Arcmin Resolution Research Articles

Climate sensitivity of spring snowpack in the Sierra Nevada

California's spring snowpack provides a critical water resource that may be greatly reduced by greenhouse warming. However, warming over the past half century has had little effect on total summer water discharge. The region's snowpack may therefore be less sensitive to temperature change than predicted by numerical models. In this study, 53 years of 1 April snow course measurements of snow‐water equivalent (SWE) from the Sierra Nevada are used in a spatially distributed covariance model of SWE sensitivity to temperature and precipitation. This model is applied at a 2.5 arc min resolution using a multivariate parameter‐surface interpolation scheme and Parameter‐elevation Regressions on Independent Slopes (PRISM) climate grids. Total modeled SWE volume has a greater covariance to precipitation than to temperature. Increasing precipitation and temperature from 1950 to 2002 has led to an increase in SWE at high elevations and a loss at low elevations, resulting in little or no overall change in SWE volume. The covariance model predicts a 6–10% decrease in total SWE volume per °C. However, sensitivity is both highly dependent on concurrent change in precipitation and spatially variable, with the lower‐elevation watersheds in the north being the most sensitive to warming. Overall, climate sensitivity is much less than that predicted by numerical models. This difference may result from inadequate treatment of elevation and precipitation in climate models.

Gravity Requirements for Compensation of Ultra-Precise Inertial Navigation

Precision inertial navigation depends not only on the quality of the inertial sensors (accelerometers and gyros), but also on the accuracy of the gravity compensation. With a view toward the next-generation inertial navigation systems, based on sensors whose errors contribute as little as a few metres per hour to the navigation error budget, we have analyzed the required quality of gravity compensation to the navigation solution. The investigation considered a standard compensation method using ground data to predict the gravity vector at altitude for aircraft free-inertial navigation. The navigation effects of the compensation errors were examined using gravity data in two gravimetrically distinct areas and a navigation simulator with parameters such as data noise and resolution, supplemental global gravity model noise, and on-track interpolation method. For a typical flight trajectory at 5 km altitude and 300 km/hr aircraft speed, the error in gravity compensation contributes less than 5 m to the position error after one hour of free-inertial navigation if the ground data are gridded with 2 arcmin resolution and are accurate to better than 5 mGal.

Very high resolution interpolated climate surfaces for global land areas

We developed interpolated climate surfaces for global land areas (excluding Antarctica) at a spatial resolution of 30 arc s (often referred to as 1-km spatial resolution). The climate elements considered were monthly precipitation and mean, minimum, and maximum temperature. Input data were gathered from a variety of sources and, where possible, were restricted to records from the 1950–2000 period. We used the thin-plate smoothing spline algorithm implemented in the ANUSPLIN package for interpolation, using latitude, longitude, and elevation as independent variables. We quantified uncertainty arising from the input data and the interpolation by mapping weather station density, elevation bias in the weather stations, and elevation variation within grid cells and through data partitioning and cross validation. Elevation bias tended to be negative (stations lower than expected) at high latitudes but positive in the tropics. Uncertainty is highest in mountainous and in poorly sampled areas. Data partitioning showed high uncertainty of the surfaces on isolated islands, e.g. in the Pacific. Aggregating the elevation and climate data to 10 arc min resolution showed an enormous variation within grid cells, illustrating the value of high-resolution surfaces. A comparison with an existing data set at 10 arc min resolution showed overall agreement, but with significant variation in some regions. A comparison with two high-resolution data sets for the United States also identified areas with large local differences, particularly in mountainous areas. Compared to previous global climatologies, ours has the following advantages: the data are at a higher spatial resolution (400 times greater or more); more weather station records were used; improved elevation data were used; and more information about spatial patterns of uncertainty in the data is available. Owing to the overall low density of available climate stations, our surfaces do not capture of all variation that may occur at a resolution of 1 km, particularly of precipitation in mountainous areas. In future work, such variation might be captured through knowledge-based methods and inclusion of additional co-variates, particularly layers obtained through remote sensing. Copyright © 2005 Royal Meteorological Society.

