  We visualize plasma bubbles driven by 0.67 PW laser pulses in plasma of
density $n_e \approx 5\times10^{17}$ ${\rm cm}^{-3}$ by imaging Faraday
rotation patterns imprinted on linearly-polarized probe pulses of wavelength
$\lambda_{pr} = 1.05 \mu$m and duration $\tau_{pr} = 2$ ps or $1$ ps that cross
the bubble's path at right angles. When the bubble captures and accelerates
tens to hundreds of pC of electron charge, we observe two parallel streaks of
length $c\tau_{pr}$ straddling the drive pulse propagation axis, separated by
$\sim45$ $\mu$m, in which probe polarization rotates by $0.3^\circ$ to more
than $5^\circ$ in opposite directions. Accompanying simulations show that they
result from Faraday rotation within portions of dense bubble side walls that
are pervaded by the azimuthal magnetic field of accelerating electrons during
the probe transit across the bubble. Analysis of the width of the streaks shows
that quasi-monoenergetic high-energy electrons and trailing lower energy
electrons inside the bubble contribute distinguishable portions of the observed
signals, and that relativistic flow of sheath electrons suppresses Faraday
rotation from the rear of the bubble. The results demonstrate favorable scaling
of Faraday rotation diagnostics to $40\times$ lower plasma density than
previously demonstrated.
