Definition of the Electro-Magnetic StateThe electro-magnetic state in a certain volume is defined if we know the electric field E and the magnetic field H as functions of space and time. However, because of the first Maxwell equationthe magnetic field variations are defined by the electric current (comprising the current density i and the displacement current aD/Ot). Hence a description in terms of currents is possible, and often physically more interesting than a description in terms of magnetic fields. The displacement current is of importance only for frequencies of the order of the plasma frequency or higher. As in this paper we describe stationary or slowly varying phenomena, we shall neglect it. Measurements of Magnetic Fields and CurrentsIn astrophysics no direct measurements of electric fields have yet been made. All conclusions about electric fields have been reached in an indirect way. The magnetic fields are much better known, because it is possible to measure them by means of the Zeeman effect or the Faraday rotation. However, up to quite recently it was essentially the longitudinal effects which were measured. Of the six electro-magnetic vector components only one, viz. the magnetic component along the line of sight, was measured, so no real understanding of the electro-magnetic state has been possible. Important progress in the knowledge of solar electro-magnetic conditions was recently made by SEVERNY (1964SEVERNY ( , 1965 with systematic measurements of the transverse Zeeman effect. In active regions, including sunspots, he has sometimes found a remarkable rotation of the transverse component H~ and a change of its magnitude from one point to another.If the z-axis of an orthogonal coordinate system points along the line of sight we can calculate the current component iz from = 4rcL ay J

Aims. We investigate the structure and velocity of cold molecular pillars, "elephant trunks", in expanding H ii regions.Methods. The trunks are seen in silhouette against the bright background in our Hα images. All trunks are filamentary, and show signs of being twisted. Four such trunks in NGC 7822, IC 1805, the Rosette Nebula, and DWB 44 were selected, and then mapped mainly in 12 CO and 13 CO. We determine the mass and density of the trunks. Most of the mass is concentrated in a head facing the central cluster, and in sub-filaments forming the body of the trunk that is connected to V-shaped filaments to the outer expanding shell. Results. We discovered that all four trunks rotate as rigid bodies (to a first approximation) about their major axes, and that at least two trunks are stretching along their major axes, meaning that the massive heads are lagging behind in the general expansion of the H ii regions. The rotational periods are of the order of a few million years -similar to the age of the clusters. Rotation, then, is responsible for the twisted appearance of many elephant trunks, since they are rooted in the outer shells. The trunks carry surprisingly large amounts of angular momentum, 3 × 10 48 −2 × 10 50 kg m 2 s −1 , with corresponding rotational energies of up to ∼10 37 J. However, we estimate the total magnetic energies to be even larger. The trunks continuously reshape, and the formation of twined, and in many cases helical, sub-filaments can be understood as a consequence of electromagnetic and inertia forces inside the trunks. A theory based on the concept of magnetically twisted trunks is developed further, where the initial angular momentum is a consequence of the twisting of parent filaments containing mass condensations. Our results also suggest a new process of removing angular momentum from parent molecular clouds.

Observations have shown that field‐aligned currents above auroras reach such magnitudes that space‐charge regions of high potential drops can be expected. Effects of the electric fields associated with such a space‐charge region are acceleration of auroral electrons and horizontal motions in the visual aurora.

A flare model based on force-free currents in the solar atrnosphere is considered. The energy of the flare is supposed to be stored as magnetic energy in the current system, tf the current density exceeds a certain critical limit an over-voltage may arise in the circuit which will give rise to a rapid release of the stored energy. At the end of the paper some results yielded by the model are compared with observational evidence of flares.

No abstract