First detection of polarization of the submillimetre diffuse galactic dust emission by Archeops

We present the first determination of the Galactic polarized emission at 353 GHz by Archeops. The data were taken during the Arctic night of February 7, 2002 after the balloon--borne instrument was launched by CNES from the Swedish Esrange base near Kiruna. In addition to the 143 GHz and 217 GHz frequency bands dedicated to CMB studies, Archeops had one 545 GHz and six 353 GHz bolometers mounted in three polarization sensitive pairs that were used for Galactic foreground studies. We present maps of the I, Q, U Stokes parameters over 17% of the sky and with a 13 arcmin resolution at 353 GHz (850 microns). They show a significant Galactic large scale polarized emission coherent on the longitude ranges [100, 120] and [180, 200] deg. with a degree of polarization at the level of 4-5%, in agreement with expectations from starlight polarization measurements. Some regions in the Galactic plane (Gem OB1, Cassiopeia) show an even stronger degree of polarization in the range 10-20%. Those findings provide strong evidence for a powerful grain alignment mechanism throughout the interstellar medium and a coherent magnetic field coplanar to the Galactic plane. This magnetic field pervades even some dense clouds. Extrapolated to high Galactic latitude, these results indicate that interstellar dust polarized emission is the major foreground for PLANCK-HFI CMB polarization measurement.

Neutral hydrogen surveys for high-redshift galaxy clusters and protoclusters

We discuss the possibility of performing blind surveys to detect large-scale features of the Universe using 21-cm emission. Using instruments with ∼5–10 arcmin resolution currently in the planning stage, it should be possible to detect virialized galaxy clusters at intermediate redshifts using the combined emission from their constituent galaxies, as well as less overdense structures, such as protoclusters and the ‘cosmic web’, at higher redshifts. Using semi-analytic methods, we compute the number of virialized objects and those at turnaround which might be detected by such surveys. We find that there is a surprisingly large number of objects even using small (∼5 per cent) bandwidths and elaborate on some issues pertinent to optimizing the design of the instrument and the survey strategy. The main uncertainty is the fraction of neutral gas relative to the total dark matter within the object. We discuss this issue in the context of the observations which are currently available.

First detection of polarization of the submillimeter diffuse Galactic dust emission by Archeops

We present the first determination of the Galactic polarized emission at 353 GHz by Archeops. The data were taken during the Arctic night of February 7, 2002 after the balloon-borne instrument was launched by CNES from the Swedish Esrange base near Kiruna. In addition to the 143 and 217 GHz frequency bands dedicated to CMB studies, Archeops had one 545 GHz and six 353 GHz bolometers mounted in three polarization sensitive pairs that were used for Galactic foreground studies. We present maps of the I, Q, U Stokes parameters over 17% of the sky and with a 13 arcmin resolution at 353 GHz (850 μm). They show a significant Galactic large scale polarized emission coherent on the longitude ranges [100°,120°] and [180°,200°] with a degree of polarization at the level of 4–5%, in agreement with expectations from starlight polarization measurements. Some regions in the Galactic plane (Gem OB1, Cassiopeia) show an even stronger degree of polarization in the range 10–20%. Those findings provide strong evidence for a powerful grain alignment mechanism throughout the interstellar medium and a coherent magnetic field coplanar to the Galactic plane. This magnetic field pervades even some dense clouds. Extrapolated to high Galactic latitude, these results indicate that interstellar dust polarized emission is the major foreground for PLANCK-HFI CMB polarization measurement.

Archeops: a prefiguration of Planck-HFI

Archeops is a balloon-borne experiment that has mapped the CMB anisotropies with a 10 arcmin resolution and a sky coverage of 30%, leading after a first analysis to measurements of the angular power spectrum between ℓ∼15 and ℓ∼350. Considering the use of bolometers cooled down at 100 mK, and its scanning strategy, Archeops can also be considered as a benchmark for the high-frequency instrument of Planck. In this paper, I present the experiment and the major steps of the data analysis, and the first Archeops results published on the angular power spectrum.

