Observation of MKN 501 at gamma-ray energies     

L. Bassani  R. C. Butler  A. J. Dean  G. Di Cocco  A. Della Ventura  F. Perotti  G. Villa 《Astrophysics and Space Science》1982,82(2):311-316

During a balloon flight in September 1979 of the MISO low-energy -ray telescope, the BL Lac-object MkN 501 was studied in the hard X-ray range above 30 keV and in the low energy -ray range up to 19 MeV. No statistically significant X- and -ray fluxes were detected. The implications of the upper limits obtained are discussed in the light of the relativistic jet theories recently proposed.  相似文献   

Massive configurations with constant densities     

M. C. Durgapal  A. K. Pande  K. Pandey 《Astrophysics and Space Science》1982,88(2):469-475

Two new classes of solutions with constant observed proper and rest mass densities are described. Unlike the well-known solution of constant coordinate mass density, these solutions pertain to realistic physical situations. For these solutions, the various relevant parameters, viz. the redshifts (dP/d)₀ and binding coefficients have been calculated.  相似文献   

Search for variability in hydrogen-poor stars     

C. Bartolini  A. Bonifazi  F. D'antona  F. Fusi Pecci  L. Oculi  A. Piccioni  R. Serra 《Astrophysics and Space Science》1982,83(1-2):287-310

The evolutionary status of hydrogen-poor stars is analyzed.Photoelectric observations of six objects are reported as a first step of a long-term project devoted to search for variability of a large sample of hydrogen-poor stars.The observed stars show phenomena of microvariability with an amplitude of the order of 0^m. 1 or less.Two extreme helium stars have been examined: a period in the range of 0^d.162-0^d.164 has been found for BD+ 10⁰2179, andP=0^d.1079962 for BD+13⁰3324.The mass-losing O subdwarf (sdO) BD+37⁰ 443 presents short-term fluctuations with a time-scale of several minutes and long-term variations on a scale of months. The standard sdO BD+75⁰325 is probably non-variable, although light variations of very small amplitude (m0.03) with a time-scale of about 1 hr might be present. The high gravity sdO BD+25⁰4655, which is very close to the white dwarf stage, also presents variability on a time-scale of about 13 minutes, and might be an analog of the recently discovered pulsating sdO, or hot white dwarf, PG 1159-035.The variability of the intermediate helium star HD 37776 is finally confirmed.  相似文献   

Unusual coronal activity following the flare of 6 November 1980     

Z. Švestka  B. R. Dennis  M. Pick  A. Raoult  C. G. Rapley  R. T. Stewart  B. E. Woodgate 《Solar physics》1982,80(1):143-159

For almost 30 hr after the major (gamma-ray) two-ribbon flare on 6 November 1980, 03:30 UT, the Hard X-Ray Imaging Spectrometer (HXIS) aboard the SMM satellite imaged in > 3.5 keV X-rays a gigantic arch extending above the active region over the limb. Like a similar configuration on 22 May 1980, this arch formed the lowest part of a stationary post-flare radio noise storm recorded at metric wavelengths at Nançay and Culgoora. 6.5 hr after the flare a coronal region below the arch started quasi-periodic pulsations in X-ray brightness, observed by several SMM instruments. These brightness variations had no response in the chromosphere (H), very little in the transition layer (O v), but they clearly correlated with similar variations in brightness at 169 MHz. There were 13 pulses of this kind, with apparent periodicity of about 20 min, until another flare occurred in the active region at 15:00 UT. All the brightenings appeared within a localized area of about 30000 km² in the northern part of the active region, but they definitely did not occur all at the same place.The top of the X-ray arch, at an altitude of 155 000 km, was continuously and smoothly decaying, taking no part in the striking variations below it. Therefore, the area variable in brightness does not seem to be the footpoint of the arch, as we supposed for similar variations on 22 May. More likely, it is a separate region connected directly with the source of the radio storm; particles accelerated in the storm may be dumped into the low corona and cause the X-ray enhancements. The X-ray arch was enhanced by two orders of magnitude in 3.5–5.5 keV X-ray counts and the temperature increased from 7.3 × 10⁶ to 9 × 10⁶ K when the new two-ribbon flare occurred at 15:00 UT. Thus, it is possible that energy is brought into the arch via the upper parts of the reconnecting flare loops - a process that can continue for hours.  相似文献   

