Current interruption in a collisionless plasma by non-linear electrostatic waves

The existence of current-interrupting non-linear electrostatic waves in the form of negative solitons is demonstrated. Self-consistent, non-linear electrostatic potentials are constructed assuming that a current may be interrupted by trapping current-carrying electrons in such a potential. A significant fraction of the current-carrying electrons is trapped by the potential if the electron thermal velocity is much less than the electron streaming velocity. In one class of solutions, the negative solitons, the current may be reduced to a fourth of its initial value in the limit of high ion-electron temperature ratio.     

Generation of density inhomogeneities by magnetohydrodynamic waves

We show that, in a cold plasma, one of the slow waves of the linear system is a Jordan mode, for which the density grows linearly with time. Although this mode is not present if the temperature is finite, slow waves still generate large density perturbations when the thermal sound speed is small compared with that of the fast and Alfvén waves. Numerical calculations show that non-linear steepening of a fast wave with finite but modest amplitude can readily excite this mode as long as the angle between its direction of propagation and the magnetic field is neither too large nor too small. This produces persistent inhomogeneities with a large density contrast. We suggest that this mechanism is responsible for the clumps identified in CO maps of the Rosette molecular cloud and similar ones in other giant molecular clouds. The same process may also be responsible for the formation of dense cores in the clumps.     

The local density of matter mapped by Hipparcos

We determine the velocity distribution and space density of a volume-complete sample of A and F stars, using parallaxes and proper motions from the Hipparcos satellite. We use these data to solve for the gravitational potential vertically in the local Galactic disc, by comparing the Hipparcos measured space density with predictions from various disc models. We derive an estimate of the local dynamical mass density of 0.102±0.010 pc⁻³, which may be compared with an estimate of 0.095 M_⊙ pc⁻³ in visible disc matter. Our estimate is found to be in reasonable agreement with other estimates by Crézé et al. and Pham, also based on Hipparcos data. We conclude that there is no compelling evidence for significant amounts of dark matter in the disc.     

Protostellar angular momentum transport by spiral density waves

The gravitational collapse of molecular clouds or cloud cores is expected to lead to the formation of stars that begin their lives in a state of rapid rotation. It is known that, in at least some specific cases, rapidly rotating, slf-gravitating bodies are subject to instabilities that cause them to assume ellipsoidal shapes. In this paper we investigate the consequences of such instabilities on the angular momentum evolution of a star in the process of formation from a collapsing cloud, and surrounded by a protostellar disk, with a view toward applications to the formation of the Solar System. We use a specific model of star formation to demonstrate the possibility that such a star would become unstable, that the resulting distortion of the star would generate spiral density waves in the circumstellar disk, and that the torque associated with these waves would regulate the angular momentum of the star as it feeds angular momentum to the disk. We conclude that the angular momentum so transported to the disk would not spread the disk to, say, Solar System dimensions, by the action of the spiral density waves alone. However, a viscous disk could effectively extract stellar angular momentum and attain Solar System size. Our results also indicate that viscous disks could feed mass and angular momentum to a growing protostar in such a manner that distortions of the star would occur before gravitational torques could balance the influx of angular momentum. In other situations (in which the viscosity was small), a gap could be cleared between the disk and star.     

Equatorial ionospheric plasma density bubbles observed by ESRO-4

It is shown that the ESRO-4 satellite, in a polar orbit, was able to observe equatorial plasma bubbles using the fixed-bias total ion current probe. Experimental conditions were suitable for observations during a period at the vernal equinox 1973, when 21 plasma density depletion events were identified, extending across the equator. Statistical analysis suggests that the bubble cross-sections, transformed on to the zero-dip surface, were elongated, with an axial ratio of the order of 7:1, and were tilted upwards by about 10° to the east. The observations were made selectively near the F-layer peak; no movement of bubbles was measured.     

