Probing chromospheric heating with millimeter interferometry

Abstract:

Observations at visible and ultraviolet wavelengths have shown that solar active regions host different kinds of small-scale, transient, bright structures that are believed to be heating events resulting from the release of magnetic energy in the low atmosphere of the Sun, especially at the early stages of flux emergence. It is of great scientific interest to be able to accurately infer temperatures and formation heights of the most localized events, which are still matter of debate, in the hope of learning about the evolution of active regions where occasionally more energetic phenomena lead to much larger outbursts that propagate across the Solar System. The millimeter (mm) continuum is a new complementary diagnostic for chromospheric heating that is now available thanks to the Atacama Large Millimeter/submillimeter Array (ALMA).

We report on the first ALMA 3 mm observations of small-scale heating events in a solar active region. In contrast with the low-amplitude brightness temperature variations in the quiet-Sun, the interferometric maps show that the active region consists of long, warm, fibril-like structures that connect magnetic concentrations of opposite polarity and often flare up along with compact, flickering mm-bursts -- reminiscent of ultraviolet bursts -- with brightness temperatures of up to 14000 K at 1.2" scales. These events also show simultaneous EUV emission observed by the Solar Dynamics Observatory (SDO). We find a weak correlation between the photospheric bright patches and the 3 mm continuum brightness and, in particular, we do not detect any mm counterpart of Ellerman bombs which confirms that they are photospheric phenomena.

Our observations and modelling highlight the diagnostic capabilities of ALMA for local heating in solar active regions and emphasize the need for coordinated observations with IRIS and DKIST in the future.