Observations of the Orion Nebula in Hα, [NII] and Hβ lines     

C. T. Hua  R. Louise 《Astrophysics and Space Science》1982,88(2):477-485

Monochromatic photographs of the Orion Nebula taken through narrow bandpass interference filters (Δλ=10 Å) centred on Hα, Hβ and [NII] lines are presented. Ratio contours of Hα/[NII] and Hα/Hβ are derived. They enable a detailed study of the point-to-point variation in ionization structure and temperature throughout the nebula. Dust located within the ionized gas is studied from the Hα/Hβ ratio which varies from point to point over the nebula. Its strongest concentration, apart in the obvious ‘dark bay’, occurs in a shell surrounding the exciting stars, with about 2′ of diameter. Close to Θ¹ Ori the Hα/Hβ ratio, corrected for interstellar reddening, is about 3.0 in good agreement with the predicted value (Brocklehurst, 1971). To account for these measures, the following arguments are proposed:

1. Dust grains are completely or partially destroyed in region close to the exciting stars.
2. Radiation pressure and stellar wind push the remaining dust up to some equilibrium distance outwards. The consequence of this action is obviously the formation of a ‘dust mantle’ which is seen as a ring in projection.

  相似文献   

Seasonal variations of atmospheric sulfur dioxide and dimethylsulfide concentrations at Amsterdam Island in the southern Indian Ocean     

J. P. Putaud  N. Mihalopoulos  B. C. Nguyen  J. M. Campin  S. Belviso 《Journal of Atmospheric Chemistry》1992,15(2):117-131

Daily measurements of atmospheric sulfur dioxide (SO₂) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37°50 S–77°30 E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (²²²Rn) as continental tracer. Average monthly SO₂ concentrations range from less than 0.2 to 3.9 nmol m^-3 (annual average = 0.7 nmol m^-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO₂ concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO₂ concentrations within ±30%. Comparing between computed and measured SO₂ concentrations allowed us to estimate a yield of SO₂ from DMS oxidation of about 70%.  相似文献   

Boundary-layer parameterizations for applied dispersion modeling over urban areas   总被引：3，自引：0，他引：3  

Steven R. Hanna  Joseph C. Chang 《Boundary-Layer Meteorology》1992,58(3):229-259

Many applied dispersion models require the knowledge of boundary-layer parameters such as sensible heat flux,Q _H , friction velocity,u _*, and turbulent energy components, _w and _v . Formulas are suggested for calculating these parameters over a wide variety of types of ground surfaces, based on simple observations of wind speed near the ground and fractional cloud cover, and specification of constants such as roughness length, albedo, and soil moisture availability. Observations ofu _*,Q _H , _w , and _v during field experiments in St. Louis and Indianapolis are used to test the formulas for urban sites. Relative errors of about ±20% in the predictions are seen to occur whenu _*,Q _H , _w , and _v are large. However, when these quantities are small (e.g.,u _* < 0.2 m/s), the errors in the predictions are as large as the mean value of the quantity itself.In addition, it is concluded from studies of available field data and theories that the magnitude of _w is not well-known at elevations above about 100m during the late afternoon and night. Some simple parameterizations for _w . are suggested that are consistent with the observed steady decrease in ground-level concentration in the afternoon and the sudden increase in concentration that can occur a few hours after sunset due to wind shears associated with a low-level jet, for continuous plumes emitted from moderate to tall stacks.  相似文献   

Interaction between atmospheric flows and obstacles: Experiments in a rotating channel     

