Maxwell currents beneath thunderstorms

Analyses of single station measurements of the Maxwell current density (Jm) made under Florida thunderstorms during the summer of 1981 have been completed. The results of these analyses indicate that: (1) Jm is usually dominated by the displacement current component when the electric field is close to zero; (2) Jm is steady with time in the intervals between lightning flashes; (3) Jm is not altered significantly by lightning; and (4) the average value of Jm changes slowly and over time scales that are comparable to those required for storm development. Maps have also been derived of the surface Maxwell current density for a number of the Florida TRIP (76-78) storms using field mill data to estimate Jm from the displacement current density. Studies show that these maps provide a good indication of the location and relative intensity of the storm current generators, and area-integrations of the current contours provide estimates of the total storm currents

Lightning Imaging Sensor on International Space Station

No abstract availabl

Principal Component Analysis of the Time- and Position-Dependent Point Spread Function of the Advanced Camera for Surveys

We describe the time- and position-dependent point spread function (PSF) variation of the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) with the principal component analysis (PCA) technique. The time-dependent change is caused by the temporal variation of the $HST$ focus whereas the position-dependent PSF variation in ACS/WFC at a given focus is mainly the result of changes in aberrations and charge diffusion across the detector, which appear as position-dependent changes in elongation of the astigmatic core and blurring of the PSF, respectively. Using >400 archival images of star cluster fields, we construct a ACS PSF library covering diverse environments of the $HST$ observations (e.g., focus values). We find that interpolation of a small number ($\sim20$) of principal components or ``eigen-PSFs'' per exposure can robustly reproduce the observed variation of the ellipticity and size of the PSF. Our primary interest in this investigation is the application of this PSF library to precision weak-lensing analyses, where accurate knowledge of the instrument's PSF is crucial. However, the high-fidelity of the model judged from the nice agreement with observed PSFs suggests that the model is potentially also useful in other applications such as crowded field stellar photometry, galaxy profile fitting, AGN studies, etc., which similarly demand a fair knowledge of the PSFs at objects' locations. Our PSF models, applicable to any WFC image rectified with the Lanczos3 kernel, are publicly available.Comment: Accepted to PASP. To appear in December issue. Figures are degraded to meet the size limit. High-resolution version can be downloaded at http://acs.pha.jhu.edu/~mkjee/acs_psf/acspsf.pd

The Principal Axis of the Virgo Cluster

Using accurate distances to individual Virgo cluster galaxies obtained by the method of Surface Brightness Fluctuations, we show that Virgo's brightest ellipticals have a remarkably collinear arrangement in three dimensions. This axis, which is inclined by 10 to 15 degrees from the line of sight, can be traced to even larger scales where it appears to join a filamentary bridge of galaxies connecting Virgo to the rich cluster Abell 1367. The orientations of individual Virgo ellipticals also show some tendency to be aligned with the cluster axis, as does the jet of the supergiant elliptical M87. These results suggest that the formation of the Virgo cluster, and its brightest member galaxies, have been driven by infall of material along the Virgo-A1367 filament.Comment: 8 pages, 4 figures, accepted for publication in ApJ Letter

ALDF Data Retrieval Algorithms for Validating the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS)

A linear algebraic solution is provided for the problem of retrieving the location and time of occurrence of lightning ground strikes from in Advanced Lightning Direction Finder (ALDF) network. The ALDF network measures field strength, magnetic bearing, and arrival time of lightning radio emissions and solutions for the plane (i.e.. no Earth curvature) are provided that implement all of these measurements. The accuracy of the retrieval method is tested using computer-simulated data sets and the relative influence of bearing and arrival time data on the outcome of the final solution is formally demonstrated. The algorithm is sufficiently accurate to validate NASA's Optical Transient Detector (OTD) and Lightning Imaging System (LIS). We also introduce a quadratic planar solution that is useful when only three arrival time measurements are available. The algebra of the quadratic root results are examined in detail to clarify what portions of the analysis region lead to fundamental ambiguities in source location. Complex root results are shown to be associated with the presence of measurement errors when the lightning source lies near an outer sensor baseline of the ALDF network. For arbitrary noncollinear network geometries and in the absence of measurement errors, it is shown that the two quadratic roots are equivalent (no source location ambiguity) on the outer sensor baselines. The accuracy of the quadratic planar method is tested with computer-generated data sets and the results are generally better than those obtained from the three station linear planar method when bearing errors are about 2 degrees

Global Electric Circuit Diurnal Variation Derived from Storm Overflight and Satellite Optical Lightning Datasets

We have combined analyses of over 1000 high altitude aircraft observations of electrified clouds with diurnal lightning statistics from the Lightning Imaging Sensor (LIS) and Optical Transient Detector (OTD) to produce an estimate of the diurnal variation in the global electric circuit. Using basic assumptions about the mean storm currents as a function of flash rate and location, and the global electric circuit, our estimate of the current in the global electric circuit matches the Carnegie curve diurnal variation to within 4% for all but two short periods of time. The agreement with the Carnegie curve was obtained without any tuning or adjustment of the satellite or aircraft data. Mean contributions to the global electric circuit from land and ocean thunderstorms are 1.1 kA (land) and 0.7 kA (ocean). Contributions to the global electric circuit from ESCs are 0.22 kA for ocean storms and 0.04 kA for land storms. Using our analysis, the mean total conduction current for the global electric circuit is 2.0 kA

