THE LARGE-SCALE DENSITY STRUCTURE OF THE SOLAR CORONA AND THE HELIOSPHERIC CURRENT SHEET

We have investigated the three-dimensional distribution of the polariz ation-brightness product (pB) and then quantitatively determined the e lectron density distribution relative to the inferred heliographic cur rent sheet during the declining phase of solar cycle 20 (1973-1976). T he current sheet is taken as the center of the bright, dense structure s from combined synoptic pB data from ground-based K-coronameter and t he white-light coronagraph aboard Skylab. Analyses of pB scans as a fu nction of minimum distance from the current sheet (theta(min)) over th e radial distance range 1.13 to 5.0 R. (from Sun center) led to the fo llowing new results: (1) a quantitative description of pB obtained aro und the inferred neutral line is given by the following equation: pB(r ho, theta(min)) = pB(p)(rho) + [pB(cs)(rho) - pB(p)(rho)]e(-theta min2 /w2(r)), where rho is the shortest distance to the line of sight from the Sun center, pB(cs)(rho) and pB(p)(rho) are the observed polarized brightness at the current sheet and the poles, respectively, and w(r) is the half-width of the distribution; (2) the electron density obtain ed by inverting the pB data is given by N(r, theta(mg)) = N-p(r) + [N- cs(r) - N-p(r)]e(-theta mg2/w2(r)), where N(r, theta(mg)) is the numbe r of free electrons per cm(3), N-cs(r) and N-p(r) are the electron den sities at the current sheet and the poles, respectively, and theta(mg) is the magnetic latitude. Here theta(mg) is given by theta mg = sin(- 1) [-cos theta sin alpha sin (phi -phi(o)) + sin theta cis alpha], whe re theta and phi are heliographic latitude and longitude, alpha is the tilt angle of the dipole axis with the rotation axis, and phi(o) is t he intersecion of the heliomagnetic and heliographic equators; (3) dur ing the period studied (the last third of the solar cycle), the mean p B at the current sheet and above the polar holes is approximately inde pendent of the phase of the solar cycle; and (4) the organization of p B data about the neutral line allows inference of the boundary of the polar coronal holes. The usefulness of one-dimensional white-light den sity constraint in solar wind modeling has already been demonstrated b y Habbal et al. The present three-dimensional model should prove very useful in better understanding of the global hydromagnetic structure o f the corona and the solar wind, relating as it does to the magnetic s tructure of the corona, as opposed to heliocentric coordinates. For ex ample, the density model could provide constraints on coronal temperat ure, flow velocity, and magnetic structure subject to a suitable analy sis of geometric effects, which in turn would provide constraints on e nergy balance in the coronal expansion.