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 .