Science Goals

Primary Science Objectives

Search for the origin of low-energy in the electron spectrum between 10-300 MeV

Resolving electrons and positrons is vital to understanding both electron origin and propagation. AESOP-Lite will be capable of charge sign separation at these energies. The observation of the low-energy electron spectrum will address the following questions:

  • What causes the negative slope of the spectrum below 100 MeV?
  • What is the source of these low energy electrons?

Past observations of electrons in the energy range 10 MeV to 5 GeV

Although the addition of drifts in modulation theory has provided charge-sign dependent component yielding unified coefficients for both nuclei and electrons with energies greater than 300 MeV, yet it cannot predict the observed negative slope at low-energies. Based on solar electron observation, it is expected the electron diffusion at low energies have much longer mean free paths than predicted. However incorporating this effect, the magnitude is still not enough to explain the observations

Resolving the positron abundance below 200 MeV

Scenario 1
Positron excess would herald the discovery of primary positrons at this energy regime, presumably accelerated from ambient supernova material or perhaps dark matter annihilation.
Scenario 2
If the abundance has a similar composition to that at higher energy, then electron diffusion model at low energies is inconsistent with standard diffusion theory.
Scenario 3
If the composition is below that expected from Galactic propagation, the source of the electron turn-up is likely local to the heliosphere.

1 AU baseline for Voyager electrons


View larger. | Artist’s concept of the paths of the Voyager 1 and 2 spacecraft on their journey through our solar system and out into interstellar space. Image via NASA, ESA, and Z. Levay (STScI).


In August 2012, Voyager 1 made the historic entry into interstellar space.

AESOP-Lite will be the only instrument that provides overlapping electron energies, to serve as a calibration at 1 AU, and better probe the propagation mechanisms of within heliosphere.

Secondary Science Objectives

Secondary goals of the AESOP-Lite mission is to measure the time variation of electrons and positrons magnetically trapped in the geomagnetic field.

Diurnal variability of the geomagnetic cutoff has been observed mainly through observations of electrons.


Schematic illustration of the geomagnetic diurnal effect for an observer near the magnetic polar caps.


At night magnetic fields lines are drawn out into the geotail, lowering the cutoff. Below approximately 100 MeV the return albedo flux is higher than the primary cosmic ray electron flux, leading to higher observed fluxes during the day. Protons and heavies interacting in the atmosphere are the source of the return albedo electron flux.


Different electron energy ranges measured by the LEE payload, launched May 16,
2009 from Esrange, Sweden, accumulated roughly 100 hours at altitudes up to
141,000 feet before termination in Northern Canada. Time profile of the electron
the flux clearly reveals diurnal variation and its latitude dependence.



These transitions are not well predicted by even the best models of today and an
improved understanding of what is actually happening is the essence of this proposed

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