Photoacoustic Tomography in a Rectangular Reflecting Cavity

Almost all known image reconstruction algorithms for photoacoustic and thermoacoustic tomography assume that the acoustic waves leave the region of interest after a finite time. This assumption is reasonable if the reflections from the detectors and surrounding surfaces can be neglected or filtered out (for example, by time-gating). However, when the object is surrounded by acoustically hard detector arrays, and/or by additional acoustic mirrors, the acoustic waves will undergo multiple reflections. (In the absence of absorption they would bounce around in such a reverberant cavity forever). This disallows the use of the existing free-space reconstruction techniques. This paper proposes a fast iterative reconstruction algorithm for measurements made at the walls of a rectangular reverberant cavity. We prove the convergence of the iterations under a certain sufficient condition, and demonstrate the effectiveness and efficiency of the algorithm in numerical simulations.Comment: 21 pages, 6 figure

Buying Time 2000: Television Advertising in the 2000 Federal Elections

Summarizes a study of political television advertising in the 2000 federal primaries and elections with a focus on the use of the issue ad loophole to evade campaign finance laws. Questions the standard used to differentiate issue ads from election ads

Pioneer Venus spacecraft charging model

Five environmental models were constructed to represent the solar wind and the upper, middle, and lower ionosphere of Venus. The spacecraft structure was modeled with over 140 passive electrical elements representing structural elements of the spacecraft. Electron, ion, secondary electron, and photocurrents to the spacecraft from the plasma were calculated, ignoring sheath effects. In all but one case, potentials of interest were less than 1 volt. Potential differences between widely separated points on the equipment shelf were less than 1 mV. The one area of concern is the solar panel potential when the orbiter is passing through the bowshock region

Canadian national sport organisations’ use of the web for relationship marketing in promoting sport participation

Sport participation development requires a systematic process which involves knowledge creation, dissemination and interactions between National Sport Organisations, participants, clubs and associations as well as other agencies. Using a relationship marketing approach (Grönroos, 1997, Gummesson, 2002, Olkkonen, 1999), this paper addressed the question ‘How do Canadian NSOs use the Web, in terms of functionality and services offered, to create and maintain relationships with sport participants and their sport delivery partners?’ Ten Canadian NSOs’ websites were examined: functionality was analysed using Burgess and Cooper’s (2000) eMICA model, while NSOs’ utilisation of the Internet to establish and maintain relationships with sport participants was analysed using Wang, Head and Archer’s (2000) relationship-building process model for the Web. It was found that Canadian NSOs were receptive to the use of the Web, but their information-gathering and dissemination activities, which make-up the relationship-building process, appear sparse, and in some cases are lagging behind the voluntary sector in the country

The evolution and star formation of dwarf galaxies in the Fornax Cluster

We present the results of a spectroscopic survey of 675 bright (16.5<Bj<18) galaxies in a 6 degree field centred on the Fornax cluster with the FLAIR-II spectrograph on the UK Schmidt Telescope. We measured redshifts for 516 galaxies of which 108 were members of the Fornax Cluster. Nine of these are new cluster members previously misidentified as background galaxies. The cluster dynamics show that the dwarf galaxies are still falling into the cluster whereas the giants are virialised. Our spectral data reveal a higher rate of star formation among the dwarf galaxies than suggested by morphological classification: 35 per cent have H-alpha emission indicative of star formation but only 19 per cent were morphologically classified as late-types. The distribution of scale sizes is consistent with evolutionary processes which transform late-type dwarfs to early-type dwarfs. The fraction of dwarfs with active star formation drops rapidly towards the cluster centre. The star-forming dwarfs are concentrated in the outer regions of the cluster, the most extreme in an infalling subcluster. We estimate gas depletion time scales for 5 dwarfs with detected HI emission: these are long (of order 10 Gyr), indicating that active gas removal must be involved if they are transformed into gas-poor dwarfs as they fall further into the cluster. In agreement with our previous results, we find no compact dwarf elliptical (M32-like) galaxies in the Fornax Cluster.Comment: To appear in Monthly Notices of the Royal Astronomical Societ

Using Transit Timing Observations to Search for Trojans of Transiting Extrasolar Planets

Theoretical studies predict that Trojans are likely a frequent byproduct of planet formation and evolution. We examine the sensitivity of transit timing observations for detecting Trojan companions to transiting extrasolar planets. We demonstrate that this method offers the potential to detect terrestrial-mass Trojans using existing ground-based observatories. We compare the transit timing variation (TTV) method with other techniques for detecting extrasolar Trojans and outline the future prospects for this method.Comment: submitted to ApJL, 12 pages, 2 figure

