Comet Hale-Bopp: Evolution Of Jets And Shells During March 1997

Hundreds of high quality CCD images obtained by the GAT group with the 33.5 cm reflector of the Sozzago Astronomical Observatory (SAS) provide a detailed insight into the temporal evolution of comet Hale-Bopp. The images cover the time period from May 1996 until May 1997 and allowed us to analyze various near nucleus phenomena. Among them are jet structures that were visible from August 1996 until May 1997 and shell structures that could be resolved from February 1997 to May 1997. We concentrate on the morphology of these phenomena during March 1997 and investigated them by radial masking techniques that reveal their precise shape and extent. In particular, polar coordinate representations could be used advantageously to derive numerical parameters of these features. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and shape of the solar corona during the total solar eclipse on March 29, 2006

The structure and the shape of the solar corona during the total solar eclipse on March 29, 2006 were studied. The corona was of the intermediate pre-minimal type, it had northern and southern polar ray systems above the polar coronal holes and six streamers of different brightness in the middle and low heliographic latitudes. All the coronal structural features were found to have counterparts at the photosphere-chromosphere level on the limb and near it. The corona’s photometric flattening index was equal to 0.17.     

Jupiter's polar clouds and waves from Cassini and HST images: 1993-2006

For a variety of reasons, Jupiter's polar areas are probably the less observed regions of the planet. To study the dynamics and cloud vertical structure in the polar regions of the planet (latitudes 50° to 80° in both hemispheres) we have used images of Jupiter obtained from the ultraviolet to near infrared (258 to 939 nm) by the Cassini Imagining Science Subsystem (ISS) in December 2000. The temporal coverage was complemented with archived images from the Hubble Space Telescope (1993-2006) in a similar spectral range. The zonal wind velocities have been measured at three Cassini ISS wavelengths (CB2, MT3 and UV1, corresponding to 750, 890 and 258 nm) sounding different altitude levels. The three eastward jets detected in CB2 images (lower cloud) go to zero velocity when measured in the UV1 filter (upper haze). A radiative transfer analysis has been performed to characterize the vertical structure of cloud and hazes distribution at the poles. We also present a characterization (phase speed, amplitude and zonal wavenumber) of the previously detected circumpolar waves at 67° N and S at 890 nm and at about 50° N and −57° S at 258 nm that are a permanent phenomenon in Jupiter with some variability in its structure during the analyzed period. From the ensemble of data analyzed we propose the waves are Rossby waves whose dynamic behavior constrains plausible values for their meridional and vertical wavenumbers. This work demonstrates the long-term nature of Jupiter's polar waves, providing a dynamical and vertical characterization which supports a detailed analysis of these phenomena in terms of a Rossby wave model.     

The Bars Database on Barred Galaxies

During 1996 and 1997, the Canary Island Observatories' `International Time' project on Barred Galaxies obtained photometric images in broad- and narrow-band, and intermediate to high spectral resolution spectra in the optical and near IR of a selected set of a dozen barred galaxies. For some of these, 2D H_α kinematics was also obtained. The project team has been working on the analysis of these data in the intervening time, but one of the more useful aspects of the project has been to provide a database on barred galaxies more complete than most in terms of the observational material on each object. Those who may wish to use this database should contact the BARS project (instructions are given below). This revised version was published online in July 2006 with corrections to the Cover Date.     

CHROMOSPHERIC AND CORONAL STRUCTURE OF POLAR PLUMES – I. Magnetic Structure and Radiative Energy Balance

The Multi-Spectral Solar Telescope Array (MSSTA), a rocket-borne solar observatory, was successfully launched from White Sands Missile Range, New Mexico, on May 13, 1991 at 19:05 UT. The telescope systems onboard the MSSTA obtained several full disk solar images in narrow bandpasses centered around strong soft X-ray, EUV, and FUV emission lines. Each telescope was designed to be sensitive to the coronal plasmas at a particular temperature, for seven temperatures ranging from 20000 K to 4000000 K. We report here on the images obtained during the initial flight of the MSSTA, and on the chromospheric and coronal structure of polar plumes observed over both poles of the Sun. We have also co-aligned the MSSTA images with Kitt Peak magnetograms taken on the same day. We are able to positively identify the magnetic structures underlying the polar plumes we analyze as unipolar. We discuss the plume observations and present a radiative energy balance model derived from them.     

