Non Linear Current Response of a Many-Level Tunneling System: Higher Harmonics Generation

The fully nonlinear response of a many-level tunneling system to a strong alternating field of high frequency $\omega$ is studied in terms of the Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent tunneling current $I(t)$ is calculated exactly and its resonance structure is elucidated. In particular, it is shown that under certain reasonable conditions on the physical parameters, the Fourier component $I_{n}$ is sharply peaked at $n=\frac {\Delta E} {\hbar \omega}$, where $\Delta E$ is the spacing between two levels. This frequency multiplication results from the highly nonlinear process of $n$ photon absorption (or emission) by the tunneling system. It is also conjectured that this effect (which so far is studied mainly in the context of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from [email protected], submitted to Phys.Rev.

The Student Nitric Oxide Explorer

The Student Nitric Oxide Explorer (SNOE) is a small scientific spacecraft designed for launch on a Pegasus™ XL launch vehicle for the USRA Student Explorer Demonstration Initiative. Its scientific goals are to measure nitric oxide density in the lower thermosphere and analyze the energy inputs to that region from the sun and magnetosphere that create it and cause its abundance to vary dramatically. These inputs are energetic solar photons in the EUV and X -ray spectral regions, and energetic electrons that are accelerated into the polar regions, where they cause auroral disturbances and displays. Both of these phenomena are aspects of solar variability; thermospheric nitric oxide responds to that variability and in turn determines key temperature and compositional aspects of the thermosphere and ionosphere through its radiative and chemical properties. The SNOE ( snowy ) spacecraft and its instrument complement is being designed, built, and operated entirely at the University of Colorado, Laboratory for Atmospheric and Space Physics (CU/LASP). The spacecraft is a compact hexagonal structure, 37 high and 39 across its widest dimension, weighing approximately 220 Ibs. It will be launched into a circular orbit, 550±50 km altitude, at 97.5° inclination for sun-synchronous precession at 10:30-22:30 solar time. It will spin at 5 rpm with the spin axis normal to the orbit plane. It carries three instruments: An ultraviolet spectrometer to measure nitric oxide altitude profiles, a two-channel ultraviolet photometer to measure auroral emissions beneath the spacecraft, and a five-channel solar soft X-ray photometer. The spacecraft structure is aluminum, with a center platform section for the instruments and primary components and truss work to hold the solar arrays. Power is regulated using switched arrays and a partial shunt. The attitude determination and control system uses a magnetometer, two torque rods, and two horizon crossing indicators to measure spin rate and orientation. Attitude control is implemented open-loop by ground commands. The command and data handling system is implemented using a single spacecraft microprocessor that handles all spacecraft and communications functions and instrument data. The communications system is NASA compatible for downlink using the Autonomous Ground Services station at Poker Flat; all mission operations, data processing, and analysis will be performed using a project operations control center (POCC) at the LASP Space Technology Research building

Global desertification: building a science for dryland development

In this millennium, global drylands face a myriad of problems that present tough research, management, and policy challenges. Recent advances in dryland development, however, together with the integrative approaches of global change and sustainability science, suggest that concerns about land degradation, poverty, safeguarding biodiversity, and protecting the culture of 2.5 billion people can be confronted with renewed optimism. We review recent lessons about the functioning of dryland ecosystems and the livelihood systems of their human residents and introduce a new synthetic framework, the Drylands Development Paradigm (DDP). The DDP, supported by a growing and well-documented set of tools for policy and management action, helps navigate the inherent complexity of desertification and dryland development, identifying and synthesizing those factors important to research, management, and policy communities