Propagation of cosmic ray nuclei above 50 GeV/amu

It is now well-known that the relative abundance of secondary cosmic ray nuclei which are produced by spallation in interstellar space depends on energies. The mechanism responsible for this energy dependence are not well understood because of the scarcity of data on the cosmic ray composition at energy above 50 GeV/amu. The University of Chicago experiment on the Spacelab-2 flown by the Space Shuttle in 1985 is aimed at the measurement of the elemental composition in this unexplored energy regime. Our latest results on the energy dependence of the secondary to primary abundance ratio using the pairs of elements B,C and N,O, are discussed.     

Neutron spectra in the atmosphere from interactions of primary cosmic rays

Spectra of neutrons from interactions of primary cosmic rays in the earth's atmosphere are calculated with the Monte Carlo model fluka for various depths down to sea level. We discuss the environmental models describing the primary cosmic ray spectrum and details of the calculations. Neutron energy spectra are presented for different depths in the atmosphere and for different geographical locations. By comparing results of calculations to measurements on neutron spectra it is shown that fluka may serve as an important tool for the estimation of the radiation environment in the atmosphere.     

The composition and distribution of anomalous cosmic rays

The composition of anomalous cosmic rays (ACR), is thought to reflect that of the neutral atoms in the very local interstellar medium, such as helium, nitrogen and neon. Recent observations in the outer heliosphere have provided the first unambiguous evidence for ACR argon, carbon and hydrogen, as well, and a method has been developed to relate the ACR abundances to those of the interstellar medium. The observations also indicate persistent negative latitudinal gradients, opposite to that observed by Pioneer 11 during the previous minimum in solar activity. These and other results are consistent with the presence of gradient and curvature drift during solar minimum periods when the tilt of the interplanetary neutral sheet is small.     

The origin of cosmic rays as viewed from their source composition

Based on the fact that the chemical composition of cosmic rays in their sources is similar to that of the interstellar clouds or grains which are relatively rich in refractory and siderophile elements as compared to the chemical composition of the solar atmosphere, it is shown that cosmic ray source matter can be identified as the dusts or grains observed in the envelope of red supergiants or the matter originally ejected from supernova explosions and that it must have passed through a state as reaching to 1000K or less in temperature.     

The composition of cosmic rays at high energies

Measurements of the composition of the cosmic rays at high energies, and of the energy spectra of the individual components provide the basis for the understanding of the sources, of the acceleration mechanism, and of the galactic containment of these particles. We briefly review the presently available information, and we describe a recent measurement on the Space Shuttle that we performed in order to substantially extend the range of energies in which the elemental composition is known. We present and discuss the results, and we also summarize and discuss recent data on the electron component of cosmic rays. The body of data now available contains several features that are difficult to explain within current models of galactic shock acceleration and “leaky box” containment. We emphasize the need for further measurements, and we briefly discuss possible opportunities for future work.     

Galactic cosmic ray composition and energy spectra.

Galactic cosmic ray nuclei represent a significant risk to long-duration spaceflight outside the magnetosphere. We review briefly existing measurements of the composition and energy spectra of heavy cosmic ray nuclei, pointing out which species and energy ranges are most critical to assessing cosmic ray risks for spaceflight. Key data sets are identified and a table of cosmic ray abundances is presented for elements from H to Ni (Z = 1 to 28). Because of the 22-year nature of the solar modulation cycle, data from the approaching 1998 solar minimum is especially important to reducing uncertainties in the cosmic ray radiation hazard. It is recommended that efforts to model this hazard take advantage of approaches that have been developed to model the astrophysical aspects of cosmic rays.     

Geomagnetically trapped heavy ions resulting from the anomalous cosmic rays

Blake and Freisen painted out that if the anomalous cosmic rays are only singly charged they can penetrate deeply into the Earth's magnetosphere well below the cutoff for stripped ions. Ions which reach low altitude can be stripped by the residual atmosphere and become stably trapped. An experiment has been developed which will be able to detect the intensities of such stably trapped heavy ions as a function of magnetospheric position. In anticipation of the flight of this experiment, the fluxes and spectra of the trapped heavy ions have been calculated based upon the anomalous component observed in the IPM.     

The radial distribution of gamma rays and cosmic rays in the outer Galaxy

The radial distribution of the high-energy (70 MeV-5 GeV) gamma-ray emissivity in the outer Milky Way is derived. The kinematics of HI are used to construct column-density maps in three galacto-centric distance ranges in the outer Galaxy. These maps are used in combination with COS-B gamma-ray data to determine gamma-ray emissivities in these distance ranges. A steep negative gradient of the emissivity for the 70 MeV-150 MeV energy range is found in the outer Galaxy. The emissivity for the 300 MeV-5 GeV range is found to be approximately constant (within 20%) and equal to the local value out to large (20 kpc) galacto-centric distances. These results imply a hardening of the gamma-ray spectrum with increrasing distance and for R > 16 kpc the spectrum is shown to be consistent with a π°-decay spectrum with the intensity expected from the local measurement of the cosmic-ray nuclei spectrum. The energy-dependent decrease is interpreted as a steep gradient in the cosmic-ray electron density and a near constancy of the nuclear component. The galactic origin of electrons with energies up to several hundreds of MeV is confirmed, while for cosmic-ray nuclei with energies of a few GeV either confinement in a large galactic halo or an extragalactic origin is suggested by the data.     

