  The mechanisms responsible for heating the extended atmospheres of early-M
spectral-type supergiants are poorly understood. So too is the subsequent role
these mechanisms play in driving the large mass-loss rates of these stars. Here
we present ALMA long (i.e., $\sim$16 km) baseline 338 GHz (0.89 mm) continuum
observations of the free-free emission in the extended atmosphere of the M2
spectral-type supergiant Betelgeuse. The spatial resolution of 14 mas
exquisitely resolves the atmosphere, revealing it to have a mean temperature of
2760 K at $\sim$1.3 R$_{\star}$, which is below both the photospheric effective
temperature ($T_{\textrm{eff}} = 3690$ K) and the temperatures at $\sim$$2
$R$_{\star}$. This is unambiguous proof for the existence of an inversion of
the mean temperature in the atmosphere of a red supergiant. The emission is
clearly not spherically symmetric with two notable deviations from a uniform
disk detected in both the images and visibilities. The most prominent asymmetry
is located in the north-east quadrant of the disk and is spatially resolved
showing it to be highly elongated with an axis-ratio of 2.4 and occupying
$\sim$$5\%$ of the disk projected area. Its temperature is approximately 1000 K
above the measured mean temperature at 1.3 R$_{\star}$. The other main
asymmetry is located on the disk limb almost due east of the disk center and
occupies $\sim$$3\%$ of the disk projected area. Both emission asymmetries are
clear evidence for localized heating taking place in the atmosphere of
Betelgeuse. We suggest that the detected localized heating is related to
magnetic activity generated by large-scale photospheric convection.