Multi-frequency polarimetry of the Galactic radio background around 350 MHz

With the Westerbork Synthesis Radio Telescope (WSRT), multi-frequency polarimetric images were taken of the diffuse radio synchrotron background in a region centered on (l,b) = (161,16). The observations were done simultaneously in 5 frequency bands from 341 MHz to 375 MHz, with 5 arcmin resolution. Ubiquitous structure on arcminute and degree scales in the polarized intensity and polarization angle, combined with no observed structure in total intensity, indicates that the structure in the polarized radiation must be due to Faraday rotation and depolarization mostly in the warm nearby Galactic interstellar medium (ISM). Beam depolarization most likely creates "depolarization canals" of one beam wide, while depth depolarization is responsible for creating most of the structure on scales larger than a beam width. Rotation measures RM are in the range -17 < RM < 10 rad/m2 with a non-zero average of about -3.4 rad/m2. The gradient and average RM are consistent with a regular magnetic field of about 1 uG which has a pitch angle of p = -14 degrees. 13 Extragalactic sources in the field have |RM| < 13 rad/m2, with an estimated intrinsic source contribution of 3.6 rad/m2. The RMs of the extragalactic sources show a gradient (with a sign reversal) that is about 3 times larger than the gradient in the RMs of the diffuse emission, and that is approximately in Galactic latitude. This difference is ascribed to a vastly different effective length of the line of sight. The observations are interpreted in terms of a single-cell-size model of the warm ISM which contains gas and magnetic fields, with a polarized background.

The Small Magellanic Cloud in the far infrared

The ISOPHOT experiment onboard the ISO satellite generated a complete view of the Small Magellanic Cloud (SMC) at 170 µm with 1.5 arcmin resolution. The map is analysed using an automated photometry program enabling accurate photometric characterization of the far infrared (FIR) emitting regions. An integrated FIR luminosity of 8 .5×10 7 Lis obtained, leading to a star formation rate of SFRFIR = 0.015 M� /yr. With an average dust temperature of , the total dust mass follows to MD = 3.7 × 10 5 M� . In this paper, the sources detected at 170 µm are compared with those obtainable from the IRAS satellite data. For this purpose, the 12 µm, 25 µm, 60 µm, and 100 µm IRAS high resolution (HiRes) maps of the SMC are re-examined using the same method. In contrast to former studies, this provides an all-band ISO/IRAS source catalog which is no longer based on eyeball classification, but relies on an algorithm which is capable of automated, repeatable photometry, even for irregular sources. In the mid infrared IRAS bands numerous bright FIR emitting regions in the SMC are detected and classified: 73 sources are found at 12 µm, 135 at 25 µm (most of them with Fν 30 K) for the first time. A comparison with earlier IRAS results suggests that many source flux densities in those studies have been under- or overestimated because of non-standardized fitting methods. Many sources with flux densities up to 40 Jy listed in former catalogs cannot be identified in our data.

Very High Energy Particle Astronomy with All-Sky Survey High Resolution Air-Shower Detector (ASHRA)

The ASHRA detector consists of 3 observational stations installed on the grid of 30-40km distance on the mountains in Hawaii, which ensures a large target mass for neutrinos and a large effective aperture for UHECR. An observational station is composed of 12 wide-angle high-precision telescopes, which can completely cover all-sky view. In realizing the telescope design, we newly apply the following techniques matured in the other fields: 1) Baker-Nunn optics optimized to keep better than 1 arcmin resolution in 50° field of view, 2) Electro-static lens image-intensifier tube (IIT) with the resolution matched with that of the above optics, 3) Gated IIT with a fast image shutter, and 4) CMOS image sensor which reads triggered images out of the above IIT. These novel techniques allow us an excellent opportunity for simultaneous observation of air fluorescence and Cherenkov lights with 1 arcmin. resolution in entirely all sky. This will open a new field, Observational Particle Astrophysics by continuously observing TeV gamma-rays, VHE-neutrinos, Knee-CR, and UHECR with the energies from TeV to ZeV in all sky.

The science of AGILE: part II

In this paper (Part II) we discuss the expected scientific performance of the AGILE mission, focusing on the large FOV, the spatial resolution and PSF of the γ-ray imager, and the crucial capability of simultaneous hard-X and γ-ray imaging with ∼ 1 – 3 arcmin resolution.

