A number of radio-loud ultra cool dwarf stars ( UCD ) exhibit both continuous broadband and highly polarized pulsed radio emission .
In order to determine the nature of the emission and the physical characteristics in the source region , we have made multi-epoch , wideband spectral observations of TVLMÂ 0513-46Â and 2MÂ 0746+20 .
We combine these observations with archival radio data to fully characterize both the temporal and spectral properties of the radio emission .
The continuum spectral energy distribution can be well modeled using gyrosynchrotron emission from mildly relativistic electrons in a dipolar field .
The pulsed emission exhibits a variety of time-variable characteristics , including frequency drifts , frequency cutoffs , and multiple pulses per period .
For 2MÂ 0746+20Â we determine a pulse period consistent with previously determined values .
We modeled locations of pulsed emission using an oblique rotating magnetospheric model with beamed electron cyclotron maser ( ECM ) sources .
The best-fit models have narrow ECM beaming angles aligned with the local source magnetic field direction , except for one isolated burst from 2MÂ 0746+20 .
For TVLMÂ 0513-46 , the best-fit rotation axis inclination is nearly orthogonal to the line of sight .
For 2MÂ 0746+20Â we found a good fit using a fixed inclination i = 36 \arcdeg , determined from optical observations .
For both stars the ECM sources are located near feet of magnetic loops with radial extents 1.2 R _ { s } -2.7 R _ { s } and surface fields 2.2 - 2.5 kG .
These results support recent suggestions that radio over-luminous UCDs have a global ‘ weak field ’ non-axisymmetric magnetic topologies .