  New results on the short-term galactic cosmic ray (GCR) intensity variation
(Forbush decrease) in December 2006 measured by the PAMELA instrument are
presented. Forbush decreases are sudden suppressions of the GCR intensities
which are associated with the passage of interplanetary transients such as
shocks and interplanetary coronal mass ejections (ICMEs). Most of the past
measurements of this phenomenon were carried out with ground-based detectors
such as neutron monitors or muon telescopes. These techniques allow only the
indirect detection of the overall GCR intensity over an integrated energy
range. For the first time, thanks to the unique features of the PAMELA magnetic
spectrometer, the Forbush decrease commencing on 2006 December 14, following a
CME 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 daily
averaged GCR proton intensity was used to investigate the rigidity dependence
of the amplitude and the recovery time of the Forbush decrease. Additionally,
for the first time, the temporal variations in the helium and electron
intensities during a Forbush decrease were studied. Interestingly, the temporal
evolutions of the helium and proton intensities during the Forbush decrease
were found in good agreement, while the low rigidity electrons (< 2 GV)
displayed a faster recovery. This difference in the electron recovery is
interpreted as a charge-sign dependence introduced by drift motions experienced
by the GCRs during their propagation through the heliosphere.
