L-shell

Abstract: This paper presents a summary of all 27 available sensitive Zeeman measurements of magnetic field strengths in molecular clouds together with other relevant physical parameters. From these data input parameters to magnetic star formation theory are calculated, and predictions of theory are compared with observations. Results for this cloud sample are the following: (1) Internal motions are supersonic but approximately equal to the Alfv?n speed, which suggests that supersonic motions are likely MHD waves. (2) The ratio of thermal to magnetic pressures ?p ? 0.04, implying that magnetic fields are important in the physics of molecular clouds. (3) The mass-to-magnetic flux ratio is about twice critical, which suggests but does not require that static magnetic fields alone are insufficient to support clouds against gravity. (4) Kinetic and magnetic energies are approximately equal, which suggests that static magnetic fields and MHD waves are roughly equally important in cloud energetics. (5) Magnetic field strengths scale with gas densities as |B| ?? with ? ? 0.47; this agrees with the prediction of ambipolar diffusion driven star formation, but this scaling may also be predicted simply by Alfv?nic motions. The measurements of magnetic field strengths in molecular clouds make it clear that magnetic fields are a crucial component of the physics governing cloud evolution and star formation.

Abstract: Magnetic clouds are defined as regions;with a radial dimension roughly-equal0.25 AU (at 1 AU) in which the magnetic field strength is high and the magnetic field direction changes appreciably by means of rotation of one component of B> nearly parallel to a plane. The magnetic field geometry in such a magnetic cloud is consistent with that of a magnetic loop, but it cannot be determined uniquely. Forty-five clouds were identified in interplanetary data obtained near earth between 1967 and 1978; at least one cloud passed the earth every 3 months. Three classes of clouds were identified, corresponding to the association of a cloud with a shock, a stream interface, or a CME. There are approximately equal numbers of clouds in each class, and the field and plasma parameters in each class are similar, suggesting that the three types of clouds might be different manifestations of a single phenomenon (e.g., a coronal transient). Interface-associated clouds may have been swept up by corotating streams. Shock-associated clouds move faster than the other two types, which are basically slow flows. The magnetic pressure inside the clouds is higher than the ion pressure, and the sum is higher than the pressure of the material outsidemore » of the cloud. This implies that the magnetic clouds were expanding even at 1 AU, and the average expansion speed is estimated to be of the order of half the ambient Alfven speed.« less

Abstract: We present a new analysis of the transport of cosmic rays in a turbulent magnetic field that varies in all three spatial dimensions. The analysis utilizes a numerical simulation that integrates the trajectories of an ensemble of test particles from which we obtain diffusion coefficients based on the particle motions. We find that the diffusion coefficient parallel to the mean magnetic field is consistent with values deduced from quasi-linear theory, in agreement with earlier work. The more interesting and less understood transport perpendicular to the average magnetic field is found to be enhanced (above the classical scattering result) by the random walk, or braiding, of the magnetic field. The value of κ⊥ obtained is generally larger than the classical scattering value but smaller than the quasi-linear value. The computed values of κ⊥/κ∥, for a representation of the interplanetary magnetic field, are 0.02-0.04; these values are of the same general magnitude as those assumed in recent numerical simulations of cosmic-ray modulation and transport in the heliosphere, and give reasonable agreement with spacecraft observations of cosmic rays. Some consequences of these results for the interpretation of heliospheric observations are discussed.

Abstract: In this paper a number of problems are considered which are related to the gravitational stability of cosmical masses of infinite electrical conductivity in which there is a prevalent magnetic field. In Section I the virial theorem is extended to include the magnetic terms in the equations of motion, and it is shown that when the magnetic energy exceeds the numerical value of the gravitational potential energy, the configuration becomes dynamically unstable. It is suggested that the relatively long periods of the magnetic variables may be due to the magnetic energy of these stars approaching the limit set by the virial theorem. In Section II the adiabatic radial pulsations of an infinite cylinder along the axis of which a magnetic field is acting is considered. An explicit expression for the period is obtained. Section III is devoted to an investigation of the stability for transverse oscillations of an infinite cylinder of incompressible fluid when there is a uniform magnetic field acting in the direction of the axis. It is shown that the cylinder is unstable for all periodic deformations of the boundary with wave lengths exceeding a certain critical value, depending on the strength of the field. The wave length of maximum instability is also determined. It is found that the magnetic field has a stabilizing effect both in increasing the wave length of maximum instability and in prolonging the time needed for the instability to manifest itself. For a cylinder of radius R = 250 parsecs and p = 2 × 10-24 gm/cm3 a magnetic field in excess of 7 × l0-6 gauss effectively removes the instability. In Section IV it is shown that a fluid sphere with a uniform magnetic field inside and a dipole field outside is not a configuration of equilibrium and that it will tend to become oblate by contracting in the direction of the field. Finally, in Section V the gravitational instability of an infinite homogeneous medium in the presence of a magnetic field is considered, and it is shown that jeans's condition is unaffected by the presence of the field.