Simultaneous ALMA–Hinode–IRIS Observations on Footpoint Signatures of a Soft X-Ray Loop-like Microflare

Microflares have been considered to be among the major energy input sources to form active solar corona. To investigate the response of the low atmosphere to events, we conducted an Atacama Large Millimeter/submillimeter Array (ALMA) observation at 3 mm, coordinated with Interface Region Imaging Spectrograph (IRIS) and Hinode observations, on 2017 March 19. During the observations, a soft X-ray loop-type microflare (active region transient brightening) was captured using the Hinode X-ray telescope in high temporal cadence. A brightening loop footpoint is located within narrow fields of view of ALMA, IRIS slit-jaw imager, and Hinode spectropolarimeter. Counterparts of the microflare at the footpoint were detected in Si iv and ALMA images, while the counterparts were less apparent in C ii and Mg ii k images. Their impulsive time profiles exhibit the Neupert effect pertaining to soft X-ray intensity evolution. The magnitude of thermal energy measured using ALMA was approximately 100 times smaller than that measured in the corona. These results suggest that impulsive counterparts can be detected in the transition region and upper chromosphere, where the plasma is thermally heated via impinging nonthermal particles. Our energy evaluation indicates a deficit of accelerated particles that impinge the footpoints for a small class of soft X-ray microflares. The footpoint counterparts consist of several brightening kernels, all of which are located in weak (void) magnetic areas formed in patchy distribution of strong magnetic flux at the photospheric level. The kernels provide a conceptual image in which the transient energy release occurs at multiple locations on the sheaths of magnetic flux bundles in the corona.