Models of interstellar dust alignment assume that dust grains are elongated, but none of these models explain why dust grains should be elongated. On the other hand, models of interstellar dust grain ...growth assume that dust grains are spherical and not elongated. We show that when dusty plasma effects and the dipole moment of water molecules are together taken into account, ice grains in interstellar space should be prolate ellipsoids and not spheres. Dusty plasma analysis shows that an ice grain is charged to a negative potential that has magnitude nearly equal to that of the electron temperature. Several different mechanisms causing deviation from sphericity are identified; these mechanisms involve the interaction of the dipole moment of water molecules with electric fields associated with ice grain charging. These mechanisms include the focusing of water molecule trajectories, the migration of water molecules in a quasi-liquid layer on the grain surface toward regions where the electric field is strongest, the enhancement of this migration by the bombardment of energetic protons that gain energy upon falling into the ice grain negative potential, and mutual repulsion by electric charges having the same sign. The aspect ratio is established shortly after the ice grain is formed, and then is maintained as the grain grows.
A practical method is proposed for determining the wave vector of waves from single‐spacecraft measurements. This wave vector knowledge can then be used to remove the space‐time ambiguity produced by ...frequency Doppler shift associated with spacecraft motion. The method involves applying the Wiener‐Khinchin theorem to cross correlations of the current and magnetic field oscillations and to autocorrelations of the magnetic field oscillations. The method requires that each wave frequency component map to a unique wave vector, a condition presumed true in many spacecraft measurement situations. Examples validating the method are presented.
Key Points
I present a method for measuring wave vector using a single spacecraft
The method resolves space‐time ambiguity of single space‐craft measurements
The plasma‐frame wave frequency can be deduced from a moving spacecraft
An analytic model of the time-dependent electric and magnetic fields of an astrophysical jet is presented. These fields satisfy the time-dependent Faraday's law and describe a jet with increasing ...length. The electric field contains both electrostatic and inductive parts. The electrostatic part corresponds to the rate of injection of toroidal magnetic flux, while the sum of the electrostatic and inductive parts results in the electric field parallel to the magnetic field being zero everywhere. The pinch force associated with the electric current provides a peaked pressure on the jet axis and a pressure minimum at the radius where the poloidal magnetic field reverses direction.
Ions and neutrals in the weakly ionized plasma of an accretion disc are tightly bound because of the high ion–neutral collision frequency. A cluster of a statistically large number of ions and ...neutrals behaves as a fluid element having the charge of the ions and the mass of the neutrals. This fluid element is effectively a metaparticle having such an extremely small charge-to-mass ratio that its cyclotron frequency can be of the order of the Kepler angular frequency. In this case, metaparticles with a critical charge-to-mass ratio can have zero canonical angular momentum. Zero canonical angular momentum metaparticles experience no centrifugal force and spiral inwards towards the central body. Accumulation of these inward spiralling metaparticles near the central body produces radially and axially outward electric fields. The axially outward electric field drives an out-of-plane poloidal electric current along arched poloidal flux surfaces in the highly ionized volume outside the disc. This out-of-plane current and its associated magnetic field produce forces that drive bidirectional astrophysical jets flowing normal to and away from the disc. The poloidal electric current circuit removes angular momentum from the accreting mass and deposits this removed angular momentum at near infinite radius in the disc plane. The disc region is an electric power source (
$\boldsymbol {E}\cdot \boldsymbol {J} <0$
) while the jet region is an electric power sink (
$\boldsymbol {E}\cdot \boldsymbol {J}>0$
).
It is shown that the low frequency plasma wave equation can be obtained much more directly than by the previously used method of solving for the determinant of a matrix involving the three components ...of the electric field vector. The more direct method uses a two‐dimensional current density vector space that is precisely equivalent to the previously used three‐dimensional electric field vector space. Unlike the electric field, the current density is restricted by the quasi‐neutrality condition to a two‐dimensional vector space. Comparison with previously obtained dispersion relations is provided and a method is presented for obtaining exact analytic solutions for the three roots of the cubic dispersion relation. The commonly used kinetic Alfvén dispersion relation is shown to be valid only for near‐perpendicular propagation in a low beta plasma. It is shown that at a cross‐over point where the perpendicular wave phase velocity equals the ion acoustic velocity, the coupling between Alfvén and fast modes vanishes and the Alfvén mode reverts to its cold form even in situations where the Alfvén velocity is smaller than the electron thermal velocity. A method is prescribed by which measurement of wave electric current density completely eliminates the space‐time ambiguity previously believed to be an unavoidable shortcoming of single‐spacecraft frequency measurements.
Key Points
Transparent derivation of complicated dispersion relation
Previously unknown decoupling of Alfven wave from fast wave
Demonstration of error and limitations of previous models
Analytic solutions are presented for the orbit of a charged particle in the combination of a uniform axial magnetic field and parabolic electrostatic potential. These trajectories are shown to ...correspond to the sum of two individually rotating vectors with one vector rotating at a constant fast frequency and the other rotating in the same sense but with a constant slow frequency. These solutions are related to Penning trap orbits and to stochastic orbits. If the lengths of the two rotating vectors are identical, the particle has zero canonical angular momentum in which case the particle orbit will traverse the origin. If the potential has an inverse dependence on distance from the source of the potential, the particle can impact the source. Axis-encircling orbits are where the length of the vector associated with the fast frequency is longer than the vector associated with the slow frequency. Non-axis-encircling orbits are the other way around.
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