A model for thinning of the plasma sheet

A one-dimensional model for thinning of the plasma sheet is developed on the basis of launching a fast mode MHD rarefaction wave propagating in the tailward direction along the plasma sheet. Behind the rarefaction wave the pressure is reduced, leading to thinning of the plasma sheet and also to an Earthward plasma flow with a speed on the order of the sound speed a₀. The plasma sheet thickness is reduced by a factor of 2 if an Earthward plasma flow speed of 0.8a₀ is induced. The predictions of the model are in reasonable agreement with observations.     

Generalized polytropes: Stellar structure with a boltzmann factor

In order to study the structure of a chemically homogeneous star in equilibrium, a density profile of the form T ^N exp(–µm(–)/kT) is suggested. As for polytropes, qualitative aspects of the resulting stellar model can be discussed analytically. In particular it is shown that one reobtains forN=3 Eddington's standard model, whereas forN<3 nearly polytropic models result. WhenN>3, the effective polytropic index does vary appreciably over the star. Numerical results indicate that the proposed density profile is quite reasonable in view of the simplicity of the model. From a comparison of the degree of precision of a polytropic approximation with that of the newly proposed model it follows that the new approximation is definitely better than the polytropic one. It is suggested that the model may be useful to study the structure of stellar clouds, clusters and (spherical) galaxies.Now at Department of Applied Mathematics, Queen Mary College, University of London, England.     

The spectrum of the Ap star HR 465 near the rare-earth minimum phase

Four spectra of the long-period Ap star HR 465 were taken in 1969–70 near its rare-earth minimum phase and analysed. Equivalent widths or upper limits were obtained for lines of 23 elements. Lines of Cr were very strong. The relative abundances of the iron-peak elements were derived by a differential curve-of-growth analysis using Gem as the comparison star. The Sc/Fe, Ti/Fe, V/Fe, and Ni/Fe abundance ratios were found to be abnormally low, while the Cr/Fe ratio was abnormally high (0.08). The equivalent widths in 1969–70 were compared with those measured by Aller on a spectrum taken in 1960 near the rare-earth maximum phase. Ionized lines of Sc, Ti, V, Te, Nb, Mo, and the rare-earths all vary out of phase with the Cri and Crii lines. Although Mo and Cr occupy corresponding positions in the periodic table, the Mo/Cr abundance ratio was apparently at least 180 times higher in 1960 than in 1969–70.     

Gamma radiation and photospheric white-light flare continuum

Recent gamma-ray observations of solar flares have provided a better means for estimating the heating of the solar atmosphere by energetic protons. Such heating has been suggested as the explanation of the continuum emission of the white-light flare. We have analyzed the effects on the photosphere of high-energy particles capable of producing the intense gamma-ray emission observed in the 1978 July 11 flare. Using a simple energy-balance argument and taking into account hydrogen ionization, we have obtained the following conclusions:

1. Heating near τ₅₀₀₀ = 1 in the input HSRA model atmosphere is negligible, even for very high fluxes of energetic particles.
2. Energy deposition increases with height for the inferred proton spectra, and does not depend strongly upon the assumed angle of incidence. The computed energy inputs fall in the range 10–100 ergs (cm³ s)^?1 at the top of the photosphere.
3. H^? continuum dominates for column densities as small as 10²² cm^?3, but at greater heights hydrogen ionizes sufficiently for the higher continua to dominate the energy balance.
4. The total energy deposited in the ‘photospheric’ region of H^? dominance could be within a factor of 3 of the necessary energy deposition, by comparison with the white-light flare of 1972 August 7, but the emergent spectrum is quite red so that the intensity excess in the visible band is insufficient to explain the observations.

In summary, it remains energetically possible, within observational limits, that high-energy protons could cause sufficient heating of the upper photosphere to produce detectable excess continuum, but emission from the vicinity of τ = 1 is not significant.     

The empirical determination of damping constants in the solar photosphere

We determine empirical damping constants for 73 selected Fe i lines following the method of Gurtovenko and Kondrashova (1980), employing high-quality observations and the accurate list of Fe i oscillator strengths by Gurtovenko and Kostik (1980).The results show: (i) No increase of the enhancement factor to van der Waals broadening with excitation potential, as predicted by Edmunds (1975), and with the frequency of the transition (Figure 1); (ii) a substantial part of the commonly-used enhancement factor for weaker lines is not due to collisional damping (Figure 2), but to a misrepresentation of the inhomogeneous structure of the deep photosphere. This false damping effect is not seen in the stronger lines which yield an average damping constant : 1.3₆ 1.5 ₆.     

