Scales Of Months Research Articles

Patch dynamics of coral reef macroalgae under chronic and acute disturbance

The patch dynamics (colonisation rate, growth rate, and extinction rate) are quantified for two dominant species of macroalgae on a Caribbean forereef in Belize: Lobophora variegata (Lamouroux) and Dictyota pulchella (Hornig and Schnetter). Measurements were taken on time scales of days, weeks, months, and years during which three hurricanes occurred. All patches were followed on naturally occurring ramets of dead Montastraeaannularis. The first hurricane (Mitch) caused massive coral mortality and liberated space for algal colonisation. The cover of Lobophora increased throughout the study and herbivores did not appear to limit its cover within a 4 year time frame. In contrast, the cover of D. pulchella fluctuated greatly and showed no net increase, despite an increase in parrotfish biomass and settlement space. Variation in the overall percent cover of an alga is not indicative of the underlying patch dynamics. The steady rise in the cover of Lobophora took place despite a high turnover of patches (12–60% of patches per year). The patch dynamics of Dictyota were slower (7–20%), but a greater patch density and threefold higher lateral growth rate led to greater fluctuations in total cover. The dynamics of algal patches are size-specific such that larger patches are less likely to become extinct during hurricanes.

Inorganic and organic nitrogen cycling in the Southern California Bight

On the basis of mass balance calculations performed for nitrogen (N) uptake experiments in the Southern California Bight (SCB), it has been suggested that a significant portion of dissolved inorganic N (DIN) uptake results in the production of dissolved organic N (DON). To investigate this process, the fate of ammonium (NH 4 +) and nitrate (NO 3 −) uptake was quantified within the euphotic zone at three coastal stations in the SCB using 15N tracer techniques. Several trends in the fate of DIN and the production of DON were observed. First, production of particulate N (PN), from both NH 4 + and NO 3 −, was quantitatively more important in near surface waters, while DON release dominated within the nitracline. Second, the percentage of gross N uptake released as DON was generally higher when NO 3 −, rather than NH 4 +, was the substrate. Third, the percentage of N released as DON was higher at night, relative to the day. Fourth, rates of DON release were significantly correlated to NH 4 + regeneration, suggesting that similar mechanisms are responsible for both processes—presumably grazing. The results of this study indicate that the DON pool is a sink for DIN uptake on the time scale of hours. One implication of this finding is that new production estimates based on 15NO 3 − uptake rates will likely underestimate particle flux out of the surface layer because the rate of NO 3 − uptake is underestimated due to loss of DO 15N during the incubation. On time scales of months to years, however, the N that is taken up as NO 3 − and released as DON will likely contribute to export flux via incorporation of the dissolved phase during seasonal mixing into sinking particles or transport. The export of DON on these time scales argues for the use of gross uptake rates to calculate f-ratios.

Moist Teleconnection Mechanisms for the Tropical South American and Atlantic Sector*

