012991   John A J;Maharaj S D (Astrophysics and Cosmology Research Unit, School of Mathematical Sciences, KwaZulu-Natal Univ, Private Bag X54001, Durban 4000, South Africa, Email: johnan@ukzn.ac.za) : Relativistic stellar models. Pramana J Phys 2011, 77(3), 461-8.

Obtain a class of solutions to the Einstein-Maxwell equations describing charged static spheres. Upon specifying particular forms for one of the gravitational potentials and the electric field intensity, the condition for pressure isotropy is transformed into a hypergeometric equation with two free parameters. For particular parameter values we recover uncharged solutions corresponding to specific neutron star models. We find two charged solutions in terms of elementary functions for particular parameter values. The first charged model is physically reasonable and the metric functions and thermodynamic variables are well behaved. The second charged model admits a negative energy density and violates the energy conditions.

3 illus, 18 ref

012990   Goodman J;Slepian Z (Princeton Observatory Univ, Peyton Hall, Princeton, NJ 08544, USA, Email: jeremy@astro.rpinceton.edu) : Chance and Chandra. Pramana J Phys 2011, 77(1), 107-18.

A few examples are given of Chandra's work on statistical and stochastic problems that relate to open questions in astrophysics, in particular his theory of dynamical relaxation in systems with inverse-square interparticle forces. The roles of chaos and integrability in this theory require clarification, especially for systems having a dominant central mass. After this prelude, a hypothetical form of bosonic dark matter with a simple but nontrivial statistical mechanics is discussed. This makes for a number of eminently falsifiable predictions, including some exotic consequences for dynamical friction.

2 illus, 34 ref

012989   Dadhich N (Inter Centre for Astronomy & Astrophysics Univ, Post Bag 4, Pune-411 007, Email: nkd@iucaa.ernet.in) : On the enigmatic Λ-a true contant of spacetime. Pramana J Phys 2011, 77(3), 433-7.

Had Einstein followed the Bianchi differential identity for the derivation of his equation of motion for gravitation, A would have emerged as a true new constant of spacetime on the same footing as the velocity of light? It is then conceivable that he could have perhaps made the most profound prediction that the Universe may suffer accelerated expansion some time in the future! Further we argue that its identification with the quantum vacuum energy is not valid as it should have to be accounted for like the gravitational field energy by enlarging the basic framework of spacetime and not through a stress tensor. The acceleration of the expansion of the Universe may indeed be measuring its value for the first time observationally.

11 ref

012988   Brandenburg A (NORDITA, AlbaNova Center Univ, Roslagstullsbacken 23, SE 10691 Stockholm, Sweden) : Chandrasekhar-Kendall functions in astrophysical dynamos. Pramana J Phys 2011, 77(1), 67-76.

Some of the contributions of Chandrasekhar to the field of magnetohydrodynamics are highlighted. Particular emphasis is placed on the Chandrasekhar-Kendall functions that allow a decomposition of a vector field into right- and left-handed contributions. Magnetic energy spectra of both contributions are shown for a new set of helically forced simulations at resolutions higher than what has been available so far. For a forcing function with positive helicity, these simulations show a forward cascade of the right-handed contributions to the magnetic field and nonlocal inverse transfer for the left-handed contributions. The speed of inverse transfer is shown to decrease with increasing value of the magnetic Reynolds number.

5 illus, 56 ref

012987   Blandford R;McKinney J V;Zakamska N (Kavli Institute of Particle Astrophysics & Cosmology, Stanford, CA 94025, USA, Email: zakamska@pha.jhu.edu) : Hydrodynamic and hydromagnetic stability of black holes with radiative transfer. Pramana J Phys 2011, 77(1), 53-66.

Subrahmanyan Chandrasekhar (Chandra) was just eight years old when the first astro-physical jet was discovered in M87. Since then, jets have been uncovered with a wide variety of sources including accretion disks orbiting stellar and massive black holes, neutron stars, isolated pulsars, x-ray bursts, protostars and planetary nebulae. This talk will be primarily concerned with collimated hydromagnetic outflows associated with spinning, massive black holes in active galactic nuclei. Jets exhibit physical processes central to three of the major research themes in Chandrasekhar's research career - radiative transfer, magnetohydrodynamics and black holes. Rela-tivistic jets can be thought of as 'exhausts' from both the hole and its orbiting accretion disk, carrying away the energy liberated by the rotating spacetime and the accreting gas that is not radiated. However, no aspect of jet formation, propagation and radiation can be regarded as understood in detail. The combination of new γ-ray, radio and optical observations together with impressive advances in numerical simulation make this a good time to settle some long-standing debates.

3 illus, 31 ref

012986   Binney J (Rudolf Peierls Centre for Theoretical Physics, Oxford Univ, 1 Keble Road, Oxford OX1 3NP, UK, Email: binney@thphys.ox.ac.uk) : Extracting science from surveys of our galaxy. Pramana J Phys 2011, 77(1), 39-52.

Our knowledge of the Galaxy is being revolutionized by a series of photometric, spec-troscopic and astrometric surveys. Already an enormous body of data is available from completed surveys, and data of ever-increasing quality and richness will accrue at least until the end of this decade. To extract science from these surveys, we need a class of models that can give probability density functions in the space of the observables of a survey - we should not attempt to 'invert' the data from the space of observables into the physical space of the Galaxy. Currently just one class of model has the required capability, the so-called 'torus models'. A pilot application of torus models to understand the structure of the Galaxy's thin and thick discs has already produced two significant results: a major revision of our best estimate of the Sun's velocity with respect to the local standard of rest, and a successful prediction of the way in which the vertical velocity dispersion in the disc varies with distance from the Galactic plane.

2 illus, 35 ref

012985   Bhattacharya D (Inter Centre for Astronomy and Astrophysics Univ, , Post Bag No. 4, Ganeshkhind, Pune-411 007, Email: dipankar@iucaa.ernet.in) : Beyond the Chandrasekhar limit: structure and formation of compact stars. Pramana J Phys 2011, 77(1), 29-37.

The concept of limiting mass, introduced by Chandrasekhar in case of white dwarfs, plays an important role in the formation and stability of compact objects such as neutron stars and black holes. Like white dwarfs, neutron stars have their own mass limit, and a compact configuration would progress from one family to the next, more dense one once a mass limit is crossed. The mass limit of neutron stars depends on the nature of nuclear forces at very high density, which has so far not been determined conclusively. This article reviews how observational determinations of the properties of neutron stars are starting to impose significant constraints on the state of matter at high density.

2 illus, 1 table, 31 ref

012984   Bhatt H C (Indian Institute of Astrophysics, , II Block, Koramangala, Bangalore-560 034, Email: hcbhatt@iiap.res.in) : Disks around young stellar objects. Pramana J Phys 2011, 77(1), 19-28.

By 1939, when Chandrasekhar's classic monograph on the theory of Stellar Structure was published, although the need for recent star formation was fully acknowledged, no one had yet recognized an object that could be called a star in the process of being born. Young stellar objects (YSOs), as pre-main-sequence stars, were discovered in the 1940s and 1950s. Infrared excess emission and intrinsic polarization observed in these objects in the 1960s and 1970s indicated that they are surrounded by flattened disks. The YSO disks were seen in direct imaging only in the 1980s. Since then, high-resolution optical imaging with HST, near-infrared adaptive optics on large ground-based telescopes, mm and radiowave interferometry have been used to image disks around a large number of YSOs revealing disk structure with ever-increasing detail and variety. The disks around YSOs are believed to be the sites of planet formation and a few such associations have now been confirmed. The observed properties of the disk structure and their evolution, that have very important consequences for the theory of star and planet formation, are discussed.

4 illus, 65 ref

012983   Antia H M (Astronomy and Astrophysics Dep, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005, Email: antia@astro.tifr.res.in) : Hydrodynamic stability and stellar oscillations. Pramana J Phys 2011, 77(1), 3-18.

Chandrasekhar's monograph on Hydrodynamic and hydromagnetic stability, published in 1961, is a standard reference on linear stability theory. It gives a detailed account of stability of fluid flow in a variety of circumstances, including convection, stability of Couette flow, Rayleigh-Taylor instability, Kelvin-Helmholtz instability as well as the Jean's instability for star formation. In most cases he has extended these studies to include effects of rotation and magnetic field. In a later paper he has given a variational formulation for equations of non-radial stellar oscillations. This forms the basis for helioseismic inversion techniques as well as extension to include the effect of rotation, magnetic field and other large-scale flows using a perturbation treatment.

