JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. A12, PAGES 13,641-13,646, DECEMBER 1, 1987 The Evolution of a Coronal Streamer Prior to Mass Ejection RICHARD WOLFSON1 High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado CHARLES CONOVER2 Department of Physics, Middlebury College, Vermont RAINER M. E. [LLING 3 High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado We have developed a model describing the quasistatic evolution of a coronal helmet streamer as it is inflated with excess mass.The model producesa sequence of magnetostaticequilibria for a magnetic field configuration that includesa current sheetin the equatorial plane as well as distributed volume currents. The equilibrium solutions are given as Legendre series, and the height at which the current sheet starts is adjusted iteratively to maintain constant flux in the closed-field region. We have fit our model to SMM coronagraph/polarimeter observations showing the slow growth of a coronal streamer prior to its disruption in associationwith an eruptive prominence and mass ejection on August 18, 1980. The results suggest that the early phase of this event is consistent with the quasi-static evolution of the corona in response to the slow addition of mass to the closed-fieldregion of the streamer. 1. INTRODUCTION On August 18, 1980, a coronal mass ejection was observed with the coronagraph/polarimeter on the solar maximum mis- sion (SMM). This event was associatedwith the eruption of a prominence under a large, well-defined helmet streamer, and resulted in disruption of the streamer. The event has been describedby llling and Hundhausen[1986] (hereafter IH), with subsequent analysis by Athay and llling [1986]. IH identified three phases of the event: first, the large helmet streamer ap- peared to brighten and grow slowly during the 2 days prior to the mass ejection; second,the prominence eruption and mass ejection occurred over an approximately 6-hour period on August 18; and, third, the region refilled with material and eventually appeared to reform a helmet streamer. Our interest here is with the first--or early--phase, which IH suggested might be describedby quasistaticmodels of coronal mag- netostatic equilibrium. In coronagraph images, the early phase shows an obvious brightening and outward distension of the helmet streamer; see Figure 1. Since the process occurs very slowly (about 5 km/s, compared with the coronal Alfv•n speed of several hundred km/s), it is reasonable to model the early phase as a sequence of magnetostaticequilibria. A number of models for such quasi-static coronal evolution have been developed. Hundhausen et al. [1981] (hereafter HHZ) presented an ana- lytic model for a corona in which the Lorentz forces of a non-potential magneticfield are balanced by transverse pres- x On leave from Middlebury College Department of Physics,Ver- mont. 2Now at Department of Physics, University of Virginia, Charlottesville. 3Now at Ball Aerospace, Boulder,Colorado. Copyright 1987 by the AmericanGeophysical Union. Paper number 7A9165. 0148-0227/87/007 A-9165 $02.00 sure gradients. A sequence of magnetostatic equilibria gener- ated by this model can be interpreted as representingthe quasi-staticevolution of the corona in response to the addi- tion of excessmass. In the model of HHZ, that excessmass is added throughout the entire corona, with greater excess den- sity toward the equatorial plane. Later, Wolfson and Gold [1985] solved a nonlinear version of the HHZ equation, giving a corona in which excessmass is confined below a specified field line. They were unable to find solutions beyond a limiting value of the excess mass, and interpreted this to mean that coronal evolution might change from a quasistatic process to a rapid event like a massejection.This interpreta- tion was corroborated by calculations showing that the ab- sence of solutions occurred when the excess mass reached a value comparable to that observedin coronal mass ejections. In another adaptation of HHZ's work, Wolfson [1985] showedhow Legendreseries solutionscould be obtained for a nonpotential corona including an equatorial current sheet starting at a specified height above the coronal base and ex- tending to infinity. Since they involve both closed and open magnetic field lines, these solutions give rise to streamerlike structures. The present model is based largely on these series solutions,although we have also attempted a calculation com- bining current-sheet boundary conditionswith the nonlinear method of Wolfson and Gould [1985]. 2. MODEL AND SOLUTION PROCEDURE Our model assumes an axisymmetric corona in mag- netostatic equilibrium, with no azimuthal field component. Under these conditions the vector potential has only a single component' A = A•. HHZ combined Ampere's law with the equation of static force balanceto obtain a single equation for A' 1 c•2(ARsinO) c• ( 1 c•(ARsinO)) sin 0 c3R 2 +•-• R 2sin 0 30 = --4rcR 2 sin Oe-Slg/c21aRF(AR sin 0) (1) 13,641