Activity Complexes and A Prominent Poleward Surge During Solar Cycle 24

TL;DR

This study investigates a significant poleward surge during solar cycle 24, linking it to activity complexes and their impact on polar magnetic field evolution through observational analysis and flux transport modeling.

Contribution

It introduces an automated method to identify and analyze activity complexes and demonstrates their role in generating poleward surges affecting polar fields.

Findings

The surge was mainly caused by two activity complexes during specific Carrington rotations.

Simulations accurately reproduced the surge characteristics and its impact on polar field evolution.

Without the flux emergence from ACs, the polar field would have remained low or reversed polarity.

Abstract

Long-lasting activity complexes (ACs), characterised as a series of closely located, continuously emerging solar active regions (ARs), are considered generating prominent poleward surges from observations. The surges lead to significant variations of the polar field, which are important for the modulation of solar cycles. We aim to study a prominent poleward surge during solar cycle 24 on the southern hemisphere, and analyse its originating ACs and the effect on the polar field evolution. We automatically identify and characterize ARs based on synoptic magnetograms from the Solar Dynamic Observatory. We assimilate these ARs with realistic magnetic configuration into a surface flux transport model, and simulate the creation and migration of the surge. Our simulations well reproduce the characteristics of the surge and show that the prominent surge is mainly caused by the ARs belonging to two ACs during Carrington Rotations 2145-2159 (December 2013-January 2015). The surge has a strong influence on the polar field evolution of the southern hemisphere during the latter half of cycle 24. Without the about one-year-long flux emergence in the form of ACs, the polar field around the cycle minimum would have remained at a low level and even reversed to the polarity at cycle 23 minimum. Our study also shows that the long-lived unipolar regions due to the decay of the earlier emerging ARs cause an intrinsic difficulty of automatically identifying and precisely quantifying later emerging ARs in ACs.