AbstractTeleconnections have traditionally been studied for the case of dry dynamical response to a given diabatic heat source. Important anomalies often occur within convective zones, for instance, in the observed remote response to El Niño. The reduction of rainfall and teleconnection propagation in deep convective regions poses theoretical challenges because feedbacks involving convective heating and cloud radiative effects come into play. Land surface feedbacks, including variations of land surface temperature, and ocean surface layer temperature response must be taken into account. During El Niño, descent and negative precipitation anomalies often extend across equatorial South America and the Atlantic intertropical convergence zone. Analysis of simulated mechanisms in a case study of the 1997/98 El Niño is used to illustrate the general principals of teleconnections occurring in deep convective zones, contrasting land and ocean regions. Comparison to other simulated events shows similar behavior. Tropospheric temperature and wind anomalies are spread eastward by wave dynamics modified by interaction with the moist convection zones. The traditional picture would have gradual descent balanced by radiative damping, but this scenario misses the most important balances in the moist static energy (MSE) budget. A small “zoo” of mechanisms is active in producing strong regional descent anomalies and associated drought. Factors common to several mechanisms include the role of convective quasi equilibrium (QE) in linking low-level moisture anomalies to free tropospheric temperature anomalies in a two-way interaction referred to as QE mediation. Convective heating feedbacks change the net static stability to a gross moist stability (GMS) M. The large cloud radiative feedback terms may be manipulated to appear as a modified static stability Meff, under approximations that are quantified for the quasi-equilibrium tropical circulation model used here. The relevant measure of Meff differs between land, where surface energy flux balance applies, and short time scales over ocean. For the time scale of an onsetting El Niño, a mixed layer ocean response is similar to a fixed sea surface temperature (SST) case, with surface fluxes lost into the ocean and Meff substantially reduced over ocean-enhancing descent anomalies. Use of Meff aids analysis of terms that act as the initiators of descent anomalies. Apparently modest terms in the MSE budget can be acted on by the GMS multiplier effect, which yields substantial precipitation anomalies due to the large ratio of the moisture convergence to the MSE divergence. Advection terms enter in several mechanisms, with the leading effects here due to advection by mean winds in both MSE and momentum balances. A Kelvinoid solution is presented as a prototype for how easterly flow enhances moist wave decay mechanisms, permitting relatively small damping terms by surface drag and radiative damping to produce the substantial eastward temperature gradients seen in observations and simulations and contributing to precipitation anomalies. The leading mechanism for drought in eastern equatorial South America is the upped-ante mechanism in which QE mediation of teleconnected tropospheric temperature anomalies tends to produce moisture gradients between the convection zone, where low-level moisture increases toward QE, and the neighboring nonconvective region. Over the Atlantic ITCZ, the upped-ante mechanism is a substantial contributor, but on short time scales several mechanisms referred to jointly as troposphere/SST disequilibrium mechanisms are important. While SST is adjusting during passive SST (coupled ocean mixed layer) experiments, or for fixed SST, heat flux to the ocean is lost to the atmosphere, and these mechanisms can induce descent and precipitation anomalies, although they disappear when SST equilibrates. In simulations here, cloud radiative feedbacks, surface heat fluxes induced by teleconnected wind anomalies, and surface fluxes induced by QE-mediated temperature anomalies are significant disequilibrium contributors. At time scales of several months or longer, remaining Atlantic ITCZ rainfall reductions are maintained by the upped-ante mechanism.

XUV Photometer System (XPS): Overview and Calibrations

The solar soft X-ray (XUV) radiation is highly variable on both short-term time scales of minutes to hours due to flares and long-term time scales of months to years due to solar cycle variations. Because of the smaller X-ray cross sections, the solar XUV radiation penetrates deeper than the extreme ultraviolet (EUV) wavelengths and thus influences the photochemistry and ionization in the mesosphere and lower thermosphere. The XUV Photometer System (XPS) aboard the Solar Radiation and Climate Experiment (SORCE) is a set of photometers to measure the solar XUV irradiance shortward of 34 nm and the bright hydrogen emission at 121.6 nm. Each photometer has a spectral bandpass of about 7 nm, and the XPS measurements have an accuracy of about 20%. The XPS pre-flight calibrations include electronics gain and linearity calibrations in the laboratory over its operating temperature range, field of view relative maps, and responsivity calibrations using the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute of Standards and Technology (NIST). The XPS in-flight calibrations include redundant channels used weekly and underflight rocket measurements from the NASA Thermosphere-Ionosphere-Mesosphere-Energetics-Dynamics (TIMED) program. The SORCE XPS measurements have been validated with the TIMED XPS measurements. The comparisons to solar EUV models indicate differences by as much as a factor of 4 for some of the models, thus SORCE XPS measurements could be used to improve these models.

