1. Cosmic Ray Electron Spectrum in 2009
32nd ICRC -- 北京
Cosmic Ray Electron Spectrum in 2009
John Clem
And
Paul Evenson
August 15, 2011

2. Low Energy Electron First Flew in 1967
LEE detects electrons with
Plastic scintillators T1, T3 and G (anticoincidence)
Gas Cherenkov detector T2.
It measures the 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

3. For Decades the Cosmic Ray Electron “Standard Candle” Has Been LEE
Magnetic Polarity
Time profile of helium and electron observations at a rigidity of 1.2 GV

4. In August of 2002 LEE flew on a 60mcf balloon reaching float at 161kft (0.9 mbar).
The high altitude provided a low background environment allowing measurements of primary electrons with energies as low as 20MeV even at solar maximum (particle flux minimum).

5. 2009 LEE Flight Technical Accomplishments
Launched 17 May 2009
Sweden to Canada using the MIP
Monitoring via Iridium
Two Commands Sent: Power On and Power Off
Complete Data Recovery from Internal Onboard Recording

6. Key Scientific Result: First Complete Electron Spectrum
This same, rather busy figure will appear on several slides while I discuss various aspects of it

7. Key Scientific Result: First Complete Electron Spectrum
The 2002 flight, to above one millibar, showed a heavily modulated spectrum
In 2009 the payload only reached 2 millibars, but the modulation was at a record minimum
A complete spectrum from about 20 MeV to several GeV was measured for the first time. 7

8. Huber (1998) Thesis Conclusions Regarding Low Energy (<200 MeV) Spectrum are Strengthened
The spectrum exhibits a persistent, negative slope
Sensitivity to modulation is much less than might be expected by extrapolation from higher energy electrons and comparison with similar energy nucleons
There is little or no evidence for charge sign dependence of the behavior. 8

9. Low Energy Turn Up Not Jovian
Nearly power law behavior of the spectrum now stands clearly distinct from the Jovian spectrum.
Possibility that these electrons come from Jupiter via simple diffusion is now essentially eliminated
Accelerated Jovian (“anomalous”) electrons are not excluded (Moraal, Jokipii and Mewaldt 1991). 9

10. Comparison With Voyager
We also have, for the first time, direct measurements of the electron spectrum in the outer heliosphere from the Voyager spacecraft
Voyager 1 data suggest significant disagreement with the estimated “interstellar” electron spectrum heretofore used in our analysis (the solid line in the figure)
Whether this is a problem with the construction of the spectrum or an indication that the particles are accelerated in the heliosphere remains to be determined. 10

11. Voyager 1 Fits a Simple Picture
1 AU spectrum is lower and softer (i.e. more steeply negative).
Lower is easy to understand as a radial gradient
Softer makes sense because as the energy increases one approaches the region of extreme electron modulation (200 – 800 MeV).
Spectrum at 1 AU in 2002 is also both lower and softer than the spectrum in 2009, consistent with this qualitative picture. 11

12. Voyager 2 is Enigmatic
The amazing thing is not only the great difference in the fluxes of electrons measured by the two Voyagers, but that Voyager 2 measures a spectrum that is below that of the electrons at 1 AU.
The relation between the two Voyager spectra also violates the “lower – softer” pattern, as the Voyager 2 spectrum is lower than the other spectra but also harder
Maybe the previous speaker has already given the solution to this puzzle? 12

13. Measurement of the positron abundance would discriminate among different models for the turn-up
A deficit of positrons would point to acceleration in the heliosphere.
If the low energy electrons contain a typical galactic abundance of positrons the answer to the turn-up must lie in transport theory.
Positron abundance higher than predicted by secondary production would constitute a discovery of primary cosmic ray positrons, presumably accelerated from ambient supernova material.
This would add a significant dimension to the discussion of a possible excess in positrons at high energy and the attendant concern with, e.g., a “dark matter” origin.
Unfortunately we are unlikely to have positron abundance measurements in this energy range in the near future, but getting them should receive high priority. 13

14. Conclusions
Definitive observations of low energy electrons both in the inner and outer heliosphere simultaneously now pose two intriguing questions.
What causes the negative slope of the spectrum?
Why is the spectrum so variable within the outer heliosphere while it is apparently rather stable at 1 AU?
Indeed these two questions lead to a third, possibly more fundamental, namely
Just where are these things really coming from anyway?