  We present coronal density profiles derived from low-frequency (80-240 MHz)
imaging of three type III solar radio bursts observed at the limb by the
Murchison Widefield Array (MWA). Each event is associated with a white light
streamer at larger heights and is plausibly associated with thin extreme
ultraviolet rays at lower heights. Assuming harmonic plasma emission, we find
average electron densities of 1.8 x10^8 cm^-3 down to 0.20 x10^8 cm^-3 at
heights of 1.3 to 1.9 solar radii. These values represent roughly 2.4-5.4x
enhancements over canonical background levels and are comparable to the highest
streamer densities obtained from data at other wavelengths. Assuming
fundamental emission instead would increase the densities by a factor of 4.
High densities inferred from type III source heights can be explained by
assuming that the exciting electron beams travel along overdense fibers or by
radio propagation effects that may cause a source to appear at a larger height
than the true emission site. We review the arguments for both scenarios in
light of recent results. We compare the extent of the quiescent corona to model
predictions to estimate the impact of propagation effects, which we conclude
can only partially explain the apparent density enhancements. Finally, we use
the time- and frequency-varying source positions to estimate electron beam
speeds of between 0.24 and 0.60 c.
