The nature of a dusty ring in Virgo

We detected a ring-like distribution of far-infrared (FIR) emission in the direction of the centre of the Virgo cluster (VC). We studied this feature in the FIR, radio and optical domains, and deduced that the dust within the feature reddens the galaxies in the direction of the VC but does not affect stars within the Milky Way. This is likely to be a dusty feature in the foreground of the VC, presumably in the Galactic halo. The H  i distribution follows the morphology of the FIR emission and shows peculiar kinematic behaviour. We propose that a highly supersonic past collision between an H  i cloud and the Galactic H  i formed a shock that heated the interface gas to soft X-ray temperatures. H  i remnants from the projectile and from the shocked Galactic H  i rain down on to the disc as intermediate-velocity gas.
Our finding emphasizes that extragalactic astronomy must consider the possibility of extinction by dust at high galactic latitude and far from the Galactic plane, which may show structure on 1° and smaller scales. This is particularly important for studies of the VC, e.g. in the determination of the Hubble constant from Cepheids in cluster galaxies.     

Chemical Ozone Loss in the Arctic Winter 1994/95 as Determined by the Match Technique

The chemically induced ozone loss inside the Arctic vortex during the winter 1994/95 has been quantified by coordinated launches of over 1000 ozonesondes from 35 stations within the Match 94/95 campaign. Trajectory calculations, which allow diabatic heating or cooling, were used to trigger the balloon launches so that the ozone concentrations in a large number of air parcels are each measured twice a few days apart. The difference in ozone concentration is calculated for each pair and is interpreted as a change caused by chemistry. The data analysis has been carried out for January to March between 370 K and 600 K potential temperature. Ozone loss along these trajectories occurred exclusively during sunlit periods, and the periods of ozone loss coincided with, but slightly lagged, periods where stratospheric temperatures were low enough for polar stratospheric clouds to exist. Two clearly separated periods of ozone loss show up. Ozone loss rates first peaked in late January with a maximum value of 53 ppbv per day (1.6 % per day) at 475 K and faster losses higher up. Then, in mid-March ozone loss rates at 475 K reached 34 ppbv per day (1.3 % per day), faster losses were observed lower down and no ozone loss was found above 480 K during that period. The ozone loss in hypothetical air parcels with average diabetic descent rates has been integrated to give an accumulated loss through the winter. The most severe depletion of 2.0 ppmv (60 %) took place in air that was at 515 K on 1 January and at 450 K on 20 March. Vertical integration over the levels from 370 K to 600 K gives a column loss rate, which reached a maximum value of 2.7 Dobson Units per day in mid-March. The accumulated column loss between 1 January and 31 March was found to be 127 DU (36 %).     

Intramonthly Indices of the Pacific/North American Teleconnection Pattern and Temperature Regimes over the United States

Summary Previous work has examined relationships between the planetary scale Pacific North American (PNA) teleconnection pattern and regional scale mean temperature over portions of the United States, but analyses have been restricted to monthly values of these two variables. It is not clear if this is the most appropriate scale for relating PNA patterns to temperature. In this study, PNA indices are related to various aspects of temperature over a range of time scales spanning one month. To carry this out, standardized PNA indices are calculated for 6 time intervals ranging from 5 to 30 days over a period of 27 winter seasons. Standardized regional scale temperature anomalies are assessed over various regions of the United States and used to compute mean, maximum, and minimum temperature anomalies over the same 6 time intervals. Correlation matrices are then constructed to provide relationship characteristics between the PNA indices and temperature measures within the intramonthly time scale. Only the southeastern and northwestern regions of the United States display significant relationships between the PNA indices and temperature. Over the Southeast, temperature anomalies on short time scales relate most strongly to PNA indices computed over time intervals of 25 to 30 days; this is especially the case for the minimum temperature anomaly. The strongest relationships, however, are observed between the PNA index and temperature anomalies computed on a 20–25 day time interval. Over the Northwest, the relationships are not as strong and somewhat more varied. Most notably, moderate connections are found between the 30 day PNA index and minimum temperature anomalies computed over a 25 day period. Received August 18, 1997Revised November 28, 1997     