Variation In Abundance And Size-Structure Of Populations Of The Small Chiton, Acanthochitona Retrojecta

Hierarchically-nested sampling designs were used to test hypotheses about the population ecology of the small chiton Acanthochitona retrojecta (Pilsbry) in coralline algal turf on rocky shores near Sydney, Australia. Spatial sampling in January 1998 demonstrated that these small chitons were more abundant in mid- than in low-shore areas and more abundant on shores north of compared with those south of Sydney Harbour. Densities of A. retrojecta were extremely high, often exceeding 3500/m2. Temporal sampling showed there was little variation in the total abundance of A. retrojecta at scales of weeks and months. Nevertheless, there were significantly more juvenile chitons in June and July, which was offset by a reduction in the abundance of adults at the same time. The lengths of chitons ranged from 0.59 mm to 9.56 mm and size-frequency distributions demonstrated that A. retrojecta recruits continuously throughout the year. Several aspects of the population ecology of A. retrojecta differed greatly from those of previously studied species of larger chitons. These differences may be a function of the unique biology of A. retrojecta or be related to its unusually small size. More information is needed on other species of small chitons before general patterns of difference can be properly evaluated.

H.E.S.S. observations of PKS 2155-304

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (~45 sigma) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2 sigma). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

Electromagnetic Instability of a Homogeneous Plasma in Interstellar Space

A study is made of an electromagnetic instability of a homogeneous plasma with an arbitrary velocity distribution in interstellar space on long time scales typical of gas-dust clouds without a significant magnetic field. It is shown that such instabilities develop on time scales of a few months or years and that it is only a narrow class of nearly spherical solutions that conforms to the stability requirements.

Dynamics of the circumstellar gas in the Herbig Ae stars BF Orionis, SV Cephei, WW Vulpeculae and XY Persei

We present high resolution (lambda / Delta_lambda = 49000) echelle spectra of the intermediate mass, pre-main sequence stars BF Ori, SV Cep, WW Wul and XY Per. The spectra cover the range 3800-5900 angstroms and monitor the stars on time scales of months and days. All spectra show a large number of Balmer and metallic lines with variable blueshifted and redshifted absorption features superimposed to the photospheric stellar spectra. Synthetic Kurucz models are used to estimate rotational velocities, effective temperatures and gravities of the stars. The best photospheric models are subtracted from each observed spectrum to determine the variable absorption features due to the circumstellar gas; those features are characterized in terms of their velocity, v, dispersion velocity, Delta v, and residual absorption, R_max. The absorption components detected in each spectrum can be grouped by their similar radial velocities and are interpreted as the signature of the dynamical evolution of gaseous clumps with, in most cases, solar-like chemical composition. This infalling and outflowing gas has similar properties to the circumstellar gas observed in UX Ori, emphasizing the need for detailed theoretical models, probably in the framework of the magnetospheric accretion scenario, to understand the complex environment in Herbig Ae (HAe) stars. WW Vul is unusual because, in addition to infalling and outflowing gas with properties similar to those observed in the other stars, it shows also transient absorption features in metallic lines with no obvious counterparts in the hydrogen lines. This could, in principle, suggest the presence of CS gas clouds with enhanced metallicity around WW Vul. The existence of such a metal-rich gas component, however, needs to be confirmed by further observations and a more quantitative analysis.

Ancient coastal wells of Caesarea Maritima, Israel, an indicator for relative sea level changes during the last 2000 years

During the detailed excavations of ancient Caesarea, Israel, East Mediterranean, 64 coastal water wells have been examined that date from the early Roman period (with the oldest occurring in the 1st century AD), up to the end of the Crusader period (mid-13th century AD). The depths of these coastal water wells establish the position of the ancient water table and therefore the position of sea level for the first century AD up to 1300 AD. The connection between the coastal water table and changes in sea level has been established from modern observations in several wells on time scales of days and months and this is used to reconstruct sea level during historical time. The results indicate that during the Byzantine period, sea level at Caesarea was higher by about 30 cm than today. The Late Moslem and Crusader data shows greater fluctuations but the data sets are also much smaller than for the earlier periods. The consistency of the data indicates that the near-coastal well data from Caesarea provides a reliable indicator of sea-level change, with an accuracy of about 10–15 cm. These results are consistent with observations for earlier periods and, with comparisons to model-predicted glacio-hydro isostatic sea-level change, indicate that ocean volumes have been constant for much of the past 2000 years. The well data is also consistent with an absence of significant vertical tectonic movement of the coast at Caesarea over about 2000 years.

