3D Coronal Density Reconstruction and Retrieving the Magnetic Field Structure during Solar Minimum

Measurement of the coronal magnetic field is a crucial ingredient in understanding the nature of solar coronal phenomena at all scales. We employed STEREO/COR1 data obtained during a deep minimum of solar activity in February 2008 (Carrington Rotation CR 2066) to retrieve and analyze the three-dimensional (3D) coronal electron density in the range of heights from 1.5 to 4 R_⊙ using a tomography method. With this, we qualitatively deduced structures of the coronal magnetic field. The 3D electron-density analysis is complemented by the 3D STEREO/EUVI emissivity in the 195 Å band obtained by tomography for the same CR. A global 3D MHD model of the solar corona was used to relate the reconstructed 3D density and emissivity to open/closed magnetic-field structures. We show that the density-maximum locations can serve as an indicator of current-sheet position, while the locations of the density-gradient maximum can be a reliable indicator of coronal-hole boundaries. We find that the magnetic-field configuration during CR 2066 has a tendency to become radially open at heliocentric distances greater than 2.5 R_⊙. We also find that the potential-field model with a fixed source surface is inconsistent with the boundaries between the regions with open and closed magnetic-field structures. This indicates that the assumption of the potential nature of the coronal global magnetic field is not satisfied even during the deep solar minimum. Results of our 3D density reconstruction will help to constrain solar coronal-field models and test the accuracy of the magnetic-field approximations for coronal modeling.     

OBSERVATIONS OF FAINT,OUTLYING LOOP SYSTEMS IN LARGE FLARES

Faintly visible, darkened regions in H lying outside but adjacentto bright flare emissionwere found to occur in 10 of 31 major flares investigated. Without exception, the darkenings occur over magnetically neutral areas, and these are usually bordered by ridges ofoppositely-poled field, where one border is shared in common with a flare ribbon. Thedarkenings probably result from the formation of faint, outlying loop systems, similar topost-flare loops seen in absorption, but which are connected to magnetic features outsidethe flare and are unresolved or only marginally resolved in patrol images. Simple modelsfor post-flare loops incorporating the results of statistical equilibrium calculations readilydemonstrate that darkenings of several percent (consistent with our photometric measurements) can be produced by loop structures of cross-sectional diameter 10² km (unresolved by patrol instruments) and containing gas at densities 5 × 10¹⁰–5 × 10¹¹ cm^-3 andtemperatures 8000–15000 K. Outlying loop systems might be formed by magnetic fieldreconnection, analogous to the mechanism ascribed to eruptive two-ribbon flares, butassociated with field structures adjacent to the flare. Alternatively, these outlying loopsystems may not erupt but become visible as a result of heating and chromospheric evaporation at the footpoints shared with the flare ribbon. In either case, the observations presented here have interesting implications for both the spatial scale and the topology of thecoronal magnetic fields in which eruptions occur.     

Jumps in pulsar angular velocity and their relaxation with allowance for pinning and depinning of quantum vortex filaments

The dynamics of the rotating two-component system in the core of a neutron star is considered. Equations of motion are derived with allowance for the pinning and depinning of neutron vortices, and general solutions of these equations are found for relatively small changes in the star's angular velocity. It is shown that these solutions can describe both a jump in a pulsar's angular velocity and its subsequent relaxation. The characteristic pinning and depinning times are estimated qualitatively from observational data for jumps in the angular velocity of the Vela pulsar.Translated from Astrofizika, Vol. 39, No. 4, pp. 593–604, October–December, 1996.