High-latitude Field Research Articles

Kinematics of metal-poor giants in an inner-halo field, with implications for disk formation

A sample of approximately 100 predominantly metal-weak giants, identified in a high-latitude field towards the galactic center using an automated objective-prism survey technique, is presented. Abundances and radial velocities have been measured for these giants, whose distances from the Sun range from 1 to 18 kpc. While the extremely metal-weak stars in the field have halo kinematics, the majority of the stars with intermediate abundance have thick disk kinematics, despite the fact that their average distance from the galactic plane is 3 kpc. The most satisfactory explanation for this effect is that the inner halo is moderately flattened, and the metal-weak stars of the thick disk have a scale height of about 2 kpc. It is suggested that the thick disk may have formed in a dissipational collapse, rather than in a separate event such as the accretion of a small satellite galaxy.

The ionospheric response to perturbation electric fields during the onset phase of geomagnetic storms

The generation and propagation of ionospheric storms are studied by analyzing EISCAT radar, and vertical-sounding and total-electron-content data obtained under different geophysical conditions. Both, case studies as well as the average storm pattern of percentage deviations of different ionospheric parameters from their corresponding reference values such as total electron content, F2-layer critical frequency foF2, F2-layer height hmF2, and slab thickness τ indicate the action of a perturbation electric field during the first few hours during the onset phase of geomagnetic storms. Considering the onset phase of the storm on July 28–29, 1987 evidence has been found that high-latitude electric fields may penetrate to lower latitudes before the ring current has developed. In most cases this process is accompanied by a positive phase in the upper ionosphere and F2-layer ionization. Different mechanisms are assumed to be responsible for the daytime and nighttime behaviour, respectively. The negative phase propagates equatorward with velocities in the order of 70–350 m s−1 following a strong heating of the thermosphere and ionosphere due to the auroral electrojet.

A spectroscopic survey of faint compact objects

We present results from a low-resolution (73 A) spectroscopic survey of faint compact objects conducted with the Low Dispersion Survey Spectrograph at the Anglo-Australian Telescope. The sample is a random subset of all compact sources with 22.5≤b J ≤23.5 in three high-latitude fields. Additional data on compact sources with 21≤b J ≤22.5 have been selected from the higher resolution (13 A) survey previously published by Colless et al. (1990), and a subset of these brighter objects has been studied at both dispersions.

Multiple electron species dynamics parallel to a high‐latitude field line

The parallel dynamics of the plasma along high‐latitude field lines is a critical feature of the ionosphere‐magnetosphere coupling problem. We have created a time‐dependent, large scale simulation of these dynamics in a region from the ionosphere well into the magnetosphere. Including multiple electron species has allowed us to dynamically simulate the diode‐like response of the field‐line plasma to the parallel currents coupling the ionosphere and magnetosphere. Downward (return) currents flow with small resistance, while upward currents produce kilovolt‐sized potential drops along the field due to the effect of the converging magnetic field on the high energy magnetospheric electrons.

Imaging Large– and Small–Scale Structures of the Far Ultraviolet Background with the Faust Instrument

The FAUST telescope is an ultraviolet survey instrument that features a wide 8° field of view, ~1′ angular resolution, and a photon counting detector. Operating in the 1400–1800 Å band, it will be sufficiently sensitive to detect blue mv =17 objects in a single 20 minute night. The instrument is part of the ATLAS-1 shuttle mission, presently scheduled for flight in May 1991. A substantial number of high galactic latitude fields will be investigated, with particular emphasis on studies of the origin of the diffuse far UV background.