4 illus, 42 ref

012982   Amery G;Moodley J;Londal J P (Astrophysics and Cosmology Research Unit, School of Mathematical Sciences, KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa, Email: AmeryG1@ukzn.ac.za) : Isometric embeddings in cosmology and astrophysics. Pramana J Phys 2011, 77(3), 415-28.

Recent interest in higher-dimensional cosmological models has prompted some significant work on the mathematical technicalities of how one goes about embedding spacetimes into some higher-dimensional space. We survey results in the literature (existence theorems and simple explicit embeddings); briefly outline our work on global embeddings as well as explicit results for more complex geometries; and provide some examples. These results are contextualized physically, so as to provide a foundation for a detailed commentary on several key issues in the field such as: the meaning of 'Ricci equivalent' embeddings; the uniqueness of local (or global) embeddings; symmetry inheritance properties; and astrophysical constraints.

40 ref

012981   Ahluwalia H S (Physics & Astronomy Dep, New Mexico Univ, Albuquerque, New Mexico, 87131-1156, USA, Email: hsa@unm.edu ) : Three-cycle quasi-periodicity in solar, geophysical, cosmic ray data and global climate change. Indian J Radio Space Phys 2012, 41(5), 509-19.

Discovery of a three-cycle quasi-periodicity (TCQP) in the planetary indices (Ap/aa), interplanetary magnetic field (IMF) intensity (B) and galactic cosmic rays (GCRs) has been described [Ahluwalia H S, Three activity cycle periodicity in galactic cosmic rays? Proc 25th Int Cosmic Ray Conf (Durban, South Africa), 2, 1997b, pp 109-112; Ahluwalia H S, The predicted size of cycle 23 based on the inferred three cycle quasi-periodicity of the planetary index Ap, J Geophys Res (USA), 103 (1998) pp 12103-12109; Ahluwalia H S, The development of solar cycle 23: Reply to Wilson & Hathaway's comment, J Geophys Res (USA), 104 (1999) pp 2559-2561]. It led to the development of a heuristic methodology for predicting the peak smooth sunspot numbers (SSNs) and the rise time for a new solar activity cycle. The scheme was used to predict key parameters for the SSN cycle 23. The predictions were right on the mark, one of the very few predictions to have achieved this status. The smoothed SSNs for cycle 24 have been increasing slowly since its onset in December 2008. Ahluwalia & Jackewicz [Ahluwalia H S & Jackiewicz J, Sunspot cycle 23 descent to an unusual minimum and forecasts for cycle 24 activity, Adv Space Res (UK), 50 (2012) pp 662-668] have predicted that cycle 24 will be only half as active as cycle 23 and reach its peak in May 2013 ± 6 months. In the present paper, the present status of the ascending phase of cycle 24 has been reported and its timeline has been compared with those of the previous ten cycles (10-23) of the twentieth century. Drawing from the past history, Ahluwalia & Jackewicz noted that we may be on the eve of a Dalton-like minimum. The ascending phase of cycle 24 is compared with those of cycles 5 and 14 that led to the start of two recent Grand Minima, namely the Dalton and the Gleissberg Minima, respectively. The physical significance of the findings is discussed and speculated about their implications for the global climate change and probable socioeconomic and political future happenings.

9 illus, 46 ref

013881   Singh S G;Singh K;Shrivastava G N;Singh Y K (Physics Dep, Model Science College, Rewa, Madhya Pradesh, Email: sgs81physics@gmail.com) : Correlative study of geomagnetic stroms with sun-spot numbers in solar cycle 23. Indian J scient Res 2012, 3(2), 131-4.

Stars are large scale magnetized plasma structures originating from closed magnetic field regions on the sun and are the most energetic solar events in which vast amount of solar plasma materials are expelled from the solar corona into interplanetary space. Geomagnetic storms in a major component of space weather and provide the impact for many other components of space weather. In present study, correlation between sunspot number to classified Geomagnetic storms are 0.75 for SSN to moderate 0.35 for SSN to super and 0.81 for SSN to intense GMS. So we have concluded that good correlation between SSN and intense GMS but general correlation between SSN and super GMS.

2 illus, 1 table, 5 ref

013880   Ouattara F;Zoundi C;Fleury R (Ecole Normale Suprieure (E N S), Koudougou Univ, BP376, Koudougou, Burkina Faso, Email: fojals@yahoo.fr) : Comparison between CODG TEC and GPS based TEC observations at Koudougou station in Burkina Faso. Indian J Radio Space Phys 2012, 41(6), 617-23.

The paper presents the comparison between total electron content (TEC) map given by CODG model and Rinex TEC obtained from Koudougou GPS station, which started its operations in December 2008. The TEC observations were carried out at Koudougou station during 5-10 December 2008 by ionosphere single layer model (SLM). The observation of TEC profiles showed noon bite out profile with predominance of morning or afternoon peak and night time peak. CODG TEC profiles highlighted dome and reverse profiles. There is no night time peak in TEC map profiles. The comparison between GPS based TEC observation at Koudougou station and TEC map shows that CODG model does not estimate Rinex TEC at Koudougou station.

5 illus, 28 ref

013879   Kane R P (Instituto Nacional de Pesquisas Espaciais-INPE, , C P 515, Sao Jose dos Campos, Brazil, Email: kane@dge.inpe.br) : Strong solar flares, weak geo-effectiveness. Indian J Radio Space Phys 2012, 41(6), 575-8.

The link between solar flares (particularly, the strong X-ray flares) and geophysical effects is: (a) solar flare; (b) associated coronal mass ejection (CME); (c) the interplanetary CME (ICME), which follows and engulfs the Earth; and (d) its geo-effectiveness (geomagnetic storms represented by the index Dst occurring only if there is a large negative Bz component of the ICME magnetic field B). For cosmic ray Forbush decreases, only a large B matters, while negative Bz magnitude is irrelevant. During 2011, there were some X-ray flare events. It was noticed that for all these, the last link, namely a substantial negative Bz, was absent and hence, only very weak geomagnetic storms resulted. Cosmic ray Forbush decreases also occurred, but magnitudes were small. Thus, in the early rising phase of a sunspot cycle, even though some local conditions could be favourable for producing strong solar flares (even X-ray flares), the overall solar activity is not adequate enough (for example in 2011) to produce strong coronal mass ejections. Next year, in 2012, there was a strong X-ray flare on 6 March. The interplanetary data are missing; but Dst depression and Forbush decrease were substantial, indicating strong CME activity, probably due to an enhanced general solar activity.

1 illus, 1 table, 14 ref

013878   Adekoya B J;Chukwuma V U;Bakare N O;David T W (Atmospheric/Ionospheric Physics Group, Physics Dep, Olabisi Onabanjo Univ, Ago Iwoye, Nigeria, Email: adekoyabolarinwa@yahoo.com) : Effects of geomagnetic storm on middle latitude ionospheric F2 during storm of 2-6 april 2004. Indian J Radio Space Phys 2012, 41(6), 606-16.

Study of the intense geomagnetic storm on 2-6 April 2004 has been carried out using ionospheric data obtained from ionosonde station in the mid-latitude stations with emphasis on the F2 region. Ionospheric storms represent an extreme form of space weather with important effects on ground and space-based technological systems. These phenomena are driven by highly variable solar and magnetospheric energy inputs to the Earth's upper atmosphere. The mid-latitude ionospheric stations are classified into two, depending on their geographic coordinates and ionospheric storm variation (i.e. positive and negative phases of ionospheric storm). The low-mid latitude are characterized with geographic latitude between 20°N<60> low-mid <60> 36°N, while high-mid latitude are <62>36°N. The ionospheric storms are classified as positive storms (P-storm) or negative storms (N-storm) depending on whether maximum electron density variation is positive or negative following the onset of geomagnetic storms (small fluctuations of less than +20% are neglected). The ionospheric storm produced at low-mid latitude was observed to be negative (N) ionospheric storm at the main phase, and also the declining phase of Dst was simultaneously depleted. The recovery phase is more pronounced with positive (P) ionospheric storm with noticeable effect than the initial and main phase. The D(foF2) response of high-mid latitude was observed with positive storm in the main phase. The initial phase responded with severe negative ionospheric storm compared to low-mid latitude with positive ionospheric storm. During the main phase of the geomagnetic storm, positive and negative phases of ionospheric storm prevailed in the daytime and at night.

