Magnetoacoustic surface waves at a single interface

The occurrence of magnetoacoustic surface waves at a single magnetic interface one side of which is field-free is explored for the case of non-parallel propagation. Phase-speeds and penetration depths of the waves are investigated for various Alfvén speeds, sound speeds and angles of propagation to the applied field. Both slow and fast magnetoacoustic surface waves can exist depending on the values of sound speeds and propagation angle. The fast waves penetrate more than the slow waves.The parallel propagation of fast and slow magnetoacoustic surface waves on a magnetic-magnetic interface is investigated. The slow surface wave is unable to propagate below a critical sound speed. In a low -plasma, only the fast mode exists (0 0).     

Geology and tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra area,Venus: Results from Arecibo image data

New radar images obtained from the Arecibo Observatory (resolution 1.5–4.0 km) for portions of the southern hemisphere of Venus show that: the upland of Phoebe Regio contains the southern extension of Devana Chasma, a rift zone extending 4200 km south from Theia Mons and interpreted as a zone of extension; Alpha Regio, the only large region of tessera within the imaged area, is similar to tessera mapped elsewhere on the planet and covers a smaller percentage of the surface than that observed in the northern high latitudes; the upland made of Ushas, Innini and Hathor Montes consists of three distinct volcanic constructs; Themis Regio is mapped as an ovoid chain of radar-bright arcuate single and double ring structures, edifices and bright lineaments. This area is interpreted as a region of mantle upwelling and on the basis of apparent split and separated features, a zone of localized faulting and extension. Linear zones of deformation in Lavinia Planitia are characterized by lineament belts that are often locally elevated, are similar to ridge belts mapped in the northern high latitudes and are interpreted to be characterized mainly by compression; radar-bright lava complexes within Lavinia Planitia are unique to this part of the planet and are interpreted to represent areas of eruption of high volumes of extremely fluid lava; the upland of Lada Terra is bound to the north by a linear deformation zone interpreted as extensional, is characterized by large ovoids and coronae, is interpreted to be associated with an area of mantle upwelling, and is in contrast to the northern high latitude highland of Ishtar Terra. Regions of plains in the southern hemisphere cover about 78%; of the mapped area and are interpreted to be volcanic in origin. Located within the area imaged (10–78 S) are 52 craters interpreted to be of impact origin ranging from 8 to 157 km in diameter. On the basis of an overall crater density of 0.94 craters/10⁶ km², it is determined that the age of this part of the Venus surface is similar to the 0.3 to 1.0 billion year age calculated for the equatorial region and northern high latitudes. The geologic characteristics of the portion of the Venus southern hemisphere imaged by Arecibo are generally similar to those mapped elsewhere on the planet. This part of the planet is characterized by widespread volcanic plains, large volcanic edifices, and zones of linear belt deformation. The southern hemisphere of Venus differs from northern high latitudes in that tessera makes up only a small percentage of the surface area and the ovoid chain in Themis Regio is unique to this part of the planet. On the basis of the analysis presented here, the southern hemisphere of Venus is interpreted to be characterized by regions of mantle upwelling on a variety of scales (ovoids, region made up of Ushas, Innini and Hathor Montes), upwelling and extension (Themis Regio) and localized compression (lineament belts in Lavinia Planitia).     

Results of TV imaging of Phobos (Experiment VSK-Fregat)

From February to March 1989 the Phobos 2 spacecraft took 37 TV images of Phobos at a distance of 190-1100 km. These images complement Mariner-9 and Viking data by providing higher-resolution coverage of a large region West of the crater Stickney (40-160 degrees W) and by providing disk-resolved measurements of surface brightness at a greater range of wavelengths and additional phase angles. These images have supported updated mapping and characterization of large craters and grooves, and have provided additional observations of craters' and grooves' bright rims. Variations in surface visible/near-infrared color ratio of almost a factor of 2 have been recognized; these variations appear to be associated with the ejecta of specific large impact craters. Updated determinations of satellite mass and volume allow calculation of a more accurate value of bulk density, 1.90 +/- 0.1 g cm-3. This is significantly lower than the density of meteoritic analogs to Phobos' surface, suggesting a porous interior perhaps containing interstitial ice.     

Earthquake hazard of the northern parts of the Bohemian Massif and Western Carpathians

The new procedure of earthquake hazard evaluation developed by Kijko and Sellevoll is tested and applied for the border region of Czechoslovakia and Poland. The new method differs from the conventional approach. It incorporates the uncertainty of earthquake magnitudes, and accepts mixed data containing only large historical events and recent, complete catalogues. Seismic hazard has been calculated for nine regions determined in the border area. In the investigated area, data of historical catalogues are uncertain or, in many cases, the epicentral intensities are unknown. Thus, a number of assumptions have to be adopted in data preparation of catalogues since the year 1200. The calculated values of parameters b in the Gutenberg-Richter frequency-intensity relation as well as the return periods, seem to be reasonable and are generally confirmed by the results obtained from catalogues for the last 80–130 years.     

