The Tilted Solar Dipole as Observed and Modeled during the 1996 Solar Minimum

Abstract

We examine the tilt of the solar magnetic dipole away from the rotational axis near the 1996 solar minimum. A persistent tilted dipole may result from an MHD instability acting on the toroidal bands in the solar interior. Nonaxisymmetric eruption of sunspots has been predicted by dynamo theory and observed in sunspot location patterns. The decay of follower spots and the poleward migration of flux could create polar caps that are slightly misaligned with the north-south rotational axis. To investigate this, we analyze the coronal streamer geometry observed with LASCO-C2 and the center of gravity of the polar caps defined by coronal hole boundaries in EIT images and the unipolar magnetic regions in KPVT magnetograms. We model the coronal hole boundaries and neutral line locations by potential field source surface (PFSS) modeling using Kitt Peak magnetograms. Our results are consistent with an observed tilt of 5°-10° in the heliospheric current sheet at solar minimum and the idea of persistent off-axis magnetic polar caps for CRs 1911-1919. The coronal holes show a stable azimuthal angle for CRs 1911-1919 with a rotation rate slightly less than the Carrington rate. The PFSS modeling is able to recreate the observed coronal hole geometry and predict the maximum extent of the heliospheric current sheet as observed by streamer locations. A 6° observed tilt of the polar caps during this time is consistent with the analytical value provided from the PFSS dipole terms. However, the determination of a tilt of the magnetic polar caps is dominated by noise. The LASCO coronal streamer geometry traces out an 10° tilt of the solar dipole from the equatorial plane during CRs 1915-1919.