Abstract
The precision measurements of galactic cosmic-ray protons from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics and the Alpha Magnetic Spectrometer are reproduced ...using a well-established three-dimensional numerical model for the period 2006 July–2019 November. The resulting modulation parameters are applied to simulate the modulation for cosmic antiprotons over the same period, which includes the times of minimum modulation before and after 2009, the 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 in their charge sign and consequent drift pattern. The lowest proton flux was in 2014 February–March, but the lowest simulated antiproton flux is found to have been in 2015 March–April. These simulated fluxes are used to predict the proton-to-antiproton ratios as a function of rigidity. The trends in these ratios contribute to clarifying, 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 is found to be in 2015 March–April.
Abstract
A well-established, comprehensive 3D numerical modulation model is applied to simulate galactic protons, electrons, and positrons from 2011 May to 2015 May, including the solar magnetic ...polarity reversal of Solar Cycle 24. The objective is to evaluate how simulations compare with corresponding Alpha Magnetic Spectrometer observations for 1.0–3.0 GV and what underlying physics follows from this comparison to improve our understanding of 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 adiabatic energy changes at lower rigidity. In order to complete the simulations for oppositely charged particles, antiproton modeling results are also obtained. Together, observations and 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 particles’ mean free paths (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 time after the polarity reversal.
Abstract
Simultaneous and continuous observations of galactic cosmic-ray electrons (
e
−
) and positrons (
e
+
) 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
e
−
and
e
+
with the purpose of reproducing the observed ratio
e
+
/
e
−
for a period that 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, evolved and how the corresponding charge-sign dependent modulation subsequently occurred. The end result of our effort is the detailed reproduction of
e
+
/
e
−
from 2006 to 2015, displaying both qualitative and quantitative agreement with the main observed features. Particularly, we determine how much particle drift is needed to explain the time dependence exhibited by the observed
e
+
/
e
−
during each solar activity phase, especially during the polarity reversal phase, when no well-defined magnetic polarity was found.
Abstract
Time-dependent energy spectra of galactic cosmic rays (GCRs) carry crucial information regarding their origin and propagation throughout the interstellar environment. When observed at the ...Earth, after traversing the interplanetary medium, such spectra are heavily affected by the solar wind and the embedded solar magnetic field permeating the inner sectors of the heliosphere. The activity of the Sun changes significantly over an 11 yr solar cycle—and so does the effect on cosmic particles; this translates into a phenomenon called solar modulation. Moreover, GCR spectra during different epochs of solar activity provide invaluable information for a complete understanding of the plethora of mechanisms taking place in various layers of the Sun’s atmosphere and how they evolve over time. The High-Energy Particle Detector (HEPD-01) has been continuously collecting data since 2018 August, during the quiet phase between solar cycles 24 and 25; the activity of the Sun is slowly but steadily rising and is expected to peak around 2025/2026. In this paper, we present the first spectra for ∼50–250 MeV galactic protons measured by the HEPD-01 instrument—placed on board the CSES-01 satellite—from 2018 August to 2022 March over a one-Carrington-rotation time basis. Such data are compared to the ones from other spaceborne experiments, present (e.g., EPHIN, Parker Solar Probe) and past (PAMELA), and to a state-of-the-art three-dimensional model describing the GCRs propagation through the heliosphere.
