Medium energy neutral atom (MENA) imager for the IMAGE mission

The Medium Energy Neutral Atom (MENA) imager was developed in response to the Imaging from the Magnetopause to the Aurora for Global Exploration (IMAGE) requirement to produce images of energetic neutral atoms (ENAs) in the energy range from 1 to 30 keV. These images will be used to infer characteristics of magnetospheric ion distributions. The MENA imager is a slit camera that images incident ENAs in the polar angle (based on a conventional spherical coordinate system defined by the spacecraft spin axis) and utilizes the spacecraft spin to image in azimuth. The speed of incident ENAs is determined by measuring the time-of-flight (TOF) from the entrance aperture to the detector. A carbon foil in the entrance aperture yields secondary electrons, which are imaged using a position-sensitive Start detector segment. This provides both the one-dimensional (1D) position at which the ENA passed through the aperture and a Start time for the TOF system. Impact of the incident ENA on the 1D position-sensitive Stop detector segment provides both a Stop-timing signal and the location that the ENA impacts the detector. The ENA incident polar angle is derived from the measured Stop and Start positions. Species identification (H vs. O) is based on variation in secondary electron yield with mass for a fixed ENA speed. The MENA imager is designed to produce images with 8°×4° angular resolution over a field of view 140°×360°, over an energy range from 1 keV to 30 keV. Thus, the MENA imager is well suited to conduct measurements relevant to the Earth's ring current, plasma sheet, and (at times) magnetosheath and cusp.     

Impact of loading effects on determination of the International Terrestrial Reference Frame

The International Terrestrial Reference Frame (ITRF), as a realization of the International Terrestrial Reference System (ITRS), is represented by a set of station positions and linear velocities. They are intended to be used as regularized coordinates to which some corrections should be added to access instantaneous coordinates. The latest ITRS realization is the ITRF2005, which has integrated time series of station positions to form long-term solutions for the four space geodetic techniques. Currently, a purely linear model is used to parameterize station displacements in the estimation process, plus occasional discontinuities in case of earthquakes or equipment changes. However the input data have been derived without applying surface loading models and so surface loading effects are supposed to be embedded in the coordinates as measured quantities. We evaluate the effect of applying a posteriori loading corrections, which include the effect of atmospheric, non-tidal ocean, and continental water loading, to time series of positions estimated from Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Global Positioning System (GPS) data. We notice that they reduce about 50% or more of the annual signals in the translation and scale parameter time series of the SLR and VLBI techniques, except in SLR Z translation. In general, the estimated secular frame definition is negligibly affected and estimated positions and velocities are not significantly modified for stations that have accumulated a large number of observations. A multi-technique combination of such derived frames allows concluding that, for some cases, loading model corrections might degrade co-located station coordinates almost as much as they benefit them. However, most significant improvement of the estimated secular coordinates is observed for stations with less than 100 estimated positions as demonstrated with a multi-technique combination.     

Numerical integration of stochastic differential equations: A parallel cosmic ray modulation implementation on Africa’s fastest computer

Three-dimensional studies of the transport and modulation of cosmic ray particles in turbulent astrospheres require large-scale simulations using specialized scientific codes. Essentially, a multi-dimensional Fokker-Planck type equation (a parabolic diffusion equation) must be integrated numerically. One such approach is to convert the relevant transport equation into a set of stochastic differential equations (SDEs), with the latter much easier to handle numerically. Due to the growing demand for high performance computing resources, research into the application of effective and suitable numerical algorithms to solve such equations is needed. We present a case study of the performance of a custom-written FORTRAN SDE numerical solver on the CHPC (Centre for High Performance Computing) Lengau cluster in South Africa for a realistic test problem with different set-ups. It is shown that SDE codes can scale very well on large parallel computing platforms. Finally, we consider an extremely computationally expensive application of the SDE approach to cosmic ray modulation, studying the behaviour of galactic cosmic ray proton latitude gradients and relative amplitudes in a physics-first manner. This is done using a modulation code that employs diffusion coefficients derived from first principles, which in turn are functions of turbulence quantities in reasonable agreement with spacecraft observations and modelled using a two-component turbulence transport model (TTM). We show that this approach leads to reduced latitude gradients qualitatively in line with spacecraft observations of the same, without making ad hoc assumptions as to anisotropic perpendicular diffusion coefficients as are often made in many cosmic ray modulation studies.     

