Spectral Analysis of the September 2017 Solar Energetic Particle Events

An interval of exceptional solar activity was registered in early September 2017, late in the decay phase of solar cycle 24, involving the complex Active Region 12673 as it rotated across the western hemisphere with respect to Earth. A large number of eruptions occurred between 4 and 10 September, including four associated with Xclass flares. The X9.3 flare on 6 September and the X8.2 flare on 10 September are currently the two largest during cycle 24. Both were accompanied by fast coronal mass ejections and gave rise to solar energetic particle (SEP) events measured by nearEarth spacecraft. In particular, the partially occulted solar event on 10 September triggered a groundlevel enhancement (GLE), the second GLE of cycle 24. A further, much less energetic SEP event was recorded on 4 September. In this work we analyze observations by the Advanced Composition Explorer (ACE) and the Geostationary Operational Environmental Satellites (GOES), estimating the SEP eventintegrated spectra above 300 keV and carrying out a detailed study of the spectral shape temporal evolution. Derived spectra are characterized by a lowenergy break at few/tens of MeV; the 10 September event spectrum, extending up to 1 GeV, exhibits an additional rollover at several hundred MeV. We discuss the spectral interpretation in the scenario of shock acceleration and in terms of other important external influences related to interplanetary transport and magnetic connectivity, taking advantage of multipoint observations from the Solar Terrestrial Relations Observatory. Spectral results are also compared with those obtained for the 17 May 2012 GLE event

The Phosphorus, Sulfur, Argon, and Calcium Isotopic Composition of the Galactic Cosmic Ray Source

Galactic cosmic ray (GCR) measurements of the phosphorus, sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer aboard the Advanced Composition Explorer are reported over the energy range from ~100 to ~400 MeV nucleon^(–1). The propagation of cosmic rays through the Galaxy and heliosphere is modeled with constraints imposed by measurements. Isotopic source abundance ratios ^(31)P/^(32)S, ^(34)S/^(32)S, ^(38)Ar/^(36)Ar, and ^(44)Ca/^(40)Ca are deduced. The derived ^(31)P/^(32)S ratio is 2.34 ± 0.34 times larger than the solar system value, lending further credence to the suggestion that refractory elements are enriched in the GCRs due to the sputtering of ions off grains in the cores of superbubbles. By determining the GCR source abundances of argon (a noble gas) and calcium (a refractory), it is determined that material in grains is accelerated to GCR energies a factor of 6.4 ± 0.3 more efficiently than gas-phase material in this charge range. With this information, the dust fraction of phosphorus and sulfur in the interstellar material that is mixed with stellar ejecta to form the GCR seed material is found to be consistent with astronomical observations

Measurements of the isotopes of lithium, beryllium, and boron from ACE/CRIS

The isotopes of lithium, beryllium, and boron (LiBeB) are known in nature to be produced primarily by CNO spallation and α-α fusion from interactions between cosmic rays and interstellar nuclei. While the dominant source of LiBeB isotopes in the present epoch is cosmic-ray interactions, other sources are known to exist, including the production of ^(7)Li from big bang nucleosynthesis. Precise observations of galactic cosmic-ray LiBeB in addition to accurate modeling of cosmic-ray transport can help to constrain the relative importance among the different production mechanisms. The Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) has measured nuclei with 2 ≲ Z ≲ 30 in the energy range ~30–500 MeV/nucleon since 1997 with good statistical accuracy. We present measurements of the isotopic abundances of LiBeB and discuss these observations in the context of previous cosmic-ray measurements and spectroscopic observations

Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope

Gamma-ray observations in the medium energy range (0.50-50.0 MeV) are central to unfolding many outstanding questions in astrophysics. The challenges of medium-energy gamma-ray observations, however, are the low photon statistics and large backgrounds. We review these questions, address the telescope technology requirements, and describe our development of the 3-Dimensional Track Imaging (3-DTI) Compton telescope and its performance for a new mediumenergy gamma-ray mission. The 3-DTI is a large-volume time projection chamber (TPC) with a 2-dimensional gas micro-well detector (MWD) readout

The cosmic-ray contribution to galactic abundances of the light elements: Interpretation of GCR LiBeB abundance measurements from ACE/CRIS

Inelastic collisions between the galactic cosmic rays (GCRs) and the interstellar medium (ISM) are responsible for producing essentially all of the light elements Li, Be, and B (LiBeB) observed in the cosmic rays. Previous calculations (e.g., [1]) have shown that GCR fragmentation can explain the bulk of the existing LiBeB abundance in the present day Galaxy. However, elemental abundances of LiBeB in old halo stars indicate inconsistencies with this explanation. We have used a simple leaky-box model to predict the cosmic-ray elemental and isotopic abundances of LiBeB in the present epoch. We conducted a survey of recent scientific literature on fragmentation cross sections and have calculated the amount of uncertainty they introduce into our model. The predicted particle intensities of this model were compared with high energy (E_(ISM) = 200–500 MeV/nucleon) cosmic-ray data from the Cosmic Ray Isotope Spectrometer (CRIS), which indicates fairly good agreement with absolute fluxes for Z ≥ 5 and relative isotopic abundances for all LiBeB species

Can ^(59)Ni Synthesized in OB Associations Decay to ^(59)Co Before Being Accelerated to Cosmic-ray Energies

Observations from the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Advanced Composition Explorer (ACE) have shown that all relevant galactic cosmic-ray isotopic ratios measured are consistent with an OB-association origin of galactic cosmic rays (GCRs). Additionally CRIS measurements of the isotopic abundances of ^(59)Ni and ^(59)Co have shown that the ^(59)Ni has completely decayed into ^(59)Co, indicating a delay of >105 years between nucleosynthesis and acceleration. However, it has been suggested that shocks generated from high-velocity Wolf-Rayet winds in the OB-association environment must accelerate nuclei synthesized in nearby core-collapse supernovae on a time scale short compared to the ^(59)Ni half-life of 7.6x10^4 years. If this were the case, it would imply that OB associations could not be the source of most galactic cosmic rays. In this paper, we describe the OB-association history and environment and show that the time scales for acceleration are such that most ^(59)Ni should be expected to decay naturally in that setting, strengthening the argument that OB associations are the likely source of a substantial fraction of galactic cosmic rays

The Energy Spectra and Relative Abundances of Electrons and Positrons in the Galactic Cosmic Radiation

Observations of cosmic-ray electrons and positrons have been made with a new balloon-borne detector, HEAT (the "High-Energy Antimatter Telescope"), first flown in 1994 May from Fort Sumner, NM. We describe the instrumental approach and the data analysis procedures, and we present results from this flight. The measurement has provided a new determination of the individual energy spectra of electrons and positrons from 5 GeV to about 50 GeV, and of the combined "all-electron" intensity (e+ + e-) up to about 100 GeV. The single power-law spectral indices for electrons and positrons are alpha = 3.09 +/- 0.08 and 3.3 +/- 0.2, respectively. We find that a contribution from primary sources to the positron intensity in this energy region, if it exists, must be quite small.Comment: latex2e file, 30 pages, 15 figures, aas2pp4.sty and epsf.tex needed. To appear in May 10, 1998 issue of Ap.