Mass Composition Data Research Articles

Constraining the sources of ultra-high-energy cosmic rays across and above the ankle with the spectrum and composition data measured at the Pierre Auger Observatory

In this work we present the interpretation of the energy spectrum and mass composition data as measured by the Pierre Auger Collaboration above 6 × 1017 eV. We use an astrophysical model with two extragalactic source populations to model the hardening of the cosmic-ray flux at around 5 × 1018 eV (the so-called “ankle” feature) as a transition between these two components. We find our data to be well reproduced if sources above the ankle emit a mixed composition with a hard spectrum and a low rigidity cutoff. The component below the ankle is required to have a very soft spectrum and a mix of protons and intermediate-mass nuclei. The origin of this intermediate-mass component is not well constrained and it could originate from either Galactic or extragalactic sources.To the aim of evaluating our capability to constrain astrophysical models, we discuss the impact on the fit results of the main experimental systematic uncertainties and of the assumptions about quantities affecting the air shower development as well as the propagation and redshift distribution of injected ultra-high-energy cosmic rays (UHECRs).

Interpreting the cosmic ray spectrum and composition measurements across the ankle and up to the highest energies with the data of the Pierre Auger Observatory

In this work we present a combined fit of the energy spectrum and mass composition data above 6×1017 eV as measured by the Pierre Auger Collaboration and we show that our data can be reproduced by the superposition of two (or more) components, either Galactic or extragalactic. We use a simple astrophysical model with two extragalactic components, one dominant at high energy and the other at low energy, whose superposition generates the so-called “ankle” feature. We find that the component dominating the low (high) energy region is required to be emitted from the sources with a very soft (hard) spectrum. In our best fit scenario the sources contributing above the ankle emit a mixed and increasingly heavier mass composition, whereas a mix of protons and intermediate-mass nuclei is observed below the ankle, which reach the Earth almost una↵ected by the propagation in the intergalactic space. The possible presence of a Galactic population providing the intermediate-mass contribution at low energy is also explored. In order to evaluate our capability to constrain astrophysical models, we finally discuss the impact on the fit results of the main experimental systematic uncertainties and of the assumptions on the quantities a↵ecting propagation, both in the atmosphere and in the intergalactic medium.

Astrophysical interpretation of Pierre Auger Observatory measurements of the UHECR energy spectrum and mass composition

We present a combined fit of a simple astrophysical model of Ultra-high-energy cosmic-ray sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated with a rigidity-dependent mechanism. The fit has been performed at first for energies above 5 EeV, i.e., the region of the all-particle spectrum above the so-called “ankle” feature, and has then been extended to lower energies in order to include this feature. The fit results and their astrophysical implications in both cases are discussed.

A New View on Auger Data and Cosmogenic Neutrinos in Light of Different Nuclear Disintegration and Air-shower Models

Abstract We study the implications of ultrahigh-energy cosmic-ray (UHECR) data from the Pierre Auger Observatory for potential accelerator candidates and cosmogenic neutrino fluxes for different combinations of nuclear disintegration and air-shower models. We exploit the most recent spectral and mass composition data (2017) with a new, computationally efficient simulation code, PriNCe. We extend a systematic framework, which has been previously applied in a combined fit by the Pierre Auger Collaboration, with the cosmological source evolution as an additional free parameter. In this framework, an ensemble of generalized UHECR accelerators is characterized by a universal spectral index (equal for all injection species), a maximal rigidity, and the normalizations for five nuclear element groups. We find that the 2017 data favor a small but constrained contribution of heavy elements (iron) at the source. We demonstrate that the results moderately depend on the nuclear disintegration (Puget–Stecker–Bredekamp, Peanut, or Talys) model and more strongly on the air-shower (EPOS-LHC, Sibyll 2.3, or QGSjetII-04) model. Variations of these models result in different source evolution and spectral indices, limiting the interpretation in terms of a particular class of cosmic accelerators. Better-constrained parameters include the maximal rigidity and the mass composition at the source. Hence, the cosmogenic neutrino flux can be robustly predicted. Depending on the source evolution at high redshifts, the flux is likely out of reach of future neutrino observatories in most cases, and a minimal cosmogenic neutrino flux cannot be claimed from data without assuming a cosmological distribution of the sources.

