On Asymmetry of Magnetic Helicity in Emerging Active Regions: High Resolution Observations

TL;DR

This study uses high-resolution magnetogram data and the DAVE technique to confirm the persistent asymmetry in magnetic helicity injection between polarities in emerging active regions, highlighting the importance of data resolution and measurement parameters.

Contribution

It demonstrates the robustness of magnetic helicity asymmetry in emerging active regions using high-resolution data and evaluates the impact of measurement parameters on helicity injection rate estimation.

Findings

Magnetic helicity asymmetry is consistently observed in studied active regions.

Measurement of helicity injection rate is reliable with specific window size and time interval.

High-resolution data provides more accurate estimates of magnetic flux and helicity injection.

Abstract

We employ the DAVE (differential affine velocity estimator, Schuck 2005; 2006) tracking technique on a time series of MDI/1m high spatial resolution line- of-sight magnetograms to measure the photospheric flow velocity for three newly emerging bipolar active regions. We separately calculate the magnetic helicity injection rate of the leading and following polarities to confirm or refute the magnetic helicity asymmetry, found by Tian & Alexander (2009) using MDI/96m low spatial resolution magnetograms. Our results demonstrate that the magnetic helicity asymmetry is robust being present in the three active regions studied, two of which have an observed balance of the magnetic flux. The magnetic helicity injection rate measured is found to depend little on the window size selected, but does depend on the time interval used between the two successive magnetograms tracked. It is found that the measurement of the magnetic helicity injection rate performs well for a window size between 12x10 and 18x15 pixels, and at a time interval {\Delta}t=10 minutes. Moreover, the short-lived magnetic structures, 10-60 minutes, are found to contribute 30-50% of the magnetic helicity injection rate. Comparing with the results calculated by MDI/96m data, we find that the MDI/96m data, in general, can outline the main trend of the magnetic properties, but they significantly underestimate the magnetic flux in strong field region and are not appropriate for quantitative tracking studies, so provide a poor estimate of the amount of magnetic helicity injected into the corona.