3 illus, 36 ref

013877   Adekoya B J;Chukwuma V U (Physics Dep, Olabisi Onabanjo Univ, PMB 2002 Ago-Iwoye, Nigeria, Email: adekoyabolarinwa@yahoo.com) : Response of the mid-latitude ionospheric F2 to the total solar eclipses of solar cycle 23. Indian J Radio Space Phys 2012, 41(6), 594-605.

Study of the response of the mid-latitude ionospheric F2 to the total eclipses of solar cycle 23 is presented using five-minute resolution ionospheric foF2 data obtained from 14 mid-latitude ionosonde stations located in the path of totality of eclipse. The changes in foF2 are analysed using D(foF2), the normalised deviation of critical frequency F2 from the quite days reference. The D(foF2) variation of the ionosphere in the path of totality, as recorded by the ionosonde stations in this path, is characterized by simultaneous decrease in electron density at elapsed time of totality. The present analysis revealed that sudden Sun radiation removal by the Moon reduces the electron production processes in the ionospheric F2-layer. The impact of total solar eclipse is clearly evident simultaneously with the time of maximum occultation as decrease in the electron density in all ionospheric stations. The results, further, suggest that the maximum electron density of F2-layer redistribution processes were observed not at the time of totality of eclipse but shortly before and after the elapsed time (i.e. about 4 min to few hours). It is noteworthy that mainly response of electron density in ionospheric F2 region varies with the sun intensity. Also, an increase in maximum obscuration of eclipse causes reduction in electron density production processes and the transport processes in the F-layer are affected due to the presence of the westward electric field which may in turn lead to the decrease in foF2 of ionospheric F2 region. This result obtained was reasonably in good agreement with the earlier study of ionospheric response to total solar eclipse using different measurement and methods.

4 illus, 2 tables, 25 ref

010854   Khan S;Nayak M K;Haridas G;Sarkar P K (Indus Operation & Accelerator Physics Design Div, RRCAT, Indore-452 013, Email: saleemk@rrcat.gov.in) : Evaluation of ozone concentration for a white beam line hutch at Indus-2 synchrotron radiation source. Indian J pure appl Phys 2012, 50(11), 840-2.

Ozone generation for a white beam line of Indus-2 is estimated theoretically. The ozone production rate, saturation concentration, ventilation rate and delay time are also estimated. The saturation concentration of ozone for the white beam line (energy dispersive X-ray diffraction beam line) BL-11 was found to be 0.965 ppm for 100 cm interaction path length and 51.45 m3 hutch volume without any ventilation. The paper presents the details of the calculations and the results.

3 tables, 4 ref

010853   Chouksey S (NO, Raja Ramanna Centre for Advanced Technology, Indore-452 013, Email: chouksey@rrcat.gov.in) : Development of beamline radiation shielding hutch for indus-2 synchrotron radiation source. Indian J pure appl Phys 2012, 50(11), 782-4.

Indus-2 is an electron synchrotron radiation source. The Synchrotron Radiation (SR) photons are transported to the experimental station through twenty-six beamlines. The radiation environment around the beamline mainly comprises of Bremsstrahlung Radiation (BR) and Synchrotron Radiation (SR). Radiation shielded hutches are required for the protection for personnel against these radiation hazards. Standardization of the hutch design and fabrication process was done. This paper discusses hutch layout design, standardization of fabrication, shielding philosophy, material selection, quality checks, installation and subsequent optimization based on radiation measurements.

3 illus, 1 table, 5 ref

009767   Singh K P (Astron & Astrophys Dep, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005, Email: singh@tifr.res.in) : Grazing incidence optics for X-ray astronomy. J Optics 2011, 40(3), 88-95.

Cosmic X-ray sources are usually very weak and their detection, therefore, needs large area telescopes to gather light ad sensitive detectors to enhance quantum efficiency. Conventional telescopes for visible light use refracting or reflective optics which is impractical for X-ray wavelengths because photon energies are greater than the binding energies of the typical atomic electrons leading to a refractive index for X-rays being less than unity. Thus single surface reflectivity for near-normal incidence is negligible for X-rays. However, by Snell's can be reflected from a surface up to a critical angle (usually about a degree for energies below 10 keV) given by cosine θ = n. This is known as the grazing angle. X-ray telescopes are made to exploit the grazing incidence from a set of co-axial and confocal shells of paraboloidal and hyperboloidal mirrors. X-ry reflectors having high atomic number surfaces with low scattering are used to realize imaging capability for a telescope. I describe here various configurations required, and the various technologies used and their limitations, to make practical X-y telescopes. A soft X-ray imaging telescope (SXT) using grazing incidence has been built at TIFR for ASTROSAT - an Indian Multiwavelength Satellite designed to cover a very broad band of X-rays, UV and optical. Astrosat is planned to be launched by a Polar Satellite Launch Vehicle in 2012 into a near-Earth Equatorial orbit. I will also describe the ongoing R & D for realizing telescopes for hard X-rays above 10 keV useful for both Astronomy and medical diagnostics.

13 illus, 15 ref

007687   Unnikrishnan K (Physics Dep, N S S Hindu College, Changanacherry, Kerala-686 102) : Artificial neural network (ANN) for modelling earth's magnetic field belonging to solar minimum observed at a low latitude station Alibag. Indian J Radio Space Phys 2012, 41(3), 359-66.

Artificial neural networks (ANNs) are well suited to environmental modelling as they are nonlinear, relatively insensitive to data noise, and perform reasonably well when limited data are available. By using solar flux (F10.7), day of the year, local time, and Ap as input, an appropriate ANN has been developed to model north-south component (X component) of earth's magnetic field belonging to solar minimum period for Alibag (18.6°N, 72.87°E, geomagnetic latitude 10.37°N), a low latitude station of Indian sub-continent. For training the network three months, namely February, June, and September, were selected which represent three seasons winter, summer, and equinox, respectively of 2007 and 2008. Based on this analysis, it is observed that ANN model with 10 hidden neurons has good performance for 500 iterations. For testing the efficiency of ANN, hourly values of input and north-south component (X component) of earth's magnetic field observed during January, October 2007 and May 2008 were used. To confirm the functional aspects of this model, similar investigations were carried out for other periods, January, October 2008, and May 2007. In this study, for the first time, artificial neural networks (ANNs) are utilised to develop a geomagnetic daily variation model for an Indian sub-continent station, Alibag.

4 illus, 15 ref

007686   Pradhan D;Mitra A;De U K (Doppler Weather Radar, India Meteorological Dep, New Secretariat Building, Kolkata-700 001, Email: pradhandev1960@gmail.com) : Estimation of pressure drop and storm surge height associated to tropical cyclone using doppler velocity. Indian J Radio Space Phys 2012, 41(3), 348-58.

A study of five tropical cyclones in the Bay of Bengal has been conducted to estimate the pressure drop in the eye of the cyclone and associated storm surge height using Doppler velocity data. A new value of constant K (13.637) has been found in the empirical relation V_max = KV(P-P_c) between maximum velocity (V_max) and central pressure drop (P-P_c) in terms of maximum radial velocity measured by the Doppler Weather Radar (DWR) for coastal region of India. The present study provides an alternate method for estimating central pressure drop and expected storm surge height associated to a tropical cyclone. The study also reveals that the storm surge height estimated from Doppler velocity measurements for these cyclones is very close to the actual occurrence. The results of pressure drop estimates from Doppler velocity are in close agreement with the satellite estimates. It is also observed that DWR estimates are sometimes better than those from satellite. However, the limitation of DWR is the limited range of observation (400 km) and shorter duration of cyclone tracking.

6 illus, 8 tables, 24 ref

007685   Oyedum O D (Physics Dep, Federal Univ of Technology, Minna, Nigeria, Email: oyedumod@yahoo.com) : Duct-induced terrestrial microwave link degradation in Nigeria: minimization factors. Indian J Radio Space Phys 2012, 41(3), 339-47.