Energy spectrum of fast second order Fermi accelerators as sources of ultra-high-energy cosmic rays

Stochastic acceleration of cosmic rays in second order Fermi processes is usually considered too slow to reach ultra-high energies, except in specific cases. In this paper we present the energy spectrum obtained from second order Fermi acceleration in highly turbulent magnetic fields as e.g. found in the outskirts of AGN jets in situations where it can be sufficiently fast to accelerate particles to the highest observed energies. We parametrize the resulting non-power-law spectra and show that these can describe the cosmic ray energy spectrum and mass-composition data at the highest energies if propagation effects are taken into account.

Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory

We present a combined fit of a simple astrophysical model of UHECR sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The fit has been performed for energies above 5 ⋅ 1018 eV, i.e. the region of the all-particle spectrum above the so-called “ankle” feature. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated through a rigidity-dependent mechanism. The fit results suggest sources characterized by relatively low maximum injection energies, hard spectra and heavy chemical composition. We also show that uncertainties about physical quantities relevant to UHECR propagation and shower development have a non-negligible impact on the fit results.

Astrophysical interpretation of Pierre Auger Observatory measurements of the UHECR energy spectrum and mass composition

We present a combined fit of a simple astrophysical model of UHECR sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The fit has been performed for energies above 5 EeV, i.e. the region of the all-particle spectrum above the so-called "ankle" feature. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated with a rigidity-dependent mechanism. The fit results suggest sources characterized by relatively low maximum injection energies and hard spectral indices. The impact of various systematic uncertainties on the above result is discussed.

Cloud condensation nuclei closure study on summer arctic aerosol

Abstract. We present an aerosol – cloud condensation nuclei (CCN) closure study on summer high Arctic aerosol based on measurements that were carried out in 2008 during the Arctic Summer Cloud Ocean Study (ASCOS) on board the Swedish ice breaker Oden. The data presented here were collected during a three-week time period in the pack ice (>85° N) when the icebreaker Oden was moored to an ice floe and drifted passively during the most biological active period into autumn freeze up conditions. CCN number concentrations were obtained using two CCN counters measuring at different supersaturations. The directly measured CCN number concentration was then compared with a CCN number concentration calculated using both bulk aerosol mass composition data from an aerosol mass spectrometer (AMS) and aerosol number size distributions obtained from a differential mobility particle sizer, assuming κ-Köhler theory, surface tension of water and an internally mixed aerosol. The last assumption was supported by measurements made with a hygroscopic tandem differential mobility analyzer (HTDMA) for particles >70 nm. For the two highest measured supersaturations, 0.73 and 0.41%, closure could not be achieved with the investigated settings concerning hygroscopicity and density. The calculated CCN number concentration was always higher than the measured one for those two supersaturations. This might be caused by a relative larger insoluble organic mass fraction of the smaller particles that activate at these supersaturations, which are thus less good CCN than the larger particles. On average, 36% of the mass measured with the AMS was organic mass. At 0.20, 0.15 and 0.10% supersaturation, closure could be achieved with different combinations of hygroscopic parameters and densities within the uncertainty range of the fit. The best agreement of the calculated CCN number concentration with the observed one was achieved when the organic fraction of the aerosol was treated as nearly water insoluble (κorg=0.02), leading to a mean total κ, κtot, of 0.33 ± 0.13. However, several settings led to closure and κorg=0.2 is found to be an upper limit at 0.1% supersaturation. κorg≤0.2 leads to a κtot range of 0.33 ± 013 to 0.50 ± 0.11. Thus, the organic material ranges from being sparingly soluble to effectively insoluble. These results suggest that an increase in organic mass fraction in particles of a certain size would lead to a suppression of the Arctic CCN activity.

