Pre-emergence Signatures Of Horizontal Divergent Flows In Solar Active Regions

TL;DR

This study investigates the earliest detectable pre-emergence signatures of active regions on the Sun, focusing on Horizontal Divergent Flows using SDO/HMI data, and introduces a new cadence-independent detection method.

Contribution

The paper extends previous methods to identify pre-emergence signatures, introduces a new cadence-independent signature, and assesses their effectiveness in detecting flux emergence.

Findings

37.5% of ARs show HDF with an average lead time of 58 minutes.

New cadence-independent transient disruptions can indicate flux emergence.

Potential pre-emergence signatures detected 60-156 minutes before flux emergence.

Abstract

Solar active regions (ARs) play a fundamental role in driving many of the geo-effective eruptions which propagate into the Solar System. However, we are still unable to consistently predict where and when ARs will occur across the solar disk by identifying pre-emergence signatures in observables such as the Doppler velocity (without using Helioseismic methods). Here we aim to determine the earliest time at which pre-emergence signatures, specifically the Horizontal Divergent Flow (HDF), can be confidently detected using data from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager (SDO/HMI). Initially, we follow previous studies using the thresholding method, which searches for significant increases in the number of pixels that display a specific line-of-sight velocity. We expand this method to more velocity windows and conduct a basic parameter study investigating the effect of cadence on the inferred results. Our findings agree with previous studies with $37.5$% of ARs displaying a HDF, with average lead times between the HDF and flux emergence of $58$ minutes. We present a new potential signature of flux emergence which manifests as cadence-independent transient disruptions to the amplitudes of multiple velocity windows and recover potential pre-emergence signatures for 10 of the 16 ARs studied, with lead times of 60-156 minutes. Several effects can influence both the estimated times of both HDF and flux emergence suggesting that one may need to combine Doppler and magnetic field data to get a reliable indicator of continued flux emergence.