Antenna array beam steering using time-varying weights

The author presents a new technique of steering an array antenna by introducing time-varying phase weights. It is shown that the technique is equivalent to placing a linear phase and linear frequency offset across the array when transmitting a linear frequency modulated (LFM) waveform. The technique reduces array dispersion and increases the operating bandwidth. An element level signal generator is presented as a possible way of implementing the time-varying weights. A possible array implementation is also shown.<>     

The capabilities of a gas scintillation camera for cosmic X-ray astronomy

The capabilities of a gas scintillator camera for use in X-ray astronomy are investigated. Detailed experimental results are presented on both the position and energy resolution over the energy range 0.28 to 6 keV. The energy resolution varies from 38% to 9.5% for 0.28 keV C-K and 6 keV X-rays respectively. Position resolutions of 1.8 mm and 3.5 mm for 6 keV and 1.5 keV Al-K X-rays were obtained. Image quality and uniformity over the camera aperture are also discussed, whilst further techniques leading to improvements in position resolution are outlined. Finally applications of these cameras in conjunction with grazing incidence and coded mask X-ray optics are discussed.     

Differential Lobing in Sidelobe Suppression Systems

The sidelobe suppression function in the secondary surveillance radar system is analyzed when separate main and control antenna are used. Their vertical lobing structures are not matched with each other, so a differential lobing pattern comes out whose maxima and minima are enveloped by smooth curves depending on vertical displacement of phase centers and horizon cutoff rates. The developed analysis provides a method for the evaluation of the system performance for any antenna configuration and for the determination of design guidelines.     

Heliopause stability revisited: dispersion analysis and numerical simulations

The heliopause, a surface separating the tenuous hot heliosheath flow and the dense, strongly magnetized interstellar flow, is subject to instabilities of the Rayleigh–Taylor and Kelvin–Helmholtz types. The dynamic evolution of this discontinuity is of considerable importance for understanding the neutral atom and cosmic-ray filtration at the interface. Here, we investigate the stability of the upwind heliopause in the presence of charge exchange collisions using both an analytic (dispersion relation) approach and a numerical model that includes the interstellar magnetic field. The linear analysis yields a cubic dispersion relation that admits imaginary solutions for the full range of wavenumbers, implying that the stagnation point on the heliopause is unconditionally Rayleigh–Taylor unstable to small perturbations propagating parallel to the discontinuity surface. We confirm this result by following the nonlinear development of the instability with a time-dependent simulation using a four fluid MHD-neutral numerical code. For the typical solar wind and LISM conditions, we obtain cyclical evolution of the upwind heliopause with a period of the order of 100 years. We also identify two areas of space physics where the instability may have important implications.     

Space weather and commercial airlines

Space weather phenomena can effect many areas of commercial airline operations including avionics, communications and GPS navigation systems. Of particular importance at present is the recently introduced EU legislation requiring the monitoring of aircrew radiation exposure, including any variations at aircraft altitudes due to solar activity. With the introduction of new ultra-long-haul “over-the-pole” routes, “more-electric” aircraft in the future, and the increasing use of satellites in the operation, the need for a better understanding of the space weather impacts on future airline operations becomes all the more compelling. This paper will present the various space weather effects, some provisional results of an ongoing 3-year study to monitor cosmic radiation in aircraft, and conclude by summarising some of the identified key operational issues, which must be addressed, with the help of the science community, if the airlines want to benefit from the availability of space weather services.     

Monopolar structure of the Sun in between polar reversals and in Maunder Minimum

After a polar reversal in one hemisphere the Sun has two polar caps of the same sign, leaving it in a kind of monopolar state. It may take months before a polar reversal occurs in the other hemisphere. The situation may have been extreme in the Maunder Minimum where the northern hemisphere most probably did not have polar reversals during several cycles, while the southern hemisphere may have had some. This may affect the interplanetary field and thus the cosmic rays reaching the Earth. Using the relation between the Wolf number and the speed of the global magnetic field regions the yearly mean Wolf number has to exceed 40 in order to have polar reversals, hence per hemisphere we expect that it must exceed 20. This may be used to give a definition of a deep minimum.     

Chromospheric line emission in seismically active flares

Some flares are known to drive seismic transients into the solar interior. The effects of these seismic transients are seen in helioseismic observations of the Sun’s surface thousands of km from their sources in the hour succeeding the impulsive phase of the flare. Energetic particles impinging from the corona into the chromosphere are known to drive strong, downward-propagating shocks in active region chromospheres during the impulsive phases of flares. H observations have served as an important diagnostic of these shocks, showing intense emission with characteristic transient redshifts. In most flares no detectable transients penetrate beneath the active region photosphere. In those that do, there is a strong correlation between compact white-light emission and the signature of seismic emission. This study introduces the first known H observations of acoustically active flares, centered in the core of the line. The morphology of line-core emission H in the impulsive phase of the flare is similar to that of co-spatial line-core emission in NaD₁, encompassing the site of seismic emission but more extended. The latter shows a compact red shift in the region of seismic emission, but a similar feature is known to appear in a conjugate magnetic footpoint from which no seismic emission emanates. Radiative MHD modelling based on the profiles of chromospheric line emission during the impulsive phases of flares can contribute significantly to our understanding of the mechanics of flare acoustic emission penetrating into the solar interior and the conditions under which it occurs.     

