Global gradients for cosmic-ray protons in the heliosphere during the solar minimum of cycle 23/24

Abstract

Global gradients for cosmic-ray (CR) protons in the heliosphere are computed with a comprehensive modulation model for the recent prolonged solar minimum of Cycle 23/24. Fortunately, the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) and Ulysses/KET (Kiel Electron Telescope) instruments simultaneously observed proton intensities for the period between July 2006 and June 2009. Radial and latitudinal gradients are calculated from measurements, with the latter possible because Ulysses changed its position significantly in the heliocentric meridional plane during this period. The modulation model is set up for the conditions that prevailed during this unusual solar minimum period to gain insight into the role role that particle drifts played in establishing the observed gradients for this period. Four year-end PAMELA proton spectra were reproduced with the model, from 2006 to 2009, followed by corresponding radial profiles that were computed along the Voyager-1 trajectory, and compared to available observations. It is found that the computed intensity levels are in agreement with solar minimum observations from Voyager-1 at multiple energies. The model also reproduces the steep intensity increase observed when Voyager-1 crossed the heliopause region. Good agreement is found between computed and observed latitudinal gradients so that we conclude that the model gives a most reasonable representation of modulation conditions from the Earth to the heliopause for the period from 2006 to 2009. As a characteristic feature of CR drifts, the most negative latitudinal gradient is computed for 2009, with a value of -0.15%/degree around 600 MV. The maximum radial gradient in the inner heliosphere (as covered by Ulysses) also occurs in this range, with the highest value of 4.25%/AU in 2009.

arXiv comment17 pages, 9 figures. To be published in Solar Physics

References in corpus (9)

• Solar Modulation of Cosmic Rays
• Time dependence of the proton flux measured by PAMELA during the July 2006 - December 2009 solar minimum
• Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009
• Time dependence of the e^- flux measured by PAMELA during the July 2006 - December 2009 solar minimum
• Modulation of galactic cosmic rays during the unusual solar minimum between cycles 23 and 24
• Very local interstellar spectra for galactic electrons, protons and helium
• Time-Dependent Modulation of Cosmic Rays in the Heliosphere
• The effect of magnetic field modifications on the modulation of cosmic rays in the heliosphere
• Spatial gradients of GCR protons in the inner heliosphere derived from Ulysses COSPIN/KET and PAMELA measurements

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• Modeling of Heliospheric Modulation of Cosmic-ray Positrons in a Very Quiet Heliosphere
• New evidence for charge-sign dependent modulation during the solar minimum of 2006 to 2009
• Time and Charge-Sign Dependence of the Heliospheric Modulation of Cosmic Rays
• Study of the 27-day variations in GCR fluxes during 2007-2008 based on PAMELA and ARINA observations
• Unfolding Drift Effects for Cosmic Rays over the Period of the Sun's Magnetic Field Reversal
• Modulation of cosmic ray anti-protons in the heliosphere: simulations for a solar cycle
• Deep Learning the Forecast of Galactic Cosmic-Ray Spectra
• Anomalous cosmic rays within the inner heliosphere: Observations of helium by the High Energy Telescope onboard Solar Orbiter