Hemispheric injection of magnetic helicity by surface flux transport

¹ Department of Mathematics, University of Exeter, N Park Rd, Exeter EX4 4QF, UK
e-mail: gh378@exeter.ac.uk
² Department of Mathematical Sciences, Durham University, Durham DH1 3LE, UK
e-mail: anthony.yeates@durham.ac.uk

Received: 7 August 2019
Accepted: 2 October 2019

Abstract

Aims. We estimate the injection of relative magnetic helicity into the solar atmosphere by surface flux transport over 27 solar cycles (1700–2009).

Methods. We determine the radial magnetic field evolution using two separate surface flux transport models: one driven by magnetogram inputs and another by statistical active region insertion guided by the sunspot number record. The injection of relative magnetic helicity is then computed from this radial magnetic field together with the known electric field in the flux transport models.

Results. Neglecting flux emergence, solar rotation is the dominant contributor to the helicity injection. At high latitudes, the injection is always negative/positive in the northern/southern hemisphere, while at low latitudes the injection tends to have the opposite sign when integrated over the full solar cycle. The overall helicity injection in a given solar cycle depends on the balance between these two contributions. This net injected helicity correlates well with the end-of-cycle axial dipole moment.