A. C. Cummings

An asymmetric solar wind termination shock

Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, ∼10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the solar wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic cosmic ray intensity is lower than expected.

Voyager 1 observes low-energy galactic cosmic rays in a region depleted of heliospheric ions.

Unexpected Magnetic Highway The heliopause is thought to separate the heliosphere (the bubble of plasma and magnetic field originating at the Sun) from interstellar plasma and magnetic field. In August last year, the Voyager 1 spacecraft, which was launched 35 years ago, was 18.5 billion kilometers away from the Sun, close to the expected location of the heliopause. Krimigis et al. (p. 144, published online 27 June) report observations of energetic ions and electrons by Voyager 1 that suggest that a sharp and distinct boundary was crossed five times over ∼30 days. Burlaga et al. (p. 147, published online 27 June) found that the magnetic field direction did not change across any of the boundary crossings, indicating that Voyager 1 had not crossed the heliopause but had entered a region in the heliosphere that serves as a magnetic highway along which low-energy ions from inside stream away and galactic cosmic rays flow in from interstellar space. Stone et al. (p. 150, published online 27 June) report the spectra of low-energy galactic cosmic rays in this unexpected region. The Voyager 1 spacecraft entered an unexpected region of the heliosphere at the boundary with interstellar space. On 25 August 2012, Voyager 1 was at 122 astronomical units when the steady intensity of low-energy ions it had observed for the previous 6 years suddenly dropped for a third time and soon completely disappeared as the ions streamed away into interstellar space. Although the magnetic field observations indicate that Voyager 1 remained inside the heliosphere, the intensity of cosmic ray nuclei from outside the heliosphere abruptly increased. We report the spectra of galactic cosmic rays down to ~3 × 106 electron volts per nucleon, revealing H and He energy spectra with broad peaks from 10 × 106 to 40 × 106 electron volts per nucleon and an increasing galactic cosmic-ray electron intensity down to ~10 × 106 electron volts.

Exclusion limits on the WIMP-nucleon cross section from the cryogenic dark matter search.

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for WIMPs via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS data give limits on the spin-independent WIMP-nucleon elastic-scattering cross-section that exclude unexplored parameter space above 10 GeV c^{-2} WIMP mass and, at>84% CL, the entire 3

PET: a proton/electron telescope for studies of magnetospheric, solar, and galactic particles

\sigma

RECORD-SETTING COSMIC-RAY INTENSITIES IN 2009 AND 2010

allowed region for the WIMP signal reported by the DAMA experiment.

Enhancements of energetic particles near the heliospheric termination shock

The proton/electron telescope (PET) on SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) is designed to provide measurements of energetic electrons and light nuclei from solar, Galactic, and magnetospheric sources. PET is an all solid-state system that will measure the differential energy spectra of electrons from approximately 1 to approximately 30 MeV and H and He nuclei from approximately 20 to approximately 300 MeV/nucleon, with isotope resolution of H and He extending from approximately 20 to approximately 80 MeV/nucleon. As SAMPEX scans all local times and geomagnetic cutoffs over the course of its near-polar orbit, PET will characterize precipitating relativistic electron events during periods of declining solar activity, and it will examine whether the production rate of odd nitrogen and hydrogen molecules in the middle atmosphere by precipitating electrons is sufficient to affect O/sub 3/ depletion. In addition, PET will complement studies of the elemental and isotopic composition of energetic heavy (Z>2) nuclei on SAMPEX by providing measurements of H, He, and electrons. Finally, PET has limited capability to identify energetic positrons from potential natural and man-made sources. >

