Flares detected in ALMA single-dish images of the Sun

Abstract

Context. The millimeter and submillimeter radiation of solar flares is poorly understood. Without spatial resolution, millimeter emission cannot be easily compared to flare emission in other wavelengths. Though the Atacama Large Millimeter-submillimeter Array (ALMA) offers sufficient resolution for the first time, ALMA cannot be used on demand to observe when a flare occurs, and when used as an interferometer, its field of view is smaller than an active region. Aims. We used readily available large-scale single-dish ALMA observations of solar millimeter flares and compared them to well-known features observed in other wavelengths. The properties of these other flare emissions, correlating in space and time, could then be used to interpret the millimeter brightenings and vice versa. The aim is to obtain reliable associations limited by the time and space resolution of single-dish observations. Methods. Ordinary interferometric ALMA observations require single-dish images of the full Sun for calibration. We collected such observations at 3 mm and 1 mm and searched for millimeter brightenings during times listed in a flare catalog. Results. All of the flares left a signature in millimeter waves. We found five events with nine or more images that could be used for comparison in time and space. The millimeter brightenings are associated with a variety of flare features in cool (Hα, 304 Å), intermediate (171 Å), and hot (94 Å) lines. In several cases, the millimeter brightening peaked at the footpoint of a hot flare loop. In other cases the peak of the millimeter brightening coincided with the top or footpoint of an active Hα filament. We also found correlations with post-flare loops and the tops of a hot loop. In some images, the millimeter radiation peaked at locations where no feature in the selected lines was found. Conclusions. The wide field of view provided by the single-dish ALMA observations allowed for a complete overview of the flare activity in millimeter waves for the first time. The associated phenomena often changed in type and location during the flare. The variety of phenomena detected in these millimeter observations may explain the sometimes bewildering behavior of millimeter flare emissions previously observed without spatial resolution.