  Spatial scan observations using WFC3's IR channel exhibit time-dependent
systematics (in the form of a ramp or hook) that have been attributed to the
effects of persistence. The amplitude of these systematics is often two orders
of magnitude larger than the signal sizes of interest and, therefore, must be
carefully modelled and removed. The goal of this calibration program
(CAL-15400) is to mitigate these systematics by continuously illuminating the
detector while repeatedly reading it out during Earth occultation (termed
pre-flashing). Compared to standard observations, we are able to reduce the
amplitude of the systematic effect by a factor of $\sim$7 (from 1.30% to
-0.19%), thus confirming our hypothesis that the detector more quickly reaches
an equilibrium state when subjected to higher flux levels. Compared to the
latest modeling techniques (Zhou et al., 2017), we achieve a marginal
improvement in the white light curve precision ($\Delta$rms = -8{\pm}9 ppm);
therefore, pre-flashing is an equally effective means to mitigate WFC3's
instrument systematics. We conclude that pre-flashing does not warrant future
consideration due to the increase in the number of channel select mechanism
(CSM) motions, effort required to implement, and equivalent ability to model
instrument systematics with current techniques.
