  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 ALMA observation at 3 mm coordinated with IRIS and
Hinode observations, on March 19, 2017. During the observations, a soft X-ray
loop-type microflare (active-region transient brightening) was captured using
Hinode X-ray telescope in high temporal cadence. A brightening loop footpoint
is located within narrow field of views ALMA, IRIS slit-jaw imager, and Hinode
spectro-polarimeter. 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 non-thermal 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.
