Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium, which is referred to as a ...'secondary source'. Positrons might also originate in objects such as pulsars and microquasars or through dark matter annihilation, which would be 'primary sources'. Previous statistically limited measurements of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron+positron flux at energies between 300 and 600 GeV (ref. 8). Here we report a measurement of the positron fraction in the energy range 1.5-100 GeV. We find that the positron fraction increases sharply over much of that range, in a way that appears to be completely inconsistent with secondary sources. We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.
Protons and helium nuclei are the most abundant components of the cosmic radiation. Precise measurements of their fluxes are needed to understand the acceleration and subsequent propagation of cosmic ...rays in our Galaxy. We report precision measurements of the proton and helium spectra in the rigidity range 1 gigavolt to 1.2 teravolts performed by the satellite-borne experiment PAMELA (payload for antimatter matter exploration and light-nuclei astrophysics). We find that the spectral shapes of these two species are different and cannot be described well by a single power law. These data challenge the current paradigm of cosmic-ray acceleration in supernova remnants followed by diffusive propagation in the Galaxy. More complex processes of acceleration and propagation of cosmic rays are required to explain the spectral structures observed in our data.
We present the space spectrometer PAMELA observations of proton and helium fluxes during the 2006 December 13 and 14 solar particle events. This is the first direct measurement of the solar energetic ...particles in space with a single instrument in the energy range from ~80 MeV n--1 up to ~3 GeV n--1. For the December 13 event, measured energy spectra of solar protons and helium are compared with results obtained by neutron monitors and other detectors. Our measurements show a spectral behavior different from those derived from the neutron monitor network. No satisfactory analytical fitting was found for the energy spectra. During the first hours of the December 13 event, solar energetic particles spectra were close to the exponential form, demonstrating rather significant temporal evolution. Solar He with energy up to 1 GeV n--1 was recorded on December 13. For the December 14 event, energy of solar protons reached 600 MeV, whereas the maximum energy of He was below 100 MeV n--1. The spectra were slightly bent in the lower energy range and preserved their form during the second event. Differences in the particle flux appearance and temporal evolution of these two events may argue for special conditions leading to the acceleration of solar particles up to relativistic energies.
The PAMELA satellite experiment has measured the cosmic-ray positron fraction between 1.5
GeV and 100
GeV. The need to reliably discriminate between the positron signal and proton background has ...required the development of an ad hoc analysis procedure. In this paper, a method for positron identification is described and its stability and capability to yield a correct background estimate is shown. The analysis includes new experimental data, the application of three different fitting techniques for the background sample and an estimate of systematic uncertainties due to possible inaccuracies in the background selection. The new experimental results confirm both solar modulation effects on cosmic-rays with low rigidities and an anomalous positron abundance above 10
GeV.
A useful parametrization of the energy spectrum of galactic cosmic rays (GCR) near Earth is offered by the so-called force-field model which describes the shape of the entire spectrum with a single ...parameter, the modulation potential. While the usefulness of the force-field approximation has been confirmed for regular periods of solar modulation, it was not tested explicitly for disturbed periods, when GCR are locally modulated by strong interplanetary transients. Here we use direct measurements of protons and α-particles performed by the PAMELA space-borne instrument during December 2006, including a major Forbush decrease, in order to directly test the validity of the force-field parameterization. We conclude that (1) The force-field parametrization works very well in describing the energy spectra of protons and α-particles directly measured by PAMELA outside the Earths atmosphere; (2) The energy spectrum of GCR can be well parameterized by the force-field model also during a strong Forbush decrease; (3) The estimate of the GCR modulation parameter, obtained using data from the world-wide neutron monitor network, is in good agreement with the spectra directly measured by PAMELA during the studied interval. This result is obtained on the basis of a single event analysis, more events need to be analyzed.
Data from the Payload for Antimatter Matter Exploration and Light‐nuclei Astrophysics (PAMELA) satellite experiment were used to measure the geomagnetic cutoff for high‐energy (≳ 80MeV) protons ...during the 14 December 2006 geomagnetic storm. The variations of the cutoff latitude as a function of rigidity were studied on relatively short timescales, corresponding to spacecraft orbital periods (∼94 min). Estimated cutoff values were compared with those obtained by means of a trajectory‐tracing approach based on a dynamical empirical modeling of the Earth's magnetosphere. We found significant variations in the cutoff latitude, with a maximum suppression of ∼7° at lowest rigidities during the main phase of the storm. The observed reduction in the geomagnetic shielding and its temporal evolution were related to the changes in the magnetospheric configuration, investigating the role of interplanetary magnetic field, solar wind, and geomagnetic parameters. PAMELA's results represent the first direct measurement of geomagnetic cutoffs for protons with kinetic energies in the sub‐GeV and GeV region.
Key Points
Direct measurement of high‐energy proton cutoffs during the 14 December 2006 magnetospheric storm
Significant reduction in the geomagnetic shielding observed, with a 7 degree maximum suppression
Cutoff changes well correlated with the IMF strength, its Z component, SYM‐H, and, especially, Kp
Despite the significant progress achieved in recent years, the physical mechanisms underlying the origin of solar energetic particles (SEPs) are still a matter of debate. The complex nature of both ...particle acceleration and transport poses challenges to developing a universal picture of SEP events that encompasses both the low-energy (from tens of keV to a few hundreds of MeV) observations made by space-based instruments and the GeV particles detected by the worldwide network of neutron monitors in ground-level enhancements (GLEs). The high-precision data collected by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment offer a unique opportunity to study the SEP fluxes between ∼80 MeV and a few GeV, significantly improving the characterization of the most energetic events. In particular, PAMELA can measure for the first time with good accuracy the spectral features at moderate and high energies, providing important constraints for current SEP models. In addition, the PAMELA observations allow the relationship between low- and high-energy particles to be investigated, enabling a clearer view of the SEP origin. No qualitative distinction between the spectral shapes of GLE, sub-GLE and non-GLE events is observed, suggesting that GLEs are not a separate class, but are the subset of a continuous distribution of SEP events that are more intense at high energies. While the spectral forms found are to be consistent with diffusive shock acceleration theory, which predicts spectral rollovers at high energies that are attributed to particles escaping the shock region during acceleration, further work is required to explore the relative influences of acceleration and transport processes on SEP spectra.
The energy spectra of galactic cosmic rays carry fundamental information regarding their origin and propagation. These spectra, when measured near Earth, are significantly affected by the solar ...magnetic field. A comprehensive description of the cosmic radiation must therefore include the transport and modulation of cosmic rays inside the heliosphere. During the end of the last decade, the Sun underwent a peculiarly long quiet phase well suited to study modulation processes. In this paper we present proton spectra measured from 2006 July to 2009 December by PAMEFA. The large collected statistics of protons allowed the time variation to be followed on a nearly monthly basis down to 400 MV. Data are compared with a state-of-the-art three-dimensional model of solar modulation.
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