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We have used the Jansky VLA to observe a sample of 5 known aurorally emitting late L and T dwarfs ranging in age from 0.2-3.4 Gyr. We observed each target for seven hours, extending to higher frequencies than previously attempted for objects in this sample. We establish proportionally higher limits on maximum surface magnetic field strengths while simultaneously placing constraints on rotation periods through detections of repeating pulses. Observations at 8{12 GHz yield measurements of 3.7{4.1 kG localized field strengths (corresponding to minimum mean surface fields between 2.7{2.9 kG) on four of our targets, including the archetypal cloud variable T2.5 dwarf SIMP J01365663+0933473 recently proposed to be a possible planetary-mass object in the Carina-Near moving group. We detect a circularly polarized radio pulse at 15{16.5 GHz for the T6.5 dwarf 2MASS 10475385+2124234, corresponding to a localized 5.6 kG field strength and minimum mean surface field of 4.0 kG. For the same object, we also tentatively detect a circularly polarized radio pulse at 16.5{18 GHz corresponding to a localized 6.2 kG field strength and minimum mean surface field of 4.4 kG. We measure rotation periods between 1.44-2.88 hr for all targets, supporting i) the emerging consensus in convective dynamo models that rapid rotation may be important for producing strong dipole fields and/or ii) rapid rotation is a key ingredient for driving the current systems powering auroral radio emission. We do not detect a clear cutoff in the pulsed emission for any targets, which would correspond to a maximum local surface magnetic field strength. However, we do observe evidence of variable structure in the frequency-dependent timeseries of our targets on timescales shorter than a rotation period, suggesting a higher degree of variability in the current systems near the surfaces of brown dwarfs, where emission at the highest frequencies are expected to probe. Finally, we find that old brown dwarfs may generate fields as strong as young brown dwarfs. 

As an initial pilot study of magnetism in Y dwarfs, we have observed the three known IR variable Y dwarfs WISE J085510.83-071442.5, WISE J140518.40+553421.4, and WISEP J173835.53+273258.9 with the Karl G. Jansky Very Large Array (VLA) from 4-8 GHz to investigate the presence of quiescent radio emission as a proxy for highly circularly polarized radio emission associated with large-scale auroral currents. Measurements of magnetic fields on Y dwarfs, currently only possible by observing auroral radio emission, are essential for constraining fully convective magnetic dynamo models. We do not detect any pulsed or quiescent radio emission, down to rms noise levels of 7.2 uJy for WISE J085510.83-071442.5, 2.2 uJy for WISE J140518.40+553421.4, and 3.2 uJy for WISEP J173835.53+273258.9. The fractional detection rate of radio emission from T dwarfs is <10% and suggests that a much larger sample of deep observations of Y dwarfs is needed to rule out radio emission in the Y dwarf population. The significance of a single detection provides strong motivation for such a search.