Composition of Anomalous Cosmic Rays and Other Heliospheric Ions

We report measurements of record-setting intensities of cosmic-ray nuclei from C to Fe, made with the Cosmic Ray Isotope Spectrometer carried on the Advanced Composition Explorer in orbit about the inner Sun-Earth Lagrangian point. In the energy interval from ~70 to ~450 MeV nucleon^(–1), near the peak in the near-Earth cosmic-ray spectrum, the measured intensities of major species from C to Fe were each 20%-26% greater in late 2009 than in the 1997-1998 minimum and previous solar minima of the space age (1957-1997). The elevated intensities reported here and also at neutron monitor energies were undoubtedly due to several unusual aspects of the solar cycle 23/24 minimum, including record-low interplanetary magnetic field (IMF) intensities, an extended period of reduced IMF turbulence, reduced solar-wind dynamic pressure, and extremely low solar activity during an extended solar minimum. The estimated parallel diffusion coefficient for cosmic-ray transport based on measured solar-wind properties was 44% greater in 2009 than in the 1997-1998 solar-minimum period. In addition, the weaker IMF should result in higher cosmic-ray drift velocities. Cosmic-ray intensity variations at 1 AU are found to lag IMF variations by 2-3 solar rotations, indicating that significant solar modulation occurs inside ~20 AU, consistent with earlier galactic cosmic-ray radial-gradient measurements. In 2010, the intensities suddenly decreased to 1997 levels following increases in solar activity and in the inclination of the heliospheric current sheet. We describe the conditions that gave cosmic rays greater access to the inner solar system and discuss some of their implications.

Ionization Processes in the Heliosphere — Rates and Methods of Their Determination

The spacecraft Voyager 1 is at a distance greater than 85 au from the Sun, in the vicinity of the termination shock that marks the abrupt slowing of the supersonic solar wind and the beginning of the extended and unexplored distant heliosphere. This shock is expected to accelerate ‘anomalous cosmic rays’, as well as to re-accelerate Galactic cosmic rays and low-energy particles from the inner Solar System. Here we report a significant increase in the numbers of energetic ions and electrons that persisted for seven months beginning in mid-2002. This increase differs from any previously observed in that there was a simultaneous increase in Galactic cosmic ray ions and electrons, anomalous cosmic rays and low-energy ions. The low-intensity level and spectral energy distribution of the anomalous cosmic rays, however, indicates that Voyager 1 still has not reached the termination shock. Rather, the observed increase is an expected precursor event. We argue that the radial anisotropy of the cosmic rays is expected to be small in the foreshock region, as is observed.

MEASUREMENT OF THE SECONDARY RADIONUCLIDES 10Be, 26Al, 36Cl, 54Mn, AND 14C AND IMPLICATIONS FOR THE GALACTIC COSMIC-RAY AGE

Interstellar pickup ions accelerated by the termination shock of the solar wind dominate the anomalous cosmic-ray (ACR) intensities observed by the Voyager spacecraft in the outer heliosphere. Using a twodimensional acceleration and propagation model, we derive the relative abundances of these interstellar ACRs and determine the mass dependence of the injection/acceleration efficiency for the diffusive acceleration of H + ,H e + ,N + ,O + , and Ne + . The energy spectra of C, Na, Mg, Si, S, and Ar also exhibit ACR increases at low energies. To interpret these observations, we have developed a new set of ionization rates for 11 neutral atoms, H, He, C, N, O, Ne, Na, Mg, Si, S, and Ar at 1 AU, and a new set of filtration factors relating neutral densities in the local interstellar medium to those at the location of the solar wind termination shock. Using the injection/acceleration efficiencies and the Ar filtration factor, we estimate the density of neutral Ar to be ð3:5 � 1:6 Þ� 10 � 7 cm � 3 in the local interstellar medium. ACR C may have a significant contribution from interstellar neutral C, but the observed intensities of ACR Na, Mg, Si, and S significantly exceed that expected from interstellar neutrals, providing evidence of another source of pickup ions. One possibility discussed is the recently discovered ‘‘ inner source ’’ of singly charged ions that is thought to be solar wind atoms desorbed from interplanetary dust grains. Subject headings: acceleration of particles — atomic processes — cosmic rays — interplanetary medium — ISM: abundances — ISM: atoms

Spectral Properties of He and Heavy Ions in 3He-rich Solar Flares

The rates of the most important ionization processes acting in interplanetary space on interstellar H, He, C, O, Ne and Ar atoms are critically reviewed in the paper. Their long-term modulations in the period 1974 – 1994 are reexamined using updated information on relevant cross-sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of the solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of ∼ 3 between maximum and minimum) especially for species such as He, Ne, C for which photoionization is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations ∼ 20% around average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and indirect orbits, or simultaneous measurements of He+ and He++ pickup ions and solar wind particles can be useful tools for narrowing the uncertainties of the He photoionization rate caused by insufficient knowledge of the solar EUV flux and its variations.