Effects of non-radial magnetic field on measuring magnetic helicity transport across solar photosphere

TL;DR

This study assesses how non-radial magnetic fields influence the measurement of magnetic helicity injection rates in solar active regions, highlighting the importance of viewing angle and magnetic field assumptions.

Contribution

It provides a quantitative analysis of the impact of non-radial magnetic fields on helicity measurements using HMI/SDO data, improving accuracy in solar magnetic studies.

Findings

Non-radial magnetic fields have a small effect on tangential velocity estimates.

The effect on helicity injection rate is significant near the solar limb.

The impact is minor when only the accumulated magnetic helicity is considered.

Abstract

It is generally believed that the evolution of magnetic helicity has a close relationship with solar activity. Before the launch of SDO, earlier studies have mostly used MDI/SOHO line of sight magnetograms and assumed that magnetic fields are radial when calculating magnetic helicity injection rate from photospheric magnetograms. However, this assumption is not necessarily true. Here we use the vector magnetograms and line of sight magnetograms, both taken by HMI/SDO, to estimate the effects of non-radial magnetic field on measuring magnetic helicity injection rate. We find that: 1) The effect of non-radial magnetic field on estimating tangential velocity is relatively small; 2) On estimating magnetic helicity injection rate, the effect of non-radial magnetic field is strong when active regions are observed near the limb and is relatively small when active regions are close to disk center; 3) The effect of non-radial magnetic field becomes minor if the amount of accumulated magnetic helicity is the only concern.