ALMA observations of transient heating in a solar active region^⋆

¹ Institute for Solar Physics, Department of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
e-mail: joao.dasilva@astro.su.se
² Space Vehicles Directorate, Air Force Research Laboratory, Albuquerque, NM, USA
³ Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035-0001, USA
⁴ Lockheed Martin Solar & Astrophysics Lab., Org. A021S, Bldg. 252, 3251 Hanover Street, Palo Alto, CA 94304, USA
⁵ Rosseland Centre for Solar Physics and Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
⁶ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway

Received: 25 June 2020
Accepted: 27 August 2020

Abstract

Aims. We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA).

Methods. We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis.

Results. We report the null detection of EBs in the 3 mm continuum at ∼1.2″ spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts – reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of > 10 kK plasma launched with high plane-of-sky velocities (37 − 340 km s⁻¹) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.