Particle size distributions in Saturn's rings from voyager 1 radio occultation

Observations of microwave opacity τ[λ] and near forward scatter from Saturn's rings at wavelengths λ of 3.6 and 13 cm from the Voyager 1 ring occultation experiment contain information regarding ring particle sizes in the range of about a = 0.01 to 15 m radius. The opacity measurements τ[3.6] and τ[13] are sufficient to constrain the scale factor n(a₀) and index q of a power law incremental size distribution n(a) = n(a₀)[a₀/a]^q, assuming known minimum and maximum sizes and a many-particle-thick model. The families of such distributions are highly convergent in the centimeter-size range. Forward scatter at 3.6 cm can be used to solve for a general distribution over the radius range $\text{1 ? a ? 15}\text{m}$ by integral inversion and inverse scattering methods, again assuming a many-particle-thick slab-type radiative transfer model. Distributions n(a) valid over $\text{0.01 ? a ? 15}\text{m}$ are obtained by combining the results from the two types of measurements above. Mass distributions may be computed directly from n(a). Such distributions, partly measured and partly synthesized, have been obtained for four features in the ring system centered at 1.35, 1.51, 2.01, and 2.12 Saturn radii (R_s). The size and mass distributions both cut off sharply at a ? 4–5 m; the mass distribution peaks over the narrow size range $\text{3 ? a ? 4}\text{m}$ for all four locations. No single power law distribution is consistent with the data over the entire interval $\text{0.01 ? a ? 5}\text{m}$, although a power law-type model is consistent with the data over a limited size range of $\text{0.01 ? a ? 1}\text{m}$, where the indices q = 3.4 and 3.3 are obtained from the slab model for the features located at 1.51 and 2.01 R_s. The fractional contribution of the suprameter particles to the microwave opacity in each feature appears to be about $\text{1}\text{3}\text{,}\text{1}\text{3}\text{,}\text{2}\text{3}\text{,}\text{and}\text{1}$, respectively, with the fraction at 2.12 R_s being the least certain. The cumulative surface mass per unit area obtained for the classical slab model is approximately 11, 16, 41, and 132 g/cm² for the four features, respectively, if the particles are solid H₂O ice. Both the fractional opacity and the mass density estimates represent upper bounds implied by the assumption of a uniformly mixed set of particles in a many-particle-thick vertical profile; lower estimates would result if the rings were assumed to be nearly a monolayer or if the vertical distribution of particles were size dependent.     

Coordinated Observations of Comet Hale–Bopp between 32 and 860 GHz

Earth, Moon, and Planets - The concept of simultaneous multifrequency continuum observations, successfully tested on Comet Hyakutake, was applied to Comet Hale-Bopp, using the Heinrich Hertz...     

Spectral investigation of chromospheric fine structure

H spectra and effectively simultaneous filtergrams were taken at the Fraunhofer Observatory on Capri with the 35 cm domeless Coudé. The spatial resolution of the 19 best spectra selected for analysis was estimated to be 1–2 arc sec. The comparison of several hundred H line profiles emitted by typical chromospheric structure elements with theoretical prediction yielded strong evidence to suggest that the chromosphere consists of two parts: A lower, rather uniform layer at rest superposed by clouds (condensations of great spatial variability) which constitute the well-known structure pattern of H filtergrams. For most image points the line-of-sight velocity, optical thickness, source function and Doppler broadening of these clouds could be determined. While the values of the latter three quantities were found to be similar to what Beckers (1968) has found for limb spicules the velocity of the bright and of the dark mottles is considerably smaller than to be expected if these features were the spicules as seen on the disk. However, our results do not rule out the possibility that the spicules rise at the centers of rosettes where they are difficult to detect.Mitteilung aus dem Fraunhofer Institut No. 105.     

Measurements of cosmic-ray Li,Be and B nuclei in the energy range 100 MeV/nuc to >22 BeV/nuc

