We present the activity and kinematics of a representative volume-limited ( 20 pc ) sample of 152 late-M and L dwarfs ( M7–L8 ) photometrically selected from the Two Micron All-Sky Survey ( 2MASS ) .
Using new proper motion measurements and spectrophotometric distance estimates , we calculate tangential velocities .
The sample has a mean tangential velocity of \langle V _ { tan } \rangle = 31.5 ~ { } km~ { } s ^ { -1 } , a velocity dispersion of \sigma _ { tan } = 20.7 ~ { } km~ { } s ^ { -1 } , and a maximum tangential velocity of V _ { tan } = 138.8 ~ { } km~ { } s ^ { -1 } .
These kinematic results are in excellent agreement with previous studies of ultracool dwarfs in the local solar neighborhood .
H \alpha emission , an indicator of chromospheric activity , was detected in 63 of 81 late-M dwarfs and 16 of 69 L dwarfs examined .
We find a lack of correlation between activity strength , measured by log ( F _ { H \alpha } / F _ { bol } ) , and V _ { tan } , though velocity distributions suggest that the active dwarfs in our sample are slightly younger than the inactive dwarfs .
Consistent with previous studies of activity in ultracool dwarfs , we find that the fraction of H \alpha emitting objects per spectral type peaks at spectral type M7 and declines through mid-L dwarfs .
Activity strength is similarly correlated with spectral type for spectral types later than M7 .
Eleven dwarfs out of 150 show evidence of variability , ranging from small fluctuations to large flare events .
We estimate a flare cycle of \sim 5 % for late-M dwarfs and \sim 2 % for L dwarfs .
Observations of strong , variable activity on the L1 dwarf 2MASS J10224821+5825453 and an amazing flare event on the the M7 dwarf 2MASS J1028404 - 143843 are discussed .