The column density structure of Orion A depicted by N-PDF

We have conducted a large-field simultaneous survey of ~(12) CO,~(13) CO and C~(18)O J=1-0 emission toward the Orion A giant molecular cloud(GMC) with a sky coverage of~4.4 deg~2 using the Purple Mountain Observatory(PMO)-13.7 m millimeter-wavelength telescope.We use the probability distribution function of the column density(N-PDF) to investigate the distribution of molecular hydrogen in the Orion A GMC.The H_2 column density,derived from the ~(13)CO emission,of the GMC is dominated by a log-normal distribution in the range from~4×10~(21) to~1.5×10~(23) cm~(-2) with excesses both at the low-density and high-density ends.The excess of the low-density end is possibly caused by an extended and low-temperature(~10 K) component with velocities in the range of 5-8 km s~(-1).Compared with the northern sub-regions,the southern sub-regions of the Orion A GMC contain less gas with column density in N_(H_2) 1.25×10~(22) cm~(-2).The dispersions of the N-PDFs of the sub-regions are found to correlate with the evolutionary stages of the clouds across the Orion A GMC.The structure hierarchy of Orion A GMC is explored with the DENDROGRAM algorithm,and it is found that the GMC is composed of two branches.All structures except one in the tree have virial parameters less than 2,indicating self-gravity is important on the spatial scales from~0.3 to~4 pc.Although power-laws and departures from lognormal distributions are found at the high-density end of N-PDFs for active star-forming regions,the N-PDFs of structures in the Orion A GMC are predominantly lognormal on scales from R~0.4 to 4 pc.     

Radial heating of a galactic disc by multiple spiral density waves

We consider a differentially rotating, 2D stellar disc perturbed by two steady-state spiral density waves moving at different pattern speeds. Our investigation is based on direct numerical integration of initially circular test-particle orbits. We examine a range of spiral strengths and spiral speeds and show that stars in this time-dependent gravitational field can be heated (their random motions increased). This is particularly noticeable in the simultaneous propagation of a two-armed spiral density wave near the corotation resonance (CR), and a weak four-armed one near the inner and outer 4:1 Lindblad resonances. In simulations with two spiral waves moving at different pattern speeds, we find: (i) the variance of the radial velocity,  σ² _R   , exceeds the sum of the variances measured from simulations with each individual pattern; (ii)  σ² _R   can grow with time throughout the entire simulation; (iii)  σ² _R   is increased over a wider range of radii compared to that seen with one spiral pattern; and (iv) particles diffuse radially in real space, whereas they do not when only one spiral density wave is present. Near the CR with the stronger, two-armed pattern, test-particles are observed to migrate radially. These effects take place at or near resonances of both spirals, so we interpret them as the result of stochastic motions. This provides a possible new mechanism for increasing the stellar velocity dispersion in galactic discs. If multiple spiral patterns are present in the Galaxy, we predict that there should be large variations in the stellar velocity dispersion as a function of radius.     

Ionospheric temperature and density measurements by means of spherical double probes

Rocket-borne double probes for electric field measurements can be intermittently operated in special, diagnostic modes involving current bias and low-impedance shunts to obtain information on the properties of the ambient ionospheric plasma along the flight path. Several such modes, and the information that they can provide, are analyzed. For example, in a low-impedance mode with asymmetric bias, the attenuation ratio (i.e. signal amplitude in this mode over the signal amplitude in the electric-field measuring mode) is in a simple way related to the electron temperature of the ambient plasma. The special surface coatings (Aquadag or vitreous carbon) normally used for electric field probes provide very homogeneous surface properties, a feature which also contributes to the reliability of the electron temperature measurements. In addition to electron temperature, the modes analyzed can be used to measure electron density and to give some information on ion temperature. The data from four rocket flights from ESRANGE are discussed in the light of these results. Electron temperature was measured in three of these flights. In all cases the temperature profile is in good agreement with theoretically predicted profiles based on the CIRA 1965 reference atmosphere and the solar illumination prevailing during the respective flights (twilight). Electron density profiles obtained by means of the double probe are in good agreement with the density measured by the Langmuir probe in the two flights for which both kinds of data are available. They are also in agreement with the electron density data available from ionosondes. Finally, pulses occurring when one of the probes passed through the rocket's shadow, are used to determine the photoelectron yield of the probe coatings (Aquadag or vitreous carbon). The values obtained, (7 ± 3) × 10^?6 A/m² for Aquadag and (4 ± 2) × 10^?6 A/m² for vitreous carbon are in good agreement with expectations based on laboratory data and solar Lyman α radiation.     

Is the present stellar birthrate really determined by gas density?

Observed supernovae rates in Sb and Sc galaxies, and a recent re-examination of the mean gas density in these galactic types, implies that if the clumpiness of gas in the disks of Sb and Sc galaxies is similar, the gas density isnot the primary factor in determining the overall present stellar birthrate.     

On the electron density in an active region observed by SERTS

Emission lines from an active region, observed by SERTS, have been used to determine electron densities from theoretical curves for Mgvii, Siviii, and Siix density-sensitive line ratios. Density diagnostics of Alviii 285.46/323.52 line emissivity ratio has also been investigated.     

