Time Dependence of the Electron and Positron Components of the Cosmic Radiation Measured by the PAMELA Experiment between July 2006 and December 2015

Cosmic-ray electrons and positrons are a unique probe of the propagation of cosmic rays as well as of the nature and distribution of particle sources in our Galaxy. Recent measurements of these particles are challenging our basic understanding of the mechanisms of production, acceleration, and propagation of cosmic rays. Particularly striking are the differences between the low energy results collected by the space-borne PAMELA and AMS-02 experiments and older measurements pointing to sign-charge dependence of the solar modulation of cosmic-ray spectra. The PAMELA experiment has been measuring the time variation of the positron and electron intensity at Earth from July 2006 to December 2015 covering the period for the minimum of solar cycle 23 (2006–2009) until the middle of the maximum of solar cycle 24, through the polarity reversal of the heliospheric magnetic field which took place between 2013 and 2014. The positron to electron ratio measured in this time period clearly shows a sign-charge dependence of the solar modulation introduced by particle drifts. These results provide the first clear and continuous observation of how drift effects on solar modulation have unfolded with time from solar minimum to solar maximum and their dependence on the particle rigidity and the cyclic polarity of the solar magnetic field.

Figure 1

The positron to electron ratios relative to July–December 2006 measured at Earth by the PAMELA experiment for three different energy intervals. The colored lines provide connection among the points. Data were selected on three month time intervals between July 2006 and December 2015. For 2010 only two time intervals were considered since the instrument was switched off from April to August. The shaded area corresponds to the period with no well-defined HMF polarity [23]. The bottom panel shows the Oulu neutron monitor count rate (data taken from Ref. [24]). Data are normalized to July 2006.

Figure 2

The positron fraction derived in this work for three time periods: July 2006–December 2009, (solar minimum, as in Ref. [5]), May 2011–November 2013 (as AMS-02 results [8]), January–December 2015, along with other recent measurements: $\mathrm{HEAT}94+95$ [15], CAPRICE94 [16], AMS-01 [17], AMS-02 [8]. The results from Refs. [15, 16, 17] refer to the previous $A>0$ solar cycle.