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New results on the short-term galactic cosmic ray (GCR) intensity variation(Forbush decrease) in December 2006 measured by the PAMELA instrument arepresented. Forbush decreases are sudden suppressions of the GCR intensitieswhich are associated with the passage of interplanetary transients such asshocks and interplanetary coronal mass ejections (ICMEs). Most of the pastmeasurements of this phenomenon were carried out with ground-based detectorssuch as neutron monitors or muon telescopes. These techniques allow only theindirect detection of the overall GCR intensity over an integrated energyrange. For the first time, thanks to the unique features of the PAMELA magneticspectrometer, the Forbush decrease commencing on 2006 December 14, following aCME at the Sun on 2006 December 13 was studied in a wide rigidity range (0.4 -20 GV) and for different species of GCRs detected directly in space. The dailyaveraged GCR proton intensity was used to investigate the rigidity dependenceof the amplitude and the recovery time of the Forbush decrease. Additionally,for the first time, the temporal variations in the helium and electronintensities during a Forbush decrease were studied. Interestingly, the temporalevolutions of the helium and proton intensities during the Forbush decreasewere found in good agreement, while the low rigidity electrons (< 2 GV)displayed a faster recovery. This difference in the electron recovery isinterpreted as a charge-sign dependence introduced by drift motions experiencedby the GCRs during their propagation through the heliosphere.

Evidence of energy and charge sign dependence of the recovery time for the December 2006 Forbush event measured by the PAMELA experiment