Ammar Yasir | Federal Urdu University of Arts, Sciences and Technolgy, Karachi (original) (raw)

Papers by Ammar Yasir

Science, Jan 1, 1993

Interstellar hydrogen ionized primarily by the solar wind has been detected by the SWICS instrume... more Interstellar hydrogen ionized primarily by the solar wind has been detected by the SWICS instrument on the Ulysses spacecraft at a distance of 4.8 astronomical units from the sun. This &amp;quot;pick-up&amp;quot; hydrogen is identified by its distinct velocity distribution function, which drops abruptly at twice the local solar wind speed. From the measured fluxes of pick-up protons and singly charged helium, the number densities of neutral hydrogen and helium in the distant regions of the solar system are estimated to be 0.077 +/- 0.015 and 0.013 +/- 0.003 per cubic centimeter, respectively.

Journal of Geophysical Research, Jan 1, 1959

Journal of Geophysical Research, Jan 1, 1974

Spin-allowed Fe2+ absorption bands occur in the visible and near-infrared diffuse reflectance spe... more Spin-allowed Fe2+ absorption bands occur in the visible and near-infrared diffuse reflectance spectra of most pyroxenes. The wavelengths of the bands centered near 1 μm and 2 μm vary as functions of pyroxene composition, making possible mineralogical and chemical deductions based on spectral reflectance curves. Typically, pyroxene bands are well developed in relation to absorption features in the spectra of other rock-forming minerals and glass; thus pyroxene often dominates the spectral curves of pyroxene-bearing rocks. Telescopic spectra of the moon and some other solar system objects contain absorption features that can be interpreted in terms of pyroxenes mixed with other minerals and/or glass.

Astrophysical Journal, Jan 1, 2003

$Research paper thumbnail of Ca isotope fractionation on the Earth and other solar system materials$

Geochimica Et Cosmochimica Acta, Jan 1, 1978

It is demonstrated that differences in the ratio due to mass dependent isotope fractionation in n... more It is demonstrated that differences in the ratio due to mass dependent isotope fractionation in nature are clearly resolvable to a level of 0.5%. This precision is obtained (a) by using the double spike technique;(b) by using a mass-dependent law for correction of instrumental mass fractionation; and(c) by eliminating fractionation effects identified as due to differential elution of isotopes through ion exchange resins. We have determined the following uniform Ca isotopic composition after removing small natural fractionation effects: , , , , , where the errors correspond to the last figures shown. This yields an atomic weight of 40.076 ± 0.001. The data indicate the absence in the studied samples of detectable, distinct nuclear components in Ca similar to those observed for oxygen. In the samples studied, there is a distinct but small degree of Ca isotope fractionation. Overlapping ranges of fractionation of 2.5%. for (four atomic mass units) are observed in meteorites, lunar, and terrestrial samples. Means by which isotope fractionation could arise for Ca are discussed, but the small range of effects and the lack of systematic variations do not permit at present the identification of the mechanisms responsible for the fractionation observed in the suite of samples. Ca in the biological cycle does not show fractionation effects larger than observed for non-biogenic samples. In contrast to these results, we have observed large effects of up to 13%. for industrially purified Ca.

Icarus, Jan 1, 1979

We present a new and more accurate expression for the radiation pressure and Poynting-Robertson d... more We present a new and more accurate expression for the radiation pressure and Poynting-Robertson drag forces; it is more complete than previous ones, which considered only perfectly absorbing particles or artificial scattering laws. Using a simple heuristic derivation, the equation of motion for a particle of mass m and geometrical cross section A, moving with velocity v through a radiation field of energy flux density S, is found to be (to terms of order )

Reviews of Geophysics, Jan 1, 1989

The processes that lead to charging of dust grains in a plasma are briefly reviewed. Whereas for ... more The processes that lead to charging of dust grains in a plasma are briefly reviewed. Whereas for single grains the results have been long known, the reduction of the average charge on a grain by “Debye screening” has only recently been discovered. This reduction can be important in the Jovian ring and in the rings of Uranus. The emerging field of gravitoelectrodynamics which deals with the motion of charged grains in a planetary magnetosphere is then reviewed. Important mechanisms for distributing grains in radial distance are due to stochastic fluctuations of the grain charge and a systematic variation due to motion through plasma gradients. The electrostatic levitation model for the formation of spokes is discussed, and it is shown that the radial transport of dust contained in the spokes may be responsible for the rich radial structure in Saturn's rings. Finally, collective effects in dusty plasmas are discussed which affect various waves, such as density waves in planetary rings and low-frequency plasma waves. The possibility of charged grains forming a Coulomb lattice is briefly described.