Radio propagation at VHF and higher frequency bands are characterized by fading and anomalous enhancements associated with meteorological conditions. In particular, link disruptions on line-of-sight terrestrial microwave paths are often associated with fading losses that often depend on the height and position of the receiver as well as season of the year. Such propagation problems may be minimized by appropriate choice of the height and relative position of the transmitting and receiving antenna. Using results based on analysis of radiosonde data obtained during 1991-1999 for surface air temperature, pressure and relative humidity from the Nigerian meteorological stations of Lagos (06°28'N, 03°28'E), on the Atlantic coast of West Africa; and an inland city of Kano (12°02'N, 08°30'E), height-distance curves have been derived for estimating suitable heights and relative positions of antennas, to minimize fade losses on line-of-sight terrestrial paths during surface duct events. A detailed account of theoretical applications of such curves in both cities is also attempted.

5 illus, 1 table, 16 ref

007684   Olufemi Adebesin B (Industrial Physics Dep, College of Science & Engineering, Landmark Univ, Omu Aran, Kwara State, Nigeria, Email: f_adebesin@yahoo.co.uk) : foF2 variations during geomagnetic disturbances at the rise of solar cycle 23. Indian J Radio Space Phys 2012, 41(3), 323-31.

The present work emphasizes on foF2, the critical frequency of the F2 layer, variations during peak geomagnetic disturbances (Dst ≤ -100 nT) at the rise of solar cycle 23. Geomagnetic, interplanetary and solar wind parameters have been studied as well as foF2 data from five ionosonde stations in the Asian sector. The study shows that: (i) the observed enhancement immediately after sunset at Petropavlovsk compared with foF2 upwelling after sunrise at Magadan after the storm on 12 February 2000 revealed that there is no simultaneity in foF2 during the activity, (ii) the variation of foF2 along a line of fixed latitude, as it occurs at four stations during the 12 August 2000 storm, shows the diurnal anomaly, which depends on the season and epoch of the solar cycle, reaching maximum before sunrise, and (iii) at any latitude, factors other than solar radiance play an important role in determining the foF2 variations. The results seem to agree with earlier works. The morphology of both positive and negative phase storms is also discussed.

6 illus, 2 tables, 24 ref

007683   David T W;Chukwuma V U (Atmospheric/Ionospheric Physics Research Group, Physics Dep, Olabisi Onabanjo Univ, Ago-Iwoye, Nigeria, Email: wemidavid@yahoo.com) : Response of ionospheric foF2 over South East Asian sector to geomagnetic storm of 29 october 1973. Indian J Radio Space Phys 2012, 41(3), 332-8.

The auroral electrojet (AE) index was employed for the study of the ionospheric response to the geomagnetic storm of 29 October 1973. The interplanetary magnetic field (IMF) component, Bz, and the low latitude magnetic index, Dst, show that the event is a moderate (Dst = -64 nT, Bz = -5.8 nT) storm. The analysis from the disturbances in ionospheric foF2 during 29-31 October 1973 shows predominantly an enhancement (positive storm) at the mid and low latitude stations. In between the time of storm (14:00 hrs UT on 29 October and 05:00 hrs UT on 30 October), the upper latitudes also show some degree of enhancement. This paper concludes that the reason for the observed positive ionospheric storm over all latitudes under investigation could be due to injection of energy from the solar wind into the auroral region as a result of significant increase in the AE index which causes an uplift of the ionospheric layers to higher altitudes, where the recombination rate is small. Furthermore, this paper confirms the argument that moderate magnetic storms are capable of generating ionospheric storms which are of comparable magnitude with those resulting from intense geomagnetic storms.

2 illus, 3 tables, 22 ref

007682   Calla O P N;Purohit U;Asopa P;Kalla A;Dadhich H (NO, International Centre for Radio Science, Jodhpur-342 003, Email: opnc06@gmail.com) : Variability of estimated dielectric constant obtained using theoretical models over pacific ocean in open sea for sea surface salinity varying from lower to higher values. Indian J Radio Space Phys 2012, 41(3), 367-71.

In this paper, the values of dielectric constant (ε) obtained by Blanch & Aguaca (B&A) as well as Klein & Swift (K&S) models using variable salinity in open seas in Pacific Ocean have been studied. The values of dielectric constant derived by B&A and K&S are compared with different salinities ranging 20-50 psu. The analysis has been done on eight days: three days in February and five days in March 2011. Each of these three days of February pair with the other three days of March before Tsunami and remaining two 9ays of March taken after Tsunami pair with each other, in such a way that each pair have nearly same pass over Pacific Ocean and thus, in this way each of these four sets form the four different passes over Pacific Ocean near Japan. This analysis was initially done with Tsunami point of view over Japan which is under process but during this analysis, variability of ε has been seen using two models (B&A and K&S) for variable salinity and the analysis has been presented. This analysis shows that within certain acceptable limits both the models of K&S and B&A are useful for estimating dielectric constant of sea water and understanding the variability of estimated e with sea surface salinities. The analysis also shows that the dielectric constant goes lower to higher value for B&A and higher to lower values for K&S with increase in the values of salinity.

6 illus, 7 ref

007681   Ambresh P A;Hadalgi P M;Hunagund P V (Microwave Electronics Research Laboratory, P G Studies & Research in, Gulbarga Univ, Gulbarga-585 106, Email: ambreshpa@rediffmail.com) : Compact dual band slotted patch antenna for wireless systems. Indian J Radio Space Phys 2012, 41(3), 372-6.

This paper presents the study on a slot-etched rectangular patch antenna directly fed by the 50 E1 semi miniature A-type (SMA) co-axial connector for wireless systems. The dual frequency behaviour by etching the slots on plane patch surface is studied experimentally. The proposed patch antenna has also achieved 28% compactness with a gain of 9.74 dB. Other antenna parameters such as antenna input impedance, voltage standing wave ratio (VSWR), return loss, gain and bandwidth are presented. Details of antenna geometry, patch design and experimental results are also presented. This antenna finds applications in WiMax, radar communication, WLAN, and fixed satellite service applications covering 3-4 GHz frequency range.

9 illus, 19 ref

006652   Vineeth C;Pant T K;Hossain M M (Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum-695 022, Email: cnvins@gmail.com) : Enhanced gravity wave activity over the equatorial MLT region during counter electrojet events. Indian J Radio Space Phys 2012, 41(2), 258-63.

Reports the evidence for an enhanced gravity wave activity of 1-2 hour periodicity in the equatorial mesopause temperature during the counter electrojet (CEJ) events. The analysis based on the data from a unique dayglow photometer and Proton Precession Magnetometer over a dip equatorial station Trivandrum (8.5°N, 77°E, dip latitude 0.5°N) clearly indicates that in general, the wave activity is high during CEJ events compared to a normal day. It is very interesting to note that the wave activity at mesopause region fades out during the period of the occurrence of CEJ when the mesospheric zonal wind shows a reversal from west to east. This indicates a possible upward penetration of the gravity waves from mesosphere to lower thermosphere during such events, thereby, reversing the vertical polarization field and hence the generation of CEJ. This study presents first direct evidence for such an enhancement in the wave activity in the upper mesospheric region during CEJ events, supporting the gravity hypothesis of Raghavarao & Anandarao [Raghvarao R & Anandarao B G, Vertical winds as a plausible cause for equatorial counter electrojet, Geophys Res Lett (USA), 7 (1980) pp 357-360].

3 illus, 23 ref

006651   Veenadhari B;Sandeep Kumar;Tulasi Ram S;Singh R;Alex S (NO, Indian Institute of Geomagnetism, New Panvel, Navi Mumbai-410 218, Email: bveena@iigs.iigm.res.in) : Corotating interaction region (CIR) induced magnetic storms during solar minimum and their effects on low-latitutde geomagnetic field and ionosphere. Indian J Radio Space Phys 2012, 41(2), 306-15.