High–resolution imaging of compact high–velocity clouds

We have imaged five compact high-velocity clouds in Hi with arcmin angular resolution and km s 1 spectral reso- lution using the Westerbork Synthesis Radio Telescope. These CHVCs have a characteristic morphology, consisting of one or more quiescent, low-dispersion compact cores embedded in a diuse warm halo. The compact cores can be unambiguously identified with the cool neutral medium of condensed atomic hydrogen, since their linewidths are significantly narrower than the thermal linewidth of the warm neutral medium. Because of the limited sensitivity to diuse emission inherent to interfer- ometric data, the warm medium is not directly detected in the WSRT observations. Supplementary total-power data, which is fully sensitive to both the cool and warm components of Hi, is available for comparison for all the sources, albeit with angular resolutions that vary from 3 0 to 36 0 . The fractional Hi flux in compact CNM components varies from 4% to 16% in our sample. All objects have at least one local peak in the CNM column density which exceeds about 10 19 cm 2 when observed with arcmin resolution. It is plausible that a peak column density of 1-2 10 19 cm 2 is a prerequisite for the long-term survival of these sources. One object in our sample, CHVC 120 20 443 (Davies' cloud), lies in close projected proximity to the disk of M 31. This object is characterized by exceptionally broad linewidths in its CNM concentrations, more than 5 times greater than the median value found in the 13 CHVCs studied to date at comparable resolution. These CNM concentrations lie in an arc on the edge of the source facing the M 31 disk. The diuse Hi component of this source, seen in total-power data from the NRAO 140-foot telescope, has a positional oset in the direction of the M 31 disk. All of these attributes suggest that CHVC 120 20 443 is in a dierent evolutionary state than most of the other CHVCs which have been studied. Similarly broad CNM linewidths have only been detected in one other cloud, CHVC 110.6 07.0 466 (Wakker & Schwarz 1991) which also lies in the Local Group barycenter direction and has the most extreme radial velocity known. A distinct possibility for Davies' cloud seems to be physical interaction of some type with M 31. The most likely form of this interaction might be the ram-pressure or tidal-stripping by either one of M 31's visible dwarf companions, M 32 or NGC 205, or else by a dark com- panion with an associated Hi condensation. The compact objects located in the direction of the Local Group barycenter have an important role to play in constraining the Local Group hypothesis for the deployment of CHVCs.

HI observations of the high-velocity cloud in the direction of M 92

We present wide-field neutral hydrogen (H i) Lovell telescope multibeam, and Dominion Radio Astrophysical Observatory H i synthesis observations, of the high velocity cloud (HVC) located in the general direction of the globular cluster M 92. This cloud is part of the larger Complex C and lies at velocities between ~-80 and -130 km s-1 in the Local Standard of Rest. The Lovell telescope observations, of resolution 12 arcmin spatially and 3.0 km s-1 in velocity, fully sampling a 3.112.6° RA-Dec grid, have found that this part of HVC Complex C comprises two main condensations, lying approximately north-south in declination, separated by ~2° and being parallel to the Galactic plane. At this resolution, peak values of the brightness temperature and H i column density of ~1.4 K and ~51019 cm-2 are determined, with relatively high values of the full width half maximum velocity (FWHM) of ~22 km s-1 being observed, equivalent to a gas kinetic temperature, in the absence of turbulence and geometric effects of ~10 000 K. Each of these properties, as well as the sizes of the clouds, are similar in the two components. The DRAO observations, towards the Northern HVC condensation, are the first high-resolution H i spectra of Complex C. When smoothed to a resolution of 3 arcmin, they identify several H i intensity peaks with column densities in the range 4-71019 cm-2. Further smoothing of these data to 6 arcmin resolution tentatively indicates that parts of the HVC consist of two velocity components, of similar brightness temperature, separated by ~7 km s-1 in velocity, and with FWHM velocity widths of ~5-7km s-1. No IRAS 60 or 100 micron flux is associated with the M 92 HVC. Cloud properties are briefly discussed and compared to previous observations of HVCs.

Derivation of a global land elevation data set from satellite radar altimeter data for topographic mapping

Radar altimeter data from the 336 day cycle of the geodetic phase of ERS‐1 has been processed over all land surfaces to produce a consistent, homogenous data set for topographic mapping purposes. Several processing steps were required to achieve this, the most important of which were waveform retracking and data filtering. No slope correction was applied to the data as this was found not to improve the accuracy. The original radar altimeter data set was 145 Gb in size and a more compact, higher‐level product was required for land topography applications. Three higher‐level products were created: 1) an unaveraged data set comprising elevation, position, backscatter and a small number of auxiliary parameters; 2) an averaged data set of quasi‐regular mean elevations, position, and standard deviation at 5 arc min resolution; 3) a 5 arc min digital elevation model interpolated and extrapolated from the second data set. The accuracy of the radar altimeter data was investigated using six point data sets of both global and regional coverage, including airport runway elevations, national geodetic data, gravity station heights, and navigation beacon heights. A small number of erroneous elevations in both the radar and the point data sets were identified. After removing outliers with a difference of greater than 300 m the mean différence between the radar and point data was −0.5 m ±44 m full width at half maximum.

