We present radio observations of a comprehensive sample of 90 dwarf stars and brown dwarfs ranging from spectral type M5 to T8 .
We detect three radio active sources in addition to the six objects previously detected in quiescence and outburst , leading to an overall detection rate of about 10 \% for objects later than M7 .
From the properties of the radio emission we infer magnetic field strengths of \sim 10 ^ { 2 } G in quiescence and nearly 1 kG during flares , while the majority of the non-detected objects have B \lesssim 50 G. Depending on the configuration and size of the magnetic loops , the surface magnetic fields may approach 1 kG even in quiescence , at most a factor of few smaller than in early-M dwarfs .
With the larger sample of sources we find continued evidence for ( i ) a sharp transition around spectral type M7 from a ratio of radio to X-ray luminosity of { log } ( L _ { R } / L _ { X } ) \sim - 15.5 to \gtrsim - 12 , ( ii ) increased radio activity with later spectral type , in contrast to H \alpha and X-ray observations , and ( iii ) an overall drop in the fraction of active sources from about 30 \% for M dwarfs to about 5 \% for L dwarfs , fully consistent with H \alpha and X-ray observations .
Taken together , these trends suggest that some late-M and L dwarfs are capable of generating 0.1 - 1 kG magnetic fields , but the overall drop in the fraction of such objects is likely accompanied by a change in the structure of the chromospheres and coronae , possibly due to the increasingly neutral atmospheres and/or a transition to a turbulent dynamo .
These possibilities can be best addressed through simultaneous radio , X-ray and H \alpha observations , which can trace the effect of magnetic fields on the coronae and chromospheres in a direct , rather than a statistical , manner .
Still , a more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars .