  The nonlinear state of a high-beta collisionless plasma is investigated when
an imposed linear shear amplifies or diminishes a uniform magnetic field,
driving pressure anisotropies and hence firehose/mirror instabilities. The
evolution of the resulting microscale turbulence is considered when the shear
is switched off or reversed after one shear time (mimicking local behaviour of
a macroscopic flow), so a new macroscale configuration is superimposed on the
microscale state left behind by the previous one. There is a threshold value of
plasma beta: when $\beta\ll\Omega/S$ (ion cyclotron frequency/shear rate), the
emergence of firehose/mirror fluctuations driven unstable by shear and their
disappearance when the shear is removed/reversed are quasi-instantaneous
compared to the shear time, viz., the decay time of these fluctuations is
$\sim\beta/\Omega \ll 1/S$ (this result follows from the free decay of the
fluctuations being constrained by the same marginal-stability thresholds as
their growth). In contrast, when $\beta\gtrsim\Omega/S$ ("ultra-high" beta),
the old microscale state can only be removed on the shear timescale. In this
regime, driven firehose fluctuations grow secularly to order-unity amplitudes,
compensating for the decay of the mean field and so pinning pressure anisotropy
at marginal stability with no appreciable scattering of particles---which is
unlike what happens at moderate $\beta$. When the shear reverses, the shearing
away of this firehose turbulence compensates for the increase in the mean field
and thus prevents growth of the pressure anisotropy, stopping the system from
going mirror-unstable. Therefore, at ultra-high beta, the system stays close to
the firehose threshold, the mirror instability is largely suppressed, while the
mean magnetic energy barely changes at all. Implications for plasma dynamo and
thus the origin of cosmic magnetism are discussed.