Simulation of the interaction of galactic cosmic ray protons with meteoroids: On the production of 3H and light noble gas isotopes in isotropically irradiated thick gabbro and iron targets

Abstract— Thick spherical targets, one made of gabbro (R = 25 cm) and one made of iron (R = 10 cm), were irradiated isotropically with 1.6 GeV protons at Laboratoire National Saturne (LNS)/Saclay to simulate the interactions of galactic cosmic ray protons with meteoroids in space. At various depths, both artificial meteoroids contained a large number of high‐purity, single‐element target foils and chemical compounds of up to 28 target elements. In these individual target foils, the elemental production rates of radionuclides and noble gas isotopes were measured. Here, we report the results for the light noble gas isotopes 3,4He, 20,21,22Ne, and 36,38,39Ar for the most cosmochemically relevant target elements as well as for some meteoritic material from Jilin, Farmington, and Cape York. From 3He analyses done several years apart, 3H diffusive losses during sample storage have been obtained, and direct as well as cumulative 3He production rates for O, Mg, Al, Si, Fe, Ni, and the meteoritic material are given. Losses by diffusion of tritium from metallic Mg and Fe are found to occur on time scales of months, while metallic Al, Si, and stone meteorites are much more retentive. The production rate ratios P(3H)/P(3He)d obtained in the simulation experiments are 0.73, 1.28, and 1.16 for O, Al, and Si, respectively. These rates are based on our best knowledge about the 3H and 3He production rates and should, therefore, replace data published earlier (Leya et al. 2000a). The earlier calculations for 4He, 20,21,22Ne, and 36,38,39Ar remain valid.The new modeled correlation 3Hecum/21Ne versus 22Ne/21Ne for chondrites exposed to cosmic rays with an energy spectrum characterized by a modulation parameter of φ = 650 MeV is in fair agreement with the empirical relationship (“Berne plot”). However, for small meteorites and little shielding in larger ones, there are systematic differences that most likely are due to an underestimation of the spallogenic 22Ne/21Ne ratio by ˜2%.

Constraints on the Very High Energy Emission from BL Lacertae Objects

We present results from observations of 29 BL Lacertae objects, taken with the Whipple Observatory 10 m Gamma-Ray Telescope between 1995 and 2000. The observed objects are mostly at low redshift (z < 0.2) but observations of objects of z up to 0.444 are also reported. Five of the objects are EGRET sources and two are unconfirmed TeV sources. Three of the confirmed sources of extragalactic TeV gamma rays were originally observed as part of this survey and have been reported elsewhere. No significant excesses are detected from any of the other objects observed, on time scales of days, months or years. We report 99.9% confidence level flux upper limits for the objects for each observing season. The flux upper limits are typically 20% of the Crab flux although, for some sources, limits as sensitive as 6% of the Crab flux were derived. The results are consistent with the synchrotron-self-Compton (SSC) model predictions considered in this work.

