emitted by novae at their typical distances. A very different situation is expected with a focusing telescope, like MAX. Prospects for detectability with a future γ -ray lens telescope are presented, with a special emphasis on the important information that γ -rays would provide about the explosion mechanism and the underlying white dwarf star     

Thermal structure of Saturn from Voyager infrared measurements: Implications for atmospheric dynamics

Saturn atmospheric temperatures at the 150-mbar level retrieved from Voyager IRIS measurements indicate the presence of small-scale meridional gradients which are approximately symmetric with respect to the equator, but are superposed on a large-scale hemispheric thermal asymmetry. Under the assumption that the retrieved values at this atmospheric level represent kinetic temperatures on a constant pressure surface, it is suggested that the small-scale structure is produced by a meriodional circulation associated with the dissipative decay of the zonal winds with height, while the hemispheric asymmetry represents a thermal response to the seasonally varying insolation. The small-scale gradients are correlated with zonal winds derived from Voyager images at mid and high latitudes through the thermal wind relation; the calculated thermal wind shears suggest a decay with height of the jet system toward a state of uniform eatward flow. The existence of the approximately symmetric zonal winds and associated temperature gradients in the presence of a large-scale seasonal thermal response suggests that the jet system is driven at depths substantially below the levels where seasonally modulated insolation is important ($\text{p?0.5}\text{bar}$).     

Prospects for astronomical development in South Africa

Because of its geographical location and (astronomically) excellent climate South Africa can make an unique contribution to international astronomical research. An assessment of recent developments in telescope technology has shown that an advanced technology telescope of 4m-class can be constructed which will out-perform most existing 4m-class telescopes. Detailed consideration is given to the construction of such a telescope for optical/infrared astronomy, the new science that this will enable and the selection of a site of sufficiently good quality to justify the erection of such a telescope. If a telescope of this nature is sited in southern Africa it would provide the premier astronomical facility in Africa for decades to come.     

Prospects for detecting ultra-high-energy particles with FAST

The origin of the highest-energy particles in nature, ultra-high-energy(UHE) cosmic rays, is still unknown. In order to resolve this mystery, very large detectors are required to probe the low flux of these particles — or to detect the as-yet unobserved flux of UHE neutrinos predicted from their interactions. The‘lunar Askaryan technique' is a method to do both. When energetic particles interact in a dense medium,the Askaryan effect produces intense coherent pulses of radiation in the MHz–GHz range. By using radio telescopes to observe the Moon and look for nanosecond pulses, the entire visible lunar surface(20 million km~2) can be used as a UHE particle detector. A large effective area over a broad bandwidth is the primary telescope requirement for lunar observations, which makes large single-aperture instruments such as the Five-hundred-meter Aperture Spherical radio Telescope(FAST) well-suited to the technique. In this contribution, we describe the lunar Askaryan technique and its unique observational requirements. Estimates of the sensitivity of FAST to both the UHE cosmic ray and neutrino flux are given, and we describe the methods by which lunar observations with FAST, particularly if equipped with a broadband phased-array feed, could detect the flux of UHE cosmic rays.     

Impact of atmospheric parameters on the atmospheric Cherenkov technique

Atmospheric density profiles as well as several light absorption and scattering processes depend on geographic position and are generally time-variable. Their impact on the atmospheric Cherenkov technique in general (imaging or non-imaging) is investigated. Different density profiles lead to differences in Cherenkov light density of up to 60%. Seasonal variations at mid-latitude sites are of the order of 15–20%. The quest for improved energy calibration of Cherenkov experiments also shows the need for improved transmission calculations, taking all relevant processes into account and using realistic profiles of absorbers. Simulations including the scattering mechanisms also reveal the relevance of Rayleigh and Mie scattering for atmospheric Cherenkov experiments. Refraction and the differences between treating the atmosphere in plane-parallel or spherical geometry are also investigated.