A deep CCD survey of 12 high-latitude fields

view Abstract Citations (73) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Deep CCD Survey of 12 High-Latitude Fields Tyson, J. A. ; Seitzer, P. Abstract A CCD spot survey of 12 random high galactic latitude fields in three bands is discussed, and deep images of each field are shown. A high surface density of faint blue galaxies is revealed, reproducible in all three bands. Cumulative integration times of 22,000 s or more were obtained at prime focus of a 4 m telescope in each field. Unsmoothed surface brightness limits range from 29 mag arcsec^-2^ (10^-8^ Jy arcsec^-2^) at 0.35-0.5 microns, to 26 mag arcsec^-2^ (8 x 10^-8^ Jy arcsec^-2^) at 0.8-1 micron wavelength. This survey has a limiting magnitude which is 3 mag fainter than previous surveys. At this depth, over 150 galaxies arcmin^-2^ are found. Their 0.4-0.6 micron colors are 1.5 mag bluer than nearby galaxies. New techniques were developed for faint object imaging at 0.3-1 microns. These allow detection and photometry at a level of 0.03% of sky background over a spatial scale of 2". R band finding charts and faint B_J_, R, I photometric sequences are given for each of the 12 fields. Publication: The Astrophysical Journal Pub Date: December 1988 DOI: 10.1086/166949 Bibcode: 1988ApJ...335..552T Keywords: Astronomical Photometry; Charge Coupled Devices; Galactic Evolution; Sky Surveys (Astronomy); Astronomical Photography; Hubble Constant; Red Shift; Astronomy; COSMOLOGY; GALAXIES: PHOTOMETRY full text sources ADS | data products SIMBAD (9)

Stromgren and BV photometry of potential halo blue horizontal branch field stars

Strömgren four-colour and broadband BV photoelectric photometry has been obtained for a sample of potential halo blue horizontal branch stars in two high galactic latitude fields. The large majority of the stars observed are classified as blue horizontal branch stars on the basis of two different surface gravity indicators. Measurements of Ca K-line equivalent widths from medium-dispersion spectra of the stars confirm that most are Population II objects. No metal-rich A stars were found beyond a few kpc from the galactic disc in the study of faint blue stars.

Deep CCD survey - Galaxy luminosity and color evolution

view Abstract Citations (629) References (100) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Deep CCD Survey: Galaxy Luminosity and Color Evolution Tyson, J. A. Abstract CCD multicolor imaging of over 25 000 faint galaxies in 12 random high-latitude fields reveals strong evidence for color and luminosity evolution. Deep images in several bands from 0.36 to 1 micron enable studies of galaxies undergoing evolution at redshifts up to 7 in a zero-dust universe. Fainter than 27th B_J_ magnitude the sky becomes confusion limited in galaxies. Galaxy counts at 25th B_J_ magnitude are a factor of 5-15 above no-evolution models. The differential number counts of galaxies log N(m) are found to vary with magnitude as 0.3 I to 0.45 B_J_, ruling out a local origin of these faint galaxies. A strong blueing trend is seen for the fainter galaxies, suggesting that the UV excess from massive star formation in these evolving galaxies has been redshifted into the blue band. The extragalactic background light is computed explicitly from the number-magnitude counts. No evidence is found for 10-30 arcsec extended primeval objects down to 30 B_J_ mag arcsec^-2^; however, cleaning these "bright" objects and smoothing reveals occasional structures as large as 30 arcsec down to 32 B_J_ mag arcsec^-2^. Mild and continuous evolution of field galaxies seems to have occurred, and any global burst phase at z < 6 must have been shrouded in dust. If the universe is not optically thick in dust at redshifts of 2-3, our number-count and color data are most consistent with formation of the first galaxies beginning at redshifts larger than 4 (although significant formation could continue to the present epoch). These data are not consistent with some dark matter theories, if galaxy-sized perturbations were gaseous at formation. If these galaxies are at redshifts of 1-3, as their colors, scale, and surface brightness suggest, then a low {OMEGA} is favored. Publication: The Astronomical Journal Pub Date: July 1988 DOI: 10.1086/114786 Bibcode: 1988AJ.....96....1T Keywords: Charge Coupled Devices; Color-Magnitude Diagram; Galactic Evolution; Red Shift; Sky Surveys (Astronomy); Ultraviolet Radiation; Dark Matter; Perturbation Theory; Stellar Evolution; Stellar Mass; Astrophysics; GALAXIES: GENERAL; COSMOLOGY full text sources ADS | data products SIMBAD (8) NED (4)