Cassini VIMS observations of latitudinal and hemispheric variations in Saturn’s infrared auroral intensity

The intensity of Saturn’s infrared aurora is investigated using Cassini VIMS images acquired during October 2006–February 2009. Polar and main oval auroral regions were defined in both hemispheres, which extend between 0–10° and 10–25° co-latitude, respectively. Average intensities were computed for these regions and compared. While the northern and southern main oval regions covered a similar range of intensities, the southern main oval was on average more intense by a factor of ∼1.3. The emission from the southern polar region was usually less intense than the main oval emissions, while this was only the case for approximately half of the northern hemisphere images. The northern hemisphere polar region displayed intensities more than twice as high as those in the south and the difference between the two hemispheres was most pronounced on the dayside. In general, more intense polar emissions were accompanied by more intense main oval emissions. Possible explanations for the hemispheric and latitudinal differences are discussed in terms of particle energies and fluxes, ionospheric conductivity, temperature and magnetic field strength.     

The origin of Ganymede's polar caps

Since their discovery in Voyager images, the origin of the bright polar caps of Ganymede has intrigued investigators. Some models attributed the polar cap formation to thermal migration of water vapor to higher latitudes, while other models implicated plasma bombardment in brightening ice. Only with the arrival of Galileo at Jupiter was it apparent that Ganymede possesses a strong internal magnetic field, which blocks most of the plasma from bombarding the satellite's equatorial region while funneling plasma onto the polar regions. This discovery provides a plausible explanation for the polar caps as related to differences in plasma-induced brightening in the polar and the equatorial regions. In this context, we analyze global color and high resolution images of Ganymede obtained by Galileo, finding a very close correspondence between the observed polar cap boundary and the open/closed field lines boundary obtained from new modeling of the magnetic field environment. This establishes a clear link between plasma bombardment and polar cap brightening. High resolution images show that bright polar terrain is segregated into bright and dark patches, suggesting sputter-induced redistribution and subsequent cold trapping of water molecules. Minor differences between the location of the open/closed field lines boundary and the observed polar cap boundary may be due to interaction of Ganymede with Jupiter's magnetosphere, and our neglect of higher-order terms in modeling Ganymede's internal field. We postulate that leading-trailing brightness differences in Ganymede's low-latitude surface are due to enhanced plasma flux onto the leading hemisphere, rather than darkening of the trailing hemisphere. In contrast to Ganymede, the entire surface of Europa is bombarded by jovian plasma, suggesting that sputter-induced redistribution of water molecules is a viable means of brightening that satellite's surface.     

Developing an Advanced Automated Method for Solar Filament Recognition and Its Scientific Application to a Solar Cycle of MLSO Hα Data

We developed a method to automatically detect and trace solar filaments in Hα full-disk images. The program is able not only to recognize filaments and determine their properties, such as the position, the area, the spine, and other relevant parameters, but also to trace the daily evolution of the filaments. The program consists of three steps: First, preprocessing is applied to correct the original images; second, the Canny edge-detection method is used to detect filaments; third, filament properties are recognized through morphological operators. To test the algorithm, we successfully applied it to observations from the Mauna Loa Solar Observatory (MLSO). We analyzed Hα images obtained by the MLSO from 1998 to 2009 and obtained a butterfly diagram of filaments. This shows that the latitudinal migration of solar filaments has three trends in Solar Cycle 23: The drift velocity was fast from 1998 to the solar maximum, after which it became relatively slow. After 2006, the migration became divergent, signifying the solar minimum. About 60 % of the filaments with latitudes higher than 50^° migrate toward the polar regions with relatively high velocities, and the latitudinal migrating speeds in the northern and the southern hemispheres do not differ significantly in Solar Cycle 23.     

Two-dimensional spectroscopy of double-barred galaxies

We describe the results of our spectroscopy for a sample of barred galaxies whose inner regions exhibit an isophotal twist commonly interpreted as a secondary bar. The line-of-sight velocity fields of the ionized gas and stars and the light-of-sight velocity dispersion fields of the stars were constructed from two-dimensional spectroscopy with the 6-m Special Astrophysical Observatory telescope. We detected various types of noncircular motions of ionized gas: radial flows within large-scale bars, counterrotation of the gas and stars at the center of NGC 3945, a polar gaseous disk in NGC 5850, etc. Our analysis of the optical and near-infrared galaxy images (both ground-based and those from the Hubble Space Telescope) revealed circumnuclear minispirals in five objects. The presence of an inner (secondary) bar in the galaxy images is shown to have no effect on the circumnuclear kinematics of the gas and stars. Thus, contrary to popular belief, the secondary bar is not a dynamically decoupled galactic structure. We conclude that the so-called double-barred galaxies are not a separate type of galaxy but are a combination of objects with distinctly different morphologies of their circumnuclear regions.     

