  Ablating a target with an ultraintense laser pulse can create a cloud of
collisionless plasma. A density ramp forms, in which the plasma density
decreases and the ion's mean speed increases with distance from the plasma
source. Its width increases with time. Electrons lose energy in the ion's
expansion direction, which gives them a temperature anisotropy. We study with
one-dimensional particle-in-cell simulations the expansion of a dense plasma
into a dilute one, yielding a density ramp similar to that in laser plasma
experiments and a thermal-anisotropy-driven instability. Non-propagating
Weibel-type modes grow in the simulation with no initial magnetic field. Their
magnetic field diffuses across the the shock and expands upstream. Circularly
polarized propagating Whistler waves grow in a second simulation, in which a
magnetic field is aligned with the ion expansion direction. Both wave modes are
driven by non-resonant instabilities, they have a similar exponential growth
rates, and they can leave the density ramp and expand into the dilute plasma.
Their large magnetic amplitude should make them detectable in experimental
settings.
