Heliotropic dust rings for Earth climate engineering

This paper examines the concept of a Sun-pointing elliptical Earth ring comprised of dust grains to offset global warming. A new family of non-Keplerian periodic orbits, under the effects of solar radiation pressure and the Earth’s J₂ oblateness perturbation, is used to increase the lifetime of the passive cloud of particles and, thus, increase the efficiency of this geoengineering strategy. An analytical model is used to predict the orbit evolution of the dust ring due to solar-radiation pressure and the J₂ effect. The attenuation of the solar radiation can then be calculated from the ring model. In comparison to circular orbits, eccentric orbits yield a more stable environment for small grain sizes and therefore achieve higher efficiencies when the orbit decay of the material is considered. Moreover, the novel orbital dynamics experienced by high area-to-mass ratio objects, influenced by solar radiation pressure and the J₂ effect, ensure the ring will maintain a permanent heliotropic shape, with dust spending the largest portion of time on the Sun facing side of the orbit. It is envisaged that small dust grains can be released from a circular generator orbit with an initial impulse to enter an eccentric orbit with Sun-facing apogee. Finally, a lowest estimate of 1 × 10¹² kg of material is computed as the total mass required to offset the effects of global warming.     

Quality variation of GPS satellite clocks on-orbit using IGS clock products

Over 60% clocks on board of the GPS satellites are working longer than their designed life. Therefore realizing their stabilities in a long time scales is essential to GPS navigation and positioning plus IGS time scale maintaining. IGS clock products from 2001 to 2010 are used to analyze the GPS satellite clock qualities such as frequency stabilities and clock noise level. We find out that for the clocks of Block IIA satellites the frequency stabilities and clock noise are 10 times worse than that of the Block IIR and IIR-M satellites. Moreover, the linear relationships between frequency stabilities and clock residuals have been deduced with an accuracy of better than 0.02 ns. Specially, it is noticed that the clock of the PRN27 is instable and the relationship between the frequency stability and residuals is at least a quadratic curve. Therefore, we suggested that GPS satellite clocks should be weighted by their quality levels in application, and the observations of the Block IIA should not be used for real-time positioning which required precision better than one meter.     

Performance evaluation of IRI-2007 at equatorial latitudes and its Matlab version for GNSS applications

International Reference Ionosphere (IRI) model is the widely used empirical model for ionospheric predictions, especially TEC which is an important parameter for radio navigation and communication. The Fortran based IRI-2007 does not support real-time interactive visualization and debugging. Therefore, the source code is converted into Matlab and is validated for the purposes of this study. This facilitates easy representation of results and for near real-time implementation of IRI in the applications including spacecraft launching, now casting, pseudolite based navigation systems etc. In addition, the vertical delay results over the equatorial region derived from IRI and GPS data of three IGS stations namely Libreville (Garbon, Africa), Brasilia (Brazil, South America) and Hyderabad (India, Asia) are compared. As the IRI model does not account for plasmasphere TEC, the vertical delays are underestimated compared to vertical delays of GPS signals. Therefore, the model should be modified accordingly for precise TEC estimation.     

Low latitude nighttime ionospheric vertical E × B drifts at African region

The nighttime vertical E × B drifts velocities of the F2-region were inferred from the hourly hmF2 values obtained from ionosonde data over an African equatorial station, Ilorin (8.50^oN, 4.68^oE; dip lat. 2.95^o) during period of low solar activity. For each season, the plasma drift Vz is characterized by an evening upward enhancement, then by a downward reversal at 1900 LT till around 0000 LT, except for June solstice. This was explained using the Rayleigh–Taylor (R-T) instability mechanism. The occasional drift differences in Vz obtained by inferred and direct measurement over Ilorin and Jicamarca, respectively are reflective of the importance of chemistry and divergent transport system due to both the E region electric and magnetic fields instead of simple motions. The pre-reversal enhancement (PRE) magnitude is higher during the equinoctial months than the solsticial months over Jicamarca, highest during December solstice and the equinoctial months over Ilorin, suggesting the dominance of higher E × B fountain during equinoxes at both stations. The lowest PRE magnitude was in June solstice. The appearance of post-noon peak in NmF2 around 1700 LT is highest during the equinoctial months and lowest during the solsticial period. A general sharp drop in NmF2 around 1800 LT is distinct immediately after sunset, lowest during June solstice and highest in March equinox. Our result suggests that between 0930 and 2100 LT, the general theory that vertical drifts obtained by digisonde measurements only match the E × B drift if the F layer is higher than 300 km is reliable, but does not hold for the nighttime period of 2200–0600 LT under condition of solar minima. Hence, the condition may not be sufficient for the representation of vertical plasma drift at nighttime during solar minima. This assertion may still be tentative, as more equatorial stations needed to be studied for better confirmation.     

