João M. da Silva Santos
The landscape of active regions on the Sun is marked by fibril- and loop-like structures that trace the magnetic field lines that confine plasma up to several megameters above the solar surface. Within these environments, it is common to observe different types of small-scale, transient optical and ultraviolet brightenings, which have been linked to magnetic reconnection processes. However, many of the details of how that happens and how important they are in the energy/mass supply to the solar corona remain unclear.
We report on the first Atacama Large Millimeter/submillimeter Array (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 both long, warm, fibril-like structures that connect magnetic concentrations of opposite polarity and more compact, flickering mm-bursts with brightness temperatures of up to 14000 K at 1.2" (~850 km) 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 and, in particular, we do not detect any mm counterpart of Ellerman bombs, which constrains their formation heights.
Our observations and modelling highlight the diagnostic capabilities of ALMA for local heating in solar active regions
ALMA is a large array of radio antennas located in Chile, and it observes the Sun in three bands (0.8, 1.25 and 3 mm).
In this work we use ALMA to investigate the properties of small-scale heating events in the solar atmosphere. Our synthetic 3 mm map computed from a 3D Bifrost simulation (page 2) shows that ALMA may detect the thermal signature of magnetic reconnection in the chromosphere as a significant increase in brightness temperature above the local background. The brightness temperature is a good proxy for actual plasma temperature.
We searched for these events in ALMA observations (page 3) of an active region and found multiple transient brightenings whose mm emission correlates well with the EUV probed by SDO/AIA. The hot corona may contribute to the observed mm brightness. However, we did not find any significant emission associated with Ellerman bombs detected in the AIA 1700 Å filter, which means that the latter are rather deep phenomena in the solar atmosphere.
In the future, coordinated observations with other facilities will give us more clues about the nature of these events, and they will help us to better constrain the atmosphere where they take place, for example, using inversion codes.