# Evolution of Magnetic Helicity in Solar Cycle 24

## TL;DR

This paper introduces a new method to reconstruct and analyze the evolution of magnetic helicity density on the Sun during solar cycle 24, revealing hemispheric sign patterns and deviations from dynamo theory predictions.

## Contribution

A novel approach to reconstruct surface magnetic helicity density from vector magnetic field data and its application to solar cycle 24 data.

## Key findings

- Hemispheric helicity rule observed with sign inversion at cycle end.
- Large-scale and small-scale helicity densities show opposite signs initially, then align later.
- Magnetic helicity evolution deviates from dynamo theory expectations.

## Abstract

We propose a novel approach to reconstruct the surface magnetic helicity density on the Sun or sun-like stars. The magnetic vector potential is determined via decomposition of vector magnetic field measurements into toroidal and poloidal components. The method is verified using data from a non-axisymmetric dynamo model. We apply the method to vector field synoptic maps from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics Observatory (SDO) to study evolution of the magnetic helicity density during solar cycle 24. It is found that the mean helicity density of the non-axisymmetric magnetic field of the Sun evolves in a way which is similar to that reported for the current helicity density of the solar active regions. It has predominantly the negative sign in the northern hemisphere, and it is positive in the southern hemisphere. Also, the hemispheric helicity rule for the non-axisymmetric magnetic field showed the sign inversion at the end of cycle 24. Evolution of magnetic helicity density of large-scale axisymmetric magnetic field is different from that expected in dynamo theory. On one hand, the mean large- and small-scale components of magnetic helicity density display the hemispheric helicity rule of opposite sign at the beginning of cycle 24. However, later in the cycle, the two helicities exhibit the same sign in contrast with the theoretical expectations.