Keck adaptive optics imaging of near-Earth Asteroid 2004 XP14

We present resolved near-infrared images of near-Earth Asteroid 2004 XP14, obtained with the adaptive optics system on Keck II during July 2006. This is the first time a near-Earth asteroid has been directly imaged from the ground. Our result illustrates the capabilities of adaptive optics and complements radar and spectral observations.     

Kinematics and Dynamics of the Galactic Stellar Halo

The structure, kinematics and dynamics of the Galactic stellar halo are reviewed including evidence of substructure in the spatial distribution and kinematics of halo stars. Implications for galaxy formation theory are subsequently discussed; in particular it is argued that the observed kinematics of stars in the outer Galactic halo can be used as an important constraint on viable galaxy formation scenarios. This revised version was published online in July 2006 with corrections to the Cover Date.     

Yearly comparisons of the martian polar caps: 1999-2003 Mars Orbiter Camera observations

The Mars Global Surveyor Mars Orbiter Camera wide-angle cameras were used to obtain images of the north and south seasonal and residual polar caps between 1999 and 2003. Wide-angle red camera images were used in assembling mosaics of the north and south polar recessions and regression rates were measured and compared. There are small variations in the north polar recession between 2000 and 2002, especially between LS=7° and LS=50°, however there is no evidence for the plateau in the recession curves that has been observed in some prior years. The south polar recession changes very little from year to year, and the 2001 dust storm had little if any effect on the average cap recession that year. Albedo values of the geographic north pole were measured using wide-angle red and blue camera images, and the residual south polar cap configuration was compared between the three years observed by MOC. The albedo of the geographic north pole generally varies between 0.5 and 0.6 as measured from MOC wide-angle red camera images. There were only minor variations near the edges of the residual south polar cap between the three years examined.     

Polar faculae and sunspot cycles

The monthly number of polar faculae of the Sun were determined from white-light images at spectral band (eff) = (4100 ± 200) Å obtained at the Kislovodsk Solar Station during 1960–1994. Corrected monthly numbers were obtained with the help of the visibility function. The level of polar activity larger than 1 above the monthly running mean was calculated, and the relation between the polar faculae and sunspot cycle was studied. We confirmed earlier results (Makarov and Makarova, 1987) that the monthly number of polar faculae, NPF _m (t) correlates with the monthly sunspot area A _m (Sp)(t + T) with a time shift T 6 yr. The new polar faculae cycle began in the middle of 1991. Peculiarities of the first part of sunspot cycle 23 are discussed.Guest scientist with the University of Arizona and Zetetic Institute. Tucson, Arizona 85719, U.S.A.     

一种测定望远镜极轴偏差的方法

利用望远镜的恒动跟踪，对两颗恒星进行无斗星曝光０．５小时，扣除蒙气差影响，用球面三角方法计算出望远镜极轴指向的偏差在赤经和赤纬方向上的分量，据此对望远镜极轴进行调整，就可以方便地把望远镜极轴指向调整到较高精度。     

Predictions of MESSENGER Neutron Spectrometer measurements for Mercury's north polar region

From radar images of Mercury's poles and MESSENGER Neutron Spectrometer (NS) measurements obtained during the spacecraft's flybys of Mercury, predictions of neutron count rates and their uncertainties are calculated for Mercury's north polar region as of the end of the MESSENGER primary orbital mission. If Mercury's poles contain large amounts of water ice, as has been suggested on the basis of the radar data, then during the one-year-long orbital mission the NS should detect signals indicative of excess polar hydrogen with a significance of at least 4σ, where σ is the standard deviation derived from Poisson counting statistics. If the polar deposits are not enriched with hydrogen, but are dominated by other elements, such as sulfur, then the MESSENGER neutron measurements should be able to confirm the absence of deposits having surface concentrations in excess of 50 wt% H₂O on permanently shadowed floors of craters near Mercury's north pole. Because of the large spatial footprint of the NS data, individual polar deposits will not be spatially resolved, but longitudinal asymmetries may be detected if residual systematic uncertainties are sufficiently low.     

Infrared imaging of Venus: 8–14 micrometers

High spatial resolution 8–14 μm images of Venus were obtained on March 6, 1974. The planet was at a phase angle of 112° with the morning terminator in view. Our images confirm the existence of a previously mapped flux anomaly near the south pole and show evidence of other infrared features, presumably transient in nature. Flux differences of 2–7% were measured, corresponding to brightnes temperature variations of roughly 1–3°K. The images also confirm the difference between polar and equatorial limb darkening. Finally, we present evidence for a night-day asymmetry in the flux, with the brightness temperature greater by about 2°K on the sunlit side.     

