Ludwig Oster

Solar irradiance modulation by active regions during 1980

The solar irradiance modulation due to active regions during 1980 has been investigated in detail. Specifically, we estimate the uncertainties caused by ground-based data used as input in the modeling effort, and by our currently incomplete knowledge of the proper parameters that describe the angular variation of sunspot and facular contrasts. We conclude that the most significant uncertainties are due to errors in area measurements and, possibly, varying spot and facular brightness. A ‘standard model’ for later use is derived by a best-fit technique of the currently available ACRIM irradiance data and the predictions of our models with appropriately varied parameters. Finally, we compute the expected irradiance for the entire year of 1980.

Growth of thermal constrictions in a weakly ionized gas discharge in helium

Neutral and charged particle densities, radial velocities, and temperatures are calculated as functions of time for cylindrical discharges in helium at pressures from 1 to 1000 Torr and a radius of 1 cm. Ionization, recombination, ambipolar diffusion, specific heat of the neutrals, and variation of gas pressure in space and time are included. Ionization of excited states, energy loss by radiation, and space charge sheaths are neglected. Under most conditions, increased ionization in regions of decreased gas density causes a constriction of the discharge which is accentuated at high pressures and high recombination rates. Greater circuit resistance stabilizes the discharge. At low pressures, the electron density profile expands. Initial steep gradients in electron density cause constrictions off axis.

The electrical conductivity in sunspot regions

The basis of conductivity calculations in the presence of magnetic fields is reviewed, covering for a solar-type plasma the full temperature range between complete ionization of hydrogen and H2-molecule formation. It is shown that both the general conductivity theory and our knowledge of momentum transfer cross-sections are now developed to the point where the resulting coefficients should be accurate to about 50% and better. This removes an often cited difficulty in model calculations for sunspots. For one specific case, a comparison is carried out with previous less accurate determinations of the conductivity.

THERMAL DISSIPATION AND ITS APPLICATION TO FLARE PHENOMENA

Cooling of stellar material by thermal conduction under solar atmospheric conditions in presence and absence of magnetic field

Pulsar geometries. III. The hollow-cone model

The relations between observable and geometrical quantities relating to the neutron star (presented in Papers I and II of this series) are extended to include the hollow-cone case, which is attractive on physical grounds. The model by Ruderman and Sutherland, with its specific predictions, has been selected for a detailed comparison of theory and observation, in particular with respect to organized subpulse drifting and objects with double-humped integrated profiles. We found that the hollow-cone model is able to reproduce qualitatively the major trends of observations, but that some crucial aspects of Ruderman and Sutherlands model are at variance with fact and require refinement, especially the predicted frequency dependences. Specific suggestions about an improvement of the physical theory are made. (AIP)

Some Applications of Detailed Balancing

The principle of detailed balancing is used in a consistent manner to derive equilibrium distribution functions, such as the Maxwell-Boltzmann and Saha-Boltzmann distributions, and the Bose-Einstein and Fermi-Dirac distributions. The same technique is then applied to matter at very high temperatures at which radiation and elementary particles are in thermodynamic equilibrium, that is, to situations postulated in supernova explosions and the early stages of an evolutionary universe.

New Restriction on Models for Quasistellar Objects

THE inverse Compton effect has recently received attention as a possible source of the continuous radiation of quasi-stellar objects1–3. Hoyle et al.4 concluded that quasi-stellar objects cannot be at cosmological distances because otherwise the inverse Compton effect would force the electrons to dump their relativistic energies in times so short that no known acceleration mechanism could be responsible for their original appearance. Modifications of this argument have been discussed by several authors5–9, in particular by Woltjer10, who considered a radiation (and electron) flux restricted to a narrow cone about the radial direction.

The emission of the quiet corona at meter wavelengths.

Quiet corona emission at meter wavelengths calculated on new radiative transfer basis, obtaining new results on radiative temperature and limb brightening

Maximum constriction phase of a weakly ionized discharge in helium

Available computer results of the time development of a low‐ionization discharge in helium are utilized to discuss the radial behavior of the density and temperature profiles as a function of external parameters in detail. It is shown that during the phase of maximum constriction the final radial profile of the electron density is determined. The profile of neutral density and temperature, by contrast, is only established at later times and depends little on external parameters. New model data are then presented which allow us to draw at least qualitative scaling conclusions concerning the dependence of the discharge quantities on external parameters.

Pulsar geometries. II. Decomposition of the radiation pattern

The model proposed in Paper I is extended to describe in detail the observed time structure of pulsar emission. It is shown with computer-generated examples that all data can be fitted to small variations of the model parameters. An attempt is made to separate the effect of geometrical parameters and their variations from the effect of differences in the physical emission mechanism. For one specific example the radiation pattern is derived on the basis of observations. Finally, a preliminary attempt is made to relate our findings to a physical emission mechanism. (AIP)