Paul Evenson January 2009 1

Low Energy Electron Observations

(LEE, AESOP and the Historical Context)

Paul Evenson and John Clem

University of DelawareDepartment of Physics and Astronomy

Bartol Research Institute

Paul Evenson January 2009 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

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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

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Electrons (Re)Discovered

• “Primary” cosmic electrons were identified in 1961

• “Secondary”now takes on a new meaning

Earl, PRL 6, 125-128, 1961 Meyer & Vogt, PRL 6, 193-196, 1961

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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

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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

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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

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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

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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 2009 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.

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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

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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 2009 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

Paul Evenson January 2009 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

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

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LEE/AESOP Launch -- 1999

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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

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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

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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

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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

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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

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A Final Thought

• Long term observation programs may not always provide instant gratification, but they can be really useful