  Empirical trends in stellar X-ray and radio luminosities suggest that low
mass ultracool dwarfs (UCDs) should not produce significant radio emission.
Defying these expectations, strong non-thermal emission has been observed in a
few UCDs in the 1-10 GHz range, with a variable component often attributed to
global aurorae and a steady component attributed to other processes such as
gyrosynchrotron emission. While both auroral and gyrosynchrotron emission peak
near the critical frequency, only the latter radiation is expected to extend
into millimeter wavelengths. We present ALMA 97.5 GHz and VLA 33 GHz
observations of a small survey of 5 UCDs. LP 349-25, LSR J1835+3259, and NLTT
33370 were detected at 97.5 GHz, while LP 423-31 and LP 415-20 resulted in
non-detections at 33 GHz. A significant flare was observed in NLTT 33370 that
reached a peak flux of 4880 +/- 360 microJy, exceeding the quiescent flux by
nearly an order of magnitude, and lasting 20 seconds. These ALMA observations
show bright 97.5 GHz emission with spectral indices ranging from alpha = -0.76
to alpha = -0.29, suggestive of optically thin gyrosynchrotron emission. If
such emission traces magnetic reconnection events, then this could have
consequences for both UCD magnetic models and the atmospheric stability of
planets in orbit around them. Overall, our results provide confirmation that
gyrosynchrotron radiation in radio loud UCDs can remain detectable into the
millimeter regime.