A model of galactic cosmic rays for use in calculating linear energy transfer spectra.

The galactic cosmic rays (GCR) contain fully stripped nuclei, from Hydrogen to beyond the Iron group, accelerated to high energies and are a major component of the background radiation encountered by satellites and interplanetary spacecraft. This paper presents a GCR model which is based upon our current understanding of the astrophysics of GCR transport through interstellar and interplanetary space. The model can be used to predict the energy spectra for all stable and long-lived radioactive species from H to Ni over an energy range from 50 to 50,000 MeV/nucleon as a function of a single parameter, the solar modulation level phi. The details of this model are summarized, phi is derived for the period 1974 to present, and results from this model during the 1990/1991 CRRES mission are presented.     

On the origin of ultrahigh energy cosmic rays

The origin of cosmic rays with energy E ? 10¹⁸ eV is a long-standing problem in astrophysics. The development of ever larger detectors has brought in key experimental results in the past decade, most particularly the detection of a cut-off at the expected position for the long sought Greisen–Zatsepin–Kuzmin suppression as well as evidence for large scale anisotropies. This paper summarizes and discusses the recent achievements in this field.     

Measurements of trapped protons and cosmic rays from recent Shuttle flights.

We have flown two new charged particle detectors in five recent Shuttle flights. In this paper we report on the dose rate, equivalent dose rate, and radiation quality factor for trapped protons and cosmic radiation separately. A comparison of the integral linear energy transfer (LET) spectra with recent transport code calculations show significant disagreement. Using the calculated dose rate from the omni-directional AP8MAX model with IGRF reference magnetic field epoch 1970, and observed dose rate as a function of (averaged over all geographic latitude) and longitude, we have determined the westward drift of the South Atlantic anomaly. We have also studied the east-west effect, and observed a 'second' radiation belt. A comparison of the galactic cosmic radiation lineal energy transfer spectra with model calculations shows disagreement comparable to those of the trapped protons.     

Propagation and origin of ultra high-energy cosmic rays

Propagation of UHE protons through CMB radiation leaves the imprint on energy spectrum in the form of Greisen–Zatsepin–Kuzmin (GZK) cutoff, bump (pile-up protons) and dip. The dip is a feature in energy range 1 × 10¹⁸–4 × 10¹⁹ eV, caused by electron–positron pair production on CMB photons. Calculated for power-law generation spectrum with index γ_g = 2.7, the shape of the dip is confirmed with high accuracy by data of Akeno-AGASA, HiRes, Yakutsk and Fly’s Eye detectors. The predicted shape of the dip is robust: it is valid for the rectilinear and diffusive propagation, for different discreteness in the source distribution, for local source overdensity, deficit, etc. This property of the dip allows us to use it for energy calibration of the detectors. The energy shift λ for each detector is determined by minimum χ² in comparison of observed and calculated dip. After this energy calibration the absolute fluxes, measured by AGASA, HiRes and Yakutsk detectors remarkably coincide in energy region 1 × 10¹⁸–1 × 10²⁰ eV. Below the characteristic energy E_c ≈ 1 × 10¹⁸ eV the spectrum of the dip flattens for both diffusive and rectilinear propagation, and more steep galactic spectrum becomes dominant at E < E_c. The energy of transition E_tr < E_c approximately coincides with the position of the second knee E_2kn, observed in the cosmic ray spectrum. The dip-induced transition from galactic to extragalactic cosmic rays at the second knee is compared with traditional model of transition at ankle, the feature observed at energy 1 × 10¹⁹ eV.     

Propagation of secondary antiprotons and cosmic rays in the Galaxy

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratios (e.g., B/C) produce too few antiprotons. In the present paper, we discuss one possibility to overcome these difficulties. Using the measured antiproton flux and B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local “unprocessed” component at low energies perhaps associated with the Local Bubble, thus decreasing the measured secondary to primary nuclei ratio. The independent evidence for SN activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni (Z ⩽ 28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three “radioactive clock” isotopes in CR, ¹⁰Be, ²⁶Al, ³⁶Cl, are all consistent and indicate a halo size z_h ∼ 4 kpc based on the most accurate data taken by the ACE spacecraft.     

