Dynamics in the transition region beneath active region upflows viewed by IRIS

TL;DR

This study investigates small-scale transition region dynamics beneath active region upflows using IRIS data, revealing highly-structured, localized features that may drive or stimulate coronal upflows linked to the slow solar wind.

Contribution

It provides the first detailed statistical analysis of small-scale transition region dynamics beneath active region upflows, highlighting their fine structure and potential role in driving coronal upflows.

Findings

Transition region features are mostly less than 1 Mm^2 in size.

Bright dots with about 0.3 Mm^2 and lifetimes under 200 s are widespread.

Surge-like motions at the boundary are observed with speeds around 15 km/s.

Abstract

Coronal upflows at the edges of active regions (AR), which are a possible source of slow solar wind, have been found to connect with dynamics in the transition region. To infer at what scale transition region dynamics connect to AR upflows, we investigate the statistical properties of the small-scale dynamics in the transition region underneath the upflows at the edge of AR NOAA 11934. With observations from the Interface Region Imaging Spectragraph (IRIS), we found that the Si IV 1403\,\AA\ Doppler map consists of numerous blue-shifted and red-shifted patches mostly with sizes less than 1\,$Mm^2$. The blue-shifted structures in the transition region tend to be brighter than the red-shifted ones, but their nonthermal velocities have no significant difference. With the SWAMIS feature tracking procedure, in IRIS slit-jaw 1400\,\AA\ images we found that dynamic bright dots with an average size of about 0.3\,$Mm^2$ and lifetimes mostly less than 200\,s spread all over the region. Most of the bright dots appear to be localised, without clear signature of propagation of plasma to a long distance on the projection plane. Surge-like motions with speeds about 15 km/s could be seen in some events at the boundaries of the upflow region, where the magnetic field appear to be inclined. We conclude that the transition region dynamics connecting to coronal upflows should occur in very fine scale, suggesting that the corresponding coronal upflows should also be highly-structured. It is also plausible that the transition region dynamics might just act as stimulation at the coronal base that then drives the upflows in the corona.