Pocketqube Deorbit Times: Susceptibility to the Solar Cycle

Nowadays, as a new kind of femto-satellite with a low cost, Pocketqube has been developed to finish the space research task within the LEO region. During its lifetime the pocketqube is exposed to a high risk of collision with space debris. Taking the solar cycle as a main factor, predicting its deorbit time and evaluating its collision probability before the launch is of great importance for the mission designers to choose a right orbit and determine the proper launch time. This article presents a combined atmospheric density model based on the data from CIRA-2012 to describe the effects of the solar cycle on air density in LEO, and shows how the model is applied to calculate orbital lifetimes of pocketqubes in essentially circular equatorial orbits below 800 km altitude. Then the classical fourth order Runge-Kutta method is utilized in integrating the first order differential equations, which express the rates of change of semi major axis and eccentricity, in order to calculate the orbital lifetimes of pocketqube in LEO. The launch date within the 11-year solar cycle has been chosen as an independent variable to present the influence on lifetime prediction and probability evaluation. The result of lifetime calculation shows that the pocketqube launched at the minimum solar activity year does not necessarily get its longest lifetime. Meanwhile if the pocketqube at some specific starting altitudes is launched at the maximum solar activity year, it may remain in orbit for the longest time period. It also demonstrates how the sensitivity of pocketqube deorbit time to the launch date varies with the initial altitudes. From the figures, it can be obtained that 450 km is the altitude at which the deorbit time is most sensitive to the launch date with the percentage amplitude of 180% over its average value. Furthermore, the collision risk from space debris whose diameter is larger than 1 mm and 10 cm are evaluated by using the same method to integrate through its whole lifetime. It illustrates that for those orbits whose initial altitude is over 700 km, no matter which date is chosen to launch a pocketqube, the debris collision risk grows sharply with the starting altitude rising. Finally, by comparison with the trend of lifetime and collision risk, the interesting thing is that at some orbits with higher altitudes, like 800km, when the lifetime of the pocketqube reaches its maximum, the collision risk inversely reaches its local minimum, which can be useful for its designers to balance these two considerations

Leveraged borrowing and boom-bust cycles

Investment booms and asset "bubbles" are often the consequence of heavily leveraged borrowing and speculations of persistent growth in asset demand. We show theoretically that dynamic interactions between leveraged borrowing and persistent asset demand can generate a multiplier-accelerator mechanism that transforms a one-time technological innovation into large and long-lasting boom-bust cycles. The predictions are consistent with the basic features of investment booms and the consequent asset-market crashes led by excessive credit expansion.Asset pricing ; Credit

Resuscitating the credit cycle

This paper resuscitates the credit-cycle theory of Kiyotaki and Moore (1997) in a two-agent RBC model with conventional preferences and standard neoclassical technologies. It is shown that small transitory shocks to credit demand (or supply) can generate large, highly persistent, dampened cycles in aggregate output. Key to our results is the interaction between credit constraints and habit formation. Credit constraints based on collateralized assets mainly amplify the impact of shocks while habit formation in consumption demand mainly propagates it. Hump-shaped boom-bust cycles do not arise in the model under standard parameter values if either one of the two elements is missing.Credit

Doping a Mott Insulator: Physics of High Temperature Superconductivity

This article reviews the effort to understand the physics of high temperature superconductors from the point of view of doping a Mott insulator. The basic electronic structure of the cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. We introduce Anderson's idea of the resonating valence bond (RVB) and argue that it gives a qualitative account of the data. The importance of phase fluctuation is discussed, leading to a theory of the transition temperature which is driven by phase fluctuation and thermal excitation of quasiparticles. We then describe the numerical method of projected wavefunction which turns out to be a very useful technique to implement the strong correlation constraint, and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the t-J model, with the goal of putting the RVB idea on a more formal footing. The slave-boson is introduced to enforce the constraint of no double occupation. The implementation of the local constraint leads naturally to gauge theories. We give a rather thorough discussion of the role of gauge theory in describing the spin liquid phase of the undoped Mott insulator. We next describe the extension of the SU(2) formulation to nonzero doping. We show that inclusion of gauge fluctuation provides a reasonable description of the pseudogap phase.Comment: 69 pages, 36 fgiures. Submitted to Rev. Mod. Phy

