  Using 2D particle-in-cell (PIC) plasma simulations we study electron
acceleration by temperature anisotropy instabilities, assuming conditions
typical of above-the-loop-top (ALT) sources in solar flares. We focus on the
long-term effect of $T_{e,\perp} > T_{e,\parallel}$ instabilities by driving
the anisotropy growth during the entire simulation time, through imposing a
shearing or a compressing plasma velocity ($T_{e,\perp}$ and $T_{e,\parallel}$
are the temperatures perpendicular and parallel to the magnetic field). This
magnetic growth makes $T_{e,\perp}/T_{e,\parallel}$ grow due to electron
magnetic moment conservation, and amplifies the ratio $\omega_{ce}/\omega_{pe}$
from $\sim 0.53$ to $\sim 2$ ($\omega_{ce}$ and $\omega_{pe}$ are the electron
cyclotron and plasma frequencies, respectively). In the regime
$\omega_{ce}/\omega_{pe}\lesssim 1.2-1.7$ the instability is dominated by
oblique, quasi-electrostatic (OQES) modes, and the acceleration is inefficient.
When $\omega_{ce}/\omega_{pe}$ has grown to $\omega_{ce}/\omega_{pe}\gtrsim
1.2-1.7$, electrons are efficiently accelerated by the inelastic scattering
provided by unstable parallel, electromagnetic z (PEMZ) modes. After
$\omega_{ce}/\omega_{pe}$ reaches $\sim 2$, the electron energy spectra show
nonthermal tails that differ between the shearing and compressing cases. In the
shearing case, the tail resembles a power-law of index $\alpha_s \sim$ 2.9 plus
a high-energy bump reaching $\sim 300$ keV. In the compressing runs, $\alpha_s
\sim$ 3.7 with a spectral break above $\sim 500$ keV. This difference can be
explained by the different temperature evolutions in these two types of
simulations, suggesting a critical role played by the type of anisotropy
driving, $\omega_{ce}/\omega_{pe}$ and the electron temperature in the
efficiency of the acceleration.