Analytical solutions for the relative motion of spacecraft subject to Lorentz-force perturbations

A spacecraft capable of producing higher-than-natural electrostatic charges may achieve propellantless orbital maneuvering via the Lorentz-force interaction with a planetary magnetic field. Development of maneuver strategies for these propellantless vehicles is complicated by the fact that the perturbative Lorentz force acts along only a single line of action at any instant. Relative-motion dynamical models are developed that lead to approximate analytical solutions for the motion of charged spacecraft subject to the Lorentz force. These solutions indicate that the principal effects of the Lorentz force on a spacecraft in a circular orbit are to change the intrack position and to change the orbit plane. A rendezvous example is presented in which a spacecraft with a specific charge of ?3.81 $\times$ 10^?4 C/kg reaches a target vehicle initially 10 km away (on the same equatorial low-Earth orbit) in 1 day. Fly-around maneuvers may be achieved in low-Earth orbit with specific charges on the order of 0.001 C/kg.     

Attractive manifold-based adaptive solar attitude control of satellites in elliptic orbits

The paper presents a novel noncertainty-equivalent adaptive (NCEA) control system for the pitch attitude control of satellites in elliptic orbits using solar radiation pressure (SRP). The satellite is equipped with two identical solar flaps to produce control moments. The adaptive law is based on the attractive manifold design using filtered signals for synthesis, which is a modification of the immersion and invariance (I&I) method. The control system has a modular controller–estimator structure and has separate tunable gains. A special feature of this NCEA law is that the trajectories of the satellite converge to a manifold in an extended state space, and the adaptive law recovers the performance of a deterministic controller. This recovery of performance cannot be obtained with certainty-equivalent adaptive (CEA) laws. Simulation results are presented which show that the NCEA law accomplishes precise attitude control of the satellite in an elliptic orbit, despite large parameter uncertainties.     

Comparative survival analysis of Deinococcus radiodurans and the haloarchaea Natrialba magadii and Haloferax volcanii exposed to vacuum ultraviolet irradiation

The haloarchaea Natrialba magadii and Haloferax volcanii, as well as the radiation-resistant bacterium Deinococcus radiodurans, were exposed to vacuum UV (VUV) radiation at the Brazilian Synchrotron Light Laboratory. Cell monolayers (containing 10(5) to 10(6) cells per sample) were prepared over polycarbonate filters and irradiated under high vacuum (10(-5) Pa) with polychromatic synchrotron radiation. N. magadii was remarkably resistant to high vacuum with a survival fraction of (3.77±0.76)×10(-2), which was larger than that of D. radiodurans (1.13±0.23)×10(-2). The survival fraction of the haloarchaea H. volcanii, of (3.60±1.80)×10(-4), was much smaller. Radiation resistance profiles were similar between the haloarchaea and D. radiodurans for fluences up to 150?J m(-2). For fluences larger than 150?J m(-2), there was a significant decrease in the survival of haloarchaea, and in particular H. volcanii did not survive. Survival for D. radiodurans was 1% after exposure to the higher VUV fluence (1350?J m(-2)), while N. magadii had a survival lower than 0.1%. Such survival fractions are discussed regarding the possibility of interplanetary transfer of viable microorganisms and the possible existence of microbial life in extraterrestrial salty environments such as the planet Mars and Jupiter's moon Europa. This is the first work to report survival of haloarchaea under simulated interplanetary conditions.     

Preservation of microbial lipids in geothermal sinters

Lipid biomarkers are widely used to study the earliest life on Earth and have been invoked as potential astrobiological markers, but few studies have assessed their survival and persistence in geothermal settings. Here, we investigate lipid preservation in active and inactive geothermal silica sinters, with ages of up to 900 years, from Champagne Pool, Waiotapu, New Zealand. Analyses revealed a wide range of bacterial biomarkers, including free and bound fatty acids, 1,2-di-O-alkylglycerols (diethers), and various hopanoids. Dominant archaeal lipids include archaeol and glycerol dialkyl glycerol tetraethers (GDGTs). The predominance of generally similar biomarker groups in all sinters suggests a stable microbial community throughout Champagne Pool's history and indicates that incorporated lipids can be well preserved. Moreover, subtle differences in lipid distributions suggest that past changes in environmental conditions can be elucidated. In this case, higher archaeol abundances relative to the bacterial diethers, a greater proportion of cyclic GDGTs, the high average chain length of the bacterial diethers, and greater concentrations of hopanoic acids in the older sinters all suggest hotter conditions at Champagne Pool in the past.     

Deducing Electron Properties from Hard X-ray Observations

X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future.