It is well known that particle drift motions are suppressed by diffusive scattering as established by direct numerical simulations. The effect of constant scattering on the drift velocities of ...charged particles has always been included in numerical modulation models provided that the weak scattering drift velocity is scaled down in magnitude, although in an empirical manner as comparison between drift models and observations required. What has not yet been established is the spatial dependence of the scattering parameter (ωτ), with ω the gyro-frequency and τ a time scale defined by diffusive scattering. In this work, current knowledge about the spatial and rigidity dependence of ωτ is used to illustrate and discuss its effect on the drift coefficient in the modulation of cosmic ray Carbon in the heliosphere. This is done with a well-established numerical model which includes all four major modulation processes, also the solar wind termination shock (TS) and the heliosheath. We estimate that a reasonable range in the value of ωτ is 0⩽ωτ⩽5, applicable to modulation studies inside and outside the TS. Furthermore, it is found that the considered different scenarios for ωτ cause significant modifications to the weak scattering drift coefficient and as such on the subsequent computed differential intensities in both solar magnetic polarity cycles. For example, it is found that when ωτ decreases rapidly over the heliospheric polar regions, the resulting drift coefficient at 1AU becomes smaller at the poles compared to its value in the equatorial plane. This is contrary to the generally assumed spatial dependence of the maximal weak scattering drift coefficient. The consequent effect is that in the equatorial plane the A<0 spectra are higher than the A>0 spectra at all energies primarily because of drifts; which is unexpected from a classical drift modelling point of view. This feature persists for the equatorial plane modulation even when the explicit enhancement of perpendicular polar diffusion is neglected. Thus, scenarios of ωτ with strong decreases over the heliospheric polar regions seem unlikely for the modulation of galactic cosmic rays in the upstream region of the TS.
The global features of the modulation of galactic cosmic ray protons and helium nuclei in a very quiet heliosphere are studied with a comprehensive, three-dimensional, drift model and compared to ...proton and helium observations measured by PAMELA from 2006 to 2009. Combined with accurate very local interstellar spectra (VLIS) for protons and helium nuclei, this provides the opportunity to study in detail how differently cosmic ray species with dissimilar mass-to-charge ratio (
A
/
Z
) are modulated down to a few GV. The effects at Earth of the difference in their VLIS’s and those caused by the main modulation mechanisms are illustrated. We find that both the PAMELA proton and helium spectra are reproduced well with the numerical model, assuming the same set of modulation parameters and diffusion coefficients. A comparative study of
3
He
2
(He-3) and
4
He
2
(He-4) modulated spectra reveals that they do not undergo identical spectral changes below 3 GV mainly due to differences in their VLIS’s. This result is important to uncover and investigate the effects on the proton to total helium ratio (
p
/He) caused by the difference in their VLIS’s and those by
A
/
Z
. The computed
p
/He displays three modulation regimes, reflecting the complex interplay of modulation processes in the heliosphere. At rigidities above ∼3 GV, the
p
/He ratio at the Earth is found to deviate modestly from a value of ∼5.5, largely independent of the assumed modulation conditions. This result indicates that the PAMELA measurement of
p
/He reveals at these rigidities the shapes of their VLIS’s. Below ∼0.6 GV,
p
/He increases with decreasing rigidity from 2006 to 2009 and significant variations are predicted depending on the assumed solar modulation conditions. This result indicates that as modulation levels decreased from 2006 to 2009, the contribution of adiabatic energy changes dissipated faster for protons than for helium nuclei at the same rigidity mainly due to different slopes of their VLIS’s. The differences between modulation effects for protons and helium are found to be the consequence of how the combined interplaying modulation mechanisms in the heliosphere affect the modulated spectra based on their
A
/
Z
and particularly on their VLIS’s.
Precise time-dependent measurements of the Z = 2 component in the cosmic radiation provide crucial information about the propagation of charged particles through the heliosphere. The PAMELA ...experiment, with its long flight duration (2006 June 15-2016 January 23) and the low energy threshold (80 MeV/n) is an ideal detector for cosmic-ray solar modulation studies. In this paper, the helium nuclei spectra measured by the PAMELA instrument from 2006 July to 2009 December over a Carrington rotation time basis are presented. A state-of-the-art three-dimensional model for cosmic-ray propagation inside the heliosphere was used to interpret the time-dependent measured fluxes. Proton-to-helium flux ratio time profiles at various rigidities are also presented in order to study any features that could result from the different masses and local interstellar spectra shapes.
