Solar modulation of cosmic ray electrons and positrons measured by the PAMELA experiment during the 23rd solar minimum.

Cosmic rays (CRs) are energetic particles mainly originating outside the Solar System in extremely powerful environments like supernovae remnants (SNRs). The cosmic radiation is composed primarily of high-energy protons, helium and atomic nuclei while only a small fraction are electrons, anti-protons and positrons. During propagation through the Galaxy, CRs interact with the interstellar matter and the Galactic magnetic field. Because of these interactions CRs lose energy and change their spectral features with respect to the injection spectrum. Moreover, before reaching the Earth, CRs traverse the heliosphere, a region of space formed by the continuously outward expanding solar wind. Propagation inside the solar environment make the CR spectra decrease in intensity and vary with time following the 11-year solar cycle. During solar minimum the intensity of CRs on Earth is maximum; the situation reverses during solar maximum. Above 30 GeV the effects of solar modulation are negligible. In this work a new measurement of the time dependent Galactic CR positron and electron energy spectra between 70 MeV and 50 GeV is presented. The analysis was conducted on data collected by the space borne PAMELA experiment during the period from July 2006 to January 2009. This was a period of intense solar minimum and negative solar magnetic field polarity. Long flight duration together with high proton rejection power make the PAMELA instrument the ideal apparatus for measuring the long-term variation of CR electrons and positrons. A total of seven spectra was obtained, each measures over six months period. This solution was a compromise between the time resolution and the statistics. Precise measurement of the electron and positron spectra allows to test the numerical 3D models which describe the transport of charged particles through the heliosphere. The results discussed in this thesis are relevant since they provide long-term observation of electron and positron spectra improving both time resolution and statistical precision with respect to previous experiments. Moreover the measurement is performed down to 70 MeV, an energy region not achievable by other space-borne experiments able to perform charge sign separation like AMS-02. A big effort has been invested to achieve precise results below 200 MeV since a change in the spectral shape is expected from the propagation models. Finally, the simultaneous measure of the positron and electron spectra allow a comprehensive study of the charge-sign dependent modulation of CRs.