2008 FANFARE SELECTED TOPICS ON THE STUDY OF LOW ENERGY GALACTIC COSMIC RAYS F.B. M c Donald Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA

GALACTIC COSMIC RAYS

CONCEPTS OF COSMIC RAY MODULATON (CIRCA 1955) SHORT TERM CHANGES (COMPETING VIEWS) Early observations of geomagnetic storms and aurorae—explained in terms of “clouds” of charged particles from local active regions on the sun. Excellent correlation between geomagnetic effects and Forbush decreases led to initial explanation in terms of changes in cut-off rigidities and ring current parameters. THE HELIOSPHERE Based on the propagation of solar cosmic rays as observed by neutron monitors, it was assumed that:The sun was in a field-free cavity of ~ 100 – 200 AU in extent (L. Davis, 1955)

CONCEPTS OF COSMIC RAY MODULATON (CIRCA 1955) MODEL FOR LONG AND SHORT TERM VARIATIONS Phil Morrison proposed: Common cause for all of the non-flare cosmic-ray changes: Time (and energy) modulation of the incoming cosmic-ray beam by the random diffusion of the particles through turbulent clouds of magnetized plasma emitted from the sun. The sporadic cosmic-ray storm decreases (Forbush events) occur when the earth is immersed in a strong fresh cloud following a solar flare. The eleven-year variations in intensity and low-energy spectrum arise from the combined effect of much weaker and more diffuse clouds which travel across the solar system to the region of the outer planets.

PIONEER V: A CRUCIAL 1 ST STEP IN SOLAR SYSTEM STUDY Launched on a Thor-Able three stage rocket on 11 March Orbited around the Sun between the Earth and Venus. Provided a wealth of new data on interplanetary space including measurements of magnetic fields, cosmic radiation, electrical fields and micrometeorites. The spacecraft transmitted information until June The beach ball sized spacecraft was equipped with four paddle like solar cells. It was stabilized by slowly spinning about its axis. Dimensions D: 72 cm (26 inches; sphere) W: 254 cm (96 inches; across solar panels) Credit: Smithsonian, National Air and Space Museum

Showed that the Forbush decrease of March 31, 1960 had no possible association with the Earth and its magnetic field. Strong evidence for the passage of conducting solar plasma with a magnetic field that convectively moved the galactic cosmic rays from a volume of the inner solar system. Galactic cosmic ray gradients near Earth at solar maximum so small as to suggest that modulation of galactic intensity occurs at distances much greater than the orbit of earth. C.Y. Fan, P. Meyer, J.A. Simpson, Phys Rev Lett 5, 269, 1960 Phys Rev Lett 5, 272, 1960

That developed to understanding low energy Galactic Cosmic Rays at 1AU: 1. Adiabatic energy loss in the interplanetary medium 2. Anomalous cosmic rays 3. Particle drifts in the interplanetary magnetic field OBSTACLES

An Annotated History of Anomalous Cosmic Rays In the beginning – The Three Discovery Papers: Flat Helium Spectrum MeV/n – IMP 5 Garcia-Munoz, Mason, Simpson ApJ 182, L81, 1973 October 1972 High Intensity, Flat Oxygen Spectra 2-8 MeV/n C/O = 0.8 ± 0.4 Hovestadt, Vollmer, Gloeckler and Fan Physical Review Letters, 31, 650, 1973

An Annotated History of Anomalous Cosmic Rays Mar – Mar (1–3.8 AU) Flat He Spectra 5–60 MeV/n Large non-solar increase in Oxygen 7-25 MeV/n Nitrogen enhanced Low C/O ratio: 0.08 ± 0.03, Small but positive radial gradients McDonald, Teegarden, Trainor and Webber ApJ 187, L105, 1974 Based on this fragment of information on Anomalous Cosmic Rays (ACR’s) Fisk, Kozlousky and Ramaty (ApJ 190, L39, 1974) proposed ACR’s had their origin as interstellar neutral ions that were ionized in the region of the supersonic solar wind and were convected out to the distant heliosphere where they were accelerated to ACR energies. Composition should reflect that of local interstellar neutrals (H, Ne and Ar should also be present). ACR’s should be singly-ionized.

Anomalous Neon Detected – IMP 7,8 09/1972 – 11/1974 Von Rosenvinge and McDonald, Proc 14 th ICRC (Munich) 2, 792,1975 Prediction that ACRs could be stably trapped in the Earth’s magnetosphere Blake and Friesen 15 th ICRC (Plovdiv) 2, 341, Pesses, Jokipii and Eichler (ApJ 246, L85, 1981) Proposed ACRs accelerated at the termination shock Determination of Oxygen charge state: ~10 COSMOS flights/year with cellulose nitrate detectors. IMP8 and ICE outside the magnetosphere found mean charge state for 10 MeV/n Oxygen = (- 0.2) Adams et al. ApJ 375, L Sampex Observations in Earth’s magnetosphere also established that ACR Oxygen is predominantly singly ionized with an upper limit of 10% for higher ionization states (similar results for Nitrogen and Neon) Detection of ACR H Christian, et al., ApJ 334, 677, 1988; McDonald et al., ApJ 446, L101, 1995; Christian et al., ApJ L105, An Annotated History of Anomalous Cosmic Rays

ANOMALOUS COSMIC RAYS

PIONEER 10/11 Spacecraft Mass:258 kg Power: RTG (Plutonium Oxide) Antenna: 2.7 m Requires 34 m / 70 m dish Bit Rate: 2048 / 16 bits/s

VOYAGER Spacecraft Mass:722 kg Power: 3 RTG (Plutonium Oxide) Antenna: 3.7 m Requires 34 m / 70 m dish Bit Rate (Dlink): X-Band 160 bits/s January 15, 2008: Voyager AU 34.17° N Voyager AU27.79° S

Science Team E.C. Stone (PI), A.C. Cummings (Caltech) N. Lal, B.C. Heikkila (GSFC) F.B. McDonald (Univ. of Maryland) W.R. Webber (New Mexico State Univ.) Energetic Particle Coverage H: MeV He: MeV/n Z = 1-28 (Resolves Isotopes) E: 2.5 – 140 MeV CRS EXPERIMENT

22 YEAR HELIOMAGNETIC CYCLE

COSMIC RAY MODULATION

The Heliosheath at the nose is estimated to be AU wide. At the TS, radial velocity of the solar wind will decrease by a factor of 2.4 – 4 depending on the strength of the TS, and will continue to decrease as 1/r 2 where r is the heliocentric distance. The intensity of the transverse component of the interplanetary magnetic field jumps by the same factor and continues to increase proportional to r across the heliosheath. HELIOSHEATH

Time-dependent structure of the heliopause and heliosheath: Rayleigh-Taylor- like and Kelvin-Helmholtz-like instabilities driven by interstellar neutrals Zank, 2007

Low Energy (2 – 100 MeV) Galactic Cosmic Ray Electrons Origin: Directly Accelerated primaries Interstellar secondaries from the decay of charged pions Knock-on electrons produced by the passage of higher energy cosmic rays through the interstellar medium At energies below 200 MeV: These electrons are the source of the lower energy diffuse gamma, x-ray and synchrotron radio emission from the galaxy. May play a major role in ionizing and heating the interstellar medium. Difficult to observe at 1AU: Large Jovian electron intensity Strongly modulated In the heliosheath, their vey low rigidity should make them especially sensitive to the passage of transient disturbances.

Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV)

PIONEER 10 … 37 Years Later