Collisional Quenching of OH Radio Emission from Comet Hale-Bopp

Observations of comets in the 18-cm OH transitions offer a means to probe gas production, kinematics, and OH excitation in comets. We present initial results of OH observations of comet Hale-Bopp obtained with the NRAO 43 m antenna located in Greenbank, WV. Maps of the emission provide strong constraints on the amount of quenching of the inversion of the OH ground state Λ-doublet in the coma. Analysis of the total radio OH flux and maps of its radial brightness distribution indicate a quenched region on the order of ∼500,000 km during March and April 1997. This large value is generally consistent with previous observations of radio OH quenching in lower production rate comets when the high production rate of comet Hale-Bopp is considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

A classification of polar cap auroral arcs

Global auroral imagery obtained by DMSP satellites during the years 1972–1979 over both the northern and southern high latitude polar regions were examined to study the morphology of the discrete arcs known as polar cap arcs. Based upon their morphology, the polar cap arcs can be generally classified into three types viz. (1) the distinctly sun-aligned polar cap arcs—Type 1 arcs, (2) the morning/evening polar cap arcs expanded from the auroral oval—Type 2 arcs and (3) the hook shaped arcs connecting the polar cap arc with the oval arc (including the hitherto unreported oppositely oriented hook shaped arcs)—Type 3 arcs. Concurrent auroral electrojet indices (AE) and interplanetary magnetic field (IMF) data were used to study the occurrence of the polar cap arcs. It was found that Type 1 arcs were observed mostly during low geomagnetic activity conditions, bright Type 2 arcs during the recovery phase of the substorms and Type 3 arcs do not occur during the recovery phase of the substorm. Over both hemispheres, the polar cap arcs were observed mostly during northward IMF. Furthermore, Type 1 arcs were obeserved over the northern polar cap during mostly negative B_x periods and over the southern polar cap during mostly positive B_x periods. The latter observation suggests that these types of arcs may be non-conjugate.     

Solar Wind Sources in the Late Declining Phase of Cycle 23: Effects of the Weak Solar Polar Field on High Speed Streams

The declining phases of solar cycles are known for their high speed solar wind streams that dominate the geomagnetic responses during this period. Outstanding questions about these streams, which can provide the fastest winds of the solar cycle, concern their solar origins, persistence, and predictability. The declining phase of cycle 23 has lasted significantly longer than the corresponding phases of the previous two cycles. Solar magnetograph observations suggest that the solar polar magnetic field is also ～?2?–?3 times weaker. The launch of STEREO in late 2006 provided additional incentive to examine the origins of what is observed at 1 AU in the recent cycle, with the OMNI data base at the NSSDC available as an Earth/L1 baseline for comparisons. Here we focus on the year 2007 when the solar corona exhibited large, long-lived mid-to-low latitude coronal holes and polar hole extensions observed by both SOHO and STEREO imagers. STEREO provides in situ measurements consistent with rigidly corotating solar wind stream structure at up to ～?45° heliolongitude separation by late 2007. This stability justifies the use of magnetogram-based steady 3D solar wind models to map the observed high speed winds back to their coronal sources. We apply the WSA solar wind model currently running at the NOAA Space Weather Prediction Center with the expectation that it should perform its best at this quiet time. The model comparisons confirm the origins of the observed high speed streams expected from the solar images, but also reveal uncertainties in the solar wind source mapping associated with this cycle’s weaker solar polar fields. Overall, the results illustrate the importance of having accurate polar fields in synoptic maps used in solar wind forecast models. At the most fundamental level, they demonstrate the control of the solar polar fields over the high speed wind sources, and thus one specific connection between the solar dynamo and the solar wind character.     

The Nearby Field Galaxy Survey

We report on our observing programme to obtain integrated spectrophotometry, intermediate- and high-resolution major-axis spectra, and UBR surface photometry of a representative sample of ∼ 200galaxies in the nearby field. The main goal of this programme is to provide a comparison sample for high-redshift studies and to study the variation in star formation rates (SFRs), star formation history (SFH),excitation, metallicity, and internal kinematics over a wide range of galaxy luminosities and morphological types. In particular, we extend the work of Kennicutt (1992) to lower-luminosity systems. We present the main results of our analysis so far. In these proceedings, we condense the two atlases presented of 1) images and radial surface brightness profiles and colour profiles, and 2) images and integrated spectra into several example images, profiles and spectra, showing the general trends observed. For the original atlases we refer the reader to the electronic distribution on CDROM or as available on the Web at http://www.astro.rug.nl/∼nfgs/. This revised version was published online in July 2006 with corrections to the Cover Date.