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The distribution of acoustic power over sunspots shows an enhanced absorption near the umbra - penumbra boundary. Previous studies revealed that the region of enhanced absorption coincides with the region of strongest transverse potential field. The aim of this paper is to i) utilize the high-resolution vector magnetograms derived using Hinode SOT/SP observations and study the relationship between the vector magnetic field and power absorption and ii) study the variation of power absorption in sunspot penumbrae due to the presence of spine-like radial structures. It is found that i) both potential and observed transverse fields peak at a similar radial distance from the center of the sunspot, and ii) the magnitude of the transverse field, derived from Hinode observations, is much larger than the potential transverse field derived from SOHO/MDI longitudinal-field observations. In the penumbra, the radial structures called spines (intra-spines) have stronger (weaker) field strength and are more vertical (horizontal). The absorption of acoustic power in the spine and intra-spine shows different behavior, with the absorption being larger in the spine as compared to the intra-spine. [ABSTRACT FROM AUTHOR]

Near simultaneous coronal EUV images were used to show that the He ii λ304 network brightening is independent of coronal EUV radiation. We studied the quantitative relation between the intensity of He ii λ304 network brightening with the associated magnetic elements. An almost linear relationship was found between the He ii λ304 network brightening and the magnetic field for a field strength higher than 10 G with exceptions at neutral lines and in the intra-network. We also calculated the most probable formation height of He ii λ304 network elements using the potential extrapolation of the photospheric magnetic field. The results show that He ii λ304 network elements form at or around 3000 km above the height of the layer at which the sources of magnetic flux are deployed. [ABSTRACT FROM AUTHOR]

The precise measurement of solar magnetic fields requires an accurate measurement of the Muller matrix of the optical components in the path of the light beam, which again requires a careful measurement of the optical constants of the reflecting surfaces in the case of a 3-mirror coelostat system. Here we present a method to measure the optical constants (the real and imaginary part of the refractive index) to an accuracy of the order of 1% for bulk aluminium. This work is directed towards the measurement of instrumental polarisation at the Kodaikanal solar tower telescope, although it can be used for any metallic coated optics. [ABSTRACT FROM AUTHOR]

The steady-state solar wind solution is examined for different geometries of the flow tube that mimics a helmet streamer. Onset of non-equilibrium is seen whenever the spatial variation of the flow geometry crosses critical values. It is suggested that the dynamical response of the flow to the onset of non-equilibrium can manifest as a coronal mass ejection. [ABSTRACT FROM AUTHOR]

We have studied the time-dependent behaviour of the He i 10830 Å line. These studies show that (i) the fluctuations of the line width are uncorrelated with the equivalent-width fluctuations and (ii) the autocorrelation curves for the equivalent-width fluctuations are broader than those for line-width fluctuations. These results could be interpreted as the signatures of the eruption of density inhomogeneities, like spicules, into the altitudes of formation of the He i 10830 Å line. [ABSTRACT FROM AUTHOR]

The time-averaged equivalent width of the He I 10830 Å line is seen to be correlated with the time-averaged line width. This correlation is interpreted as evidence for the association of the chromospheric energy density with the heating of the overlying corona. [ABSTRACT FROM AUTHOR]

The difference between the magnetic tension and magnetic shear was calculated for four vector magnetograms of NOAA AR 4474. It was seen that this difference between the two independent angular measures of magnetic stress is less than 18° for more than 50% of the pixels. Magnetic tension is thus found to be fairly well correlated with magnetic shear for AR 4474. [ABSTRACT FROM AUTHOR]

The equivalent width, line depth, line width, and Doppler shift of the He i 10830 Å line were extracted from two time series of spectra. Scatter plots of time-averaged line depth, line width, and Doppler shifts, as well as the root mean square temporal fluctuation of these quantities against the time-averaged equivalent width at a few hundred spatial locations were obtained. The statistical behaviour of these line parameters and their fluctuations was used to infer plausible reasons for the fluctuations. Examination of these results showed that the line parameter fluctuations could be caused by fluctuations in the coronal UV radiation (which could drive the spicules) or by the appearance of density inhomogeneities such as spicules within the line forming domain. In either case, the data can be interpreted as representing the initial phases of spicules. [ABSTRACT FROM AUTHOR]

