with a fraction of their charge, mitigating the effects of CTI.

Finally, of particular importance for CTI effects is the detectors’ temperature.

On Gaia, the CCDs nominal operating temperature is 163 K, with some variations of several degrees observed over the focal plane.

The temperature has been found to be stable over time, albeit with low–level seasonal variations dependent on heliocentric distance in addition to abrupt variations in short intervals during specific mission phases such as the decontamination events and other disturbances to power dissipation in the focal plane array caused by operational changes (see Section 6.2).
2.2.

Radiation environment at L2 The radiation environment experienced by Gaia at L2 is the focus of Crowley (in prep).

Its analysis is based on the data acquired by the 14 Star Mappers, the devices dedicated to the stars’ on-board detections.

In particular, as part of the automated detection algorithm, Prompt Particle Events (PPEs) are identified.

These are features in the SM images that are too sharp in comparison to the Gaia Point Spread Function shape to be considered true signals from point-like sources, and are instead attributed to high-energy particles, mostly protons, either cosmic rays or originating from the Sun.

The observed PPEs rate can therefore be considered as a real-time measure of the radiation environment experienced by Gaia.

[Table: Table 1: Common silicon defect species
Type Symbol Energy (eV) Phosphorous Si-E 0.44 Divacancy V-V 0.41 Unknown 0.34 Multi-vacancy V-V-V 0.20 Oxygen Si-A 0.17
The impact of high energy particles on the detectors can cause ionizing or non-ionizing radiation. In the first instance irradiation is revealed over time as shifts in the measured CCDs baseline voltage values (Srour & McGarrity 1988), that can be compensated by hardware setting adjustments during the course of the mission. The non-ionizing radiation instead consists of the impact of high energy particles with the silicon crystal of the detectors, that can lead to the displacement of one or multiple atoms from their original positions in the lattice structure, and is therefore also known as displacement damage (van Lint 1963); the atoms knocked out of their original sites leave a vacancy, that over time can be filled by impurities already present in the detector, such as Phosphorous or Oxygen atoms (Watkins & Corbett 1964), or form a stable bonding with another vacancy (a type of defect called a Divacancy, Marshall et al. (1989)). These defects can become permanent, generating spurious energy levels between the valence and the conduction band of the semiconductor, where electrons can become temporarily trapped during the CCD readout process. The amount of time after which a trapped electron is released back into the conduction band is called the release (or emission) time, and it characterises the defect species, as it is a function of the specific trap energy level and the temperature at which the camera is operated. The trapping of electrons during the signal readout in CCDs is the cause of CTI, quantified as the fractional loss of charge during the transfer between two pixels. Common defects of silicon devices with their characteristic energy below the conduction band are listed in Table 1. On board the Gaia spacecraft, the parallel CTI level and its evolution are monitored using the same blocks of Charge Injection (CI) lines that are used to mitigate the effect of radiation damage. The pixels following the blocks of CI lines present characteristic charge trails, that are caused by the release of previously captured electrons in a stochastic process. The total charge in the trails provides a measurement of the number of active defects in the detector, and is a proxy for the CTI in the parallel direction. Figure 1 shows the measured intensity of the parallel trails, averaged over all the AF CCDs, for the entire span of the Gaia mission, with time expressed in units of On Board Mission Time (OBMT) revolutions, corresponding to approximately one satellite rotation (Lindegren et al. 2018). In the serial direction, a similar analyses is performed using the charge trails in the serial register post-scan pixels measured during dedicated serial CTI calibration data sets. These special engineering activities are described in detail in Section 3. The analysis of the charge trails in Gaia CCDs has revealed a number of significant results that were first reported by Crowley et al. (2016b) using data from the initial two years of the mission. The measured evolution of the damage is characterised by a steady, gradual increase since the beginning of the mission, that is attributed to the continuous irradiation from the Galactic Cosmic Rays (CR) flux (mostly protons and alpha particles). A second component of the damage evolution is seen as sudden steps at specific times in correspondence with significant solar activity events. The strongest of these occurred on the]