Abstract
Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species,
p
¯
,
e
±
, and nuclei (H–Si, Fe), ...which resulted in a number of breakthroughs. The most recent AMS-02 result is the measurement of the spectra of CR sodium and aluminum up to ∼2 TV. Given their low solar system abundances, a significant fraction of each element is produced in fragmentations of heavier species, predominantly Ne, Mg, and Si. In this paper, we use precise measurements of the sodium and aluminum spectra by AMS-02 together with ACE-CRIS and Voyager 1 data to test their origin. We show that the sodium spectrum agrees well with the predictions made with the
GalProp
-
HelMod
framework, while the aluminum spectrum shows a significant excess in the rigidity range from 2–7 GV. In this context, we discuss the origin of other low-energy excesses in Li, F, and Fe found earlier. The observed excesses in Li, F, and Al appear to be consistent with the local Wolf-Rayet stars hypothesis, invoked to reproduce anomalous
22
Ne/
20
Ne,
12
C/
16
O, and
58
Fe/
56
Fe ratios in CRs, while excess in Fe is likely connected with a past supernova activity in the solar neighborhood. We also provide updated local interstellar spectra (LIS) of sodium and aluminum in the rigidity range from a few megavolts to ∼2 TV. Our calculations employ the self-consistent
GalProp
-
HelMod
framework, which has proved to be a reliable tool in deriving the LIS of CR
p
¯
,
e
−
, and nuclei
Z
≤ 28.
The heliospheric modulation model HelMod is a two dimensional treatment dealing with the helio-colatitude and radial distance from Sun and is employed to solve the transport-equation for the GCR ...propagation through the heliosphere down to Earth. This work presents the current version 3 of the HelMod model and reviews how main processes involved in GCR propagation were implemented. The treatment includes the so-called particle drift effects – e.g., those resulting, for instance, from the extension of the neutral current sheet inside the heliosphere and from the curvature and gradient of the IMF –, which affect the transport of particles entering the solar cavity as a function of their charge sign. The HelMod model is capable to provide modulated spectra which well agree within the experimental errors with those measured by AMS-01, BESS, PAMELA and AMS-02 during the solar cycles 23 and 24. Furthermore, the counting rate measured by Ulysses at ±80° of solar latitude and 1–5AU was also found in agreement with that expected by HelMod code version 3.
The AMS‐01 observations (in June 1998, on board the space shuttle orbiter Discovery) have shown the presence of primary (PCR) and secondary (SCR) cosmic rays (most of them protons) at a low Earth ...orbit (about 400 km altitude). The SCRs are mostly created in interactions with the atmosphere by fast PCRs and can be trapped or become reentrant albedo particles. Some of them seem to be sufficiently energetic to populate the “penumbra region” above the local geomagnetic cutoff rigidity. A backtracking procedure of simulated protons entering the AMS‐01 spectrometer has provided the fraction of allowed (and hence forbidden) trajectories of PCRs. Consequently, it has allowed the determination of the so‐called transmission function (TF) which is able to describe the properties of the PCR transport from the Earth's magnetopause (i.e., the primary spectrum at 1 AU) to the atmosphere and finally the fluxes of the PCRs in the ten geomagnetic regions for AMS‐01 observations. In the penumbra regions, the observed spectra of the AMS‐01 geomagnetic regions have been found to be larger than those predicted for the PCRs in the penumbra region by means of the TF, i.e., some SCRs (mainly reentrant albedo protons) are also found to populate the rigidity regions above the local geomagnetic cutoff rigidity. The fraction of the secondary to overall particle flux in the penumbra region increases gradually as the geomagnetic latitude increases.
HelMod model allows one to describe how solar modulation affects the propagation of galactic cosmic rays (GCRs) through the heliosphere by solving the Parker transport equation. HelMod provides ...modulated spectra for protons and nuclei during high and low solar activity periods, in the inner and outer heliosphere, at Earth location and outside the ecliptic plane, as well as at different distances from Sun in the inner and outer heliosphere. The GCRs mainly constitute the high-energy population of the so-called space radiation environment. Modulated cosmic-ray spectra of protons and nuclei, obtained from HelMod and other solar modulation codes were compared to the AMS–02 spectra ranging from GeV up to TeV energies. Concerning other commonly used solar modulation models, HelMod modulated spectra are available over the entire rigidity range and observation period of AMS–02 mission. A quantitative comparison among models was carried out using ΔΦ, i.e., the relative difference between the experimental and simulated fluences, and ηRMS, i.e., the error-weighted root mean square of the relative difference between AMS–02 experimental data and simulated differential intensities. HelMod was found to achieve a good agreement over the full set of experimental data with, typically, ΔΦ within ±2.5% and ηRMS within 5%. Usually larger or, in a few cases, much larger values for ΔΦ and ηRMS were found for the other models.
The accurate knowledge of cosmic ion fluxes is essential for fundamental physics, deep space missions, and exploration activities in the solar system. In the HelMod-4 model the Parker transport ...equation is solved using a Monte Carlo approach to evaluate the solar modulation effect on local interstellar spectra of Galactic Cosmic Rays (GCRs). This work presents the latest updates to the HelMod-4 model parameters, focusing on the descending phase of solar cycle 24. The updates are motivated by the latest high-precision measurements from the AMS-02 detector which revealed for the first time with high accuracy the features of GCR fluxes’ evolution during a period of positive interplanetary magnetic field polarity. Furthermore, we present HelMod-4/CUDA, a GPU-accelerated approach for solving the Parker equation in the heliosphere using a stochastic differential equation method. The code is an evolution of the HelMod-4 code, porting the algorithm to GPU architecture using the CUDA programming language. This approach achieves significant speedup compared to a CPU implementation. The HelMod-4/CUDA code has been validated by comparing its results with the most precise and updated experimental GCR spectra observed during high and low solar activity periods, both in the inner and outer heliosphere, at the Earth location, and outside the ecliptic plane. The comparison shows that HelMod-4 and HelMod-4/CUDA can be equivalently used to provide solar-modulated spectra with a similar degree of accuracy in reproducing observed data.
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