Modeling of Heliospheric Modulation of Cosmic-Ray Positrons in a Very Quiet Heliosphere

Abstract

Heliospheric modulation conditions were unusually quite during the last solar minimum activity between Solar Cycles 23/24. Fortunately, the PAMELA space-experiment measured six-month averaged Galactic positron spectra for the period July 2006 to December 2009, over an energy range of 80 MeV to 30 GeV, which is important for solar modulation. The highest level of Galactic positrons was observed at Earth during the July-December 2009 period. A well-established, comprehensive three-dimensional (3D) numerical model is applied to study the modulation of the observed positron spectra. This model had been used previously to understand the modulation of Galactic protons and electrons also measured by PAMELA for the same period. First, a new very local interstellar spectrum for positrons is constructed, using the well-known GALPROP code together with the mentioned PAMELA observations. The 3D model is used to distinguish between the dominant mechanisms responsible for the heliospheric modulation of Galactic positrons, and to understand the effect of particle drift during this unusual minimum in particular, which is considered diffusion dominant, even though particle drift still had a significant role in modulating positrons. Lastly, the expected intensity of Galactic positrons during an A>0 polarity minimum, with similar heliospheric conditions than for 2006-2009, is predicted to be higher than what was observed by PAMELA for the 2006-2009 unusual minimum.

A Numerical Study of Cosmic Proton Modulation Using AMS-02 Observations

“…In this context, we note that as more precise CR spectra become available, e.g., daily fluxes or spectra over a wide energy range for a complete 11 yr solar cycle, it will become necessary to adapt an automated statistical tool to scan the full parameter space, investigating, e.g., if the rigidity dependence of all three diffusion coefficients is exactly the same, or if drift effects are changing meaningfully toward solar maximum activity; see, e.g., Martucci et al (2018) and Aslam et al (2019). Apart from proton spectra, the AMS-02 helium flux data have become available as well (Aguilar et al 2018a).…”

Current state and perspectives of Space Weather science in Italy

“…Thanks to the new-generation experiments, such as AMS-02 and PAMELA, the current solar cycle has been monitored with an unprecedented coverage in terms of multi-channel and timeresolved measurements on cosmic-ray leptons and nuclei (Adriani et al, 2016;Bindi et al, 2017;Aguilar et al, 2018aAguilar et al, , 2018bMartucci et al, 2018). The recent release of these data has generated widespread interest in both the astrophysical and Space Weather communities, leading to substantial advance in the theoretical understanding of the GCR transport and modulation in the heliosphere (Usoskin et al, 2017;Tomassetti et al, 2017Tomassetti et al, , 2018Aslam et al, 2019;Corti et al, 2019). The China Seismo-Electromagnetic Satellite (CSES) is part of a collaboration program between the China National Space Administration (CNSA) and ASI.…”

The solar modulation of protons and anti-protons

“…Neutron monitors also reported record high counts (cosmicrays.oulu.fi). Modulation studies for the period [2006][2007][2008][2009] conclude that all modulation process had played important roles during this unusual minimum period, especially particle drifts, but that diffusion was relatively dominant; see [3,4,12,13] and reviews [5,11].…”

“…Drift modulation models predict that positively charged CRs drift inward to the inner heliosphere mainly along the equator and outward via the polar regions during the negative polarity phase (A<0) so that they encounter the changing wavy HCS during their entry (as happened from 2001-2012), and that the drift direction reverses when the Sun's magnetic polarity is completely switched. For negatively charged CRs (anti-protons, electrons) the drift velocity is oppositely directed [12,13]. We are interested in how solar modulation varies from one activity minimum to the next, and how particle drift effects vary from one polarity phase to another.…”

“…We present preliminary results of a proton and anti-proton modeling study, a continuation of previous detailed studies using a comprehensive three-dimensional (3D) modulation model including drifts as applied to galactic protons [3,4], electrons [12] and positrons [13] using PAMELA observations [1,[21][22][23] during quiet heliospheric conditions (2006)(2007)(2008)(2009). We consider protons from July 2006-May 2017 for which solar rotation averaged PAMELA spectra over an energy range of ∼ 80 MeV to 50 GeV are available up to Dec 2009 [1] and Bartel rotation averaged AMS-02 spectra over an energy range of ∼ 1 GeV to 50 GeV from May 2011-May 2017 [2].…”