The corotating interaction region (CIR) induced magnetic storms which occurred during solar minimum are investigated to study their effects on equatorial and low-latitude ionosphere and geomagnetic field at Indian sector. This work was a part of Indian CAWSES campaign for the period of March - April 2006. The penetration electric fields play a crucial role in modifying equatorial electric fields during severe magnetic storms and these effects are not studied in detail for moderate magnetic storm originated by CIRs. The present paper investigates the effects of CIR related moderate storms and found that during the moderate magnetic storms, the short lived (duration of 2 - 3 h) penetration of high latitude electric fields to equatorial ionosphere exists during local morning and noon hours during main phase as inferred from changes in EEJ with the response to interplanetary electric field. The variations of EEJ are linearly correlated with the sudden changes in interplanetary electric field (IEF_Y) around morning to noon hours, which confirms existence of the penetration of electric fields.

6 illus, 39 ref

006650   Tulasi Ram S;Yamamoto M;Veenadhari B;Sandeep Kumar;Gurubaran S (Research Institute for Sustainable Humanosphere (RISH), Kyoto Univ, Uji, Japan, Email: tulasi@iigs.iigm.res.in) : Corotating interaction regions (CIRs) at sub-harmonic solar rotational periods and their impact on ionosphere-thermosphere system during the extreme low solar activity year 2008. Indian J Radio Space Phys 2012, 41(2), 294-305.

The past solar minimum year 2008 has been exceptionally quiet with more than 70% spotless days and the solar EUV irradiance and F10.7 solar flux levels steadily at low values for a prolonged period. During this unique solar minimum epoch, however, the geospace environment is recurrently intercepted by solar wind high speed streams from the low-latitude coronal holes, which are the primary contributors for recurrent geomagnetic activity at solar rotational (27-day) and its sub-harmonic (13.5 and 9-day) periodicities. In response, both the neutral and ionized upper atmospheric properties continued to ring with same periodicities. The results indicate that both the neutral and electron density response at 400 km (topside ionosphere) is dominated by the changes in temperature and/or scale height and exhibits coherent enhancements concurrently with the periodicities in geomagnetic activity. However, the equatorial electrojet (EEJ) strength does not exhibit any recurrent signatures at these periodicities.

7 illus, 61 ref

006649   Tiwari P;Tiwari D;Surve G;Nayak C (NO, Dr K S Krishnan Geomagnetic Research Laboratory, IIG, Allahabad-221 505) : Sporadic E over Allahabad during the extremely prolonged low solar activity period of 2007-2009. Indian J Radio Space Phys 2012, 41(2), 285-93.

In this paper, the characteristics of sporadic E (Es) layer echoes, observed using a Canadian Advanced Digital lonosonde (CADI) from equatorial ionization anomaly crest region of Allahabad (25.3°N, 81.5°E, and dip latitude 16.3°N) during extremely prolonged low solar activity period of 2007-2009, have been investigated. Quarter hourly ionograms have been used to study the Es characteristics, viz. the top frequency reflected by the Es layer (ftEs), the blanketing frequency (fbEs), virtual height of Es (h'Es) and their local time and seasonal variations. Also, the occurrence statistics of Es and total blanketing type Es are presented. The most notable feature of the sporadic Es layer at Allahabad is the high percentage occurrence during daytime in the summer months. Interestingly, observation shows that the occurrence of total blanketing type Es layer is more during the nighttime of summer months than in other times and other months. Height migration of h'Es shows descending pattern indicating the role of tidal winds. These results are finally compared with those reported earlier and discussed in the light of current understanding of the solar cycle and seasonal variability of the temperate latitude Es.

8 illus, 21 ref

006648   Thampi S V;Yamamoto M (NO, Physical Research Laboratory, Ahmedabad-380 009, Email: smitha@prl.res.in) : Evolution of plasma bubbles over Vietnam region observed using the CERTO beacon on board C/NOFS satellite. Indian J Radio Space Phys 2012, 41(2), 233-9.

Observation of evolution of plasma bubbles using the ground based reception of Coherent Electromagnetic Radio Tomography (CERTO) beacon signals from the Communications/Navigation Outage Forecasting System (C/NOFS) satellite on 25 January 2009 are presented. These observations are made from Bac Lieu, Vietnam (9.2°N, 105.6°E geographic, 1.7°N magnetic dip latitude). The ionosonde observations from Bac Lieu and Chumphon (10.7°N, 99.4°E, 3.3° magnetic dip latitude) show that there is a well developed equatorial spread-F (ESF) at 105°E longitude, whereas ESF was absent at 99°E longitude. Hence, the successive passes of C/NOFS reveal the evolutionary phases of plasma irregularities over 105°E longitude. This also enables to observe the corresponding VHF and UHF scintillations at their different phases of evolution and gives a measure of the spectral evolution of irregularities. The spectral characteristics obtained from successive passes bring out the signatures of the cascading process. This example shows the patchy nature of the ESF irregularities and the longitudinal variability of ESF during the deep solar minimum period.

5 illus, 24 ref

006647   Taori A;Kamalakar V;Joshi L M;Sripathi S; Patra A K (NO, National Atmospheric Research Laboratory, Gadanki-517 112, Email: alok.taori@gmail.com) : On the linkage between mesospheric gravity waves and occurrence of equatorial plasma bubble observed during the low solar activity. Indian J Radio Space Phys 2012, 41(2), 264-70.

In this paper, the linkage between the low-latitude mesospheric gravity waves and the equatorial plasma bubble (EPB) have been investigated using simultaneously Rayleigh lidar and VHF radar observations from Gadanki (13.5°N, 79.2°E) and ionosonde observations from Tirunelveli (8.7°N, 77.8°E) made between 2007 and 2009 when solar activity was undergoing a deep minima. The two sets of observations have been analysed, each comprising of one case having EPB and another without EPB observed on two nearby nights. Observations show that the occurrences of plume structures in the radar observations are found to have a close link with the amplitudes of the short period gravity waves in the mesospheric temperature. It is argued that such waves have an important role in seeding the Rayleigh-Taylor (RT) instability manifesting EPB.

6 illus, 22 ref

006646   Sripathi S (NO, Indian Institute of Geomagnetism, Plot 5, Sector 18, Navi Mumbai-410 218, Email: sripathi@iigs.iigm.res.in) : COSMIC observations of ionospheric density profiles over Indian region: ionospheric conditions during extremely low solar activity period. Indian J Radio Space Phys 2012, 41(2), 98-109.

Attempt has been made to study the seasonal, altitudinal, diurnal and latitudinal variation of low latitude electron density obtained using COSMIC radio occultation (RO) measurements over Indian longitudes during the deep solar minimum year 2008. The seasonal variation shows enhanced electron densities at vernal and autumn equinoxes compared to winter and summer seasons. The observations also suggest a shift in the time and altitude at which the peak of the electron density occurs in different seasons. An important finding is that there exists an equinoctial asymmetry in the electron density with respect to altitude and latitude, where the electron density is higher at vernal equinox compared to autumn equinox. The latitudinal and seasonal variation of peak electron density (N_mF₂) during 10:00-14:00 hrs LT indicate enhanced equatorial ionization anomaly (EIA) on either side of the magnetic equator at both vernal and autumn equinoxes compared to the other seasons. Seasonal variation of equatorial electrojet (EEJ) strength obtained from geomagnetic H-field variations also shows strong EEJ at vernal and autumn equinoxes indicating that EEJ strength indeed partly controls the EIA development. Further, the results indicate that N_mF₂ over the northern EIA crest region is correlated well with solar flux.

9 illus, 36 ref

006645   Sanjay Kumar;Singh A K (Atmospheric Research Laboratory, Physics Dep, Banaras Hindu Univ, Varanasi-221 005, Email: abhay_s@rediffmail.com) : Effect of solar flares on ionospheric TEC at Varanasi, near EIA crest, during solar minimum period. Indian J Radio Space Phys 2012, 41(2), 141-7.

Solar flare is a sudden release of energy usually near a complex group of sunspots. The interaction of solar flare radiations with constituents of ionosphere produces immediate increase in the electron density in the ionosphere. The present paper describes the effect of solar flare on ionospheric total electron contents (TEC) using global positioning system (GPS) data recorded at Varanasi, a station situated near equatorial ionization anomaly (EIA) crest region during solar minimum period. In this study, total four flares have been selected to study the effect of solar flares on ionosphere during the year 2008, which is a solar minimum period. The enhancements on ionospheric TEC have been observed during the period of solar flares. The maximum enhancement of TEC during solar flare compared to quiet mean TEC up to 24 TEC units have been observed. A significant enhancement in TEC is observed at regions around the EIA crest region during the flare in association with: (a) the flare related EUV flux enhancement and consequent increased production of ionization, and (b) changes in the equatorial electrodynamics. The magnitude of enhancement in ionospheric TEC appears to be dependent on the class of the solar flare.