The Shuttle Radar Topography Mission (SRTM): a breakthrough in remote sensing of topography

The Shuttle Radar Topography Mission (SRTM), flown on the Space Shuttle Endeavour on Flight STS-99 and launched on 11 February 2000, will produce digital elevation data of the Earth's land mass between 60 degrees north latitude and 54 degrees south latitude. This data will be at least one order of magnitude more precise in the elevation resolution, and will have postings of 30 m, representing an order of magnitude increase in the density of the postings over currently available data.     

Challenges in X-ray binary timing: current and future

Millisecond X-ray time variability studies of accreting low-magnetic-field neutron stars and stellar-mass black holes in X-ray binaries probe the motion of matter in regions of strong gravity. In these regions, general relativity (GR) is no longer a small correction to the classical laws of motion, but instead dominates the dynamics: we are studying motion in strongly curved spacetime. Such millisecond X-ray variability studies can therefore provide unique tests of GR in the strong-field regime. The same studies also constrain neutron-star parameters such as stellar mass and radius, and thereby the equation of state (EOS) of ultradense matter. I briefly review the status, and discuss the prospects for mapping out space-time near accreting stellar-mass compact objects, and measuring the EOS of dense matter, through millisecond timing, particularly with an eye towards future missions. The overwhelming consideration for timing sensitivity is collecting area: contrary to most applications, the signal-to-noise ratio for the aperiodic timing phenomena produced by accretion flows increases proportionally with count rate rather than as the square root of it. A 10 times larger instrument turns 1σ effects into 10σ effects (or does as well in 1% of the time). With the Rossi X-ray Timing Explorer (RXTE), using 0.6 m² collecting area, we have found several timing diagnostics from the accretion flow in the strong field region around neutron stars and black holes, as well as signals from neutron star surface hot spots. Combined work between RXTE and the new sensitive X-ray spectrographs onboard Chandra and XMM can already begin to clinch the geometry and physical mechanisms underlying these signals. Future instruments, larger in area by an order of magnitude and in some cases with enhanced spectral capabilities, are expected to turn these diagnostics of GR into true tests of GR. They are also expected to put strong constraints on neutron-star structure, and thereby on the EOS of supranuclear density matter.     

On maximum luminosities of X-ray bursts

In this paper we address problems associated with the observation of super-Eddington luminosities in X-ray burst sources and discuss some possible solutions.     

THE SIGNAL OF THE 11-YEAR SUNSPOT CYCLE IN THE UPPER TROPOSPHERE-LOWER STRATOSPHERE

The paper summarizes work by the authors over the past ten years on an apparent signal of the 11-year sunspot cycle in the lower stratosphere-upper troposphere. The signal appears as a basic, consistent pattern in correlations between heights of stratospheric constant-pressure levels, at least as high as 25 km, and the solar cycle in which the highest correlations are in the subtropics.The variation of the stratospheric heights in phase with the sunspot cycle are – in the areas of high correlations between the two – associated with temperature variations on the same time scale in the middle and upper troposphere. The spatial distribution of the correlations suggests that the year-to-year changes in tropical and subtropical vertical motions contain a component on the time scale of the solar cycle.In January and February the correlations with the sunspot cycle are smallest. The smallness of the correlations is owing to the fact that they are different in the east and west years of the quasi-biennial oscillation in the equatorial stratospheric winds. The correlation pattern in the east years is the same as in the other seasons and is statistically significant. In the west years the correlations are insignificant outside the arctic, and the positive correlation in the arctic in these years is related to the fact that major midwinter breakdowns of the cyclonic vortex in the west years so far have happened only at maxima in the solar cycle.Until recently reliable continuous series of analyses of the stratosphere were not available for the southern hemisphere. The U.S. National Centers for Environmental Prediction and the National Center for Atmospheric Research have now, however, issued a 23-year series of re-analyzed global data which has made it possible to detect the solar signal on the southern hemisphere. It turns out to be almost the same as that on the northern hemisphere.The correlations between total column ozone and the sunspot cycle are lowest in the equatorial regions, where ozone is produced, and in the subpolar regions, where the largest amounts are found. In the annual mean the largest correlations lie between 5° lat. and 30° lat. We suggest that this distribution of correlations is due to the fact that the subtropical heights of the constant-pressure surfaces in the ozone layer are higher in maximum than in minimum years of the sunspot cycle, and that the higher subtropical heights in the solar maxima depress the poleward transport of ozone through the subtropics and thus create an abundance of ozone.