Structure and stability of rotating fluid disks around massive objects. I. Newtonian formulation

In this paper we have presented a very general class of solutions for rotating fluid disks around massive objects (neglecting the self gravitation of the disk) with density as a function of the radial coordinate only and pressure being nonzero. Having considered a number of cases with different density and velocity distributions, we have analysed the stability of such disks under both radial and axisymmetric perturbations. For a perfect gas disk with γ= 5/3 the disk is stable with frequency (MG/r³)^1/2 for purely radial pulsation with expanding and contracting boundary. In the case of axisymmetric perturbation the critical γ_c for neutral stability is found to be much less than 4/3 indicating that such disks are mostly stable under such perturbations. On leave of absence from Government College, Jagdalpur 494005.     

Long term variations in solar flare activity

A statistical analysis of the contemporary (1954-1975) solar flare particle events has been made for the parametersF (integrated, proton fluence in cm^-2 in an event with kinetic energy above 10 MeV) andR ₀ (the characteristic rigidity). These data are compared with the long-term averaged values determined from stable- and radio-nuclide measurements of lunar samples. The analysis shows that the ancient solar flare proton spectrum was harder (higher R₀ values) compared to that observed in contemporary flares. A similar analysis can not be made for the mean long-term averaged flux (ˉJ, cm^-2 S^-1), since the contemporary averages suffer from an uncertainty due to the statistics of a single event. However, the average flux estimates for time durations 〈T〉 exceeding 10³ yr, are free from such uncertainties. The long-term averaged ˉJ values obtained over different time scales (10⁴ - 10⁶ yr) suggest a possible periodic variation in solar flare activity, with enhanced flux level during the last 10⁵ yr. The available data rule out the occurrence of giant flares, with proton fluence exceeding 10¹⁵ cm^-2 during the last million years.     

Eclipsing binary system AV Hydrae

The eclipsing binary AV Hya has been observed photoelectrically inU, B andV filters on 15 nights. The primary eclipse appeared to be total with a totality of 23 min duration. A modified period of 0^d.6 834 062 has been obtained. Geometrical elements could only be determined for partial transit case.     

General relativistic analysis of structure and stability of charged fluid disks around compact objects

In this paper we give a detailed general relativistic formulation of the study of structure and stability of charged fluid disks around compact objects like black holes neglecting the self-gravitation of the disk itself. Having presented the general equations for equilibrium as well as for perturbations we solve explicitly the cases of rigidly and differentially rotating thin disks, with constant charge density and zero pressure, confined to the equatorial plane of the black hole. By using normal mode analysis we have analysed the stability of such disks under purely radial perturbations and find that the disks are generally stable. On leave of absence from Government College, Jagadalpur 494005     

Time variability of Martian bolometric albedo

A time-sequential set of bolometric albedo maps for Mars has been constructed from Viking Infrared Thermal Mapper data. The maps provide global coverage in longitude for latitudes -60° to +60° at 1° by 1° spatial resolution. Individual maps are constructed under strict geometric constraints for a narrow range of L_s. The set of albedo maps spans a Martian year and includes maps before, during, and after the global dust storms of 1977. Transient brightenings associated with local dust storms or condensate clouds are apparent in some of the maps. During dust-free periods, bolometric albedo maps are generally similar to classical, visual albedo map of Mars. The distribution of bolometric surface albedos is bimodal with typical, clear-sky, Lambert albedos of 0.27 and 0.16 for bright and dark areas, respectively. Atmospheric effects strongly influence apparent surface albedos, especially for dark areas. Neither bright nor dark regions show measurable, long-term variations of bolometric albedos during clear periods.     

Average nocturnal and seasonal variations of sodium nightglow at 23°S, 46°W

The seasonal variation of the nocturnal average intensity, and the typical variation of the sodium airglow intensity during the night have been deduced, using a data set that has been accumulated over a period of 5 years. The maximum intensity, which occurs at the equinoxes, is about 2–3 times as large as the intensity during winter, when the intensity of the D₂ line is about 30 R. The nocturnal variation is symmetrical about midnight, where the intensity is some 30% less than just after (before) dusk (dawn) values.     

Ion-ion mutual neutralization and ion-neutral switching reactions of some stratospheric ions

Following the recent mass spectrometric observations of the ambient stratospheric positive and negative ions we have carried out co-ordinated laboratory experiments using a selected ion flow tube apparatus and a flowing afterglow apparatus for the following purposes: (i) to consider whether CH₃CN is a viable candidate molecule for the species X in the observed stratospheric ion series H⁺ (H₂O_n (X)_m and (ii) to determine the binary mutual neutralization rate coefficients α_i for the reactions ofH⁺ (H₂O₄ and H⁺(H₂O)(CH₃CN)₃ with several of the negative ion species observed in the stratosphere. We conclude from (i) that CH₃CN is indeed a viable candidate for X and from (ii) that the α_i for stratospheric ions are within the limited range (5–6) × 10^?8 cm³ s^?1.     

Ein Programm zur Periodensuche bei großen Anzahlen von Meßwerten

A method for the search of periods in large series of observational data is discribed. Like that proposed by Lafler and Kinman our procedure is based on the computation of the variance, however, the speed of computation of periodogramms is considerably higher since we do not need to size the values with regard to the phase. A FORTRAN-programm is given.     

Using Strong Solar Coronal Emission Lines as Coronal Flux Proxies

We investigate the possibility that strong EUV lines observed with the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) provide good proxies for estimating the total coronal flux over shorter wavelength ranges. We use coordinated SERTS and Yohkoh observations to obtain both polynomial and power-law fits relating the broad-band soft X-ray fluxes to the intensities of Fexvi 335 Ú and 361 Ú, Fexv 284 Ú and 417 Ú, and Mgix 368 Ú measured with SERTS. We found that the power-law fits best cover the full range of solar conditions from quiet Sun through active region, though not surprisingly the cooler Mgix 368 Ú line proves to be a poor proxy. The quadratic polynomial fits yield fair agreement over a large range for all but the Mgix line. However, the linear fits fail conspicuously when extrapolated into the quiet-Sun regime. The implications of this work for the Heii 304 Ú line formation problem are also briefly considered.     

PKS2250-41 and the Role of Jet Cloud Interactions in Powerful Radio Galaxies

We have studied the effects of the interaction between the radio jet and the ambient gas in the powerful radio galaxy PKS2250-41 (z = 0.31). Our results show that the gas has been accelerated, compressed, heated and collisionally ionized by the shock. This study helps us to understand the processes which determine the observed properties of many high redshift radio galaxies. This revised version was published online in July 2006 with corrections to the Cover Date.