4 illus, 34 ref

006644   Rajesh P K;Liu J Y;Lin C H;Chen A B;Hsu R R; Su H T (Institute of Space Science, National Central Univ, Chung-Li, Taiwan, Email: pkrajesh@jupiter.ss.ncu.edu.tw) : Airglow observation over equatorial and low-latitudes in the extreme solar minimum of 2007-2008. Indian J Radio Space Phys 2012, 41(2), 148-54.

Reports the airglow observations in the extreme solar minimum period of 2007-2008 over low-latitude and equatorial regions. The observations are carried out using Imager of Sprites and Upper Atmospheric Lightning (ISUAL), onboard FORMOSAT-2 satellite. The ISUAL takes limb images of the emissions centered at 630.0 nm, viewing towards the right when the satellite is in the ascending orbit. The images are analysed to investigate the airglow intensity in the solar minimum years and also the latitudinal enhancement peaks in the emission. The results indicate that in the quiet conditions during 2007-2008, the seasonal variations are less pronounced.

6 illus, 20 ref

006643   Prasad S N V S;Prasad D S V V D;Venkatesh K; Niranjan K;Rama Rao P V S (Physics Dep, Andhra Univ, Visakhapatnam-530 003, Email: dsvvdprasad@yahoo.com) : Diurnal and seasonal variations in sporadic E-layer (Es layer) occurrences over equatorial, low and mid latitude stations- a comparative study. Indian J Radio Space Phys 2012, 41(1), 26-38.

Simultaneous hourly ionograms, from an equatorial station Trivandrum (8.47°N, 76.91°E), a low latitude station Waltair (17.70N, 83.3°E) in the Indian sector and a mid latitude station Kokubunji (35.7°N, 139.5°E) in the Japanese sector during the high sunspot year 2001, are scaled for the study of E-region parameters, namely the virtual height of sporadic E-layer (h'Es) and the critical frequency of sporadic E-layer (foEs). The diurnal, seasonal and day-to-day variations in h'Es and foEs are studied for all the three stations. It is observed that the sporadic E-layer is absent in the early morning and post sunset hours over Trivandrum during three different seasons. The values of h'Es are more at Kokubunji as compared to those at equatorial (Trivandrum) and low latitude (Waltair) stations during the three different seasons. The critical frequency of sporadic E-layer (foEs) over Trivandrum shows higher values, compared to those over Waltair and Kokubunji during daytime hours, in three different seasons indicating that foEs decrease as the latitude increases. The seasonal variation in foEs shows higher values during daytime hours of equinox and winter months over equator (Trivandrum) compared to those over low (Waltair) and mid (Kokubunji) latitude stations. The variations in h'Es and foEs for quiet and disturbed conditions indicate that the h'Es variation during quiet days of equinoctial months at Kokubunji shows a primary peak around morning hours while it is absent at Waltair and Trivandrum. Further, the behaviour of blanketing frequency of Es layer fbEs, and foEs-fbEs are also studied.

11 illus, 30 ref

006642   Pavan Chaitanya P;Patra A K;Rao S V B (Physics Dep, S V Univ, Tirupati-517 502, Email: pavanpeddapati@gmail.com ) : Contrasting features of the F₃ layer during high and low solar activity conditions observed from Indian low latitude. Indian J Radio Space Phys 2012, 41(2), 121-9.

Observations of the F₃ layer made during the extremely low solar activity (LSA) condition during 2008-2009 and high solar activity (HSA) condition during 2002-2003 from 6.5°N magnetic latitude in the Indian sector are presented. Structure of the ionogram, displaying the F₃ layer, is found to be very distinct and clear in the LSA as compared to that in the HSA. The occurrence rate and the duration of occurrence of the F₃ layer is maximum during the summer solstice, moderate during the equinox, and minimum during the winter solstice. While there is no significant difference in the occurrence of the F₃ layer in the equinox and the summer solstice during the HSA and LSA periods; in the winter solstice, it is extremely low in the LSA when compared to those observed in the HSA. Mean f₀F₃ is as high as 13 MHz in the HSA and 7 MHz in the LSA. In general, mean f₀F₃ is found to be 4-6 MHz higher in the HSA than that of the LSA. Notably, f₀F₃ values observed during 2008-2009 are found to be 2-3 MHz lower than those reported for the previous LSA periods. While in the summer and winter solstices, mean h_mF₃ is higher in the HSA than in the LSA; in the equinox, it is higher in the LSA than in the HSA. Also there exists large variability of h_mF₃ irrespective of local time, season and solar activity levels, indicating the roles of E x B drift and meridional neutral wind on the height of the F₃ layer. In an overall sense, while the observations agree well with the model prediction, they are found to agree on some counts and differ on some other counts with those reported earlier. The similarity and dissimilarity of the observations from different low-latitude locations are discussed in the light of current understanding on the F₃ layer.

6 illus, 28 ref

006641   Parihar N;Kakad B A;Mukherjee G K (NO, Indian Institute of Geomagnetism, Navi Mumbai-410 218, Email: navindeparihar@gmail.com) : Nightglow observations of OI 630 nm emission during the beginning of solar cycle 24 over 25 N, India. Indian J Radio Space Phys 2012, 41(2), 155-61.

This paper presents the first nightglow observations of the thermospheric red line emission of atomic oxygen at 630 nm from Allahabad (25.4°N, 81.9°E), a low-latitude station in India, during October-November 2009 and 2010. These observations were made using a CCD based All-Sky Imaging system. Most of the observations presented here represent geomagnetically quiet nights. The variability of OI 630 nm emission as observed from this station has been presented and discussed.

5 illus, 20 ref

006640   Pant T K;et. al. (NO, Vikram Sarabhai Space Centre, Trivandrum-695 022) : Radio beacon for ionospheric tomography (RaBIT) onboard YOUTHSAT: preliminary results. Indian J Radio Space Phys 2012, 41(2), 162-7.

This paper presents, for the first time, a few tomograms obtained using India's own Radio Beacon for Ionospheric Tomography (RaBIT) onboard YOUTHSAT, a small satellite dedicated for the terrestrial upper atmospheric studies. The tomograms presented here, were obtained during the beginning of the solar cycle 24, and clearly demonstrate the potential of the tomography technique to investigate the large scale ionospheric processes over the Indian longitude region.

4 illus, 11 ref

006639   Otsuka Y;Shiokawa K;Nishioka M;Effendy (Solar-Terrestrial Environment Laboratory, Nagoya Univ, Nagoya, Japan, Email: otsuka@stelab.nagoya-u.ac.jp) : VHF radar observations of post-midnight F-region field-aligned irregularities over Indonesia during solar minimum. Indian J Radio Space Phys 2012, 41(2), 199-207.

VHF backscatter radar with operating frequency 30.8 MHz has been operated at Kototabang (0.20°S, 100.32°E; dip latitude 10.4°S), Indonesia since February 2006. The F-region field-aligned irregularities (FAIs) observed by this radar from February 2006 to December 2010 during a solar minimum period have been analyzed and found that FAIs appeared frequently at the post-midnight sector between May and August every year from 2006 to 2010. Five-beam measurements by the radar revealed zonal propagation of the F-region FAIs. The present paper reports, for the first time, statistics of the zonal propagation velocity of the post-midnight FAIs. Between May and August, 46% (14%) of the post-midnight FAIs propagated westward (eastward), and zonal propagation was not discernible for 40% of the post-midnight FAIs. Average velocity was approximately 50 m s^-1 westward. The post-midnight FAIs were likely associated with either plasma bubbles or medium-scale traveling ionospheric disturbances (MSTIDs).

5 illus, 1 table, 36 ref

006638   Manju G;Madhav Haridas M K;Ravindran S;Pant T K;Tulasi Ram S (Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum-695 022, Email: manju_spl@vssc.gov.in) : Equinoctial asymmetry in the occurrence of equatorial spread-F over Indian longitudes during moderate to low solar activity period 2004-2007. Indian J Radio Space Phys 2012, 41(2), 240-6.

