The tilted solar dipole: coronal streamer and polar cap geometry observed near solar minimum

Abstract

We examine the tilt of the solar magnetic dipole away from the rotational axis near solar minimum. The coronal streamer geometry as observed with LASCO-C2 at 5 R⊙ traces out an approximately 10◦ tilt of the solar dipole from the equatorial plane during Carrington Rotations 1915–1919. We also analyze the center-of-gravity of the polar caps as defined by coronal hole boundaries in EIT images. A persistent tilted dipole may result from an MHD instability acting upon the toroidal magnetic bands in the solar interior. A non-axisymmetric eruption of sunspots mapping out an m = 1 pattern in longitude has been predicted by dynamo theory and observed in sunspot location patterns. The decay of the follower spots and migration of this flux poleward could create polar caps that are slightly misaligned with the North–South rotational axis. Our results are consistent with an observed tilt of 10◦ in the heliospheric current sheet at solar minimum.