Galactic cosmic rays and cell-hit frequencies outside the magnetosphere.

An evaluation of the exposure of space travelers to galactic cosmic radiation outside the earth's magnetosphere is made by calculating fluences of high-energy primary and secondary particles with various charges traversing a sphere of area 100 microns2. Calculations relating to two shielding configurations are presented: the center of a spherical aluminum shell of thickness 1 g/cm2, and the center of a 4 g/cm2 thick aluminum spherical shell within which there is a 30 g/cm2 diameter spherical water phantom with the point of interest 5 g/cm2 from the surface. The area of 100 microns2 was chosen to simulate the nucleus of a cell in the body. The frequencies as a function of charge component in both shielding configurations reflects the odd-even disparity of the incident particle abundances. For a three-year mission, 33% of the cells in the more heavily shielded configuration would be hit by at least one particle with Z greater than 10. Six percent would be hit by at least two such particles. This emphasizes the importance of studying single high-Z particle effects both on cells which might be "at risk" for cancer induction and on critical neural cells or networks which might be vulnerable to inactivation by heavy charged particle tracks. Synergistic effects with the more numerous high-energy protons and helium ions cannot be ruled out. In terms of more conventional radiation risk assessment, the dose equivalent decreased by a factor of 2.85 from free space to that in the more heavily shielded configuration. Roughly half of this was due to the decrease in energy deposition (absorbed dose) and half to the decrease in biological effectiveness (quality factor).     

On the transport of cosmic rays in the distant heliosheath

This paper discusses the transport of energetic charged particles through a sectored magnetic field in distant regions of the inner heliosheath. As the plasma flow slows down on approach to the stagnation point on the heliopause, the distance between the folds of the current sheet decreases to the point where it becomes comparable to the cyclotron radius of a cosmic ray particle. Under these conditions a particle can effectively drift across the stack of magnetic sectors with a speed comparable with the particle’s velocity. For a random distribution of current sheet separation distances, a diffusive transport across the stack of sectors occurs instead. The proposed mechanism could have contributed to unusually high intensities of galactic cosmic rays measured by Voyager 1 in the heliosheath during 2009–2010.     

Acceleration of interplanetary pick-up ions and anomalous cosmic rays

It may not be doubted anymore that anomalous cosmic rays (ACRs) are produced in the heliosphere from interplanetary pick-up ions through their acceleration at the solar wind termination shock. However, there is no general agreement in the community of heliospheric researchers concerning the mechanism of injection of the pick-up ions into the shock acceleration. We discuss here three possible ways for pick-up ions to be involved into the acceleration process at the termination shock: (1) preacceleration of pick-up ions in the whole region from the Sun up to the termination shock by solar wind turbulences and interplanetary shock waves, (2) local preacceleration of pick-up ions in a vicinity of the termination shock by shock surfing, and (3) formation of high-velocity tails in pick-up ion spectra consisting of secondary pick-up ions which are produced in the supersonic solar wind due to ionization of energetic neutral atoms entering from the inner heliosheath.     

Solar cosmic rays in the near Earth space and the atmosphere

Solar energetic particles (SEPs) constitute a distinct population of energetic charged particles, which can be often observed in the near Earth space. SEP penetration into the Earth’s magnetosphere is a complicated process depending on particle magnetic rigidity and geomagnetic field structure. Particles in the several MeV energy range can only access to periphery of the magnetosphere and the polar cap regions, while the GeV particles can arrive at equatorial latitudes. Solar protons with energies higher than 100 MeV may be observed in the atmosphere above ∼30 km, and those with energies more than 1 GeV may be recorded even at the sea level. There are some observational evidences of SEP influence on atmospheric processes. Intruding into the atmosphere, SEPs affect middle atmosphere odd-nitrogen and ozone chemistry. Since spatial and temporal variations of SEP fluxes in the near Earth space are controlled by solar activity, SEPs may present an important link between solar activity and climate. The paper outlines dynamics of SEP fluxes in the near Earth space during the last decades. This can be useful for tracing relationship between SEPs and atmospheric processes.     

Fluctuations of cosmic rays and IMF in the vicinity of interplanetary shocks

Fluctuations of cosmic rays and interplanetary magnetic field upstream of interplanetary shocks are studied using data of ground-based polar neutron monitors as well as measurements of energetic particles and solar wind plasma parameters aboard the ACE spacecraft. It is shown that coherent cosmic ray fluctuations in the energy range from 10 keV to 1 GeV are often observed at the Earth’s orbit before the arrival of interplanetary shocks. This corresponds to an increase of solar wind turbulence level by more than the order of magnitude upstream of the shock. We suggest a scenario where the cosmic ray fluctuation spectrum is modulated by fast magnetosonic waves generated by flux of low-energy cosmic rays which are reflected and/or accelerated by an interplanetary shock.