The seasonal pattern of equatorial spread-F (ESF) was observed at the magnetic equatorial station Trivandrum and low latitude station SHAR in the Indian sector using ionosonde data. A clearly higher ESF occurrence percentage is observed during vernal equinox, compared to autumnal equinox at both locations. Post-sunset vertical drift velocities were observed to be nearly comparable at Trivandrum indicating the comparable electrodynamical forcing on the post-sunset F-layer during the two equinoxes. At the off equatorial station SHAR, post-sunset vertical drift exhibits clear asymmetric pattern, with higher values in the vernal equinox, indicating the possible role of meridional neutral winds in modulating the layer height. The meridional winds estimated during the period just prior to ESF at the two equinoxes reveal much larger poleward winds to be prevalent during autumnal equinox compared to that in vernal equinox. Large neutral winds are shown to be related directly to asymmetry factor of the equatorial ionisation anomaly (EIA) derived from single station Coherent Radio Beacon Experiment (CRABEX) data. The larger neutral winds in autumnal equinox, results in an increase in off equatorial E-region conductivity, thereby, inhibiting or weakening the F-region dynamo. This study, thus, focuses on the role of neutral dynamics, as one possible candidate producing the asymmetric occurrence pattern of ESF, during the two equinoxes over Indian longitudes.

6 illus, 31 ref

006637   Li G;Ning B;Liu L;Wan W;Hu L;Zhao B;Patra A K (Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, Email: gzlee@mail.iggcas.ac.cn) : Equinoctial and june solstitial F-region irregularities over Sanya. Indian J Radio Space Phys 2012, 41(2), 184-98.

A 47.5 MHz VHP radar with a'psak power of 24 kW has been operated at Sanya (18°N, 109°E, dip 13°N) in Hainan Island, China since February 2009. Data from the continuous observations of F-region irregularities by the VHF radar, an ionosonde and a GPS scintillation/TEC receiver during the equinoctial and June solstitial months of 2009-2010 have been analyzed. It is found that the F-region 3-m scale field aligned irregularities (FAIs) appeared frequently at post-sunset hours during equinox but initiated mostly at midnight/post-midnight in June solstice. Comparison of FAIs with GPS scintillations, TEC fluctuations and spread-F shows that the equinoctial FAIs coincided well with GPS scintillations and TEC fast depletions, which are associated with the development of equatorial plasma bubbles (EPBs). However, for the June solstitial FAIs, the simultaneous Communication/Navigation Outage Forecasting System (C/NOFS) satellite in situ density measurements show that some post-midnight FAI structures over Sanya may originate from equatorial F-region and then extend to Sanya latitude along magnetic field lines, but the others could resemble the irregularities of localized origin, not associated with the equatorial process. Both types of post-midnight FAIs during June solstice are not accompanied by GPS scintillations and TEC depletions, but are associated with the occurrence of spread-F. Possible mechanisms responsible for the occurrence of post-midnight FAIs without GPS scintillations over Sanya have been discussed.

6 illus, 38 ref

006636   Joshi L M;Patra A K;Rao S V B (NO, National Atmospheric Research Laboratory, Gadanki, Andhra Pradesh-517 112, Email: lalit@narl.gov.in) : Equatorial F-region irregularities during low and high solar activity conditions. Indian J Radio Space Phys 2012, 41(2), 208-19.

In this paper a comparative study on equatorial F-region irregularities observed during the extremely low solar activity period of 2007-2009 and high solar activity period of 2002-2003 based on the Gadanki radar and ionosonde observations has been made. Observations show that the occurrence probability of the F-region field-aligned irregularities (FAI) on night-tonight basis was 65% in the high solar activity and 30-40% in the low solar activity. During the high solar activity condition, F-region FAI extended to higher altitude (600 km) than those in the low solar activity condition (400-500 km). Intriguingly, the altitude extent of the FAI during 2008 was 100 km higher than that of 2009. The lowest altitude at which the F-region FAI were observed during the high solar activity period was 250 km while it was 200 km during the low solar activity period of 2008-2009. During the high solar activity condition, the high altitude plume structures were observed earlier than those observed in the low solar activity period. Also, in the high solar activity period, the FAI were mostly confined to pre-midnight hours while during the low solar activity condition, the FAI extended well beyond post-midnight and to dawn hours. While the duration of the ionosonde observed range-spread echoes was generally consistent with that of the radar observed F-region FAI echoes, on a few occasions, range-spread echoes were observed during the post-midnight hours when radar echoes were completely absent. Concurrent observations of Es layer displayed descending behaviour reminiscent of tidal/gravity wave associated winds. Importantly, the descending property led to disruption of Es during the evening hours when equatorial plasma bubbles are formed. Interestingly, some of the high altitude plumes appear to be linked with such Es disruption. Examination of the growth perspective of the Rayleigh-Taylor instability suggests that even though the height of F-layer in the post sunset hour was lower during the low solar activity period than that of the high solar activity period. The chance of bubble formation is near equal owing to the advantage of lower ion-neutral collision frequency in the low solar activity than in the high solar activity. Detailed observational and theoretical results, including the E-F region coupling, are presented and discussed in the light of the current understanding of the equatorial plasma bubble phenomenon.

8 illus, 18 ref

006635   Girish T E;Gopkumar G (Physics Dep, College Univ, Trivandrum-695 034, Email: tegirish5@yahoo.co.in) : Secular changes in the solar terrestrial conditions observed during sunspot minima: some implications for the earth's ionosphere. Indian J Radio Space Phys 2012, 41(2), 83-8.

Presents the evolution of solar and geomagnetic conditions, galactic cosmic ray flux, and solar energetic particle occurrences from one sunspot minima to another starting from Maunder minimum period up to the recent long deep minimum between solar cycles 23 and 24. The study seems to confirm the Gleissberg periodicity of 90-100 years for the occurrence of long deep solar minimum and lorig term trends in these parameters. Their implications on the earth's ionosphere are discussed in this paper. In addition, possible ionospheric effects of weak geomagnetic disturbances, large proton events, lightning and seismic phenomena occurring during the deep solar minima epochs are discussed.

4 illus, 25 ref

006634   Dhanya R;Gurubaran S;Sathishkumar S (Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism, Krishnapuram, Tirunelveli-627 011, Email: dhanya.ramani@gmail.com) : Planetary wave coupling of the mesosphere-lower thermosphere-ionosphere (MLTI) region during deep solar minimum 2005-2008. Indian J Radio Space Phys 2012, 41(2), 271-84.

The study investigates the role of planetary-scale waves in the mesosphere-lower thermosphere-ionosphere (MLTI) coupling during the deep solar minimum period of 2005-2008. In the absence of severe magnetic disturbances, it is expected that much of the observed variations in low latitude ionospheric current system, the equatorial electrojet (EEJ), in particular, is expected to be driven by global scale waves propagating from below. The availability of continuous wind data from MF radar at mesospheric heights and geomagnetic field variations, the latter providing a measure of the strength of the ionospheric current system provides a unique opportunity to investigate the role of planetary-scale waves in driving the variabilities in the ionospheric current system during this solar minimum epoch. The study presents interesting findings on the role of planetary waves, especially the 3.5-day ultra-fast Kelvin wave and the 6.5-day wave in the low latitude MLTI coupling.

5 illus, 30 ref

006633   Dashora N;Taori A;Patra A K (NO, National Atmospheric Research Laboratory, Gadanki-517 112, Email: ndashora@narl.gov.in) : Multi-instrument observations of winter solstice F-region irregularities during the low solar activity. Indian J Radio Space Phys 2012, 41(2), 220-32.

Coordinated observations of the nighttime ionosphere in the Indian low-latitude zone were conducted during the winter of 2008-2009 using GPS TEC/Scintillation receiver, radar, ionosonde, and airglow photometer. It has been noted that out of ten nights of observations from 19 January to 27 January 2009, F-region irregularities were observed on six nights. Three distinctly different case studies have been presented to characterize the plasma irregularities observed during the extremely low solar activity condition. Specifically, the focus is on: (i) space-time evolution of plasma bubbles, (ii) zonal drift of the depleted regions, (iii) absence of sub-kilometer scale irregularities in the presence of its higher and lower scale irregularities, and (iv) characteristics of the post-midnight F-region irregularities. It has been found that rate of change of TEC index (ROTI) and derivative of ROTI (DROTI) increase with TEC depletions, but no correlation was found between DROTI and S4 index. These observations are discussed in the light of current understanding on the plasma irregularity processes with due importance to TEC variation and scintillation, which are detrimental for GPS based communication navigation applications.