Vertical and temporal variability of redox zonation in the water column of the Cariaco Basin: implications for organic carbon oxidation pathways

Temporal and vertical variability in the concentrations of major electron acceptors and donors (O 2, NO 3 −, particulate Mn and Fe, dissolved Mn 2+ and Fe 2+, and HS −) were investigated in the water column of the Cariaco Basin between 1995 and 1999. Vertical profiles of the redox species during upwelling and non-upwelling periods reflected the zonation of aerobic respiration, denitrification, metal reduction, and sulfate reduction in the water column and varied widely over time scales of months to years. Comparisons of these profiles with distributions of organic carbon production and acetate uptake suggest that the rate of organic carbon supply was the major control on the variability. Inhibition and enrichment experiments suggested that sulfate reduction was not a dominant pathway for organic carbon oxidation near the oxic/anoxic interface. Instead, the reduction of metal oxides, elemental sulfur, and/or thiosulfate is likely to be more important. Estimates of vertical fluxes showed that vertical diffusive transport of the major electron acceptors (oxygen, nitrate, and metal oxides) only accounted for less than 10% of the oxidation of organic carbon produced near the suboxic/anoxic transition zone. Thus, horizontal oxic mid-water intrusions coupled with strong internal redox recycling of the major redox species occur to supply additional electron acceptors for organic carbon oxidation near the redox transition zone.

SUCCESSIONAL MECHANISM VARIES ALONG A GRADIENT IN HYDROTHERMAL FLUID FLUX AT DEEP‐SEA VENTS

Invertebrate communities inhabiting deep‐sea hydrothermal vents undergo substantial succession on time scales of months. Manipulative field experiments assessed the relative roles of environmental state and biotic interactions in determining temporal succession along a spatial gradient in vent fluid flux at three vent sites near 9°50′ N on the East Pacific Rise (2500 m water depth). Species colonization patterns on cubic basalt blocks (10 cm on a side) deployed by the submersible Alvin revealed both positive (facilitation) and negative (inhibition) biological interactions, in the context of established succession theory. Over a series of four cruises from 1994 to 1998, blocks were exposed to colonists for consecutive and continuous intervals in short‐term (5 + 8 = 13 mo) and longer‐term (8 + 29 = 37 mo) experiments. Colonists grouped into a mobile functional group were less abundant in the continuous interval (13 mo) than in the synchronous pooled‐consecutive intervals (5 + 8 mo) of the short‐term experiment, indicating that early colonists inhibited subsequent recruitment. Colonists grouped into a sessile functional group exhibited the opposite pattern, indicating facilitation. Similar trends, though not statistically significant, were observed in the longer‐term experiment. The character of species interactions varied along a gradient in hydrothermal fluid flux (and inferred productivity), with inhibitory interactions more prominent in zones with high temperatures, productivity, and faunal densities, and facilitative interactions appearing where temperatures, productivity, and densities were low. Analyses of primary succession on introduced basalt blocks suggest that biological interactions during early vent community development strongly modify initial patterns of settlement, even in the absence of sustained temporal change in the vent fluid flux.Corresponding Editor: R. J. Etter

Processing watershed‐derived nitrogen in a well‐flushed New England estuary

Isotopically labeled nitrate (15NO3−) was added continuously to the Rowley estuary, Massachusetts, for 22 d to assess the transport, uptake, and cycling of terrestrially derived nitrogen during a period of high river discharge and low phytoplankton activity. Isotopic enrichment of the 3.5‐km tidal prism (150,000 m3) was achieved for the 3 weeks and allowed us to construct a nitrogen mass balance model for the upper estuary. Mean δ15NO3− in the estuary ranged from 300‰ to 600‰, and approximately 75%–80% of the 15N was exported conservatively as 15NO3− to the coastal ocean. Essentially all of the 20%–25% of the 15N processed in the estuary occurred in the benthos and was evenly split between direct denitrification and autotrophic assimilation. The lack of water‐column 15N uptake was attributed to low phytoplankton stocks and short water residence times (1.2–1.4 d). Uptake of watercolumn NO3− by benthic autotrophs (enriched in excess of 100‰) was a function of NO3− concentration and satisfied up to 15% and 25% of the total nitrogen demand for benthic microalgae and macroalgae, respectively. Approximately 10% of tracer assimilated by benthic autotrophs was mineralized and released back to the water column as 15NH4+. By the end of the study, 15N storage in sediments and marsh macrophytes accounted for 50%–70% of the 15N assimilated in the estuary. These compartments may sequester watershed‐derived nitrogen in the estuary for time scales of months to years.