Icarus, Jan 1, 1990

In a previous paper (J. Laskar, Nature 338, (237–238)), the chaotic nature of the Solar System ex... more In a previous paper (J. Laskar, Nature 338, (237–238)), the chaotic nature of the Solar System excluding Pluto was established by the numerical computation of the maximum Lyapunov exponent of its secular system over 200 myr. In the present paper an explanation is given for the exponential divergence of the orbits: it is due to the transition from libration to circulation of the critical argument of the secular resonance 2 (g4 − g3) − (s4 − s3) related to the motions of perihelions and nodes of Earth and Mars. Another important secular resonance is identified: (g1 − g5) − (s1 − s2). Its critical argument stays in libration over 200 myr with a period of about 10 myr and amplitudes from 85 to 135°. The main features of the solutions of the inner planets are now identified when taking these resonances into account. Estimates of the size of the chaotic regions are determined by a new numerical method using the evolution with time of the fundamental frequencies. The chaotic regions in the inner Solar System are large and correspond to variations of about 0.2 arcsec/year in the fundamental frequencies. The chaotic nature of the inner Solar System can thus be considered as robust against small variations in the initial conditions or in the model. The chaotic regions related to the outer planet frequencies are very thin except for those of g6 which present variations sufficiently large to be significant over the age of the Solar System.

Geochimica Et Cosmochimica Acta, Jan 1, 1982

A new abundance table has been compiled, based on a critical review of all C1 chondrite analyses ... more A new abundance table has been compiled, based on a critical review of all C1 chondrite analyses up to mid-1982. Where C1 data were inaccurate or lacking, data for other meteorite classes were used, but with allowance for fractionation among classes. In a number of cases, interelement ratios from meteorites or lunar and terrestrial rocks as well as solar wind were used to check and constrain abundances. A few elements were interpolated (Ar, Kr, Xe, Hg) or estimated from astronomical data (H, C, N, O, He, Ne).For most elements, the new abundances differ by less than 20% from those of Cameron (1982a). In 14 cases, the change is between 20 and 50% (He, Ne, Be, Br, Nb, Te, I, Xe, La, Gd, Tb, Yb, Ta and Pb) and in 5 others, it exceeds 50% (B, P, Mo, W, Hg). Some important interelement ratios (, , , , , , , etc.) are significantly affected by these changes.Three tests were carried out, to see how closely C1 chondrites approximate primordial solar system abundances. (1) A plot of solar vs Cl abundances shows only 7 discrepancies by more than twice the nominal error of the solar abundance: Ga, Ge, Nb, Ag, Lu, W and Os. Most or all apparently reflect errors in the solar data or f-values. (2) The major cosmochemical groups (refractories, siderophiles, volatiles, etc.) show no significant fractionation between the Sun and C1's, except possibly for a slight enrichment of volatiles in Cl's. (3) Abundances of odd-A nuclides between A = 65 and 209 show an almost perfectly smooth trend, with elemental abundances conforming to the slope defined by isotopic abundances. There is no evidence for systematic fractionation of the major cosmochemical groups from each other. Small irregularities (10–15%) show up in the Ag-Cd-In and Sm-Eu regions; the former may be due to a ~ 15% error in the Ag abundance and the latter, to a 10–20% fractionation of Eu during condensation, to contamination of C1 chondrites with interplanetary dust during regolith exposure, or to a change from s-process to r-process dominance.It appears that the new set of abundances is accurate to at least 10%, as irregularities of 5–10% are readily detectable. Accordingly, Cl chondrites seem to match primordial solar-system matter to ⩽ 10%, with only four exceptions. Br and I are definitely and B is possibly fractionated by hydrothermal alteration, whereas Eu seems to be enriched by nebular condensation or regolith contamination.

Journal of Physics and Chemistry of Solids, Jan 1, 1998

On the basis of the empirical psudopotential method and independent praticle model, electronic an... more On the basis of the empirical psudopotential method and independent praticle model, electronic and positronic band structures are obtained. The calculated electron and positron chemical potentials give the positron and positronium work functions. The deformation potential model is used in the treatment of the contribution to the relaxation by a positron-acoustical phonon, which is needed to calculate the positron diffusion constant. Calculations of the diffusion-related quantities are presented.

Physical Review Letters, Jan 1, 1993

Spectrochimica Acta Part B-atomic Spectroscopy, Jan 1, 1980

Spectrochemical analysis originated with the work of Kirchoff and Bunsen in 1860 but found relati... more Spectrochemical analysis originated with the work of Kirchoff and Bunsen in 1860 but found relatively little application until the 1930s. Arc-spark emission and, to a lesser extent, flame emission methods then became popular. Following World War II flame emission became very popular. In 1955 the modern era of atomic absorption spectroscopy began with the work of Walsh and Alkemade and Milatz. The time since 1955 can be divided into seven year periods. The first was an induction period (1955–1962) when AA received attention from only a very few people. This was followed by a growth period (1962–1969) when most of what we see today was developed, and then by a period of relative stability (1969–1976) when AA contributed greatly to other fields. We are now in a period of great change, which started in about 1976, due to the impact of computer technology on individual laboratory instruments.

Analytical Chemistry, Jan 1, 1991