14 illus, 31 ref

006632   Das T;Roy B;DasGupta A;Paul A (S K Mitra Center for Research in Space Environment, Calcutta Univ, Calcutta-700 009, Email: ashik_paul@rediffmail.com) : Impact of equatorial ionospheric irregularities on transionospheric satellite links observed from a low-latitude station during the minima of solar cycle 24. Indian J Radio Space Phys 2012, 41(2), 247-57.

Effects of equatorial ionization density irregularities on transionospheric communication and navigation links have been studied during the abnormally prolonged minima of the 24th solar cycle from Calcutta, a station situated virtually underneath the northern crest of the equatorial ionization anomaly in the Indian longitude sector. Scintillations at L-band have been sparse during 2008-2010 with four cases of GPS scintillations being observed from Calcutta. It is interesting to note that all these cases of scintillations were observed when the irregularities were field-aligned. One of the cases occurred on 25 September 2008 when GPS SV12 link exhibited depletions in TEC of nearly 8 TEC units around 15:30 hrs UTC associated with fluctuations in carrier-to-noise (CNO) ratio at L1 frequency and position dilution of precision (PDOP). The 1-min S₄ index reached a maximum value of 0.48 [SI~11dB] on the SV12 link around 15:30 hrs UT. The geostationary FLEETSATCOM link at 250 MHz was also disrupted during the period 15:55-16:25 hrs UT with maximum S₄ of 0.69 [SI~17dBJ. The ambient ionization and strength of the equatorial electrojet was higher on that day than other days around that period. The F-region height rise around sunset was significant as indicated by a flat-topped ionization density distribution over the magnetic equator by Defense Meteorological Satellite Program (DMSP).

8 illus, 30 ref

006631   Chakrabarty D;Bagiya M S;Thampi S V;Iyer K N (NO, Physical Research Laboratory, Navrangpura, Ahmedabad-380 009, Email: dipu@prl.res.in) : Solar EUV flux (0.1-5 nm), F10.7 cm flux, sunspot number and the total electron content in the crest region of equatorial ionization anomaly during the deep minimum between solar cycle 23 and 24. Indian J Radio Space Phys 2012, 41(2), 110-20.

The daily peak total electron content (TEC), recorded from Rajkot (22.29°N, 70.74°E, 31.6°N dip angle), is analyzed to investigate the combined roles of the solar cycle effect and neutral dynamical processes on the TEC variation over the equatorial ionization anomaly (EIA) crest region during the deep minimum between solar cycle 23 and 24. It is found that the mean peak TEC level decreases monotonically during 2005-2008 in accordance with the mean solar EUV (0.1-50 nm) and F10.7 cm (2800 MHz) flux (1 sfu = 10^-22 Wm^-2 Hz^-1). The mean peak TEC level over Rajkot seems to respond to the mean EUV flux (or F10.7 cm flux) level by simultaneously registering an increasing trend from February 2009 onwards. Most importantly, the rate of increase in the mean peak TEC level during 2009 over Rajkot is greater than the corresponding rates of increase in the mean EUV and F10.7 cm flux levels indicating the important additional roles played possibly by the residual (difference between annual averages) effects of the trans-equatorial winds, compositional changes, etc. over this time scale. It is verified that the inherent solar periodicity of around six months has no consistent causal connection with the semi-annual variation in TEC. A few spectral differences are also observed between the EUV flux and F10.7 cm flux. The annually averaged peak TEC value over Rajkot is found to be well correlated (R = 0.98) with the corresponding annually averaged F10.7 cm flux during 2005-2009. Statistical analyses also indicate that the time of occurrence of maximum TEC over Rajkot changes randomly during 2005-2009 indicating the possible role played by the processes related to the residual trans-equatorial wind over the years.

5 illus, 44 ref

006630   Balan N;Chen C Y;Liu J Y;Bailey G J (Institute of Space Science, National Central Univ, Chung-Li, Taiwan, Email: b.nanan@sheffield.ac.uk) : Behaviour of the low-latitude ionosphere-plasmasphere system at long deep solar minimum. Indian J Radio Space Phys 2012, 41(2), 89-97.

The behaviour of the low-latitude ionosphere-plasmasphere system during the long deep solar minimum between solar cycles 23 and 24 is presented using the physics based model SUPIM and FORMOSAT-3/COSMIC satellite data. Using the vertical ion drift velocity measured by C/NOFS satellite, and neutral densities and wind velocities obtained from MSIS and HWM, SUPIM calculates the electron and ion (O⁺, H⁺, He⁺, N₂⁺, O₂⁺, and NO⁺) densities and temperatures and plasma fluxes within ±40° magnetic latitudes and 150-2000 km heights for Indian longitudes at equinox (F10.7 = 68, Ap = 4). FORMOSAT-3 measures the corresponding electron density up to 600 km height. The data and model show the ionosphere contracting to a thin layer during the long deep solar minimum. During daytime, the ionosphere has a half-width of only about 250 km over the equator and 150 km at EIA crests with a peak density of about 10⁶ cm and O⁺/H⁺ transition height at around 750 km. At night, the ionosphere reduces to a cold thin layer of half-width less than about 150 km at the crests with a peak density of about 10⁵ cm^-3, and transition height about 500 km where the ion densities reduce to about 10⁴ cm^-3. Plasma density in the plasmasphere (above transition height) remains nearly constant at about 10⁴ cm^-3 during both day and night, and decreases only very slowly with height. However, the temperatures increase rapidly at sunrise at all heights to reach about 2500 K (electron) and 2250 K (ion) at 0800 hrs LT at 2000 km height (at EIA crests), which decrease by a maximum of only 250 K until sunset. After sunset, both electrons and ions cool rapidly until about midnight and then decrease slowly to nearly constant temperatures at all heights to about 650 K prior to sunrise. SUPIM predictions and FORMOSAT-3 data agree also with C/NOFS and CHAMP satellite observations.

7 illus, 39 ref

006629   Ansari I A;Khan M T (Physics Dep, Aligarh Muslim Univ, Aligarh-202 002, Email: iaaamuphysics@gmail.com) : Pc4 occurrence and its dependence on Kp values at low latitudes in India. Indian J Radio Space Phys 2012, 41(1), 39-47.

Study has been undertaken for Pc4 geomagnetic pulsations (in 6.7 - 22.2 mHz frequency range) at three very low latitude stations, viz. Pondicherry (PON) (geomagnetic latitude 2.5°N, geomagnetic longitude 151.97°E), Nagpur (NAG) (geomagnetic latitude 11.72°N, geomagnetic longitude 151.93°E), Hanle (HAN) (geomagnetic latitude 23.38°N, geomagnetic longitude 151.89°E) in India employing three axis fluxgate magnetometers, established and operated by the Indian Institute of Geomagnetism (IIG), Navi Mumbai. Digital dynamic spectra (DDS) for the north-south (X), east-west (Y) and vertical (Z) components of the recorded data were constructed for each day for one year (1 January to 31 December 2005). The X and Y components of these DDS were investigated for carrying out statistical study of the dependence of diurnal and seasonal variations of occurrence of Pc4 events on Kp. The monthly variation of Pc4 occurrence has a Kp dependence range from 0 to 9-. However, the yearly Pc4 occurrence was found to be evenly distributed with magnetic activity over Kp range from 2- to 4 at all the three stations with the peak occurrence recorded at Kp = 3-. The magnitudes of durations of Pc4 occurrence decreased in the station order PON, HAN and NAG, respectively. The prominent peaks in the seasonal Pc4 occurrence were observed at Kp = 3-, 3 for all the seasons. However, additional peaks were observed at Kp = 1-, 1 and 1+ for the autumn season. It is also worth noting that Pc4 in winter was observed during intense magnetic activity when 5+ <60> Kp <60> 8+.

8 illus, 1 table, 50 ref