Evaluation of a quantitative structure–property relationship (QSPR) for predicting mid-visible refractive index of secondary organic aerosol (SOA)

In this work we describe and evaluate a simple scheme by which the refractive index (λ = 589 nm) of non-absorbing components common to secondary organic aerosols (SOA) may be predicted from molecular formula and density (g cm(-3)). The QSPR approach described is based on three parameters linked to refractive index-molecular polarizability, the ratio of mass density to molecular weight, and degree of unsaturation. After computing these quantities for a training set of 111 compounds common to atmospheric aerosols, multi-linear regression analysis was conducted to establish a quantitative relationship between the parameters and accepted value of refractive index. The resulting quantitative relationship can often estimate refractive index to ±0.01 when averaged across a variety of compound classes. A notable exception is for alcohols for which the model consistently underestimates refractive index. Homogenous internal mixtures can conceivably be addressed through use of either the volume or mole fraction mixing rules commonly used in the aerosol community. Predicted refractive indices reconstructed from chemical composition data presented in the literature generally agree with previous reports of SOA refractive index. Additionally, the predicted refractive indices lie near measured values we report for λ = 532 nm for SOA generated from vapors of α-pinene (R.I. 1.49-1.51) and toluene (R.I. 1.49-1.50). We envision the QSPR method may find use in reconstructing optical scattering of organic aerosols if mass composition data is known. Alternatively, the method described could be incorporated into in models of organic aerosol formation/phase partitioning to better constrain organic aerosol optical properties.

Two types of ion energy dispersions observed in the nightside auroral regions during geomagnetically disturbed periods

The Akebono satellite has observed two types of energy dispersion signatures of discrete ion precipitation event in the nightside auroral regions during active geomagnetic conditions. The charged particle experiments and electric and magnetic field detectors on board Akebono provide us with essential clues to characterize the source regions and acceleration and/or injection processes associated with these two types of ion signatures. The magnetic field data obtained simultaneously by the geosynchronous GOES 6 and 7 satellites and the ground magnetograms are useful to examine their relationships with geomagnetic activity. Mass composition data and pitch angle distributions show that different sources and processes should be attributed to two types (Types I and II) of energy dispersion phenomena. Type I consists of multiple bouncing ion clusters constituted by H+. These H+ clusters tend to be detected at the expansion phase of substorms and have characteristic multiple energy‐dispersed signatures. Type II consists of O+ energy dispersion(s), which is often observed at the recovery phase. It is reasonable to consider that the H+ clusters of Type I are accelerated by dipolarization at the equator, are injected in the field‐aligned direction, and bounce on closed field lines after the substorm onset. We interpret these multiple energy dispersion events as mainly due to the time‐of‐flight (TOF) effect, although the convection may influence the energy‐dispersed traces. Based on the TOF model, we estimate the source distance to be 20–30 RE along the field lines. On the other hand, the O+ energy dispersion of Type II is a consequence of reprecipitation of terrestrial ions ejected as an upward flowing ion (UFI) beam from the upper ionosphere by a parallel electrostatic potential difference. The O+ energy dispersion is induced by the E × B drift during the field‐aligned transport from the source region to the observation point.

Low-energy ions at low equatorial altitudes

Williams and Frank (1984, J. geophys. Res. 89, 3903) have reported the observation of intense low-energy ion populations in the low-altitude, equatorial magnetosphere. They reported observing two narrow energy peaks in the total ion flux that increased in energy from ∼ 8 keV to > 24 keV as the altitude decreased from ≈ 4 R E to 2.1 R E. They analyzed one pass (29 November 1977) in detail and described the difficulties caused by the fact that the analysis had to use data from the limits of the response functions of the plasma and energetic particle instruments. Because of these difficulties, we have analyzed additional data and incorporated ion mass composition data from the Lockheed mass spectrometer. The new data agree in general with the earlier results and show that complex spectral features occur in the total ion observations. The observed ions are now identified as primarily H +, O +, and He +, whose spectra also show complex structures. Observed peak energies increase with decreasing altitude and can vary by at least 30% over time. An acceleration mechanism appears to be continuously present throughout the plasmasphere. As discussed earlier by Williams and Frank (1984), the most likely mechanism for the energization of the observed ions is a resonant wave-particle interaction, probably with ion cyclotron waves generated by an anisotropic energetic ion population. It has been estimated that such a mechanism may act in a two-phase mode resulting in the energization of minor ions up to the local Alfvén energy of the ambient plasma, E A = B 2 8πN total . Although this energy (~ 10 keV at L ∼ 2.5) is similar to the peak energies observed, the radial dependence of the peak energy is not consistent with that expected for E A.