Intraseasonal oscillations in the middle atmosphere forced by gravity waves

In our Numerical Spectral Model (NSM) that incorporates Hines’ Doppler Spread Parameterization, small-scale gravity waves (GW) propagating in the east/west direction can generate the essential features of the observed equatorial oscillations in the zonal circulation and in particular the QBO extending from the stratosphere into the upper mesosphere. We report here that the NSM also produces in the zonally symmetric (wavenumber=0) meridional circulation intraseasonal oscillations on time scales of a few months. A distinct but variable meridional wind oscillation (MWO) is generated, which appears to be the counterpart to the quasi-biennial oscillation (QBO). With a vertical grid-point resolution of about 0.5 km , the NSM produces the MWO through momentum deposition of GWs propagating in the north/south direction. The resulting momentum source represents a non-linear function of the meridional winds that is of third (generally odd) order, and this enables the oscillation, as is the case for the zonal winds in the QBO. Since the meridional winds are relatively small compared to the zonal winds, however, the vertical scale-length for the MWO is much smaller, i.e., only about 10 km instead of 40 km for the QBO. Consistent with the associated increase of the viscous stress, the period of the MWO is then short compared with that of the QBO, i.e., only about 1–4 months. Measurements with the HRDI instrument on UARS show evidence of the MWO with periods between 2 and 3 months (Huang and Reber, J. Geophys. Res. (2003)). Depending on the strength of the GW forcing, the computed amplitudes of the MWO are typically 5 m/ s in the upper stratosphere and mesosphere, and the associated temperature amplitudes are about 3 K . These amplitudes may be observable with the instruments on the TIMED spacecraft. Extended computer simulations with the NSM in 2D and 3D reveal that the MWO is modulated by and in turn influences the QBO.

Circular Polarization Variability in Extragalactic Sources on Time Scales of Months to Decades

We present previously unpublished circular polarization (cp) measurements at 4.8 and 8.0 GHz made with the University of Michigan 26-meter prime focus telescope during 1978–1984 and results of a new observational program during the past two years. Based on the preliminary analysis of our recent data, eight sources were detected at 4.8 GHz with average degrees of cp ranging from ≤ 0.1% to ≥ 1%.The results are compared with observations at other frequencies and with linear polarization and total flux density variability in the sources. The behavior of the cp variability observed to date is consistent with stochastic variations produced by mode conversion in transient, opaque emitting regions in the sources. The observed sign changes observed between different epochs and different frequencies are not consistent with the hypothesis that sources maintain a fixed handedness of cp.

Combination Exchange Mechanisms for Efficient Bandwidth Allocation

The size, scale and multiple ownership of communication network resources makes it important to consider an economic framework wherein we can investigate the efficiency of network operation taking agents' incentives into account. Such a framework has been considered in the design and analysis of pricing mechanisms to regulate congestion and share bandwidth over short time scales. We consider time scales of a few months over which owners of communication links lease bandwidth to network service providers. As is well-known, economic efficiency is related to how close an allocation is to a competitive equilibrium. We first show that achieving economic efficiency through a market mechanism depends on network topology. We then show that in finite networks a competitive equilibrium may not exist. But a competitive equilibrium does exist in an idealized continuum model, in which all agents are infinitesimal compared with the size of the network. This suggests that approximate competitive equilibria with good performance may be attainable in real networks. We finally introduce a market mechanism called the combinatorial seller's bid double auction whose outcome, in the continuum model, is a competitive equilibrium. 1. Introduction. Communication networks have increased in scale and hetero- geneity. There are multiple network owners and operators, each with their own hetero- geneous endowments and privately known cost and revenue models. So, an allocation of network resources efficient from an engineering perspective need not be realized in the market. Hence, it is useful to find economic mechanisms that result in efficient resource allocations among selfish agents. The efficiency of bandwidth allocation among competing users in a communica- tion network has been studied within an economic framework before. The previous research literature focuses attention on congestion that results when aggregate user demand exceeds capacity, and proposes usage-dependent pricing as a method for con- trolling congestion (23, 24). However, several difficulties must be faced in coming up with a good price-adjustment mechanism. First, congestion occurs over a very short time scale: Aggregate traffic is bursty and exceeds nominal capacity for periods of a few seconds (10). So the pricing mechanism must react very quickly to changes in aggregate demand. Second, the 'correct' congestion price depends on how much users are willing to pay for the service, which is not known. This has prompted invention of distributed price iteration algorithms that converge to the 'correct' price (16, 22). These algorithms are not practical but permit a re-interpretation of TCP-like proto-