We report on new measurements of the spectra of Li, Be and B nuclei in the primary cosmic radiation in the energy range 100 MeV/nuc to >22 BeV/nuc. The differential spectrum of these light nuclei is found to have a maximum at 400 MeV/nuc in 1966. The L/M ratio is found to be equal to 0.25±0.01, constant over the entire energy range of the measurement. Atmospheric and solar modulation effects on the L nuclei and the L/M ratio are discussed. It is concluded that this ratio is representative of conditions in interstellar space. Using the most recently available fragmentation parameters gives a material path length of 3.6 g/cm² of hydrogen for the particles producing the L nuclei. The absence of any variation of the L/M ratio with energy places severe constraints on models for the propagation of cosmic rays. Models in which the material path length is a strong function of energy — or that exhibit an exponential path-length distribution for a fixed energy are incompatible with these results. An examination of the abundance ratios of the individual L nuclei separately reveals major discrepancies with the predictions of interstellar diffusion theory based on presently accepted fragmentation parameters. The constancy of the measured Li/M and B/M ratios with energy is not in accord with the large energy dependence of these ratios expected from the energy dependence of the fragmentation cross-sections. The low Li/M ratio and high B/M ratio to be expected if these nuclei are created at a much lower energy than we observe are also not found. This presents difficulties for theories which suggest that the passage through matter has occurred at low energies subsequently followed by considerable acceleration.The Be/M ratio in cosmic rays is anomalous in that it is 40% larger than expected on the basis of the fragmentation cross-sections. Evidence presented here on the isotopic composition of Be nuclei suggests that this discrepancy is due to an enhanced abundance of Be⁹ or Be¹⁰ in cosmic rays. This discrepancy complicates the determination of a cosmic-ray age using the decay of Be¹⁰ into B.Nevertheless the Be/B ratio is observed to remain constant at 0.42±0.03 over the energy range from 100 MeV/nuc to over 10 BeV/nuc. Unless the fragmentation parameters into the various isotopes of Be and B are such that e.g. (Be/B)<0.05 as a result of this decay, then the age of cosmic rays is either >3×10⁸ years or <10⁶ years. The further observation that the mass to charge ratio of all Be nuclei of energy 1 BeV/nuc is =2.05±0.1 suggests that Be¹⁰ is present at these energies. This supports the idea of a short lifetime.     

Time-series temperature measurements at high-temperature hydrothermal vents, East Pacific Rise 9°49′–51′N: evidence for monitoring a crustal cracking event

Temperature measurements of hydrothermal vent fluids provide an important indicator of the physical and chemical state of mid-ocean ridge crest hydrothermal and magmatic systems. Changes in vent fluid temperature and chemistry can have dramatic effects on biological communities that inhabit these unique ecosystems. In an attempt to understand temporal variability of ridge crest hydrothermal activity as it relates to geological processes at the ridge axis, six high-temperature hydrothermal vents on the East Pacific Rise crest between 9°49′N and 9°51′N were instrumented and sampled repeatedly during five years following a submarine volcanic eruption in 1991. Bio9 vent, located on the floor of the axial trough near 9°50.2′N, has the most complete record of fluid temperatures from 1991 to 1997, including a continuous temperature record of nearly three years (1994–1997). Bio9 vent fluids were 368°C in 1991, increased to an estimated temperature ≥388°C after a second volcanic event in 1992, and thereafter declined over the next 2 years reaching a temperature of 365°C in December 1993. Continuous temperature records and point measurements made by Alvin's thermocouple probe show Bio9 vent fluids were stable for 15 months at 365±1°C, until March 26, 1995. On March 26, an abrupt 7°C increase occurred over a period of eight days at this vent, and a maximum temperature of 372±1°C persisted for 14 days. The vent fluid cooled gradually over 3.5 months to 366±1°C, and for several months at the end of the recording period the temperature increased a few degrees. A continuous record of fluid temperature at this vent between November 1995 and November 1997 shows a 5±1°C increase for the two-year period. The abrupt temperature increase at Bio9 vent, and coincident changes in faunal community structure, and geochemistry of vent fluids from this area suggest that a crustal event occurred, either in the form of a cracking front in the crust or intrusion of a small dike. Based on the results of a microseismicity experiment conducted around the Bio9 vent in 1995 [Sohn et al., Trans. Am. Geophys. Union 78 (1997) F647; Sohn et al., Nature (in press)], and the identification of a small earthquake swarm which occurred on March 22, 1995 we conclude that the temperature anomaly measured at Bio9 four days following the swarm was caused by a cracking front penetrating into hot crustal rocks beneath the vent.     

The radio occultation method for the study of planetary atmospheres

Planets, the Moon and the Sun have a number of ‘atmospheres’ which may be measured by the radio occultation method, using radio links to spacecraft which are being occulted by the body as seen from Earth. Molecular atmospheres, ionospheres, magnetospheres, particulate atmospheres and several general-relativistic atmospheres can all affect radio signal charecteristics. Measured Doppler frequencies, signal amplitudes and wave polarizations contain information on these atmospheres. From such occultation measurements it is possible, for example, to derive profiles which are related to the changes with height of temperature, pressure, density, free electron concentration and the fractional volume occupied by particulate matter. Important clues for identifying molecular or particulate constituents can also be obtained. Even though one type of measurement may be sensitive to several different atmospheric characteristics, these characteristics can often be separated due to differences in their dependence on radio wavelength or on height above the surface.