Difference in the heliospheric modulation of cosmic-ray protons and electrons during the solar minimum period of 2006 to 2009

Center for Space Research, North-West University, 2520 Potchefstroom, South Africa
e-mail: marius.potgieter@nwu.ac.za

Received: 2 November 2016
Accepted: 8 January 2017

Abstract

Context. The PAMELA experiment observed galactic proton and electron spectra down to 70 MV and 400 MV, respectively, from mid-2006 to the end of 2009 during what is called an A < 0 solar magnetic polarity cycle. During this period, solar activity was at its lowest level since the beginning of the space exploration era. This provides the opportunity to study charge-sign-dependent modulation under very quiet heliospheric conditions.

Aims. Drift theory for the solar modulation of cosmic rays predicts that the intensity of protons at the Earth is expected to show a different rate of recovery towards solar minimum when compared to electrons during A < 0 cycles. These charge-sign related differences are investigated.

Methods. The solutions of a comprehensive three-dimensional drift model are compared to PAMELA spectra to authenticate the modelling approach and then to make predictions of how electrons and protons are differently modulated down to 1 MeV, based on new very local interstellar spectra.

Results. The comparison of observations and modelling provides insight into how the rigidity dependence of the three major diffusion coefficients changes during such quiet modulation conditions. How drift effects dissipate above several GeV and below 100 MeV is illustrated for both protons and electrons. The modulation that occurred at the Earth during this quiet period is shown as a function of rigidity and time. The e⁻/p ratio is computed from 10 MV to 50 GV for this period and a prediction is made for what may be observed in terms of spectra during the next A > 0 solar minimum.

Conclusions. The presence of drifts during this quiet period is established based on the presented modelling and PAMELA measurements. Drift effects for protons and electrons are quantified in terms of their rigidity and temporal development from 2006 to 2009.