Subarcsecond imaging of a solar active region filament with ALMA and IRIS

An understanding of the processes leading to the formation of fine threads in filaments or prominences in the solar atmosphere is still elusive. ALMA observations in the millimeter continuum offer a powerful diagnostic of the thermal conditions in filament fine structures. Because of their smaller spatial scales, active region (AR) filaments, in particular, could not previously be resolved in that wavelength range. We present interferometry maps of an AR filament taken with ALMA Bands 6 and 3 with significantly improved spatial resolution (0.6 arcsec at 1.25 mm) compared to previous single-dish observations, and we compare them to ultraviolet imagery provided by IRIS and SDO/AIA and photospheric magnetograms obtained by Hinode/SOT. The 1.25 mm map reveals high-contrast, dark, fine threads co-spatial with the filament body seen in the IRIS Mg II core images and in the AIA 304 passband, but there are significant opacity variations across the filament body on time scales of a few minutes. Surprisingly, the 3 mm map does not show the same dark/cool structures. In the absence of suitable models in the literature, our results underline the need for follow-up radiative transfer modeling of the radio continuum and the Mg resonance lines to constrain the thermodynamics of AR filaments.