Spontaneous spin ordering of Dirac spin liquid in a magnetic field

The Dirac spin liquid was proposed to be the ground state of the spin-1/2 Kagome antiferromagnets. In a magnetic field $B$, we show that the state with Fermi pocket is unstable to the Landau level (LL) state. The LL state breaks the spin rotation around the axis of the magnetic field. We find that the LL state has an in-plane 120$^{\circ}$ $q=0$ magnetization $M$ which scales with the external field $M\sim B^{\alpha}$, where $\alpha$ is an intrinsic calculable universal number of the Dirac spin liquid. We discuss the related experimental implications which can be used to detect the possible Dirac spin liquid phase in Herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$.Comment: rewritten for clarit

The state of commercial augmentative biological control: plenty of natural enemies, but a frustrating lack of uptake

Augmentative biological control concerns the periodical release of natural enemies. In com- mercial augmentative biological control, natural enemies are mass-reared in biofactories for release in large numbers to obtain an immediate control of pests. The history of commercial mass production of natural enemies spans a period of roughly 120 years. It has been a successful, environmentally and eco- nomically sound alternative for chemical pest control in crops like fruit orchards, maize, cotton, sugar cane, soybean, vineyards and greenhouses. Currently, aug- mentative biological control is in a critical phase, even though during the past decades it has moved from a cottage industry to professional production. Many efficient species of natural enemies have been discovered and 230 are commercially available today. The industry developed quality control guidelines, mass production, shipment and release methods as well as adequate guidance for farmers. However, augmentative biological control is applied on a frustratingly small acreage. Trends in research and application are reviewed, causes explaining the limited uptake are discussed and ways to increase application of augmentative biological control are explored

A kinematic wave theory of capacity drop

Capacity drop at active bottlenecks is one of the most puzzling traffic phenomena, but a thorough understanding is practically important for designing variable speed limit and ramp metering strategies. In this study, we attempt to develop a simple model of capacity drop within the framework of kinematic wave theory based on the observation that capacity drop occurs when an upstream queue forms at an active bottleneck. In addition, we assume that the fundamental diagrams are continuous in steady states. This assumption is consistent with observations and can avoid unrealistic infinite characteristic wave speeds in discontinuous fundamental diagrams. A core component of the new model is an entropy condition defined by a discontinuous boundary flux function. For a lane-drop area, we demonstrate that the model is well-defined, and its Riemann problem can be uniquely solved. We theoretically discuss traffic stability with this model subject to perturbations in density, upstream demand, and downstream supply. We clarify that discontinuous flow-density relations, or so-called "discontinuous" fundamental diagrams, are caused by incomplete observations of traffic states. Theoretical results are consistent with observations in the literature and are verified by numerical simulations and empirical observations. We finally discuss potential applications and future studies.Comment: 29 pages, 10 figure

Observations and Analysis of High-Resolution Magnetic Field Structures in Molecular Clouds

Recent high-angular-resolution (up to 0.7") dust polarization observations toward star forming regions are summarized. With the Sub-Millimeter Array, the emission from the dense structures is traced and resolved. The detected magnetic field morphologies vary from hourglass-like structures to isolated patches depending on the evolutionary stage of the source. These observed features have also served as a testbed to develop new analysis methods, with a particular focus on quantifying the role of the magnetic field in the star formation process.Comment: 4 pages, 2 figures; To appear in Proceedings of Magnetic Fields in the Universe: From Laboratory and Stars to Primordial Structures Aug. 21st - 27th 2011, Zakopane, Poland Eds. M. Soida, K. Otmianowska-Mazur, E.M. de Gouveia Dal Pino & A. Lazaria