Minimal joule dissipation models of magnetospheric convection

This paper gives a topical review of theoretical models of magnetospheric convection based on the concept of minimal Joule dissipation. A two-dimensional slab model of the ionosphere featuring an enhanced conductivity auroral oval is used to compute high-latitude electric fields and currents. Mathematical methods used in the modeling include Fourier analysis, fast Fourier transforms, and variational calculus. Also, conformai transformations are introduced in the analysis, which enable the auroral oval to be represented as a nonconcentric, crescent-shaped figure. Convection patterns appropriate to geomagnetic quiet and disturbed conditions are computed, the differentiating variable being the relative amount of power dissipated in the magnetospheric ring current. When ring current dissipation is small, the convection electric field is restricted to high latitudes (shielding regime), and when it is large, a significant penetration of the field to low latitudes occurs, accompanied by an increase in the ratio of the region 1 current to the region 2 current.

A spectroscopic survey of high-latitude fields and comparison to Galaxy models

The results of a spectroscopic survey of the stellar populations of high-Galactic latitude fields in SA 94 and Serpens are presented and analyzed. A set of six spectrophotometric indices is used to provide information on both the number and metallicity of evolved stars, while measurement of another 11 spectrophotometric indices provides an estimate of (Fe/H) good to 0.25 dex for the evolved stars identified in each field. A detailed comparison of the data with predictions of the standard Bahcall-Soneira Galaxy model reveals serious discrepancies. The properties of giants with roughly solar metallicity are described well by the disk giants predicted by the standard model. The sample is dominated by intermediate-metallicity giants whose properties are well described by a thick-disk component with scale height of approximately 700 pc that makes up about 5 percent of the solar neighborhood stellar density. 96 references.

Spatial variations of background magnetic field polarity distribution on the Sun

Based on an analysis of the latitude and longitude regularities in the distribution of the sign of the background magnetic field (BMF) on the solar surface for 14 years (1969–1982), a classification of BMF distributions in the form of synoptic maps, is proposed. That the low- and high-latitude BMF distributions are differing in character appears to be due to the difference in the character of the BMF rotation at low and high latitudes. It is shown that as low-latitude BMF details make contact with the high-latitude field of the same polarity, the former grow in area. The low- and high-latitude fields come into contact through high-latitude field details protruding into lower latitudes as far as 10 to 15 ° below 40 ° latitude, and they are referred to as bulges. Bulges and low-latitude fields of like polarity are moving with respect to each other along the E-W line at a mean rate of 10 to 15 ° per rotation. Bulges of the same polarity in the same hemisphere are moving with respect to each other 3–5 ° per rotation, on the average. The above-mentioned properties of the structure, interaction and rotation of the low- and high-latitude magnetic field details, taken together, provide a qualitative scheme for the formation and variation of the longitude-regular (sectoral) BMF distribution in the latitude range 40 S-40 N.

The X-ray properties of high redshift quasi-stellar objects

We describe a program aimed at characterizing the X-ray emission of high redshift QSOs. We have obtained slitless spectra of 50 high galactic latitude fields previously imaged at very high levels of sensitivity by the Einstein Observatory, generally for original goals unrelated to QSOs. Our survey, covering ∼ 17 deg2 of sky to limiting magnitude Bcont ∼ 21, has yielded ∼ 400 previously uncatalogued QSO candidates, each with sensitive new X-ray information available. About 100 of these objects, constituting a “high confidence” set of QSOs, chiefly in the redshift range 1.7 &lt; z &lt; 3 and thus complementary to previous samples with X-ray data, are used to derive the X-ray properties of high redshift QSOs. Even at these most sensitive available X-ray flux levels, only about 25% of the objects are positively detected in X-rays; thus extensive attention has been given to proper treatment of the upper-limit information. We find a mean optical-to-X-ray slope parameter for the sample of . Our results are combined with those of previous surveys to estimate the fraction of the diffuse X-ray background radiation due to QSOs. QSOs are capable of supplying the majority of the radiation, but the chief contribution comes from an annulus of intermediate redshift, moderate luminosity objects.

