Detection of the Horizontal Divergent Flow prior to the Solar Flux Emergence

TL;DR

This study detects the horizontal divergent flow (HDF) prior to solar flux emergence using SDO/HMI data, showing HDF appears about 100 minutes before flux emergence and could help predict sunspot formation.

Contribution

First observational detection of HDF prior to flux emergence, linking plasma flow patterns to early sunspot formation prediction.

Findings

HDF appears ~100 minutes before flux emergence

HDF horizontal speed estimated at 0.6-2.3 km/s

Chromospheric plages appear ~14 minutes after HDF

Abstract

It is widely accepted that solar active regions including sunspots are formed by the emerging magnetic flux from the deep convection zone. In previous numerical simulations, we found that the horizontal divergent flow (HDF) occurs before the flux emergence at the photospheric height. This Paper reports the HDF detection prior to the flux emergence of NOAA AR 11081, which is located away from the disk center. We use SDO/HMI data to study the temporal changes of the Doppler and magnetic patterns from those of the reference quiet Sun. As a result, the HDF appearance is found to come before the flux emergence by about 100 minutes. Also, the horizontal speed of the HDF during this time gap is estimated to be 0.6 to 1.5 km s^-1, up to 2.3 km s^-1. The HDF is caused by the plasma escaping horizontally from the rising magnetic flux. And the interval between the HDF and the flux emergence may reflect the latency during which the magnetic flux beneath the solar surface is waiting for the instability onset to the further emergence. Moreover, SMART Halpha images show that the chromospheric plages appear about 14 min later, located co-spatial with the photospheric pores. This indicates that the plages are caused by plasma flowing down along the magnetic fields that connect the pores at their footpoints. One importance of observing the HDF may be the possibility to predict the sunspot appearances that occur in several hours.