Chromosphere of the quiet Sun

II. Atmospheric response to small-scale magnetic flux emergence

¹ Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, Oslo, NO-0315, Norway
² Centre for Mathematical Plasma-Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001, Leuven, Belgium
³ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, Oslo, NO-0315, Norway
⁴ Sorbonne Université, Observatoire de Paris – PSL, École Polytechnique, Institut Polytechnique de Paris, CNRS, Laboratoire de Physique des Plasmas (LPP), 4 Place Jussieu, 75005, Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 March 2026
Accepted: 2 May 2026

Abstract

Context. The couplings between the photosphere, chromosphere, and corona in the quiet Sun (QS) are governed by a complex interplay between magnetic structuring, heating, mass-loading, and radiative cooling. The current constraints on how this balance responds to variations in small-scale magnetic flux are limited.

Aims. We investigate how chromospheric heating and the thermodynamic response of higher atmospheric layers vary as a function of small-scale magnetic flux emergence under QS conditions.

Methods. We performed a parametric set of 3D radiative-MHD simulations with the Bifrost code, starting from a weakly magnetised quiet-Sun reference model and injecting horizontal magnetic flux of increasing amplitude into the sub-surface convection zone. We analysed the resulting chromospheric dynamics, heating, mass-loading, and coronal response in quasi-static regimes.

Results. Chromospheric temperatures and mechanical heating rise monotonically with increasing magnetic-field strength. Although the fractional contribution of shocks decreases from 23 to 5%, reconnecting current sheets (CSs) continue to remain steady at about 50%. In contrast, the temperature at the base of the corona exhibits a non-monotonic response, reaching a maximum at intermediate magnetic amplitudes and decreasing for the strongest-field case. We show that stronger magnetic-field strength increases chromospheric heating, thereby increasing the coronal-base density through efficient mass-loading, and amplifies radiative losses. These density-driven radiative losses dominate the coronal energy balance and, thus, lead to reduced coronal-base temperatures despite increased heating.

Conclusions. Our results demonstrate the sensitivity of chromospheric structure and dynamics to small-scale flux emergence and its key role in regulating coronal thermodynamics. In particular, this study has revealed a non-monotonic thermodynamic response in the upper atmosphere: stronger heating in the chromosphere can paradoxically lead to lower coronal temperatures as increased mass-loading enhances radiative losses. This result illustrates the chromosphere’s role as a thermodynamic gatekeeper, warranting further investigations of realistic flux-emergence models, as well as surface-to-corona parametrisation across various magnetic configurations, relevant to global solar wind models and space weather forecasts.