Catastrophic landslip activity in two hard rock terrains of peninsular India

This paper presents the case histories of two catastrophic landslips in hard rock terrains with varied climatic and geological environments. The first slip is associated with a power project in very close proximity (200 m) of the Porthimund Dam (11°22N, 76°3430E), in a charnockitic terrain in the Nilgiri hills (Tamil Nadu), and the second is associated with a railroad structure (19°525N, 78°1720E), in Adilabad district (Andhra Pradesh), in a basaltic terrain.The landslip in the charnockites is attributable to: (1) a high degree of saprolitization in the charnockites, with maximum intensity in the crest portion; (2) the coincidence of a major joint pattern in a NE-SW direction, with the strike of the foliation; and (3) the poor-to-fair physical rock quality in the crest and scarp portions.The slips in the basaltic terrain are due to: (1) the partially altered, highly jointed nature of the regional trap rocks with boulder sizes varying from 20 cm to 250 cm in diameter and the debris accumulating in a precarious condition on the northeast side of the rail track, with unfavorable alignment direction; and (2) the instability created in the weak rock mass by the vibrational forces of heavily loaded running trains.The weathered state of the rock masses in both the cases, showing good agreement with their physical state, accounts for the landslips. The remedial measures suggested are also discussed.     

Re-analysis of close binary systems

Line-forming regions around close binaries with strong winds ( /4r _* v 10^–4 g cm^–2) are large in extent compared with the stars, large enough to screen them. Their orbitally-modulated Doppler shifts can overestimate the mass function, because of a larger rotational lever arm. In particular, most of the black-hole candidates need not involve companions more massive than a neutron star.The solar-wind problem is reconsidered. An extrapolation to Wolf-Rayet stars suggests that their winds are centrifugally driven. Their mass-loss rates tend to have been overestimated.Seemingly single (massive) stars can hide a (compact) companion.     

The formation of spicules in the course of the chromospheric network magnetic field reconnection

We investigate the physical processes occurring in the supergranule boundary cylinder layer (SBCL). Taking into account the Coriolis force, we obtain an expression for the component of the magnetic field and velocity in the SBCL. Within the framework of linear MHD, we consider the formation and coalescence of magnetic tubes, i.e. spicules, in the course of the reconnection of the SBCL magnetic field. The estimated number of spicules appearing on each supergranule cell is in agreement with observations. This number depends on the solar latitude : (1) if the normal component of the magnetic fieldB _z is assumed to be independent of , then the maximum number of spicules should be at = 71°; (2) ifB _z is assumed to be the component of the dipolar fieldB _z sin , then the maximum number should be at the pole: = 90°. The timescale of the formation and the coalescence of the magnetic tubes is 10–20 min, which is of the order of the observed lifetime of the spicules.     

Investigation of non-uniform heating during the decay phase of solar flares

We have analysed X-ray spectra of 13 solar flares as obtained by the Bent Crystal Spectrometer (BCS) on the Solar Maximum Mission. In particular, we have examined the observed ratio of T _Fe/T _Ca where T _Fe and T _Ca are the temperatures obtained from the Fexxv and Caxix spectra, respectively. In order to simplify the investigation we have analysed only flares which reach quasi-steady-state during the decay. It turned out that the observed ratios cannot be explained by a model consisting of a single, uniformly heated loop, with a constant or variable cross-sectional area. We propose that this problem may be solved by introducing some distribution of the heating function across the flaring loop. This model has been tested by detailed calculations.     

Water in India with reference to agriculture and population: some issues and patterns —dynamic approaches needed for development

In this paper an attempt is made to present some facets of status of water for agriculture and population in India. The issues are basic in nature which have been focused by scholars all over time and again. The materials are gathered from various sources to highlight and to consider a plea in the context of population-hydrological regions of the country to develop water potentials. Some characteristic mapping has also been conducted showing the main parameters of water utilization, need and spatial issues regarding consumptive use of water during agricultural operations as per the seasons prevalent in India. A relation is also established with the drought areas leading to assess the need for water. The human problems specially in relation to the safe drinking water is also projected in relation to distributional qualities with reference to natural regions by districts. The study does not claim a full assessment of the water issue due to paucity of statistics on water use, generation and potentials but it does indicate in many ways the geographical problems as an important study for further research by all concerned.     

Eruptive instability of magnetic arcades

Eruptive prominences trace disruptions of magnetic arcades in which they are embedded. The stability of an arcade containing an electric current filament at its axis is discussed. The model provides criteria for the onset of the eruptive instability in terms of prominence twist and overall geometry, i.e., the parameters which could be measured directly. The evolution of the eruption is analyzed, and the dependence of the acceleration and the pitch of field-lines on the height is established. The model is compared with the observations of one eruptive prominence where the development of helical structure was followed.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.