Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January Electrons (Re)Discovered “Primary” cosmic electrons were identified in 1961 “Secondary” now takes on a new meaning Earl, PRL 6, , 1961Meyer & Vogt, PRL 6, , 1961

Paul Evenson January 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 26 Al decay

Paul Evenson January Primary Acceleration of Electrons In 1964, Deshong, Hildebrand and Meyer (PRL ) showed that negative electrons dominate the flux Electrons were thus established as an independently accelerated component of cosmic rays

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January For Decades LEE has been the Cosmic Ray Electron “Standard Candle” 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 Magnetic Polarity

Paul Evenson January Charge Sign Dependent Modulation Measurement of positrons once more becomes important Electrons and helium of the same rigidity have significantly different velocity

Paul Evenson January 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

Paul Evenson January Anti Electron Sub Orbital Payload Low Energy Electron AESOP (left) is LEE with a spark chamber hodoscope First science data in 1995 AESOP 2009 is being prepared in Kiruna

Paul Evenson January Digital Optical Spark Chambers Give AESOP a Low Power Draw (<100 watts)

Paul Evenson January LEE/AESOP Launch

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January 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

Paul Evenson January A Final Thought Long term observation programs may not always provide instant gratification, but they can be really useful