Evolution of coronal streamer structure during the rising phase of solar cycle 23

White-light images of the outer corona recorded with the Large Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) show a rapid widening of the streamer belt during 1998 and early 1999. The observed streamer structure and its evolution from rotation to rotation are reproduced with a model in which the Thomson-scattering electrons are concentrated within a narrow layer centered around the heliospheric current sheet. The latitudinal spreading of the streamer belt is shown to be a consequence of the increased rate of magnetic flux emergence in the sunspot latitudes, which led to a weakening of the Sun's axisymmetric dipole moment, to a rapid growth in the nonaxisymmetric components of the coronal field, and hence to a strong tilting and warping of the plasma/current sheet.