28 research outputs found
Modulation of cosmic ray anti-protons in the heliosphere: simulations for a solar cycle
The precision measurements of galactic cosmic ray protons from PAMELA and AMS
are reproduced using a well-established 3D numerical model for the period July
2006 - November 2019. The resulting modulation parameters are applied to
simulate the modulation for cosmic antiprotons over the same period, which
includes times of minimum modulation before and after 2009, maximum modulation
from 2012 to 2015 including the reversal of the Sun's magnetic field polarity,
and the approach to new minimum modulation in 2020. Apart from their local
interstellar spectra, the modulation of protons and antiprotons differ only in
their charge-sign and consequent drift pattern. The lowest proton flux was in
February-March 2014, but the lowest simulated antiproton flux is found to be in
March-April 2015. These simulated fluxes are used to predict the proton to
anti-proton ratios as a function of rigidity. The trends in these ratios
contribute to clarify to a large extent the phenomenon of charge-sign
dependence of heliospheric modulation during vastly different phases of the
solar activity cycle. This is reiterated and emphasized by displaying so-called
hysteresis loops. It is also illustrated how the values of the parallel and
perpendicular mean free paths, as well as the drift scale, vary with rigidity
over this extensive period. The drift scale is found to be at its lowest level
during the polarity reversal period, while the lowest level of the mean free
paths are found to be in March-April 2015.Comment: 17 Pages, 7 Figures, Submitted to Astrophysical Journa
Unfolding Drift Effects for Cosmic Rays over the Period of the Sun's Magnetic Field Reversal
A well-established, comprehensive 3-D numerical modulation model is applied
to simulate galactic protons, electrons and positrons from May 2011 to May
2015, including the solar magnetic polarity reversal of Solar Cycle 24. The
objective is to evaluate how these simulations compare with corresponding AMS
observations for 1.0-3.0 GV, and what underlying physics follows from this
comparison in order to improve our understanding on how the major physical
modulation processes change, especially particle drift, from a negative to a
positive magnetic polarity cycle. Apart from their local interstellar spectra,
electrons and positrons differ only in their drift patterns, but they differ
with protons in other ways such as their adiabatic energy changes at lower
rigidity. In order to complete the simulations for oppositely charged
particles, antiproton modeling results are obtained as well. Together, the
observations and the corresponding modeling indicate the difference in the
drift pattern before and after the recent polarity reversal and clarify to a
large extent the phenomenon of charge-sign dependence during this period. The
effect of global particle drift became negligible during this period of no
well-defined magnetic polarity. The resulting low values of all particles' MFPs
during the polarity reversal contrast their large values during solar minimum
activity, and as such expose the relative contributions and effects of the
different modulation processes from solar minimum to maximum activity. We find
that the drift scale starts recovering just after the polarity reversal, but
the MFPs keep decreasing or remain unchanged for some period after the polarity
reversal.Comment: Submitted to Astrophysical Journal, 27 pages, 13 Figure
Time and Charge-Sign Dependence of the Heliospheric Modulation of Cosmic Rays
Simultaneous and continuous observations of galactic cosmic-ray electrons and
positrons from the PAMELA and AMS02 space experiments are most suitable for
numerical modeling studies of the heliospheric modulation of these particles
below 50 GeV. A well-established comprehensive three-dimensional modulation
model is applied to compute full spectra for electrons and positrons with the
purpose of reproducing the observed ratio positrons/electrons for a period
which covers the previous long and unusual deep solar minimum activity and the
recent maximum activity phase including the polarity reversal of the solar
magnetic field. For this purpose the very local interstellar spectra for these
particles were established first. Our study is focused on how the main
modulation processes, including particle drifts, and other parameters such as
the three major diffusion coefficients, had evolved, and how the corresponding
charge-sign dependent modulation had occurred subsequently. The end result of
our effort is the detailed reproduction of positron/electrons from 2006 to
2015, displaying both qualitative and quantitative agreement with the main
observed features. Particularly, we determine how much particle drifts is
needed to explain the time dependence exhibited by the observed
positron/electron during each solar activity phase, especially during the
polarity reversal phase when no well-defined magnetic polarity was found.Comment: 16 pages, 8 figures, Submitted to Astrophysical Journa
Studies of cosmic-ray solar modulation with the PAMELA experiment
The launch of the satellite-borne PAMELA instrument on the 15th June 2006 opened a new era of high-precision studies of cosmic rays. Due to its low detection energy threshold and its long operation, PAMELA was able to accurately measure the fluxes of several cosmic-ray species over a large energy range and study their time variations below a few tens of GeVs. In this presentation we will review PAMELA results on the time-dependent proton, helium and electron fluxes measured between a few tens of MeV/n and few tens of GeV/n from 2006 to 2014. Moreover, preliminary results of yearly energy spectra of deuterons, helium-3 and helium-4 nuclei below 1 GeV/n will be discussed. These measurements covered a time including the minimum phase of the 23rd solar cycle and the 24th solar maximum including the polarity reversal of the solar magnetic field. The PAMELA measurements have allowed to significantly improve the understanding of the charged-particle propagation through the Heliosphere, the charge-sign effect due to the drift motions of these particles and to calibrate state-of-the-art models of cosmic-ray transport in the Heliosphere