Paul Evenson January 2009 1 Low Energy Electron Observations (LEE, AESOP and the Historical Context)...
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![Page 1: Paul Evenson January 2009 1 Low Energy Electron Observations (LEE, AESOP and the Historical Context) Paul Evenson and John Clem University of Delaware.](https://reader036.fdocuments.in/reader036/viewer/2022070323/56649dc55503460f94ab8f34/html5/thumbnails/1.jpg)
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
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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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Paul Evenson January 2009 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
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Paul Evenson January 2009 4
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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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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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.
25
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Paul Evenson January 2009 11
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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Paul Evenson January 2009 12
Charge Sign Dependent Modulation
• Measurement of positrons once more becomes important• Electrons and helium of the same rigidity have significantly
different velocity
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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
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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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Paul Evenson January 2009 15
Digital Optical Spark Chambers Give AESOP a Low Power Draw (<100 watts)
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Paul Evenson January 2009 16
LEE/AESOP Launch -- 1999
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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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Paul Evenson January 2009 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
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Paul Evenson January 2009 22
A Final Thought
• Long term observation programs may not always provide instant gratification, but they can be really useful