The kinematics of Population II stars

Simple dynamical models are made to study the implications of recent kinematic data for Population II stars in the Galactic halo. The models assume a scale-free distribution of stars orbiting within a spherical potential with a flat rotation curve. This potential is consistent with the observed properties of the globular cluster system. A highly flattened star distribution can reproduce the velocity dispersion behavior reported by Ratnatunga and Freeman in 1985 in a variety of high-latitude fields. In particular it can reproduce the small dispersion they observed out to 25 kpc toward the Galactic pole. These data appear to require a spatial distribution which differs substantially both from that of globular clusters and from those assumed by standard models for star count data. The dynamical models require the halo giants to be on near-circular orbits, in conflict with the traditional collapse picture for the formation of Population II.

The S201 far-ultraviolet imaging survey. I - Two fields at high galactic latitude

view Abstract Citations (12) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The S 201 far-ultraviolet imaging survey. I. Two fields at high galactic latitude. Carruthers, G. R. ; Page, T. Abstract Far-ultraviolet imagery was obtained of 10 20-deg diameter fields by the S201 far-ultraviolet camera during the Apollo 16 mission. The present discussion is of two high galactic latitude fields, in Grus and Aquarius, in which exposures of up to 30 minutes duration were obtained in the 1250-1600 A wavelength range (effective wavelength 1400 A), and in which objects as faint as m(1400) = 11.8 are detected. The number versus magnitude relation for the observed objects appears to peak near m(1400) = 10, implying that relatively few such objects exist at magnitudes fainter than our sensitivity limit. Most of the stellar contribution to the ultraviolet radiation field is contributed by stars brighter than m(1400) = 6, which are mainly nearby late B and early A members of the galactic disk population. The objects fainter than m(1400) = 8, however, appear to be members of a second population of hot, subluminous objects, but further ground-based observations of these objects are needed. Publication: The Astrophysical Journal Supplement Series Pub Date: November 1983 DOI: 10.1086/190904 Bibcode: 1983ApJS...53..623C Keywords: Astronomical Photography; Early Stars; Milky Way Galaxy; Star Distribution; Ultraviolet Astronomy; A Stars; Astronomical Coordinates; Astronomical Maps; B Stars; Far Ultraviolet Radiation; Hot Stars; Stellar Magnitude; Stellar Radiation; Ultraviolet Photography; Astrophysics full text sources ADS | data products SIMBAD (23) NED (1) Related Materials (2) Part 2: 1984ApJS...54..271C Part 3: 1984ApJS...55..101C

Pc1 wave generation by sudden impulses

We have studied induction magnetometer data recorded at College, Alaska for the occurrence of ULF emissions associated with sudden impulses (SI). In this study we surveyed three years of data (1977–1979 inclusively) and found that for the 76 SI's reported in IAGA bulletins, 32 were found to be followed by ULF emissions in the College data. While the 76 SI's occurred at all local times those which were associated with ULF emissions at College peaked in occurrence near local noon. We have attempted to interpret these observations in terms of a simple model based upon a Chapman-Ferraro double-dipole model of the magnetosphere. Using this model an estimate of field compressions associated with SI's can be made and from this the increases in the temperature anisotropy and plasma beta may be estimated. This simple model predicts maximum growth rate near noon on high latitude field lines, just inside the magnetopause. Further, inspection of growth rate curves for varying plasma anisotropy and beta leads us to the conclusion that an increase in anisotropy is the primary cause of the ULF emissions observed.

Equivalent ionospheric current systems representing IMF sector effects on the polar geomagnetic field

Equivalent ionospheric current systems representing IMF sector effects on the geomagnetic field in high latitudes are examined for each of the twelve calendar months by spherical harmonic analyses of geomagnetic hourly data at 13 northern polar stations for seven years. The main feature of obtained equivalent current systems includes circular currents at about 80° invariant latitude mostly in the daytime in summer and reversed circular currents at about 70° invariant latitude mainly at night in winter. Field-aligned current distributions responsible for equivalent currents, as well as vector distributions of electric fields and ionospheric currents, are approximated numerically from current functions of equivalent current systems by taking assumed distributions of the ionospheric conductivity. Two sets of upward and downward field-aligned current pairs in the auroral region, and also a field-aligned current region near the pole show seasonal variations. Also, ionospheric electric-field propagation along geomagnetic field lines from the summer hemisphere to the winter hemisphere with auroral Hall-conductivity effects may provide an explanation for the winter reversal of sector effects.

