2008 FANFARE

SELECTED TOPICS ON THE STUDY OF LOW ENERGY GALACTIC COSMIC RAYS

F.B. McDonaldInstitute 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 ofthe 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 1ST STEP IN SOLAR SYSTEM STUDY

Launched on a Thor-Able three stage rocket on 11 March 1960.

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 26 1960.

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 Spectrum12-55 MeV/n – IMP 5

Garcia-Munoz, Mason, SimpsonApJ 182, L81, 1973

October 1972High Intensity, Flat Oxygen Spectra 2-8 MeV/nC/O = 0.8 ± 0.4

Hovestadt, Vollmer, Gloeckler and FanPhysical Review Letters, 31, 650, 1973

An Annotated History of Anomalous Cosmic Rays

Mar. 1972 – Mar. 1973 (1–3.8 AU)

Flat He Spectra 5–60 MeV/nLarge non-solar increase inOxygen 7-25 MeV/n

Nitrogen enhanced

Low C/O ratio: 0.08 ± 0.03, Small but positive radial gradients

McDonald, Teegarden, Trainor and WebberApJ 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 – 11/1974Von Rosenvinge and McDonald, Proc 14th ICRC (Munich) 2, 792,1975

• Prediction that ACRs could be stably trapped in the Earth’s magnetosphereBlake and Friesen 15th ICRC (Plovdiv) 2, 341, 1977

• 1981 Pesses, Jokipii and Eichler (ApJ 246, L85, 1981)Proposed ACRs accelerated at the termination shock

• Determination of Oxygen charge state: 1984-1988 ~10 COSMOS flights/year with cellulose nitrate detectors. IMP8 and ICE outside the magnetosphere found mean charge state for 10 MeV/n Oxygen = 0.9 + 0.3 (- 0.2)

Adams et al. ApJ 375, L45 1981

• 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 HChristian, et al., ApJ 334, 677, 1988;McDonald et al., ApJ 446, L101, 1995;Christian et al., ApJ L105, 1995.

An Annotated History of Anomalous Cosmic Rays

ANOMALOUS COSMIC RAYS

PIONEER 10/11

Spacecraft Mass: 258 kgPower: RTG (Plutonium Oxide)Antenna: 2.7 mRequires 34 m / 70 m dishBit Rate: 2048 / 16 bits/s

VOYAGER

Spacecraft Mass: 722 kgPower: 3 RTG (Plutonium Oxide)Antenna: 3.7 mRequires 34 m / 70 m dishBit Rate (Dlink): X-Band 160 bits/s

January 15, 2008: Voyager 1 105.1 AU 34.17° N Voyager 2 84.8 AU 27.79° S

Science TeamE.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: 1.8-300 MeV He: 1.8-650 MeV/n Z = 1-28 (Resolves Isotopes) E: 2.5 – 140 MeV

CRS EXPERIMENT

22 YEAR HELIOMAGNETIC CYCLE

COSMIC RAY MODULATION

COSMIC RAY MODULATION

COSMIC RAY MODULATION

COSMIC RAY MODULATION

COSMIC RAY MODULATION

• The Heliosheath at the nose is estimated to be 30-60 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/r2 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 tothe passage of transient disturbances.

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

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

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

PIONEER 10

… 37 Years Later