Low Energy Electron Observations (LEE, AESOP and the Historical Context)

1. Low Energy Electron Observations (LEE, AESOP and the Historical Context) Paul Evenson and John Clem University of Delaware Department of Physics and Astronomy Bartol Research Institute

2. Early Views of Cosmic Ray Composition • Originally, primary cosmic rays were assumed to be gamma rays because they produced air showers • In the 1930’s electromagnetic cascades were known, but before identification of pions there was no mechanism as to how they could arise from nucleons Credit:: Wikipedia

3. The Era of Hadrons • With the knowlege of pions, and the geomagnetic determination that the primary cosmic rays are positively charged, the hadronic interpretation of air showers dominated • Balloon and spacecraft instruments confirmed that the primary cosmic rays were predominantly protons with a small admixture of heavier nuclei

4. Electrons (Re)Discovered Earl, PRL6, 125-128, 1961 Meyer & Vogt, PRL 6, 193-196, 1961 • “Primary” cosmic electrons were identified in 1961 • “Secondary”now takes on a new meaning

5. Astrophysical Implications • Observed electron flux was quickly realized to be roughly consistent with “secondary” origin, i.e. production by p-p collisions and pion decay • Measuring positron abundance was critical, since this process yields a slight excess of e+ • “Secondary” electrons are not as interesting to astrophysics, as they are “easy” to explain • “Primary” positrons could come from 26Al decay

6. Primary Acceleration of Electrons • In 1964, Deshong, Hildebrand and Meyer (PRL 12 3-6) showed that negative electrons dominate the flux • Electrons were thus established as an independently accelerated component of cosmic rays

7. Pioneering Work • Through the 1960’s many people contributed to the development of electron instruments • Positrons were measured a few more times, with inconsistent results, but these never challenged the “primary” origin so interest was relatively low

8. Low Energy Electrons: < 5 GeV • Below about 5 GeV electrons are • well contained in modest calorimeters • easily identified by cascade development • abundant compared to interacting protons • Above this, none of the above is true • I thus follow the low energy trail which is important to the study of heliospheric processes

9. Cosmic Ray Electron Spectrum • Origin of the “turn-up” in the lowest energy electrons is not understood • For the rest of my talk I concentrate on the behavior of 500 MeV to 5 GeV electrons

10. Hovestadt and Meyer’s Low Energy Electron Payload • First flown in 1967, LEE detects electrons with • Plastic scintillators T1, T3 and G (anticoincidence) • Gas Cherenkov detector T2. • Measures electron energy with • Cesium iodide (T4) calorimeter • Lead glass (T5) calorimeter • Scintillator T6 assists in particle identification and energy determination by counting the number of particles that escape the calorimeter. 25

11. For Decades LEE has been the Cosmic Ray Electron “Standard Candle” Magnetic Polarity • Time profile of helium and electron observations at a rigidity of 1.2 GV • Alternation with solar magnetic polarity is probably due to “drifting” across magnetic field lines • Large symbols are LEE flights, others are spacecraft “calibrated” by LEE • LEE 2009 is now beeing prepared in Kiruna • We hope PAMELA is next

12. Charge Sign Dependent Modulation • Measurement of positrons once more becomes important • Electrons and helium of the same rigidity have significantly different velocity

13. Mechanism of Charge Sign Dependence is Controversial • Some charge sign effects clearly are a function of current sheet tilt angle • However there is also a major influence from some other process

14. Anti Electron Sub Orbital PayloadLow Energy Electron • AESOP (left) is LEE with a spark chamber hodoscope • First science data in 1995 • AESOP 2009 is being prepared in Kiruna

15. Digital Optical Spark Chambers Give AESOP a Low Power Draw (<100 watts)

16. LEE/AESOP Launch -- 1999

17. Clem et al. (1996)Our First Positron Measurement • Confirmed high positron abundance in A+, resolving earlier discrepancy • Agrees with “self consistent” model of charge sign dependence derived from • Protheroe 1982 calculation • Observed total electron A+ vs A- modulation

18. Today’s Postron Data and the 1996 Self Consistency Calculation • AESOP, AMS and others agree with the A+ calculation • AESOP agrees with the A- calculation, but with large errors • PAMELA is within (large) errors of AESOP but disagrees with the calculation

19. Today’s Data and Today’s Theory • Work being done by Bieber, Burger, Clem, Pei, Stanev, and Yuksel • Protheroe (1982) calculation enhanced with “Geminga excess” – not too important at these energies • Drift modulation calculation with a flat current sheet for two diffusion coefficients

20. Present Status of Positron Observations and Theory • New “drift model” modulation calculation with a flat current sheet for two diffusion coefficients can reproduce AMS or PAMELA but not both

21. Hopes for the Future • Compare LEE 2009 and PAMELA total electron spectra -- keep the standard candle burning • AESOP 2009 should have better deadtime and an extremely low modulation level so better comparison with PAMELA is possible • Make PAMELA last through the polarity reversal (maybe 2011) • Improve AESOP with Fermi/LAT hodoscope technology

22. A Final Thought • Long term observation programs may not always provide instant gratification, but they can be really useful