Discovery of low-redshift X-ray selected quasars - New clues to the QSO phenomenon

The identification of six X-ray sources discovered by the Einstein Observatory with X-ray quasars is reported, and the properties of these X-ray selected quasars are discussed. The four high-latitude fields of 1 sq deg each in which the Einstein imaging proportional counter detected serendipitous X-ray sources at intermediate exposures of 10,000 sec were observed by 4-m and 1.5-m telescopes, and optical sources with uv excesses and emission line spectra typical of many low-redshift quasars and Seyfert 1 galaxies were found within the 1-arcsec error boxes of the X-ray sources. All six quasars identified were found to be radio quiet, with low redshift and relatively faint optical magnitudes, and to be similar in space density, colors and magnitude versus redshift relation to an optically selected sample at the same mean magnitude. X-ray luminosity was found to be well correlated with both continuum and broad-line emission luminosities for the known radio-quiet quasars and Seyfert 1 galaxies, and it was observed that the five objects with the lowest redshifts have very similar X-ray/optical luminosity ratios despite tenfold variations in X-ray luminosity. It is concluded that photoionization by a continuum extending to X-ray energies is the dominant excitation mechanism in radio-quiet quasars.

Automated Faint Galaxy Counts and Galactic Evolution

Detection and classification of faint images by eye has traditionally encountered systematic errors faintwards of 20th mag on Schmidt plates and 22nd mag on 4-meter plates. Automated classification of Schmidt plate images has pushed the classification limit to 22 mag (Kibblewhite, et al., 1975). Automated detection and classification of faint 4-meter limit plate images has recently led to statistical studies of galaxy numbers and clustering at redshifts where cosmology and galactic evolution dominate over local effects. Here we report on some aspects of the FOCAS (Faint Object Classification and Analysis System) automated classifier (Tyson and Jarvis, 1979) and compare our results of number counts in SA57 with those of Kron, 1979. Differential galaxy counts in six high latitude fields and evidence for galaxy evolution are briefly discussed.

Equatorial electric fields during magnetically disturbed conditions 2. Implications of simultaneous auroral and equatorial measurements

Simultaneous auroral and equatorial electric field data are used along with magnetic field data to study anomalous electric field patterns during disturbed times. During some substorms, accompanied by ring current activity, the worldwide equatorial zonal electric field component reverses from the normal pattern. This is interpreted as a partial closure of high latitude field aligned currents in the dayside, low latitude ionosphere. These currents flow westward across the dayside. In several cases the zonal equatorial electric field component was nearly identical in form to the zonal auroral component, indicating the close electrical coupling between these regions. Less certain, but equally intriguing, is the evidence presented for a close relationship between the zonal equatorial electric field and the time derivative of the ring current induced magnetic field. Another class of events seems related to rapid changes of magnetospheric convection and a temporary imbalance between the field external to the plasmasphere and the shielding charges in the Alfven layer. Examples of both rapid increases and decreases are presented. The latter seems often to be related to a northward turning of the interplanetary magnetic field.

Geomagnetic activity and the high latitude magnetic field of the Sun

SMITH et al.1 using Pioneer 11 in 1976 did not observe any sector structure at the heliographic latitude of about +16°. At this latitude the field polarity was consistently directed away from the Sun. Its direction was in accord with the direction of the high-latitude magnetic field near the north pole of the Sun. This result, as well as indirect data2–4, shows that the high latitude solar field extends far into interplanetary space. The evidence for this is shown by the analysis of the yearly aa indices5,6 between 1868 and 1976. We find intensification of geomagnetic disturbances during those 11 yr periods when the extended solar magnetic field polarity in the northern hemisphere is away from the Sun.