On the Rotation Period of (90377) Sedna

We present precise, ~1%, r-band relative photometry of the unusual solar system object (90377) Sedna. Our data consist of 143 data points taken over eight nights in October 2004 and January 2005. The RMS variability over the longest contiguous stretch of five nights of data spanning nine days is only 1.3%. This subset of data alone constrain the amplitude of any long-period variations with period P to be A<1% (P/20 days)^2. Over the course of any given 5-hour segment, the data exhibits significant linear trends not seen in a comparison star of similar magnitude, and in a few cases these segments show clear evidence for curvature at the level of a few millimagnitudes per hour^2. These properties imply that the rotation period of Sedna is O(10 hours), cannot be 10 days, unless the intrinsic light curve has significant and comparable power on multiple timescales, which is unlikely. A sinusoidal fit yields a period of P=(10.273 +/- 0.002) hours and semi-amplitude of A=(1.1 +/- 0.1)%. There are additional acceptable fits with flanking periods separated by ~3 minutes, as well as another class of fits with P ~ 18 hours, although these later fits appear less viable based on visual inspection. Our results indicate that the period of Sedna is likely consistent with typical rotation periods of solar system objects, thus obviating the need for a massive companion to slow its rotation.Comment: 7 pages, 4 figures, 2.5 tables. Final ApJL version, minor changes. Full light curve data in tex

Low-Altitude Reconnection Inflow-Outflow Observations during a 2010 November 3 Solar Eruption

For a solar flare occurring on 2010 November 3, we present observations using several SDO/AIA extreme-ultraviolet (EUV) passbands of an erupting flux rope followed by inflows sweeping into a current sheet region. The inflows are soon followed by outflows appearing to originate from near the termination point of the inflowing motion - an observation in line with standard magnetic reconnection models. We measure average inflow plane-of-sky speeds to range from ~150-690 km/s with the initial, high-temperature inflows being the fastest. Using the inflow speeds and a range of Alfven speeds, we estimate the Alfvenic Mach number which appears to decrease with time. We also provide inflow and outflow times with respect to RHESSI count rates and find that the fast, high-temperature inflows occur simultaneously with a peak in the RHESSI thermal lightcurve. Five candidate inflow-outflow pairs are identified with no more than a minute delay between detections. The inflow speeds of these pairs are measured to be 10^2 km/s with outflow speeds ranging from 10^2-10^3 km/s - indicating acceleration during the reconnection process. The fastest of these outflows are in the form of apparently traveling density enhancements along the legs of the loops rather than the loop apexes themselves. These flows could either be accelerated plasma, shocks, or waves prompted by reconnection. The measurements presented here show an order of magnitude difference between the retraction speeds of the loops and the speed of the density enhancements within the loops - presumably exiting the reconnection site.Comment: 31 pages, 13 figures, 1 table, Accepted to ApJ (expected publication ~July 2012

Stability of Satellites in Closely Packed Planetary Systems

We perform numerical integrations of four-body (star, planet, planet, satellite) systems to investigate the stability of satellites in planetary Systems with Tightly-packed Inner Planets (STIPs). We find that the majority of closely-spaced stable two-planet systems can stably support satellites across a range of parameter-space which is only slightly decreased compared to that seen for the single-planet case. In particular, circular prograde satellites remain stable out to $\sim 0.4 R_H$ (where $R_H$ is the Hill Radius) as opposed to $\sim 0.5 R_H$ in the single-planet case. A similarly small restriction in the stable parameter-space for retrograde satellites is observed, where planetary close approaches in the range 2.5 to 4.5 mutual Hill radii destabilize most satellites orbits only if $a\sim 0.65 R_H$. In very close planetary pairs (e.g. the 12:11 resonance) the addition of a satellite frequently destabilizes the entire system, causing extreme close-approaches and the loss of satellites over a range of circumplanetary semi-major axes. The majority of systems investigated stably harbored satellites over a wide parameter-space, suggesting that STIPs can generally offer a dynamically stable home for satellites, albeit with a slightly smaller stable parameter-space than the single-planet case. As we demonstrate that multi-planet systems are not a priori poor candidates for hosting satellites, future measurements of satellite occurrence rates in multi-planet systems versus single-planet systems could be used to constrain either satellite formation or past periods of strong dynamical interaction between planets.Comment: 11 pages, 5 figures. Accepted for publication, ApJ