The topological association of Hα structures and magnetic fields

The topological associations between H structures and magnetic fields are examined for an active region observed on two different dates. The structures seen in the on and off band of H filtergrams are compared with the contour maps of magnetic fields at the level of magnetogram observations. Similar comparisons are made also with the configurations of force-free magnetic lines of force at various heights evaluated with the use of formulations developed previously by Nakagawa and Raadu (1972).Among the results of significance, we may note that (1) H plages could be identified with regions of magnetic field larger than ±80 G, (2) the network of bright dots seen in H -1 Å filtergrams follow closely ±80 G contours. (3) stable prominences lie along either neutral lines or valleys of magnetic fields, (4) the configuration of magnetic lines of force shows discrete domain structures suggesting bipolar nature of local magnetic fields, and (5) within a domain the configuration is governed apparently by evalutional consequences. Details of analyses are described with discussions on the limitations and possible future improvements.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Preflare current sheets in the solar atmosphere

Neutral current sheets are expected to form in the solar atmosphere when photospheric motions or the emergence of new magnetic flux causes oppositely directed magnetic fields to be pressed together. Magnetic energy may thus be stored slowly in excess of the minimum energy associated with a purely potential field and released suddenly during a solar flare. For simplicity, we investigate the neutral sheet which forms between two parallel line dipoles when either the distance between them decreases or their dipole moments increase. It is found that, when the dipoles have approached by an amount equal to a tenth of their original separation distance, the stored energy is comparable with that released in a major flare. In addition, a similarity solution for one-dimensional magnetohydro-dynamic flow within such a neutral sheet is presented; it demonstrates that rapid conversion of magnetic energy into heat is possible provided conditions at the edge of the neutral sheet are changing sufficiently quickly.     

On the topology of filaments and chromospheric fibrils near sunspots

The similarity between the spiral topology of chromospheric fibrils and filaments observed in H near sunspots and the configuration of an axisymmetric force-free magnetic field is examined. It is suggested that some of the observed features could be interpreted in terms of the configuration of lines of force of an axisymmetric force-free chromospheric magnetic field. Implications of the results of analysis to the possible interpretations of other observed topological features near a sunspot are discussed.Visiting scientist at the High Altitude Observatory.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Sponsored by the Office of Naval Research, Contract No. N 00014-67-C-0290.     

Electrostatic double layers and a plasma evacuation process

An evacuation process due to the growth of current driven instabilties in a plasma is discussed. The process, which leads to localized extreme density reductions, is related to the formation of electrostatic double layers. The initial linear phase is treated using the superposition of unstable plasma waves. In the long wavelength, non-dispersive limit, a density dip — which is initially present as a small disturbance — grows rapidly and remains localized in the plasma. The process works for a variety of plasma conditions provided a certain current density is exceeded. For a particular choice of plasma parameters the non-linear development is followed, by solving the coupled Vlasov-Poisson equations by finite difference methods. The evacuation process is found to work even more effectively in the non-linear phase and leads to an extreme density reduction within the dip. It is suggested that the growth of such structures produces weak points within the plasma that can lead to the formation of double layers.     

Differential rotation and the structure and energy content of coronal magnetic fields

It is argued that differential rotation of the photospheric magnetic fields will induce currents in the corona. The work done against surface magnetic stresses will increase the energy content of the coronal magnetic field. The electrical conductivities are high and the foot points of field lines move with the differential rotation. The force-free field equations are solved with this constraint to obtain a minimum estimate of the energy increase for a quadrupole field. During a solar rotation the magnetic energy increases by 25%. Local release of this energy in the corona would have a significant effect. The expansion of field lines as a result of the differential rotation should increase the amount of flux and the field strength in the solar wind region.The National Center for Atmospheric Research is sponsored by the National Science Foundation.