We have used NSF ’ s Karl G. Jansky Very Large Array ( VLA ) to observe a sample of five known radio-emitting late L and T dwarfs ranging in age from \sim 0.2–3.4 Gyr .
We observed each target for seven hours , extending to higher frequencies than previously attempted and establishing proportionally higher limits on maximum surface magnetic field strengths .
Detections of circularly polarized pulses at 8–12 GHz yield measurements of 3.2–4.1 kG localized magnetic fields on four of our targets , including the archetypal cloud variable and likely planetary-mass object T2.5 dwarf SIMP J01365663+0933473 .
We additionally detect a pulse at 15–16.5 GHz for the T6.5 dwarf 2MASS 10475385+2124234 , corresponding to a localized 5.6 kG field strength .
For the same object , we tentatively detect a 16.5–18 GHz pulse , corresponding to a localized 6.2 kG field strength .
We measure rotation periods between \sim 1.47–2.28 hr for 2MASS J10430758+2225236 , 2MASS J12373919+6526148 , and SDSS J04234858-0414035 , supporting ( i ) an emerging consensus that rapid rotation may be important for producing strong dipole fields in convective dynamos and/or ( ii ) rapid rotation is a key ingredient for driving the current systems powering auroral radio emission .
We observe evidence of variable structure in the frequency-dependent time series 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 .
Finally , we find that age , mass , and temperature together can not account for the strong magnetic fields produced by our targets .