  The Planck High Frequency Instrument (HFI) has been surveying the sky
continuously from the second Lagrangian point (L2) between August 2009 and
January 2012. It operates with 52 high impedance bolometers cooled at 100mK in
a range of frequency between 100 GHz and 1THz with unprecedented sensivity, but
strong coupling with cosmic radiation. At L2, the particle flux is about 5
$cm^{-2} s^{-1}$ and is dominated by protons incident on the spacecraft.
Protons with an energy above 40MeV can penetrate the focal plane unit box
causing two different effects: glitches in the raw data from direct interaction
of cosmic rays with detectors (producing a data loss of about 15% at the end of
the mission) and thermal drifts in the bolometer plate at 100mK adding
non-gaussian noise at frequencies below 0.1Hz. The HFI consortium has made
strong efforts in order to correct for this effect on the time ordered data and
final Planck maps. This work intends to give a view of the physical explanation
of the glitches observed in the HFI instrument in-flight. To reach this goal,
we performed several ground-based experiments using protons and $\alpha$
particles to test the impact of particles on the HFI spare bolometers with a
better control of the environmental conditions with respect to the in-flight
data. We have shown that the dominant part of glitches observed in the data
comes from the impact of cosmic rays in the silicon die frame supporting the
micro-machinced bolometric detectors propagating energy mainly by ballistic
phonons and by thermal diffusion. The implications of these results for future
satellite missions will be discussed.
