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

TL;DR

This study uses high-resolution ALMA and IRIS observations to investigate the thermal structure of an active region solar filament, revealing complex temperature and density variations in fine structures.

Contribution

First high-resolution imaging of an active region filament in the millimeter range, combining ALMA, IRIS, and other data to analyze its thermal and fine-structure properties.

Findings

Dark filament threads have low brightness temperatures below 5000 K.

Visibility of filament structures varies across the filament and over time.

ALMA Band 3 maps do not clearly show the filament, indicating non-isothermal conditions.

Abstract

Quiescent filaments appear as absorption features on the solar disk when observed in chromospheric lines and at continuum wavelengths in the millimeter (mm) range. Active region (AR) filaments are their small-scale, low-altitude analogues, but they could not be resolved in previous mm observations. This spectral diagnostic can provide insight into the details of the formation and physical properties of their fine threads, which are still not fully understood. Here, we shed light on the thermal structure of an AR filament using high-resolution brightness temperature ($T_{\rm b}$) maps taken with ALMA Band 6 complemented by simultaneous IRIS near-UV spectra, Hinode/SOT photospheric magnetograms, and SDO/AIA extreme-UV images. Some of the dark threads visible in the AIA 304 {\AA} passband and in the core of Mg II resonance lines have dark ($T_{\rm b}<5000$K) counterparts in the 1.25 mm maps, but their visibility significantly varies across the filament spine and in time. These opacity changes are possibly related to variations in temperature and electron density in filament fine structures. The coolest $T_{\rm b}$ values ($<$5000 K) coincide with regions of low integrated intensity in the Mg II h and k lines. ALMA Band 3 maps taken after the Band 6 ones do not clearly show the filament structure, contrary to the expectation that the contrast should increase at longer wavelengths based on previous observations of quiescent filaments. The ALMA maps are not consistent with isothermal conditions, but the temporal evolution of the filament may partly account for this.