S. Alessio  L. Briatore  E. Ferrero  A. Longhetto  C. Giraud  O. Morra 《Boundary-Layer Meteorology》1992,60(3):235-241

We performed an experimental study using scale models in a hydrodynamic rotating channel, concerning the interactions between fluid flows and obstacles of different shapes. The study was meant to analyze the characteristics of the wakes observed on the lee side of quasi-bidimensional obstacles, in a neutral atmosphere.The obstacles were half-cylinders (with aspect ratio 0.87), placed transversally on the channel bottom and totally submerged in the fluid. We call them quasi-bidimensional since their width was a little smaller than the channel width, thus allowing the flow to partially go round their edges.The simulations were performed in the rotating hydraulic channel of ICG-CNR in Turin, and included various conditions of rotation period and flow speed. An interesting behaviour of the wakes was found on the lee side of subsynoptic-scale obstacles, modelled in conditions of Reynolds-Rossby similitude. More precisely, if a given threshold of flow velocity is exceeded, wake size is constant and is fully determined by the height of the obstacle.  相似文献   

Stable isotope (O and C) and pollen trends in eastern Lake Erie,evidence for a locally-induced climatic reversal of Younger Dryas age in the Great Lakes basin     

C F M Lewis  T W Anderson 《Climate Dynamics》1992,6(3-4):241-250

A cool period from about 11000 to 10 500 BP (11 to 10.5 ka) is recognized in pollen records from the southern Great Lakes area by the return of Picea and Abies dominance and by the persistence of herbs. The area of cooling appears centred on the Upper Great Lakes. A high-resolution record (ca. 9 mm/y) from a borehole in eastern Lake Erie reveals, in the same time interval, this pollen anomaly, isotope evidence of meltwater presence (a — 3 per mil shift in ¹⁸O and a +1.1 per mil shift in ¹³C), increased sand, and reduced detrital calcite content, all suggesting concurrent cooling of Lake Erie. The onset of cooling is mainly attributed to the effect of enhanced meltwater inflow on the relatively large upstream Main Lake Algonquin during the first eastward discharge of glacial Lake Agassiz. Termination of the cooling coincides with drainage of Lake Algonquin, and is attributed to loss of its cooling effectiveness associated with a substantial reduction in its surface area. It is hypothesized that the cold extra inflow effectively prolonged the seasonal presence of lake ice and the period of spring overturn in Lake Algonquin. The deep mixing would have greatly increased the thermal conductive capacity of this extensive lake, causing suppression of summer surface lakewater temperatures and reduction of onshore growing-degree days. Alternatively, a rapid flow of meltwater, buoyed on sediment-charged (denser) lakewater, may have kept the lake surface cold in summer. Other factors such as wind-shifted pollen deposition and possible effects from the Younger Dryas North Atlantic cooling could have contributed to the Great Lakes climatic reversal, but further studies are needed to resolve their relative significance.Contribution to Climo Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate ProgramGeological Survey of Canada Contribution 58 890  相似文献   

A time- and magnitude-predictable model for generation of shallow earthquakes in the Aegean area     

B. C. Papazachos 《Pure and Applied Geophysics》1992,138(2):287-308

Repeat times of strong shallow mainshocks have been determined by the use of instrumental and historical data for 68 seismogenic sources in the Aegean and surrounding area (34°N–43°N, 18°E–30°E). For 49 of these sources at least two interevent times (three mainshocks) are available for each source. By using the repeat times for these 49 sources the following relation has been determined: $$\log T_t = 0.36M_{\min } + 0.35M_p + a$$ whereT _t is the repeat time, measured in years,M _p the surface wave magnitude of the preceding mainshock,M _min the magnitude of the smallest earthquake considered and “a” parameter which varies from source to source. A multilinear correlation coefficient equal to 0.89 was determined for this relation. By using the same repeat times for the 49 seismogenic sources, the following relation has been determined between the magnitude,M _f , of the following mainshock andM _min andM _p . $$M_f = 0.95M_{\min } - 0.49M_p + m$$ wherem is a constant which varies from source to source. A multilinear correlation coefficient equal to 0.80 was found for this relation. The model expressed by these two relations is represented by a scheme of a time variation of stress under constant tectonic loading. In this scheme, the maximum stress values during the different seismic cycles fluctuate around a value, τ₁, in a relatively narrow stress interval, expressing the high correlation coefficient of the relation between LogT andM _p . On the contrary, the minimum stress values fluctuate around a value, τ₂, in a much broader stress interval. However, each of these minimum stress values becomes lower or higher than τ₂ if the previous one is higher or lower than τ₂, respectively, expressing the negative correlation betweenM _f andM _p .  相似文献