The High‐Resolution IRAS Galaxy Atlas

An atlas of the Galactic plane (-4.7 deg < b < 4.7 deg) plus the molecular clouds in Orion, Rho Oph, and Taurus-Auriga has been produced at 60 and 100 micron from IRAS data. The Atlas consists of resolution-enhanced coadded images having 1 arcmin -- 2 arcmin resolution as well as coadded images at the native IRAS resolution. The IRAS Galaxy Atlas, together with the DRAO HI line / 21 cm continuum and FCRAO CO (1-0) line Galactic plane surveys, both with similar (approx. 1 arcmin) resolution, provide a powerful venue for studying the interstellar medium, star formation and large scale structure in our Galaxy. This paper documents the production and characteristics of the Atlas.

Galactic HI Mapping and Optical–IR Studies

AbstractHI structure is seen at every scale down to the angular resolution limit of all survey telescopes. I present several examples of the structures at high Galactic latitudes with the 12 arcmin beam of the Loveli Telescope, the 2 arcmin resolution of the DRAO synthesis, as well as other indicators of finer scale structure in the interstellar medium (ISM).

H I “Tails” from Cometary Globules in IC 1396

IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized by a single O6.5V star and containing bright-rimmed cometary globules. We have made the first arcmin resolution images of atomic hydrogen toward IC 1396, and have found remarkable ``tail''-like structures associated with some of the globules and extending up to 6.5 pc radially away from the central ionizing star. These HI ``tails'' may be material which has been ablated from the globule through ionization and/or photodissociation and then accelerated away from the globule by the stellar wind, but which has since drifted into the ``shadow'' of the globules. This report presents the first results of the Galactic Plane Survey Project recently begun by the Dominion Radio Astrophysical Observatory.

Regional recovery of the gravity field from satellite gravity gradiometer and gravity vector data using collocation

The method of least squares collocation has been used to investigate the regional recovery of the gravity field from satellite gravity gradiometer (SGG) data and gravity vector data derived from, e.g., satellite‐to‐satellite tracking (SST). We chose a region centered over the European Alps where the gravity anomalies showed a (large) standard deviation of 60 mGal. Mean gravity anomalies were used to generate SGG data at a satellite altitude of 200 km. As SGG data we used the second‐order derivatives in radial direction, across‐track, and the mixed radial/cross‐track derivative, all assumed to have an associated noise equal to 0.01 EU (Eötvös unit =EU, 1 EU=10−9 s−2). As gravity data we used the three components of the gravity vector, assuming that the associated errors were uncorrelated noise with standard deviation equal to 1 mGal. The satellite data in the test area were used in combination to predict 0.5° mean gravity anomalies and geoid heights. Ground truth data were used to investigate the quality of the recovery. The difference between observed and computed values have a standard deviation equal to 22 mGal in the best case. The use of the gravity vector data gave no improvement when added to the SGG data. The use of topographic information with 5 arc min resolution (ETOPO5U) gave, after removal of severe errors, a 17‐mGal standard deviation of observed minus computed values. This is a much smaller improvement than expected but is due to errors in the topographic values.

Feasibility of 1 arcmin resolution gamma-ray air-Čerenkov multiple telescope experiment

In this paper we discuss the feasibility of a ground based detector exploiting the technique of Čerenkov light stereo imaging of air showers for the detection of discrete cosmic sources of γ-rays with energy threshold of 100 GeV, angular resolution (HWHM) of 1 arcmin ( 1 3 mrad), cosmic ray background rejection ≥ 99%. This experiment will make possible a deep survey of the sky with sensitivity of 1 100 of the Crab in 45 h of exposure.

Imaging the Sun at 21 cm: Budgetting the S-component

On fourteen days in July and August 1992 and June 1993, we used the 7-element synthesis radio telescope at the Dominion Radio Astrophysical Observatory to make full-disc, arc-min resolution images of the Sun at 21 cm, with the objective of budgetting the contributions to the slowly-varying component of solar radio emission. This instrument has the advantage that the mapping field at this wavelength is about 2.5° wide. However, it has also the severe disadvantage that with only 12 hours to record each image, the brightness distribution is severely undersampled. This difficulty, along with solar rotation and declination motion during each observation, required development of special image correction procedures.