  Since the launch of ESA's Gaia satellite in December 2013, the 106
large-format scientific CCDs onboard have been operating at L2. Due to a
combination of the high-precision measurement requirements of the mission and
the predicted proton environment at L2, the effect of non-ionizing radiation
damage on the detectors was early identified pre-launch as potentially imposing
a major limitation on the scientific value of the data. In this paper we
compare pre-flight radiation-induced Charge Transfer Inefficiency (CTI)
predictions against in-flight measurements, focusing especially on charge
injection diagnostics, as well as correlating these CTI diagnostic results with
solar proton event data. We show that L2-directed solar activity has been
relatively low since launch, and radiation damage (so far) is less than
originally expected. Despite this, there are clear cases of correlation between
earth-directed solar coronal mass ejection events and abrupt changes in CTI
diagnostics over time. These sudden jumps are lying on top of a rather constant
increase in CTI which we show is primarily due to the continuous bombardment of
the devices by high-energy Galactic Cosmic Rays. We examine the possible
reasons for the lower than expected levels of CTI as well as examining the
effect of controlled payload heating events on the CTI diagnostics.
Radiation-induced CTI in the CCD serial registers and effects of ionizing
radiation are also correspondingly lower than expected, however these topics
are not examined here in detail.