Quiet time mass composition at near‐geosynchronous altitudes

Mass composition data acquired from the near‐geosynchronous SCATHA spacecraft during magnetically quiet times are analyzed. The time intervals over which data were included in the study span some 4 months in the spring and summer of 1979. This allows a reasonable coverage in both L shell and magnetic local time. At the higher L shells, L > 6.5, the mass composition data are consistent with sunward convection of plasma sheet particles. Protons and alpha particle fluxes peak near 90° pitch angle. There is evidence that the alpha particle spatial distribution has a sharp inner edge near L = 6.5. At lower L values, the proton characteristics change. The density of protons above 1 keV decreases, while the lower‐energy protons show an increase in density. The oxygen ions show a similar change, in that there is a large increase in the lower‐energy oxygen ions from high L to low L, especially in the dusk and midnight local time sectors. This suggests that the ionosphere may be continuously supplying plasma to the inner magnetosphere even during magnetically quiet times.

Energetic ion mass composition as observed at near‐geosynchronous and low altitudes during the storm period of February 21 and 22, 1979

Mass composition data acquired during the storm period of February 21 and 22, 1979, are presented and analyzed. The data were obtained from the near‐geosynchronous SCATHA spacecraft and the polar‐orbiting S3‐3 spacecraft at altitudes below 8000 km. The data from both spacecraft show that significant amounts of ionospheric plasma were observed to be injected around the main phase of the two storms on February 21, 1979. At geosynchronous altitudes the increase in ionospheric plasma was found to be significant in both number density and energy density. Moreover, multiple dispersionlike signatures in the particle spectrograms were observed during the second storm, indicating that this plasma was recently injected into the magnetosphere. At lower altitudes the S3‐3 data also showed significant enhancements of ionospheric plasma, as determined from number density data. It was found that the density enhancement in the plasma population moved to progressively lower L shells during the recovery phase of the storms. As it is unlikely that the plasma is injected at the point of observation, at least during the recovery phase, we consider drift effects to be responsible for this signature. We hence summarize some of the simpler convection theory, specifically addressing the dependence of the boundaries between drift regimes as a function of L shell. To do this, a steady state convection model has been employed, but we assume that this “steady state” only applies during the recovery phase of the storm. Since we consider shielding to be important only later in the recovery phase, it is further assumed that the cross‐tail electric field is uniform. On comparing the data with the convection boundaries we find that we can usually choose a cross‐tail electric field strength which models the particle signatures quite closely. The major feature present in the particle spectra is an energy‐dependent minimum which, we presume, marks those ions that either have been lost or drift so slowly that they are not observed at the spacecraft. As a consequence of the comparison of the S3‐3 particle signatures with the predicted convection boundaries, we find that the apparent movement to lower L shells of the density enhancement during the recovery phase is due to time of flight effects on a low‐energy plasma population at these L shells (below L = 4). Time of flight also implies that these ions were in the morning local time sector at the time of the main phase of the storms. At the same time that these low‐energy ions drifting eastward are observed, large numbers of ions convecting westward are also seen. This plasma population contains a large amount of ionospheric plasma. Furthermore, the ionospheric plasma as labeled by singly charged oxygen appears to have been injected over quite a large range in local time during the first storm. However, the proton signatures imply that most of the protons observed at the higher L shells were confined to the nightside sector during the main phase of the storm. While there are many similar features associated with the second storm in the S3‐3 data, the oxygen ions display a signature consistent with injection only in the nightside magnetosphere. This is in agreement with the SCATHA observations where the particle spectra show multiple dispersion signatures.