  We report on the first continuous, 80 day optical monitoring of the
transitional millisecond pulsar PSR J1023+0038 carried out in mid-2017 with
Kepler in the K2 configuration, when an X-ray subluminous accretion disk was
present in the binary. Flares lasting from minutes to 14 hr were observed for
15.6% of the time, which is a larger fraction than previously reported on the
basis of X-ray and past optical observations, and more frequently when the
companion was at the superior conjunction of the orbit. A sinusoidal modulation
at the binary orbital period was also present with an amplitude of ~16%, which
varied by a few percent over timescales of days, and with a maximum that took
place 890 +/- 85 s earlier than the superior conjunction of the donor. We
interpret these phenomena in terms of reprocessing of the X-ray emission by an
asymmetrically heated companion star surface and/or a non-axisymmetric outflow
possibly launched close to the inner Lagrangian point. Furthermore, the
non-flaring average emission varied by up to ~ 40% over a time scale of days in
the absence of correspondingly large variations of the irradiating X-ray flux.
The latter suggests that the observed changes in the average optical luminosity
might be due to variations of the geometry, size, and/or mass accretion rate in
the outer regions of the accretion disk.
