Active region upflows in various coronal structures and their coupling to the lower atmosphere

¹ ETH-Zürich, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
² Physikalisch-Meteorologische Observatorium Davos/World Radiation Center, Dorfstrasse 33, 7260 Davos Dorf, Switzerland
³ Southwest Research Institute, Boulder, CO 80302, USA
⁴ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁵ Adnet Systems, Inc., NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771, USA
⁶ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁷ Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401A, 79110 Freiburg, Germany
⁸ Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway
⁹ RAL Space, UKRI STFC Rutherford Appleton Laboratory, Harwell, Didcot OX11 0QX, UK
¹⁰ Key Laboratory of Modern Astronomy and Astrophysics, School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China
¹¹ Now at European Space Agency, European Space Astronomy Center, Camino Bajo del Castillo, s/n Urbanización Villafranca del Castillo, Villanueva de la Cañada, 28692 Madrid, Spain

^⋆ Corresponding author: yingjie.zhu@pmodwrc.ch

Received: 21 May 2025
Accepted: 31 July 2025

Abstract

Context. Plasma upflows with a Doppler shift exceeding −10 km s⁻¹ at active region (AR) boundaries are considered potential sources of the nascent slow solar wind. These upflows are often located at the footpoints of large-scale fan-like loops and show temperature-dependent Doppler shifts with redshifts in the transition region and blueshifts in the lower corona.

Aims. We investigate the driving mechanisms of a pair of coronal upflow regions on the western and eastern peripheries of an AR, which have different magnetic topologies and surroundings. It is aimed to explore how these upflows couple to the lower atmosphere.

Methods. Using observations of the Fe XII 19.51 nm line from Hinode, we identified two upflow regions at the western and eastern boundaries of a decaying AR. Context images for the two regions were obtained by the High Resolution Imager (HRI) telescope of the Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter mission. Other instruments on Solar Orbiter and other observatories provide diagnostics to the lower atmosphere. Potential Field Source Surface (PFSS) extrapolations were used to examine the magnetic field configuration associated with the AR upflows.

Results. The eastern upflow region, located over the AR moss, displays small-scale dynamic fibril structures, whereas the western region hosts fan-like loops. We found blueshifted Ne VIII emission at the eastern site, in contrast to redshifted Ne VIII profiles in the west. Magnetic field extrapolations reveal a pseudostreamer topology connecting both these regions. Moreover, low transition-region lines show systematically reduced redshift below the eastern footpoint.

Conclusions. The observations support the scenario in which both upflows are driven by pressure imbalances created by coronal reconnection, leading to a continuous upflow above approximately 0.6 MK (i.e., Ne VIII line formation temperature). Meanwhile, mass flows in the lower transition region beneath the eastern upflow region appear to respond passively to the pressure-driven coronal upflows.