Rate of helicity production by solar rotation

Abstract

In recent years, solar observers have discovered a striking pattern in the distribution of coronal magnetic structures: northern hemisphere structures tend to have negative magnetic helicity, while structures in the south tend to have positive magnetic helicity. This hemispheric dependence extends from photospheric observations to in situ measurements of magnetic clouds in the solar wind. Understanding the source of the hemispheric sign dependence, as well as its implications for solar and space physics has become known as the solar chirality problem. Rotation of open fields creates the Parker spiral which carries outward 1047 Mx2 of magnetic helicity (in each hemisphere) during a solar cycle. In addition, rough estimates suggest that each hemisphere sheds on the order 1045 Mx2 in coronal mass ejections each cycle. Both the α effect (arising from helical turbulence) and the Ω effect (arising from differential rotation) should contribute to the hemispheric chirality. We show that the Ω effect contribution can be captured in a surface integral, even though the helicity itself is stored deep in the convection zone. We then evaluate this surface integral using solar magnetogram data and differential rotation curves. Throughout the 22 year cycle studied (1976–1998) the helicity production in the interior by differential rotation had the correct sign compared to observations of coronal structures ‐ negative in the north and positive in the south. The net helicity flow into each hemisphere over this cycle was approximately 4 × 1046 Mx2. For comparison, we estimate the α effect contribution; this may well be as high or higher than the differential rotation contribution. The subsurface helicity can be transported to the corona with buoyant rising flux tubes. Evidently only a small fraction of the subsurface helicity escapes to the surface to supply coronal mass ejections.