  Recently unanticipated magnetic activity in ultracool dwarfs (UCDs, spectral
classes later than M7) have emerged from a number of radio observations. The
highly (up to 100%) circularly polarized nature and high brightness temperature
of the emission has been interpreted as an effective amplification mechanism of
the high-frequency electromagnetic waves, the electron cyclotron maser
instability (ECMI). In order to understand the magnetic topology and the
properties of the radio emitting region and associated plasmas in these
ultracool dwarfs and interpret the origin of radio pulses and their radiation
mechanism, we built an active region model, based on the rotation of the UCD
and the ECMI mechanism. ECMI mechanism is responsible for the radio bursts from
the magnetic tubes and the rotation of the dwarf can modulate the integral of
flux with respect to time. The high degree of variability in the brightness and
the diverse profile of pulses can be interpreted in terms of a large-scale hot
active region with extended magnetic structure existing in the magnetosphere of
TVLM 513-46546. We suggest the time profile of the radio light curve is in the
form of power law in the model. The radio emitting region consists of
complicated substructure. With this model, we can determine the nature (e.g.
size, temperature, density) of the radio emitting region and plasma. The
magnetic topology can also be constrained. We compare our predicted X-ray flux
with Chandra X-ray observation of TVLM 513-46546. Although the X-ray detection
is only marginally significant, our predicted flux is significantly lower than
the observed flux. We suggest more observations at multi-wavelength will help
us understand the magnetic field structure and plasma behavior on the ultracool
dwarf.