Wave perturbations in the MLT at high northern latitudes in winter,observed by two SATI instruments

To investigate the Mesosphere and Lower Thermosphere (MLT) region, several ground-based instruments called SATI (Spectral Airglow Temperature Imager) were designed and built to measure airglow emission and temperature in the upper mesosphere. One SATI instrument was installed at Resolute Bay (74.7°N, 94.9°W) and has monitored the polar MLT region since November, 2001. In October 2007 another SATI instrument was installed at Eureka (80.0°N, 86.3°W) at the Polar Environment Atmospheric Research Laboratory (PEARL) as part of the Canadian Network for the Detection of Atmospheric Change (CANDAC) project. SATI is a spatial scanning Fabry–Perot spectrometer measuring column emission rates for several rotational lines of OH and O₂ airglow at 87 and 94 km height. The rotational temperatures are inferred from the ratios of these lines. The measurements are divided into 12 sectors with an annular field of view. The phase differences between the sectors yield information on the horizontal atmospheric wave direction and wavelength. Horizontal perturbations of 2–8 h period have correlatively been observed and investigated at both locations. Short-periodic oscillations identified as gravity waves with periods between 2 and 8 h propagate in southward and eastward directions, but in opposite directions in some cases. The wave propagation characteristics are often different at the two locations; the relationship with the lower mean wind is considered.     

Utilization of CAM,CAF, MAX,and FAX for space radiation analyses using HZETRN

To estimate astronaut health risk due to space radiation, one must have the ability to calculate various exposure-related quantities that are averaged over specific organs and tissue types. Such calculations require computational models of the ambient space radiation environment, particle transport, nuclear and atomic physics, and the human body. While significant efforts have been made to verify, validate, and quantify the uncertainties associated with many of these models and tools, relatively little work has focused on the uncertainties associated with the representation and utilization of the human phantoms. In this study, we first examine the anatomical properties of the Computerized Anatomical Man (CAM), Computerized Anatomical Female (CAF), Male Adult voXel (MAX), and Female Adult voXel (FAX) models by comparing the masses of various model tissues used to calculate effective dose to the reference values specified by the International Commission on Radiological Protection (ICRP). The MAX and FAX tissue masses are found to be in good agreement with the reference data, while major discrepancies are found between the CAM and CAF tissue masses and the reference data for almost all of the effective dose tissues. We next examine the distribution of target points used with the deterministic transport code HZETRN (High charge (Z) and Energy TRaNsport) to compute mass averaged exposure quantities. A numerical algorithm is presented and used to generate multiple point distributions of varying fidelity for many of the effective dose tissues identified in CAM, CAF, MAX, and FAX. The point distributions are used to compute mass averaged dose equivalent values under both a galactic cosmic ray (GCR) and solar particle event (SPE) environment impinging isotropically on three spherical aluminum shells with areal densities of 0.4 g/cm², 2.0 g/cm², and 10.0 g/cm². The dose equivalent values are examined to identify a recommended set of target points for each of the tissues and to further assess the differences between CAM, CAF, MAX, and FAX. It is concluded that the previously published CAM and CAF point distributions were significantly under-sampled and that the set of point distributions presented here should be adequate for future studies involving CAM, CAF, MAX, or FAX. It is also found that the errors associated with the mass and location of certain tissues in CAM and CAF have a significant impact on the mass averaged dose equivalent values, and it is concluded that MAX and FAX are more accurate than CAM and CAF for space radiation analyses.     

On high-altitude electric fields in the auroral downward current region and their coupling to ionospheric electric fields

The downward field-aligned current region plays an active role in magnetosphere–ionosphere coupling processes associated with aurora. A quasi-static electric field structure with a downward parallel electric field forms at altitudes between 800 km and 5000 km, accelerating ionospheric electrons upward, away from the auroral ionosphere. Other phenomena including energetic ion conics, electron solitary waves, low-frequency wave activity, and plasma density cavities occur in this region, which also acts as a source region for VLF saucers. Results are presented from high-altitude Cluster observations with particular emphasis on the characteristics and dynamics of quasi-static electric field structures. These, extending up to altitudes of at least 4–5 Earth radii, appear commonly as monopolar or bipolar electric fields. The former occur at sharp boundaries, such as the polar cap boundary whereas the bipolar fields occur at softer boundaries within the plasma sheet. The temporal evolution of quasi-static electric field structures, as captured by the pearls-on-a-string configuration of the Cluster spacecraft, indicates that the formation of electric field structures and of ionospheric plasma density cavities are closely coupled processes. A related feature of the downward current is a broadening of the current sheet with time, possibly related to the depletion process. Preliminary studies of the coupling of electric fields in the downward current region, show that small-scale structures are typically decoupled from the ionosphere, similar to what has been found for the upward current region. However, exceptions are also found where small-scale electric fields couple perfectly between the ionosphere and Cluster altitudes. Recent FAST results indicate that the degree of coupling differs between sheet-like and curved structures, and that it is typically partial. The electric field coupling further depends on the current–voltage relationship, which is highly non-linear in the downward current region, and still unrevealed, as to its specific form.