OBSERVATIONAL ASPECTS OF THE THREE-DIMENSIONAL CORONAL STRUCTURE OVER A SOLAR ACTIVITY CYCLE

Abstract

Solar rotational tomography is applied to almost eleven years of Large Angle Spectrometric Coronagraph C2/Solar and Heliospheric Observatory data, revealing for the first time the behavior of the large-scale coronal density structures, also known as streamers, over almost a full solar activity cycle. This study gives an overview of the main results of this project. (1) Streamers are most often shaped as extended, narrow plasma sheets. The sheets can be extremely narrow at times (⩽0.14 × 106 km at 4 R☉). This is over twice their heliocentric angular thickness at 1 AU. (2) At most times outside the height of solar maximum, there are two separate stable large helmet streamer belts extending from mid-latitudes (in both north and south). At solar minimum, the streamers converge and join near the equator, giving the impression of a single large helmet streamer. Outside of solar minimum, the two streamers do not join, forming separate high-density sheets in the extended corona (one in the north, another in the south). At solar maximum, streamers rise radially from their source regions, while during the ascending and descending activity phases, streamers are skewed toward the equator. (3) For most of the activity cycle, streamers share the same latitudinal extent as filaments on the disk, showing that large-scale stable streamers are closely linked to the same large-scale photospheric magnetic configuration, which give rise to large filaments. (4) The poleward footpoints of the streamers are often above crown polar filaments and the equatorial footpoints are above filaments or active regions (or above the photospheric neutral lines which underlie these structures). The high-density structures arising from the equatorial active regions either rise and form the equatorial footpoints of mid-latitude quiescent streamers, or form unstable streamers at the equator, not connected to the quiescent streamer structure at higher latitude (so there are often three streamer sheets sharing the same extended longitudinal region). (5) Comparison between the tomography results and a potential field source surface model shows that streamers are not necessarily associated with a magnetic polarity reversal, but rather are regions containing field lines arising from widely separated sources at the Sun. We call these convergence sheets. (6) There is considerable differential rotation of streamers at high latitudes, which makes comparison between disk and coronal structure complicated. The presence of differential rotation has implications for many areas of coronal and heliospheric research.