Temperature tides determined with meteor radar

Abstract. A new analysis method for producing tidal temperature parameters using meteor radar measurements is presented, and is demonstrated with data from one polar and two mid-latitude sites. The technique further develops the temperature algorithm originally introduced by Hocking (1999). That earlier method was used to produce temperature measurements over time scales of days and months, but required an empirical model for the mean temperature gradient in the mesopause region. However, when tides are present, this temperature gradient is modulated by the presence of the tides, complicating extraction of diurnal variations. Nevertheless, if the vertical wavelengths of the tides are known from wind measurements, the effects of the gradient variations can be compensated for, permitting determination of temperature tidal amplitudes and phases by meteor techniques. The basic theory is described, and results from meteor radars at Resolute Bay (Canada), London (Canada) and Albuquerque (New Mexico, USA) are shown. Our results are compared with other lidar data, computer models, fundamental tidal theory and rocket data. Phase measurements at two mid-latitude sites (Albuquerque, New Mexico, and London, Canada) show times of maximum for the diurnal temperature tide to change modestly throughout most of the year, varying generally between 0 h and 6 h, with an excursion to 12 h in June at London. The semidiurnal tide shows a larger annual variation in time of maximum, being at 2–4 h in the winter months but increasing to 9 h during the late summer and early fall. We also find that, at least at mid-latitudes, the phase of the temperature tide matches closely the phase of the meridional tide, and theoretical justification for this statement is given. We also demonstrate that this is true using the Global Scale Wave Model (Hagan et al., 1999). Median values for the temperature amplitudes for each site are in the range 5 to 6 Kelvin. Results from a more northern site (Resolute Bay) show less consistency between the wind tides and the temperature tides, supporting suggestions that the temperature tides may be zonally symmetric at these high latitudes (e.g. Walterscheid and Sivjee, 2001).Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides) – Radio science (signal processing)

A dynamical study of the circumstellar gas in UX Orionis

We present the results of a high spectral resolution () study of the circumstellar (CS) gas around the intermediate mass, pre-main sequence star UX Ori. The results are based on a set of 10 échelle spectra covering the spectral range 3800–5900 Å, monitoring the star on time scales of months, days and hours. A large number of transient blueshifted and redshifted absorption features are detected in the Balmer and in many metallic lines. A multigaussian fit is applied to determine for each transient absorption the velocity, v, dispersion velocity, , and the parameter R, which provides a measure of the absorption strength of the CS gas. The time evolution of those parameters is presented and discussed. A comparison of intensity ratios among the transient absorptions suggests a solar-like composition of the CS gas. This confirms previous results and excludes a very metal-rich environment as the cause of the transient features in UX Ori. The features can be grouped by their similar velocities into 24 groups, of which 17 are redshifted and 7 blueshifted. An analysis of the velocity of the groups allows us to identify them as signatures of the dynamical evolution of 7 clumps of gas, of which 4 represent accretion events and 3 outflow events. Most of the events decelerate at a rate of tenths of m s-2, while 2 events accelerate at approximately the same rate; one event is seen experiencing both an acceleration and a deceleration phase and lasts for a period of few days. This time scale seems to be the typical duration of outflowing and infalling events in UX Ori. The dispersion velocity and the relative aborption strength of the features do not show drastic changes during the lifetime of the events, which suggests they are gaseous blobs preserving their geometrical and physical identity. These data are a very useful tool for constraining and validating theoretical models of the chemical and physical conditions of CS gas around young stars; in particular, we suggest that the simultaneous presence of infalling and outflowing gas should be investigated in the context of detailed magnetospheric accretion models, similar to those proposed for the lower mass T Tauri stars.

