On the temporal variation of the solar continuous brightness fluctuations

From a sequence of white-light photographs of solar granulation at the centre of the disk, obtained by Spectro-Stratoscope on May 17, 1975, two-dimensional spatial power spectra of photospheric intensity fluctuations were deduced. These show periodicities of 1000 s, 250–450 s (5-min oscillation), and shorter ones in the range 30–120 s. The reality of the shorter periods, however, seems to be questionable.The weighted mean wavenumber of the spatial power spectra and rms of the intensity fluctuation (I _rms) are also computed, showing the same periodicities as the power.Mitteilungen aus dem Kiepenheuer-Institut Nr. 188.On leave of absence from the Institute of Geophysics, University of Tehran.     

Interstellar clouds and the formation of stars

Part I gives a survey of the drastic revision of cosmic plasma physics which is precipitated by the exploration of the magnetosphere throughin situ measurements. The pseudo-plasma formalism, which until now has almost completely dominated theoretical astrophysics, must be replaced by an experimentally based approach involving the introduction of a number of neglected plasma phenomena, such as electric double layers, critical velocity, and pinch effect. The general belief that star light is the main ionizer is shown to be doubtful; hydromagnetic conversion of gravitational and kinetic energy may often be much more important.In Part II the revised plasma physics is applied to dark clouds and star formation. Magnetic fields do not necessarily counteract the contraction of a cloud; they may just as well pinch the cloud. Magnetic compression may be the main mechanism for forming interstellar clouds and keeping them together.Part III treats the formation of stars in a dusty cosmic plasma cloud. Star formation is due to an instability, but it is very unlikely that it has anything to do with the Jeans instability. A reasonable mechanism is that the sedimentation of dust (including solid bodies of different size) is triggering off a gravitationally assisted accretion. A stellesimal accretion analogous to the planetesimal accretion leads to the formation of a star surrounded by a very low density hollow in the cloud. Matter falling in from the cloud towards the star is the raw material for the formation of planets and satellites.The study of the evolution of a dark cloud leads to a scenario of planet formation which is reconcilable with the results obtained from studies based on solar system data. This means that the new approach to cosmical plasma physics discussed in Part I logically leads to a consistent picture of the evolution of dark clouds and the formation of solar systems.     

A new model for X-ray emission from NGC 4151

We present an analysis of the reported spectral features of NGC 4151 in X-rays. It is shown that the origin of X-rays from the source is inconsistent with a single production mechanism. We suggest a new two-component model in which soft X-rays arise from the black-body emission of a tiny hot nucleus withT2×10⁷ K and the hard X-ray photons are generated in an extended region by inverse Compton scattering of electrons with the infrared photons.     

Observations of cosmic gamma-ray bursts

The occasional occurrence of brief but intense bursts of cosmic gamma-rays was disclosed through a systematic search of data acquired from the Vela satellites. Confirmation of the nature of the events and additional detail of their characteristics has subsequently been provided by 15 other groups of experimenters with instruments on 13 spacecraft. Thirty-nine such events have been identified from data spanning a period of four and a half years. The record of intensity as a function of time varies considerably for different events, with total durations ranging from 0.1 to 60s. Time-integrated flux density ranges from 10^–6 to 10^–3 erg cm^–2. Spectral measurements have been accomplished by several groups of experimenters, showing a broad maximum in the energy distribution at about 150 keV. The distribution of source directions implies either near galactic or extragalactic locations. The existing data are not sufficient to distinguish between the various models proposed to explain the phenomenon; no model is completely consistent with all observed characteristics.Work performed under the auspices of the United States Energy Research and Development Administration.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

The atmosphere of Io from Pioneer 10 radio occultation measurements

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10⁴ elcm^?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na⁺ and 495 K if N₂⁺ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10³ elcm^?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N₂^? and 131 K for Na⁺. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10¹¹?10¹² cm³, corresponding to a surface pressure of less than 10^?8 to 10^?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.     

The absorption of soft X-rays from the Crab nebula

The X-ray spectrum of the Crab nebula has been determined in the energy range 0.5 10 keV using thin window proportional counters carried aboard a Centaur IIA rocket launched from TERLS, India. The spectrum can be well represented by a power law with an exponent?2.1 beyond 2 keV. The absorption of the soft X-ray component below 2 keV is clearly seen in the experiment. Attempts to understand quantitatively the spectral features in terms of interstellar absorption lead to a column density of hydrogen in the iirection of the Crab nebula of 3.5×10²¹ H atoms cm^?2, if we adopt a revised version of the interstellar absorption coefficients of Brown and Gould to include the contributions of heavier elements, especially of iron. This value of density is a factor of 2 higher than the density obtained from 21 cm radio observations, but falls well within the range of values for atomic and total hydrogen deducible from UV measurements with satellites and the measured visual extinction coefficients for the Crab nebula. It is concluded that it is not necessary to consider anomalous abundance of elements like carbon or neon either in the source or in the interstellar medium as suggested by some authors. The absorption of X-rays in the interstellar dust in the light of current dust models is presented.     

The quadratic Zeeman effect in the Caii H and K lines

The effects of a large magnetic field on the H and K lines of Caii are investigated incorporating both the linear and quadratic Zeeman effects for fields in the range 10⁶–10⁸ G. For these fields the linear term is the dominant one, resulting in a Paschen-Back effect with a smaller quadratic Zeeman shift superimposed on all components. Due to the partial cancellation of the quadratic effect between the upper and lower states of the lines, a much smaller quadratic Zeeman shift occurs than in the lines of other atoms previously studied at similar fields.     

Solar-interplanetary modeling: 3-D solar wind solutions in prescribed non-radial magnetic field geometries

A model is presented which describes the 3-dimensional non-radial solar wind expansion between the Sun and the Earth in a specified magnetic field configuration subject to synoptically observed plasma properties at the coronal base. In this paper, the field is taken to be potential in the inner corona based upon the Mt. Wilson magnetograph observations and radial beyond a certain chosen surface. For plasma boundary conditions at the Sun, we use deconvoluted density profiles obtained from synopticK-coronameter brightness observations. The temperature is taken to be 2 × 10⁶ K at the base of closed field lines and 1.6 x 10⁶K at the base of open field lines. For a sample calculation, we employ data taken during the period of the 12 November 1966 eclipse. Although qualitative agreement with observations at 1 AU is obtained, important discrepancies emerge which are not apparent from spherically symmetric models or those models which do not incorporate actual observations in the lower corona. These discrepancies appear to be due to two primary difficulties - the rapid geometric divergence of the open field lines in the inner corona as well as the breakdown in the validity of the Spitzer heat conduction formula even closer to the Sun than predicted by radial flow models. These two effects combine to produce conductively dominated solutions and lower velocities, densities, and field strengths at the Earth than those observed. The traditional difficulty in solar wind theory in that unrealistically small densities must be assumed at the coronal base in order to obtain observed densities at 1 AU is more than compensated for here by the rapid divergence of field lines in the inner corona. For these base conditions, the value ofβ(ratio of gas pressure to magnetic pressure) is shown to be significantly greater than one over most of the lower corona - suggesting that, for the coronal boundary conditions used here, the use of a potential or force-free magnetic field configuration may not be justified. The calculations of this paper point to the directions where future research on solar-interplanetary modelling should receive priority:

1. better models for the coronal magnetic field structure
2. improved understanding of the thermal conductivity relevant for the solar wind plasma.