The launching of the LEE (Low Energy Electrons) instrument in 1968 by the University of Chicago marked the beginning of a series of balloon observations of cosmic ray electrons. Regular balloon flights were launched from Northern Canada up until 1979. In 1984, the project was transferred to the Bartol Research Institute, which has been a part of the University of Delaware since 2000. The last flight of LEE was in 2011. The data from these balloon flights has been used to study solar modulation of electrons with energies up to ~ 20GeV.
To the left is a mock up of LEE. It briefly detects incoming electrons with 3 plastic scintilators labeled T1, T3, and G as well as the gas Cherenkov detector T2. Through the use of a cesium iodide (T4) and a leadglass calorimeter (T5) it measures the electron energy. Scintillator T6 also assists in particle identification and energy determination by counting the number of particles that escape the calorimeter. However, since LEE lacks a magnetic system it is unable to distinguish between negative and positive electrons. Despite this, LEE still provides one of the more precise measurements of the electron spectrum over an extended period of time.
Measurements taken by the LEE payload in 2009 and 2011 show an inconsistency between the electron spectrum prediction from propagation models below 200 MeV. Part of the motivation for AESOP-Lite is measuring the positron abundance in this energy range. This is crucial in order to probe the source and modulation of the low-energy spectrum.
The figure above shows one of the more crucial results from the most recent LEE flights. It depicts the estimate of the interstellar electron spectrum based on recent Voyager data compared with PAMELA (a Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) and LEE observations during 2009. Measurements from the 2011 LEE flight flight were also added for comparison.
2009 Launch of LEE and AESOP
References for Images
Figure 1: Clem, John. “AESOP and LEE Balloon Instruments.” AESOP and LEE:Balloon Payloads, Bartol Research Institute – University of Delaware, http://www.bartol.udel.edu/gp/balloon/.
1 “A general time-dependent stochastic method for solving Parker’s transport equation in spherical coordinates”,C. Pei, J. W. Bieber, R. A. Burger, and J. Clem, Journal of Geophysical Research, 115, A12107, doi:10.1029/2010JA015721, 2010.
2 “Balloon-borne observations of the galactic positron fraction during solar minimum negative polarity”, J. Clem and P. Evenson, Journal of Geophysical Research, 114, A10108, doi:10.1029/2009JA014225, 2009.
3 “Observations of Cosmic Ray Electrons and Positrons during the Early Stages of the A- Magnetic Polarity Epoch”, J. Clem and P. Evenson, Journal of Geophysical Research, 109, A07107, doi:10.1029/2003JA010361, 2004.
4 “Cosmic electron gradients in the inner heliosphere,”, J. Clem, B. Heber and P. Evenson, Geophysical Research Letters, 29 (23), 2096, doi:10.1029/2002GL015532, 2002.
5 “Positron Abundance in Galactic Cosmic Rays,” J. Clem and P. Evenson, Astrophysical Journal, 568, 216, 2002.
6 “Charge Sign Dependence of Cosmic Ray Modulation Near a Rigidity of 1 GV,” J.M Clem, P. Evenson, D. Huber, R. Pyle, C. Lopate and J. Simpson, Journal of Geophysical Research, volume 105, 23099, 2000.
7 “Solar Modulation of Cosmic Electrons,” J. Clem, D.P Clements, J. Esposito, P. Evenson, D. Huber, J.L’Heureux, P. Meyer and C. Constantin, Astrophysical Journal, 464, 507-515, 1996.