A note on the kinematics of the tachyon fluid

It is shown that the kinematical parameters associated with the congruence formed by tachyonic motion can be defined in the manner of Greenberg, but not that of Ehlers. The space-like counterpart of Raychaudhuri's equation has also been obtained.     

Chromospheric evolution and the flare activity of super-active region NOAA 6555

Super-active region NOAA 6555 was highly flare productive during the period March 21st–27th, 1991 of its disk passage. We have st udied its chromospheric activity using high spatial resolution Hα filtergrams taken at Udaipur along with MSFC vector magnetograms. A possible relationship of flare productivity and the variation in shear has been explored. Flares were generally seen in those subareas of the active region which possessed closed magnetic field configuration, whereas only minor flares and/or surges occurred in subareas showing open magnetic field configuration. Physical mechanisms responsible for the observed surges are also discussed.     

Geo-effectiveness of solar wind extremes

Examples of extreme events of solar wind and their effect on geomagnetic conditions are discussed here. It is found that there are two regimes of high speed solar wind streams with a threshold of ∼ 850 km s^-1. Geomagnetic activity enhancement rate (GAER) is defined as an average increase in Ap value per unit average increase in the peak solar wind velocity (Vp) during the stream. GAER was found to be different in the two regimes of high speed streams with +ve and-ve IMF. GAER is 0.73 and 0.53 for solar wind streams with +ve and -ve IMF respectively for the extremely high speed streams (< 850 km s^-1). This indicates that streams above the threshold speed with +ve IMF are 1.4 times more effective in enhancing geomagnetic activity than those with -ve IMF. However, the high speed streams below the threshold with -ve IMF are 1.1 times more effective in enhancing geomagnetic activity than those with +ve IMF. The violent solar activity period (October–November 2003) of cycle 23 presents a very special case during which many severe and strong effects were seen in the environment of the Earth and other planets; however, the z-component of IMF (Bz) is mostly positive during this period. The most severe geomagnetic storm of this cycle occurred when Bz was positive.     

Giant Meterwave Radio Telescope observations of an M2.8 flare: Insights into the initiation of a flare–coronal mass ejection event

We present the first observations of a solar flare with the GMRT. An M2.8 flare observed at 1060 MHz with the GMRT on 17 November 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronagraph. Towards the start of the eruption, we find evidence for reconnection above the prominence. Subsequently, we find evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the flare and prominence eruption.     

Solar radio burst (type III)

This article describes the observations of a type III radio burst observed at 103 MHz simultaneously by the two radio telescopes situated at Rajkot (22.3°N, 70.7°E) and Thaltej (23°N, 72.4°E). This event occurred on September 30, 1993 at about 0430 UT and lasted for only half a minute. The event consisted of several sharp spikes in a group. The rise and fall time of these are comparable, however the peaks of individual spikes varied by a factor of four. The comparison of these observations with the data of solar radio spectrograph HiRAS indicates that this was a metric radio burst giving highest emission at about 103 MHz.     

Age estimates of coastal terraces in the Andaman and Nicobar Islands and their tectonic implications

The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr^− 1 in the Little Andaman to 2.80 mm yr^− 1 in South Andaman and 2.45 mm yr^− 1 in the North Andaman. Our radiocarbon dates converge on  600 yr and  1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region.     

Deciphering potential groundwater zone in hard rock through the application of GIS

Remote Sensing and Geographic Information System has become one of the leading tools in the field of hydrogeological science, which helps in assessing, monitoring and conserving groundwater resources. It allows manipulation and analysis of individual layer of spatial data. It is used for analysing and modelling the interrelationship between the layers. This paper mainly deals with the integrated approach of Remote Sensing and geographical information system (GIS) to delineate groundwater potential zones in hard rock terrain. The remotely sensed data at the scale of 1:50,000 and topographical information from available maps, have been used for the preparation of ground water prospective map by integrating geology, geomorphology, slope, drainage-density and lineaments map of the study area. Further, the data on yield of aquifer, as observed from existing bore wells in the area, has been used to validate the groundwater potential map. The final result depicts the favourable prospective zones in the study area and can be helpful in better planning and management of groundwater resources especially in hard rock terrains.     

Sample Disturbance in Soft and Sensitive Clays: Analysis and Assessment

Despite the sophisticated methods employed to obtain high quality samples, they are prone to disturbance, due to stress release, let apart, due to sampling tools and techniques adopted. Critical reappraisal of different methods suggest that any method of sample quality assessment involving strength and deformation parameters would enable easier implementation and correct other engineering parameters for estimated sample disturbance. In this technical paper, sample disturbance index, using merely the slopes of compression paths, (representing mechanical response), in the pre- and post-yield stress regimes under odeometric loading conditions is proposed. This method is based on single oedometer test results on samples of unknown degrees of disturbance during sampling and handling. The unique feature of this approach is that the ideal rigid plastic material behavior is taken as reference. This eliminates the need for any additional experimental data or any calculations from the relationships formulated with any set of assumptions.     

Optical interferometric observations of a transient event of 1986 March 13 in the coma of Comet Halley

During the recent apparition of Comet Halley in 1985-86 a transient ionic event in the form of a blob of H₂ O⁺ emission was recorded in thecoma at ～ 0^h UT on 1986 March 13. Observations were carried out using a special IHW filter for H₂ O⁺ emission at 7000 å/175 å, a 35 cm telescope, a Fabry-Perot interferometer and an image intensifier camera from Gurushikhar, Mt Abu. (24?39’N,72? 47’E). A Fabry-Perot inter-ferogram in Ha taken a few minutes later at the same location reveals strong hydrogen emission (Hα) associated with the blob. The velocity field in the blob is structured with relative velocities upto ～ 35km s^?1. The event is interpreted as arising due to the sector boundary crossing of the interplanetary magnetic field by the comet     

Attenuation relations for strong seismic ground motion in the Himalayan region

Strong motion data from various regions of India have been used to study attenuation characteristics of horizontal peak acceleration and velocity. The strong ground motion data base considered in the present work consists of various earthquakes recorded in the northern part of India since 1986 with magnitudes 5.7 to 7.2. Using these data, relations for horizontal peak acceleration and velocity, which are $$\begin{gathered} log_{10} a = 1.14 + 0.31M + 0.65log_{10} R \hfill \\ log_{10} v = 0.571 + 0.41M + 0.768log_{10} R \hfill \\ \end{gathered} $$ have been proposed wherea is the peak horizontal acceleration in cm/sec²,v is the peak horizontal velocity in mm/sec,M is body wave magnitude, andR is the hypocentral distance in km. The proposed relations are in reasonable agreement with the small amount of strong ground motion data available for the northern part of India. The present results will be useful in estimating strong ground motion parameters and in the earthquake resistant design in the Himalayan region.