The long time in orbit of the PAMELA mission (almost 10 years) has allowed a continuous monitoring of the ever-changing solar activity. The resulting modulation of the galactic cosmic rays (GCRs) has been studied in time to follow the significant global variations in their intensity and in their energy as a function of position inside the heliosphere.
Moreover, short term variations like Solar Events and Forbush decreases, have been widely studied by PAMELA, particularly focusing on spectral evolution as a function of both energy and time; evidences of a charge-sign dependence, introduced by drift motions experienced by the GCRs during their propagation through the heliosphere, where found in the recovery time of protons, Helium and electrons during the event of December 2006.
Solar Modulation
PAMELA has started to operate during the very long and peculiar 23rd solar minimum, with a A– polarity, when the magnetic dipole projection on the solar rotational axis and the rotational axis itself are anti-parallel, but also spent a significant part of its data-taking period in the rising phase of the 24th cycle (with A+ polarity).
The last very stable solar activity minimum represents an ideal condition to study solar modulation, allowing to gain a better understanding of the mechanisms dominating the charged particle transport in the solar environment (diffusion, convection, adiabatic energy losses and, in particular, drift). PAMELA studied solar modulation of various particle species, like protons and electrons. Modulation for Helium nuclei, positron etc will be published soon.
Figure 1 shows the low energy galactic proton absolute flux measured by PAMELA between June 2006, instrument launch date, and December 2009, when the solar activity reached its minimum (top) and the same absolute flux measured between January 2010 and February 2014, the maximum of the activity cycle (bottom).
The complete time profiles of rigidity intervals from 0.4 GV to 30 GV, normalized to July 2006, measured by PAMELA between July 2006 and May 2014 is shown in the topmost panel of Figure 2. As a comparison, the normalized Oulu neutron monitor count rate (mid panel) and the average daily number of sunspot (bottom panel) are also shown.