Solar modulation of cosmic ray positrons in a very quiet heliosphere

TL;DR

This study models and analyzes the solar modulation of cosmic ray positrons during a very quiet solar cycle, comparing predictions with PAMELA data to understand charge-sign-dependent effects and the role of drift processes.

Contribution

It presents a comprehensive 3D drift modulation model for positrons during a quiet solar cycle, incorporating new local interstellar spectra and detailed comparisons with observational data.

Findings

First meaningful modulation factor computed for positrons from 50 GeV to 1 MeV.

Electron to positron ratios analyzed over time and rigidity during the quiet period.

Model validated against PAMELA spectra, enhancing understanding of charge-sign-dependent modulation.

Abstract

Since the beginning of the space exploration era, solar activity was observed at its lowest level during 2006 to 2009. During this period, the PAMELA space experiment observed spectra for galactic cosmic rays, specifically for protons, electrons and positrons over a wide energy range, during what is called an A < 0 solar magnetic polarity cycle. Drift theory predicts a difference in the behaviour for these oppositely charge particles during A < 0 cycles. An opportunity was thus created to study the predicted charge-sign-dependent modulation, also now for very quiet heliospheric conditions. A comprehensive three-dimensional, drift modulation model has been used to study the solar modulation for cosmic rays in detail with extensive comparison to the observed PAMELA spectra for the mentioned period. First, this was done for protons and secondly for electrons, as already published, to test and to authenticate the modelling approach and then to come to a better understanding and appreciation of the underlying physics, such as diffusion and drift theory. The results were also used to make predictions of how cosmic rays are differently modulated down to low energies (1 MeV) for the two magnetic polarity cycles of the Sun, and what role drifts play in this process. All computed solutions are based on new very local interstellar spectra, now also done for positrons. This report is focussed on detailed aspects of the solar modulation of positrons during the extraordinary quiet solar modulation period from 2006 to 2009. For the first time, a meaningful modulation factor in the heliosphere is computed for positrons, from 50 GeV down to 1 MeV, as well as the electron to positron ratios as a function of time and rigidity for the mentioned period.