TWO COMPONENTS OF COSMIC-RAY MODULATION

The long-term modulation of galactic cosmic rays is investigated from 1957 up to 1992 analysing the dynamic and the quasi-stationary components, separately. It has been found that the dynamic component is characterized by the presence of two peaks at the maximum phase of each solar activity cycle. We infer that the time interval between the two peaks corresponds to a period (well-related to the polar heliomagnetic reversal) in which somewhat decreased activity occurs for intense and long-lasting solar events. In fact, a contemporary dip in the magnetic energy released from the Sun was observed, in agreement with the suggested double maximum displayed by the basic features of the 11-year solar-activity cycle (Gnevyshev, 1977, and references therein). Moreover, the dynamic component of cosmic-ray modulation often shows a multi-structured profile in both peaks of activity, fairly well-connected with the pattern of the green corona brightness. On the other hand, analysing the quasi-stationary long-term trend of cosmic-ray intensity we pick out a good relationship between periods of enhanced cosmic-ray modulation and the area expansion of coronal intensity levels. The relevance of our results for solar-terrestrial forecasting is underlined.     

On the Nonlinear Thermal Evolution of Solar Corona Structures

The thermal evolution of structures is investigated for different values of the size of structure. A simplified cooling function and a constant heating mechanism are assumed. Analytical criteria for thermal instability are obtained. It is found that the response of the thermal structure not only depends on the amplitude of the disturbances, but also on whether the disturbance increases or decreases the initial steady temperature. Additionally, the evolution of the structure is examined numerically by using a time-dependent code under the assumption that the inertia terms are small. In particular, the analytical criteria obtained for thermal instability are verified.     

Photopolarimetric studies of comet Austin

Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope (VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the 1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990. The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of 0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C₂ bands are quite strong; C₃ emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C₂ emission. Molecular band polarization for CN, C₃, C₂ and H₂ O+ have been calculated. It has been found that emission polarization in CN, C₂ and C₃ is between 1–7% (phase angle between 107.4–109 degrees). For CN and C₂ the polarization values are close to the theoretically predicted values, but for C₃ the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley.     

Periodical light variations of t tauri stars as a result of disk accretion

I argue that temperatures of spots, responsible for observed periodical light variations of T Tauri stars (TTS), are not known with reliable accuracy to discriminate between chromospheric and accretion theories of TTS 's phenomenon. The hypothesis is set up that spots on classical TTS (CTTS) are due to heating of stellar surface by radiation from a collisional accretion shock, whereas spots on weak line TTS (WTTS), at least in some cases, are connected with a collisionless accretion shock rather than chromospheric activity. Possible scenarios of WTTS interaction with circumstellar matter are discussed.     

Spectrophotometry of comet Levy (1990c)

Two spectrophotometric scans of comet Levy (1990c) have been analysed to estimate Haser model column densities and production rates of CN andC ₂ molecules.     

Finite-element analysis of time-dependent large-deformation problems

An updated Lagrangian finite-element formulation has been developed for time-dependent problems of soil consolidation involving finite deformations. Large plastic strains as well as rotations occur in such problems and nominal stress measures are introduced in the formulation to redefine stresses. This leads to corrective terms for equilibrium and yield violations in addition to geometric stiffening terms in the governing integral equations. The soil is considered to be either a linear elastic or an elastoplastic, critical-state material. Some simple numerical examples are studied to validate the formulation, followed by a detailed analysis of the problem of penetration of a pile into soil. The results of this problem are viewed with emphasis on the physical interpretation and practical significance.     

Nonlinear ion-acoustic solitons in a warm plasma with adiabatic positive and negative ions and hot non-isothermal electrons

The propagation of an ion-acoustic soliton in a collisionless plasma with adiabatic positive and negative ions (with equal ion temperature) and hot non-isothermal electrons is studied by use of the renormalization method introduced by Kodama and Taniuti in the reductive perturbation method. The basic set of fluid equations describing the system is reduced to a Korteweg-de Vries (K-dV)-type equation for the first-order perturbed potential and to a linear inhomogeneous differential equation to the second-order of the perturbed potential. A stationary solution of the coupled equations is obtained.