We investigate the effects of radiative heat losses and thermal conductivity on the hydromagnetic surface waves along a magnetic discontinuity in a plasma of infinite electrical conductivity. We show that the effects of radiative heat losses on such surface waves are appreciable only when values of the plasma pressure on the two sides of the discontinuity are substantially different. Overstability of a surface wave requires that the medium in which it gives larger first-order compression should satisfy the criterion of Field (1965). Possible applications of the study to magnetic discontinuities in solar corona are briefly discussed. [ABSTRACT FROM AUTHOR]

The effect of applying external pressure fluctuations on slender flux tubes is studied as a nonlinear initial value problem. Large amplitude velocity oscillations are seen to be produced when the frequency of the imposed fluctuations matches the natural frequency of the tube. Radiative cooling does not significantly damp these resonantly built-up oscillations. The absence of observational evidence for such a resonant response of the tubes is used to put a constraint on the length of tubes. [ABSTRACT FROM AUTHOR]

The observations of Stokes line profiles require an accurate knowledge of the instrumental polarisation caused by optical components in the path of the light beam. In this context we present a theoretical approach to the instrumental polarisation caused by the 3-mirror coelostat system of the Kodaikanal Solar Tower. [ABSTRACT FROM AUTHOR]

The transient response of the solar wind to changes in geometry is examined. An initial stationary flow in a configuration that diverges as r is assumed. This state corresponds to the usual solar wind solution. The effect on the flow through a tube whose area A(r, t) diverges faster than r, with the degree of divergence increasing in time, is considered. The asymptotic form of A(r, t) is chosen to mimic the form inferred in coronal holes. A detailed parameter study relating the form of A(r, t) to the pattern of flow in the tube is presented. It is observed that in the limit of large time (large compared to τ, the time constant for change in geometry of a flow tube) the solutions obtained from a time-dependent analysis can depend upon τ. For sufficiently large τ, the asymptotic solution is the same as the steady state solution obeying the correct boundary conditions and possessing a smooth sonic transition. However, if the geometry changes rapidly enough, solutions exhibiting shock-like discontinuities can also exist. This is essentially a new feature that emerges from the present investigation. Finally, it is suggested that this study may be useful in describing flows in evolving coronal holes. [ABSTRACT FROM AUTHOR]

A time dependent model for the flow of gas in a spicule is studied. In this model, the flow occurs in a magnetic flux sheath. Starting from hydrostatic equilibrium, the flux sheath is allowed to collapse normal to itself. The collapse induces a flow of gas along the magnetic field and this flow is identified as a spicule. A variety of sheath geometries and velocity patterns for the normal flow have been studied. It is observed that a large curvature in the field geometry and a large initial value for the normal flow are necessary to achieve spicule-like velocities. The duration for which a large velocity of normal flow is required is much shorter than the average lifetime of a spicule. It is proposed that the initial rapid collapse occurs during an 'impulsive spicule' phase and it is the subsequent gradual relaxation of the flow which is observed as a spicule. [ABSTRACT FROM AUTHOR]

When a sound wave is incident on a magnetic flux sheath, it causes fluctuations in the mean magnetic field of the sheath. We have calculated the space-average of the longitudinal component of these fluctuations and plotted this against the frequency of the incident sound wave. The main result is the presence of local maxima and minima in the response curve. If such maxima and minima could be detected in any actual observation then these would provide an estimate of the thickness of these magnetic structures. [ABSTRACT FROM AUTHOR]

The presence of finite current sheets at the boundaries of a magnetic flux sheath will lead to a somewhat reduced transmission of the energy of an incident acoustic wave. [ABSTRACT FROM AUTHOR]

The recently reported pre-flare activity in H-alpha filaments (Sivaraman, K. R., Rausaria, R. R., and Aleem, S. M.: 1992, Solar Phys. 138,353) is shown to be unrelated to changes in magnetic shear. An alternative interpretation for these observations is suggested. [ABSTRACT FROM AUTHOR]

The observed effects of solar flares and interplanetary sector crossings seem to indicate that particle precipitation in the Earth's upper atmosphere decreases cyclonic activity in the troposphere. As an extrapolation to longer term effects, it is suggested that the recurrence of prolonged periods of enhanced solar wind particle precipitation in the upper atmosphere during alternate solar minima could cause the recurrence of extreme droughts. [ABSTRACT FROM AUTHOR]

In the absence of genuine forcing terms, there is no resonance between linear fast mhd and gravito-acoustic waves. [ABSTRACT FROM AUTHOR]