Star Formation at z~6: The UDF-Parallel ACS Fields

We report on the i-dropouts detected in two exceptionally deep ACS fields (B_{435}, V_{606}, i_{775}, and z_{850} with 10 sigma limits of 28.8, 29.0, 28.5, and 27.8, respectively) taken in parallel with the UDF NICMOS observations. Using an i-z>1.4 cut, we find 30 i-dropouts over 21 arcmin^2 down to z_AB=28.1, or 1.4 i-dropouts arcmin^{-2}, with significant field-to-field variation (as expected from cosmic variance). This extends i-dropout searches some ~0.9^m further down the luminosity function than was possible in the GOODS field, netting a ~7x increase in surface density. An estimate of the size evolution for UV bright objects is obtained by comparing the composite radial flux profile of the bright i-dropouts (z<27.2) with scaled versions of the HDF-N + HDF-S U-dropouts. The best-fit is found with a (1+z)^{-1.57_{-0.53} ^{+0.50}} scaling in size (for fixed luminosity), extending lower redshift (1<z<5) trends to z~6. Adopting this scaling and the brighter i-dropouts from both GOODS fields, we make incompleteness estimates and construct a z~6 LF in the rest-frame continuum UV (~1350 A) over a 3.5 magnitude baseline, finding a shape consistent with that found at lower redshift. To evaluate the evolution in the LF from z~3.8, we make comparisons against different scalings of a lower redshift B-dropout sample. Though a strong degeneracy is found between luminosity and density evolution, our best-fit model scales as (1+z)^{-2.8} in number and (1+z)^0.1 in luminosity, suggesting a rest-frame continuum UV luminosity density at z~6 which is just 0.38_{-0.07} ^{+0.09}x that at z~3.8. Our inclusion of size evolution makes the present estimate lower than previous z~6 estimates.Comment: 5 pages, 5 figures, accepted for publication in the Astrophysical Journal Letters, labelling to the left-hand axis of Figure 4 correcte

The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency

We study the rich globular cluster (GC) system in the center of the massive cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude I_814=29 with >90% completeness and sample the brightest ~5% of the GC system. Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we estimate a total population of N(GC_total) = 162,850 GCs within a projected radius of 400kpc. As many as half may comprise an intracluster component. Even with the sizable uncertainties, which mainly result from the uncertain GCLF parameters, this is by far the largest GC system studied to date. The specific frequency S_N is high, but not uncommon for central galaxies in massive clusters, rising from S_N~5 near the center to ~12 at large radii. Passive galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass profiles of the GCs, stars, intracluster gas, and lensing-derived total mass, and we compare the mass fractions as a function of radius. The estimated mass in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar mass of the Milky Way. The shape of the GC mass profile appears intermediate between those of the stellar light and total cluster mass. Despite the extreme nature of this system, the ratios of the GC mass to the baryonic and total masses, and thus the GC formation efficiency, are typical of those in other rich clusters when comparing at the same physical radii. The GC formation efficiency is not constant, but varies with radius, in a manner that appears similar for different clusters; we speculate on the reasons for this similarity in profile.Comment: 13 pages, 11 figures; accepted for publication in Ap

The Next Generation Virgo Cluster Survey (NGVS). XVIII. Measurement and Calibration of Surface Brightness Fluctuation Distances for Bright Galaxies in Virgo (and Beyond)

We describe a program to measure surface brightness fluctuation (SBF) distances to galaxies observed in the Next Generation Virgo Cluster Survey (NGVS), a photometric imaging survey covering $104~deg^2$ of the Virgo cluster in the ${u}^*,g,i,z$ bandpasses with the Canada-France Hawaii Telescope. We describe the selection of the sample galaxies, the procedures for measuring the apparent $i$-band SBF magnitude $\bar{i}$, and the calibration of the absolute $\bar{M}_i$ as a function of observed stellar population properties. The multi-band NGVS data set provides multiple options for calibrating the SBF distances, and we explore various calibrations involving individual color indices as well as combinations of two different colors. Within the color range of the present sample, the two-color calibrations do not significantly improve the scatter with respect to wide-baseline, single-color calibrations involving $u^{*}$. We adopt the ${u}^*{-}z$ calibration as reference for the present galaxy sample, with an observed scatter of 0.11 mag. For a few cases that lack good ${u}^*$ photometry, we use an alternative relation based on a combination of $g{-}i$ and $g{-}z$ colors, with only a slightly larger observed scatter of 0.12 mag. The agreement of our measurements with the best existing distance estimates provides confidence that our measurements are accurate. We present a preliminary catalog of distances for 89 galaxies brighter than $B_T\approx13.0$ mag within the survey footprint, including members of the background M and W Clouds at roughly twice the distance of the main body of the Virgo cluster. The extension of the present work to fainter and bluer galaxies is in progress.Comment: ApJ accepte