  After the focal plane of the HFI instrument of the Planck mission (launched
in May 2009) reached its operational temperature, we observed thermal
signatures of interactions of cosmic rays with the Planck satellite, located at
the L2 Lagrange point. When a particle hits a component of the bolometers (e.g.
thermometer, grid or wafer) mounted on the focal plane of HFI, a thermal spike
(called glitch) due to energy deposition is measured. Processing these data
revealed another effect due to particle showers of high energy cosmic rays:
High Coincidence Events (HCE), composed of glitches occurring coincidentally in
many detectors followed by a temperature increase from the nK to the uK. A flux
of about 100 HCE per hour has been estimated. Two types of HCE have been
detected: fast and slow. For the first type, the untouched bolometers reached,
within a few seconds, the same temperature as those which were "touched". This
can be explained by the storage of the energy deposited in the stainless steel
focal plane. The second type of HCE is not fully understood yet. These effects
might be explained by an extra conduction due to the helium released by
cryogenic surfaces and creating a temporary thermal link between the different
stages of the HFI.
