Evolution of solar surface inflows around emerging active regions

TL;DR

This study investigates the evolution of surface inflows around emerging solar active regions, revealing early converging flows prior to emergence and their dependence on active region properties, using observational data and empirical modeling.

Contribution

It provides the first detailed empirical analysis of inflow evolution around emerging active regions and models their dependence on flux and latitude.

Findings

Converging inflows of 20-30 m/s appear one day before emergence.

Inflow velocities reach about 50 m/s during early emergence.

Inflows extend up to 7 degrees from active region center within six days.

Abstract

Solar active regions are associated with Evershed outflows in sunspot penumbrae, moat outflows surrounding sunspots, and extended inflows surrounding active regions. The latter have been identified on established active regions by various methods. The evolution of these inflows and their dependence on active region properties as well as their impact on the global magnetic field are not yet understood. We aim to understand the evolution of the average inflows around emerging active regions and to derive an empirical model for these inflows. We analyze horizontal flows at the surface of the Sun using local correlation tracking of solar granules observed in continuum images of SDO/HMI. We measure average flows of a sample of 182 isolated active regions up to seven days before and after their emergence onto the solar surface with a cadence of 12 hours. We investigate the average inflow properties with respect to active region characteristics of total flux and latitude. We fit a model to these observed inflows for a quantitative analysis. We find that converging flows of around $20$ to $30$ m/s are first visible one day prior to emergence, in agreement with recent results. These converging flows are present independently of active region properties of latitude or flux. We confirm a recently found prograde flow of about $40$ m/s at the leading polarity during emergence. We find that the time after emergence when the latitudinal inflows increase in amplitude depends on the flux of the active region, ranging from one to four days after emergence and increasing with flux. The largest extent of the inflows is up to about $7 \pm 1^\circ$ away from the center of the active region within the first six days after emergence. The inflow velocities have amplitudes of about $50$ m/s.