A full solar cycle of proton and helium measurements

Abstract

Time-dependent energy spectra of galactic cosmic rays (GCRs) carry fundamental information regarding their origin and propagation. When observed at the Earth, these spectra are significantly affected by the solar wind and the embedded solar magnetic field that permeates the heliosphere, changing significantly over an 11-year solar cycle. Energy spectra of GCRs measured during different epochs of solar activity provide crucial information for a thorough understanding of solar and heliospheric phenomena. The PAMELA experiment had collected data for almost ten years (15 June 2006 -23 January 2016), including the minimum phase of solar cycle 23 and the maximum phase of solar cycle 24. Here, we present spectra for protons and helium-nuclei measured by the PAMELA instrument from 2006 to 2014. Time profiles of the proton-to-helium flux ratio at various rigidities were also investigated, allowing the study of all characteristic features resulting from their different mass-to-charge ratio and the difference in the shape of their respective local interstellar spectra. The force-field approximation of the solar modulation was used to relate these dependencies to the different shapes of the local interstellar proton and helium-nuclei spectra.

Time and Charge-Sign Dependence of the Heliospheric Modulation of Cosmic Rays

“…Adriani et al (2009a) reported the first results of PAMELA on the e + fraction over an energy range of 1.5 -100 GeV for the period July 2006 to February 2008. As a continuation, observations of anti-protons (Adriani et al 2009b(Adriani et al , 2010, protons and several isotopes were reported with a varying time resolution (Adriani et al 2011a(Adriani et al , 2013a(Adriani et al , 2016b, also Carrington rotation and longer averaged helium (Adriani et al 2011a;Marcelli et al 2020), 6-months and longer averaged electrons (Adriani et al 2011b(Adriani et al , 2015 and positrons (Adriani et al 2013b(Adriani et al , 2016aMunini, 2015).…”

“…This particular model includes gradient, curvature and HCS drift, and have been applied successively earlier and also recently to the modulation studies of GCRs such as protons (Potgieter et al 2014;, electrons (Potgieter et al 2015;, positrons Aslam et al 2019a), and helium nuclei (Ngobeni et al 2020). All these studies are related to observations by the PAMELA experiment for the unusual solar minimum of 2006 to 2009 (Adriani et al 2013aMunini et al 2015Munini et al , 2017Marcelli et al 2020). Corti et al (2019) applied the same numerical code to study the modulation of galactic protons and helium observed by the AMS 02 (Aguilar et al 2018a) for 2011 to 2017.…”

“…For positrons, however, a different approach must be taken, utilizing the availability of sophisticated and comprehensive galactic propagation models (GAL-PROP) along with the 3D numerical modulation model to construct new self-consistent VLIS for GCRs as acknowledged and described in detail by Bisschoff et al (2019); see also Bisschoff & Potgieter (2014 and references therein. These self-consistent VLIS based on GALPROP models for different GCR particles together with the mentioned observations, where available, have been used extensively in recent modeling studies of GCR protons (Aslam et al 2019a;Ngobeni et al 2020), electrons and positrons (Aslam et al 2019b), and helium (Marcelli et al 2020;Ngobeni et al 2020). Positron and electron VLIS computed with this approach are discussed next.…”