LEE and AESOP balloon filling up

Balloon observations with the AESOP (Anti-Electron Sub Orbital Payload) instrument began in 1997 when it flew on September 1st with LEE as a single balloon payload.  It flew again with LEE in 1998, 1999, 2000, 2002, 2006, 2009, and 2011.  The main advantage of AESOP over LEE was the addition of a permanent magnet and a spark chamber hodoscope.  These enabled AESOP to determine the charge sign of the incoming electron whereas LEE could not.  This made the primary goal of the AESOP instrument to investigate the charge-sign dependence in solar modulation.


Figure 1

The AESOP chambers contained 5 parallel aluminum plates connected, in alternating order, to ground and to a high voltage pulser.  The medium between the plates was a slow moving noble gas mixture of neon and helium.  As a charged particle transversed a chamber it left behind an ion trial in the gas.  The purpose of the scintilator detectors, mounted above and below the chamber, was to detect coincidence light pulses from the resulting ionization track.  When this occurred, a 10,000 volt pulser was triggered.  In the presence of a high electric field, the ions in the gas were accelerated toward the plate surface.  This produced more ions with each ion-atom collision.  These multiple collisions formed an ion cascade which ultimately resulted in a high voltage breakdown very near the original ion trail.  The breakdown in each gap produced a bright red vertical spark which was digitized and recorded using a linear CCD camera.


Figure 2                                                                          Figure 3

Charge sign dependent solar modulation of electrons was first definitively observed by AESOP.  In the figures above, the left panel illustrates the dramatic drop in positron abundance related to the solar polarity reversal detected by AESOP.  As seen in the panel on the right, subsequent PAMELA (a Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) and AMS (Alpha Magnetic Spectrometer) observations confirmed the reduced positron abundance in the present solar polarity state compared to consensus of determinations in the 1990’s.


LEE and AESOP payload in flight

References for Images

Figure 1: Clem, John. “AESOP and LEE Balloon Instruments.” AESOP and LEE:Balloon  Payloads, Bartol Research Institute – University of Delaware,

Figure 2: Clem, J., and P. Evenson (2009), Balloon-borne observations of the galactic positron fraction during solar minimum negative polarityJ. Geophys. Res.114, A10108, doi:10.1029/2009JA014225.

Figure 3: M. Aguilar et al. (AMS Collaboration), First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5-350 GeV, Phys. Rev. Lett 110, 141102 – Published 3 April 2013


“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.

“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.

“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.

“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.

“Positron Abundance in Galactic Cosmic Rays,” J. Clem and P. Evenson, Astrophysical Journal, 568, 216, 2002.

“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.

“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.


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