MonitoringRXTEobservations of Markarian 348: the origin of the column density variations

ABSTRACT We analyze 37 RXTE observations of the type 2 Seyfert galaxy Mrk348 obtainedduring a period of 14 months. We confirm the spectral variability previous reportedby Smith et al., in the sense that the column density decreases by a factor of ∼3 asthe count rate increases. Column density variations could possibly originate either dueto the random drift of clouds within the absorption screen, or due to photoionizationprocesses. Our modeling of the observed variations implies that the first scenario ismore likely. These clouds should lie in a distance of >2 light years from the source,having a diameter of a few light days and a density of > 10 7 cm −3 , hence probablyresiding outside the Broad Line Region.Key words:galaxies:active-quasars:general-X-rays:general 1 INTRODUCTIONMonitoring observations are a powerful tool in the study ofthe nuclear environment of type 2 Seyfert galaxies. Althoughthe presence of X-ray flux variability in Seyfert-2 galaxies iswell established (eg Georgantopoulos & Papadakis 2001 andreferences therein), our understanding of their spectral vari-ations remains limited. Nevertheless, monitoring of the X-ray spectra of a few bright Seyfert-2 galaxies have revealedsome intriguing results. For example, Warwick et al. (1988)first reported variations of the column density in ESO 103-G35. Using EXOSAT data, they found a decrease in thecolumn density by a factor of ∼ 1.7 in a period of 90 days.Warwick et al. (1993) found column density variations inNGC 7582 by a factor of ∼ 3 over an interval of about 4years, using Ginga data. They attributed these variationsto motions of clouds near the central source. Investigationof the spectrum of NGC 7582 based on ASCA data by Xueet al. (1998) confirmed the existence of significant columndensity variations (by a factor of ∼ 2) over a timescale of2 years. Variability analysis of column density variations inlarge samples of Seyfert-2 galaxies has been performed byMalizia et al. (1997) and Risaliti, Elvis & Nicastro (2002)using mainly literature data. They find variation of the col-umn density in time scales of a few months up to severalyears.Systematic monitoring observations only became feasi-ble with the RXTE mission. In particular, Georgantopouloset al. (1999), Georgantopoulos & Papadakis (2001), Smith,Georgantopoulos & Warwick (2001) present monitoring ob-servations of several Seyfert-2 galaxies (Mrk3, ESO 103-G35,IC 5063, NGC 4507, NGC 7172 and Mrk 348) spanning timeperiods from about seven days to seven months. They foundstatistically significant spectral variations in all cases. Insome objects the variations appear to be caused by intrinsicpower-law slope changes whereas in others column densityvariations dominate.In this paper we present an analysis of 37 RXTE ob-servations of the Seyfert 2 galaxy Mrk348. Previously Smithet al. (2001) have reported the results from 12 RXTE ob-servations covering a period of six months. Here we use anexpanded sample of 37 RXTE monitoring observations ofMrk 348 (including the 12 reported by Smith et al. 2001),spanning a time interval of 14 months, to investigate furtherthe nature of the X-ray spectral variability exhibited by thissource. More specifically, we investigate whether photoion-ization of the absorbing screen or alternatively motion of