THE SHAPE OF THE OUTER CORONA DURING CYCLE-21

The Naval Research Laboratory (NRL) Solwind coronagraph recorded the o uter corona at elongations 2.5 R-. to 10 R-. during the 6 1/2-year int erval from March 1979, before solar maximum, to the beginning of solar minimum in September 1985. During the minimum period, when the solar magnetic field was dipole-like, the observed corona consisted of the e quatorial streamer belt that is characteristic of solar minimum, and t hat is interpreted as an edgewise view of a nearly flat current sheet or coronal disk lying near the plane of the heliographic equator. The observed disk was a radial projection from the magnetic neutral line t hat was computed for the 2.5 R-. source surface surrounding the Sun. A t earlier times, shortly after solar maximum, the observed corona ofte n consisted of a single coronal disk similar to that at solar minimum, but strongly tilted to the heliographic equator. Again this disk proj ected from a tilted magnetic neutral line that was computed for the 2. 5 R-. source surface. Solar rotation allowed this coronal disk to be v iewed in all aspects. In the edgewise view it appeared as a tilted str eamer belt. In the broadside view the more flower-like pattern of sola r maximum was observed. The latter view was interpreted as a non-unifo rm distribution of coronal material in the thin coronal disk. There we re many intervals during the declining phase of the solar cycle when t he computed magnetic neutral line at 2.5 R-. remained relatively simpl e but was not the source of an observable coronal disk. This latter re sult was probably because of the limitations of plane-of-sky observati ons, combined with short-term changes in the corona. Altogether, a sin gle coronal disk, either flat or somewhat convoluted, was recognizable during only one third of the 6 1/2 year lifetime of the coronagraph.