Very local interstellar spectra (vLIS’s) for protons and total Helium (He) were observed in situ by Voyager 1 below about 340 MeV/
since it had moved across the heliopause (HP). Together with high ...precision PAMELA and AMS observations at the Earth, we previously reported on new vLIS calculated from 1 MeV to 100 GeV for protons, electrons, total He, Oxygen, Carbon, and Boron. We now follow this up to report on the vLIS’s for positrons and for the isotopes He-3, He-4, and Deuteron (H-2) by combining computations with the galactic propagation code, GALPROP, and our 3D modulation model for GCRs in the heliosphere as done previously. Similarly, we also have computed updates of the vLIS’s for antiprotons and antideuteron. We assume that the modulation processes between the HP and the Earth for protons and other GCR nuclei are essentially similar, which is also the case for electrons and positrons, except for particle drifts of oppositely charged particles. The procedures followed to obtain these updates are summarized and a compilation of the mentioned vLIS’s is presented.
Abstract
Time-dependent energy spectra of galactic cosmic rays (GCRs) carry fundamental information regarding their origin and propagation. When observed at the Earth, these spectra are significantly ...affected by the solar wind and the embedded solar magnetic field that permeates the heliosphere, changing significantly over an 11 yr solar cycle. Energy spectra of GCRs measured during different epochs of solar activity provide crucial information for a thorough understanding of solar and heliospheric phenomena. The PAMELA experiment collected data for almost 10 years (2006 June 15–2016 January 23), including the minimum phase of solar cycle 23 and the maximum phase of solar cycle 24. In this paper, we present new spectra for helium nuclei measured by the PAMELA instrument from 2010 January to 2014 September over a three-Carrington-rotation time basis. These data are compared to the PAMELA spectra measured during the previous solar minimum, providing a picture of the time dependence of the helium-nuclei fluxes over a nearly full solar cycle. Time and rigidity dependencies are observed in the proton-to-helium flux ratios. The force-field approximation of the solar modulation was used to relate these dependencies to the shapes of the local interstellar proton and helium-nuclei spectra.
Observations made with the two Voyager spacecraft confirmed that the solar wind decelerates to form the heliospheric termination shock. Voyager 1 crossed this termination shock at ∼94
AU in 2004, ...while Voyager 2 crossed it in 2007 at a different heliolatitude, about 10
AU closer to the Sun. These different positions of the termination shock confirm the dynamic and cyclic nature of the shock’s position. Observations from the two Voyager spacecraft inside the heliosheath indicate significant differences between them, suggesting that apart from the dynamic nature caused by changing solar activity there also may exist a global asymmetry in the north–south (polar) dimensions of the heliosphere, in addition to the expected nose–tail asymmetry. This relates to the direction in which the heliosphere is moving in interstellar space and its orientation with respect to the interstellar magnetic field. In this paper we focus on illustrating the effects of this north–south asymmetry on modulation of galactic cosmic ray Carbon, between polar angles of 55° and 125°, using a numerical model which includes all four major modulation processes, the termination shock and the heliosheath. This asymmetry is incorporated in the model by assuming a significant dependence on heliolatitude of the thickness of the heliosheath. When comparing the computed spectra between the two polar angles, we find that at energies
E
<
∼1.0
GeV the effects of the assumed asymmetry on the modulated spectra are insignificant up to 60
AU from the Sun but become increasingly more significant with larger radial distances to reach a maximum inside the heliosheath. In contrast, with
E
>
∼1.0
GeV, these effects remain insignificant throughout the heliosphere even very close to the heliopause. Furthermore, we find that a higher local interstellar spectrum for Carbon enhances the effects of asymmetric modulation between the two polar angles at lower energies (
E
<
∼300
MeV). In conclusion, it is found that north–south asymmetrical effects on the modulation of cosmic ray Carbon depend strongly on the extent of the geometrical asymmetry of the heliosheath together with the assumed value of the local interstellar spectrum.
PDF