Vector magnetic field evolution,energy storage,and associated photospheric velocity shear within a flare-productive active region

The evolution of vector photospheric magnetic fields has been studied in concert with photospheric spot motions for a flare-productive active region. Over a three-day period (5–7 April, 1980), sheared photospheric velocity fields inferred from spot motions are compared both with changes in the orientation of transverse magnetic fields and with the flare history of the region. Rapid spot motions and high inferred velocity shear coincide with increased field alignment along the B _L= 0 line and with increased flare activity; a later decrease in velocity shear precedes a more relaxed magnetic configuration and decrease in flare activity. Crude energy estimates show that magnetic reconfiguration produced by the relative velocities of the spots could cause storage of 10³² erg day^–1, while the flares occurring during this time expended 10³¹ erg day^–1.Maps of vertical current density suggest that parallel (as contrasted with antiparallel) currents flow along the stressed magnetic loops. For the active region, a constant-, force-free magnetic field (J = B) at the photosphere is ruled out by the observations.Presently located at NASA/MSFC, Huntsville, Ala. 35812, U.S.A.     

Tunneling and interference of Alfvén waves

The propagation and interference of Alfvén waves in magnetic regions is studied. A multilayer approximation of the standard models of the solar atmosphere is used. In each layer, there is a linear law of temperature variation and a power law of Alfvén velocity variation. The analytical solutions of a wave equation are stitched at the layer boundaries. The low-frequency Alfvén waves (P > 1 s) are able to transfer the energy from sunspots into the corona by tunneling only. The chromosphere is not a resonance filter for the Alfvén waves. The interference and resonance of Alfvén waves are found to be important to wave propagation through the magnetic coronal arches. The transmission coefficient of Alfvén waves into the corona increases sharply on the resonance frequences. To take into account the wave absorption in the corona, a method of equivalent schemes is developed. The heating of a coronal arch by Alfvén waves is discussed.     

On geological processes on venus: Analysis of the relationship between altitude and degree of surface roughness

Aiming to study the relationship between Venus surface heights and surface roughness, the Pioneer Venus surface altitude map and map of r.m.s. slope in m-dkm scale have been analy sed for the Beta and Ishtar regions using a system of digital image processing. To integrate the data obtained, the results of geomorphological analysis of Venera 9 and 10 TV panoramas as well as gamma-spectrometric and photometric measurements were used. The analysis gives proof that Venera 9 and 10 landing sites represent geologic-morphologic situations typical of Venus, thus enabling the results of observations made at landing sites to be extended to large provinces. Apparently this conclusion is also applicable to the Venera 8 landing site. No strong relationship exists between the roughness of the surface and its altitude or the amount of a regional slope; neither for the Beta nor for the Ishtar region. A weak direct correlation observable for roughness-altitude pairs for the Beta region and roughness-altitude, roughness-slope pairs for the Ishtar region are quite obviously a consequence of regional roughness control, i.e. of an overall character of geological structure. On Venus the factors contributing to higher surface roughness on the m-dkm scale are, obviously, mostly volcanic and tectonic in their nature whilst those responsible for smoothing-out of the surface are chiefly exogenic. The rate of exogenic transformation of the Cytherean surface may be fairly high. On Venus, similarly as on the Earth, active tectono-magmatic processes have possibly taken place in recent geological epochs. One of the places where they are manifest is an extensive zone running from north to south across the Beta, Phoebe and Themis highlands. Within its limits occur both the process of basaltic shield-type volcanism and areal basalt effusions at low hypsometric levels accounting for the formation of lowland plains at the expense of ancient rolling plains. The basalts of the shield volcano Beta show some differences in composition compared to those of areal effusions at low hypsometric levels. The overall character of Cytherean tectonics in the recent geologic epoch is apparently block-type with a predominance of vertical movements. Against the background of the sinking of some of the blocks the other ones are rising and, possibly, such compensation upheavals have been responsible for the formation of the Ishtar region.     

Some factors influencing variations in the rate of natural increase of population in western Maharashtra

This paper analyzes the impact of various factors on variations in the rate of natural increase of population in Western Maharashtra, India. By using district level data, coefficients of correlation have been calculated between the rate of natural increase, the birth rate and the death rate on the 1 hand and different factors affecting them on the other. The analysis shows that in Western Maharashtra the birth rate is high in response to the high death rate and the high infant mortality rate. It is also revealed that the rate of natural increase of the population declines with an increase in the proportion of working females as well as the proportion of educated males. All this signifies that the region under study is in the 2nd stage of the "demographic transition."     

Seasonal variations of atmospheric sulfur dioxide and dimethylsulfide concentrations at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric sulfur dioxide (SO₂) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37°50 S–77°30 E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (²²²Rn) as continental tracer. Average monthly SO₂ concentrations range from less than 0.2 to 3.9 nmol m^-3 (annual average = 0.7 nmol m^-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO₂ concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO₂ concentrations within ±30%. Comparing between computed and measured SO₂ concentrations allowed us to estimate a yield of SO₂ from DMS oxidation of about 70%.