Hannes Alfvén

Existence of Electromagnetic-Hydrodynamic Waves

IF a conducting liquid is placed in a constant magnetic field, every motion of the liquid gives rise to an E.M.F. which produces electric currents. Owing to the magnetic field, these currents give mechanical forces which change the state of motion of the liquid. Thus a kind of combined electromagnetic-hydro-dynamic wave is produced which, so far as I know, has as yet attracted no attention.

Fractionation and condensation in space

Abstract Observations in space suggest that the primordial condensation of solids in our solar system took place from a low density, partially excited gas, and that the gas temperatures were much higher than the temperatures of the solid grains growing from this medium. Laboratory simulation of such condensation processes has provided information on the characteristics of the ensuing solids. The materials in meteorites have properties which in some cases permit, in others strongly suggest or require the assumption that they are primary and largely unaltered solids grown in extreme thermal disequilibrium with the surrounding gas phase.

Experiments with plasma rings

The construction of a coaxial plasma gun is described. At its output end the gun is provided with a radial magnetic field, which is trapped by the plasma. The plasma which is shot out from the gun is studied by photographic and magnetic methods. It is demonstrated that the gun produces magnetized plasma rings with the same basic structure as the rings obtained in toroidal pinch experiments. When the the plasma rings are formed, the magnetic field lines from the gun break, a result which is of interest from a theoretical point of view.

Double Layers and Circuits in Astrophysics

As the rate of energy release in a double layer with voltage ¿V is P ¿ I¿V, a double layer must be treated as a part of a circuit which delivers the current I. As neither double layer nor circuit can be derived from magnetofluid models of a plasma, such models are useless for treating energy transfer by means of double layers. They must be replaced by particle models and circuit theory. A simple circuit is suggested which is applied to the energizing of auroral particles, to solar flares, and to intergalactic double radio sources. Application to the heliospheric current systems leads to the prediction of two double layers on the suns axis which may give radiations detectable from Earth. Double layers in space should be classified as a new type of celestial object (one example is the double radio sources). It is tentatively suggested that X-ray and ¿-ray bursts may be due to exploding double layers (although annihilation is an alternative energy source). A study of how a number of the most used textbooks in astrophysics treat important concepts such as double layers, critical velocity, pinch effects, and circuits is made. It is found that students using these textbooks remain essentially ignorant of even the existence of these concepts, despite the fact that some of them have been well known for half a century (e.g., double layers, Langmuir, 1929; pinch effect, Bennet, 1934).

Asteroidal jet streams

Detailed studies of the asteroidal belt are of importance for clarifying whether the asteroids are fragments of an exploded planet or represent an intermediate state in the accretion of planets.A study of the Hirayama family Flora shows that it contains three groups of bodies travelling in almost identical orbits, thus constituting three jet streams.It is shown that the formation of jet streams is difficult to reconcile with the exploded planet view.In order to decide whether the formation of jet streams is a general phenomenon in the asteroidal belt, it is necessary to determine the orbits of a large number of small asteroids.

ORIGIN AND EVOLUTION OF THE SOLAR SYSTEM. II.

Abstract(7)Formation of celestial bodies. The basic concepts of the accretional process are discussed, and the inadequacy of the contractional model is pointed out. A comparison is made between the general pre-planetary state on the one hand and the present state in the asteroidal region on the other. A model for accretion of resonance-captured grains leading to the formation of resonance-captured planets and satellites is suggested.(8)Spin and accretion. The relation between the accretional process and the spin of planets is analyzed.(9)Accretion of planets and satellites. It is shown that jet streams are a necessary intermediate stage in the formation of celestial bodies. The time sequence of planet formation is analyzed, and it is shown that the newly accreted bodies have a characteristic internal heat structure; the cases of the Earth and the Moon are considered in detail. A region of high initial temperature is found at 0.4 of the present Earth radius, whereas the culminating temperature of the Moon is near its present surface. An accretional heat wave is found to proceed outwards, and may produce the observed differentiation features.

The Plasma Universe

For millennia we have based our views of the universe on observations in the narrow visual octave of the electromagnetic spectrum, 400–800 nm, supplemented during the last half‐century by infrared and radio observations. During the last decade, however, space research has opened the full spectrum, including the entire infrared region and the ultraviolet, x‐ray and γ‐ray regions (see the photo on the cover and figure 1).

On the origin of the asteroids

Abstract Photometric measurements of asteroids show that they rotate with periods of 5–10 hr. The rotational period does not vary in a systematic way with the size of the asteroid, and it is of the same order as the periods of the ordinary planets. It is pointed out that it is difficult to reconcile this fact with the current hpothesis that the asteroids are fragments of a “broken-up planet”. Instead one must assume that there must be some process by which all planets-including asteroids-obtain this period of rotation when they are formed by condensation of interplanetary material.

The early history of the Moon and the Earth

Abstract Arguments are given for the view that the Moon originally was a planet, which was captured by the Earth. According to Gerstenkorn the capture took place in a retrograde orbit, which by the tides was changed to a polar orbit and later to the present direct orbit. During this process the Moon reached the Roche limit and a break-up took place. The results for the Earth (possibly the formation of continents) and for the Moon (possibly the formation of the lunar craters) are discussed.

Jet streams in space

Ifviscosity is taken into account, Keplerian motion of a large number of grains in a gravitational field has a tendency to lead to the formation ofjet streams.In order to treat problems of this kind it is advantageous to present celestial mechanics by a simple perturbation approach which is developed in Sections 2–7.Inelastic collisions between a number of grains will tend to make their orbits similar. This leads to the formation of jet streams. Their properties are treated in Sections 8–10.Finally, in Section 11, we discuss the possible application of the jet stream theory to meteor streams, to asteroidal jet streams, and to the cosmogonic accretion process.