Spiral density waves illustrated by the superposition of periodic stellar orbits

Plane, periodic stellar orbits in the spiral gravitational field of the Galaxy superimposed on the axisymmetric background potential are studied in the epicyclic approximation. The superposition of such orbits is illustrated to demonstrate the response of the stellar system to an imposed spiral density perturbation.     

Quenching of the beam-plasma instability by large-scale density fluctuations in 3 dimensions

A model is presented to explain the highly variable yet low level of Langmuir waves measured in situ by spacecraft when electron beams associated with type III solar bursts are passing by; the low level of excited waves allows the propagation of such streams from the Sun to well past 1 AU without catastrophic energy losses. The model is based, first, on the existence of large-scale density fluctuations that are able to efficiently diffuse small-k beam-unstable Langmuir waves in phase space, and, second, on the presence of a significant isotropic non-thermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The strength of the model lies in its ability to predict various levels of Langmuir waves depending on the parameters. This feature is consistent with the high variability actually observed in the measurements. The calculations indicate that, for realistic parameters, the most unstable, small k modes are fully stabilized while some oblique mode with higher k and lower growth rate might remain unstable; thus a very broad range of levels of Langmuir waves is possible from levels of the order of enhanced spontaneous emission to the threshold level for nonlinear processes. On the other hand, from in situ measurements of the density fluctuations spectrum by ISEE-1 and 2 in the vicinity of the Earth, it is shown that measured 100 km scale fluctuations may be too effective in quenching the instability. If such strong density fluctuations are common in the solar wind, we show they must be highly anisotropic in order to allow the build-up of Langmuir waves to the observed mV m^–1 range. Moreover, the anisotropy must be such that the strongest variations of density occur in a plane perpendicular to the magnetic field.     

Cosmic ray modulation produced by radial density gradients in the interplanetary medium

A new technique for the calculation of cosmic ray daily variations, given the radial number density gradient, is introduced. The technique is applied to selected gradients to account for the average, enhanced and asymmetrical diurnal variations and different Forbush decrease profiles. A small semidiurnal variation is predicted and it is concluded that the profiles of Forbush decreases are strongly dependent on the radial approach velocity of the shock.     

Interstellar electron density

We impose the requirement that the spatial distribution of pulsars deduced from their dispersion measures using a model of the galactic electron density (n _e ) should be consistent with cylindrical symmetry around the galactic centre (assumed to be 10 kpc from the Sun). Using a carefully selected subsample of the pulsars detected by the II Molonglo Survey (II MS), we test a number of simple models and conclude that (i) the effective mean 〈n_e〉) for the whole galaxy is 0.037_-0.012 ^+0.020 cm^-3, (ii) the scale height of electrons is greater than 300 pc and probably about 1 kpc or more, and (iii) there is little evidence for variation of n_e with galactic radius R_GC for R_Gc ≳ 5 kpc. Further, we make a detailed analysis of the contribution to n_e from H II regions. Combining the results of a number of relatively independent calculations, we propose a model for the galactic electron density of the formn _e (z) = 0.030 + 0.020 exp (- |z|/70) cm^-3 where z(pc) is the height above the galactic plane and the second term describes the contribution from H II regions. We believe the statistical uncertainties in the parameters of this model are quite small.     

Determination of refraction anomalies by global inclinations of airstratas of identical density

This paper presents formulas for the calculation of the refraction anomalies caused by the inclination of atmospheric boundary layers. Anomalies were calculated for a few zenith distances for several different atmospheric models. It was established, that near Earth the atmospheric boundary layers have the global inclination in meridian plane near one minute of arc from North to South. They are calculated with standard deviation ±0.2′–±0.35′. The tilts are decreased gradually with the altitude and equal nearly 0 on the heights 8–10 km. Then direction of inclination is changed on opposite (from South to North) and maximum 1′ reaches on the heights 15–18 km. Next inclinations slowly decrease and equal 0 on the heights 28–30 km. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The formation of primordial stars triggered by the evolution of large-scale density perturbations

This paper summarizes the basic idea of coupling of density perturbation modes in Newtonian cosmology and the formation of primordial stars. The way of deriving the second-order differential equation governing the growth of two coupled adiabatic density perturbation modes in the matter era is briefly discussed. A Jeans criterion for the growth of short wavelength perturbations is given. A mechanism is proposed for the cosmological origin of Population III stars.