Secondary electrons and positrons in near earth orbit

Abstract

The secondary e + and e − populations in near earth's orbit have been calculated by simulation. The results are in very good agreement with the recent AMS measurements. The e + over e − flux ratio for particles below the geomagnetic cutoff appears to be due to the geomagnetic East-West effect.The study of particle populations in the earth environment has been a field of intense research activity for more than 50 years [1]. After a period of latence, the field is regaining interest with the new generation of ambitious experimental projects. They should bring major improvements to the instrumental accuracy of the measurements and orders of magnitudes to the statistics of counting, with respect to previous experiments (see the references quoted in [2,3] for the current and historical context). Some new measurements of particle flux close to earth have been performed recently by the AMS experiment, providing a large sample of new data. These measurements open new prospects for high precision studies of the cosmic ray (CR)-atmosphere interactions and the dynamics of particles in the magnetosphere. This is of particular importance for the atmospheric neutrino issue, in the current context of this research field, since the knowledge of the (electron and muon) neutrino flux produced in the same decay chain of pions in the atmosphere is necessary for interpreting the results of the underground neutrino experiments [4]. Consequently, a good understanding of the electron and positron flux is highly relevant.The proton distributions measured below the earth geomagnetic cutoff (GC) by AMS [2] have been successfully interpreted recently in terms of interactions of the (proton) cosmic ray (CR) flux with the atmosphere [5]. This work is referred to as I in the following. Extending the work reported in I on protons, the present paper addresses the interpretation of the flux of positrons and electrons measured by AMS below the cutoff (subGC) [3] in the same phenomenological framework.The lepton distributions observed by AMS showed a few remarkable features, of which the main two follow. First, the kinetic energy spectra have a strong subGC (then secondary) component (figure 1), similar to those observed in the proton flux. Second, the ratio of the e + over e − flux is large in the equatorial region (≈ 4) and decreasing towards higher latitudes (≈ 1). The measured spectra extended from the kinetic energy threshold of the spectrometer at ≈0.15 GeV, up to about 30 GeV for e − and 3 GeV for e + . At low energy, the subGC secondary spectral yield decreases rapidly from a maximum around the low energy limit of the spectrometer up to the GC energy, around 15 GeV/c for equatorial latitudes (electrons) where 1

The Alpha Magnetic Spectrometer (AMS) on the International Space Station: Part I – results from the test flight on the space shuttle

“…After the AMS data had become available, many physicists [62][63][64][65][66][67] have made detailed Monte Carlo simulations of the second spectrum phenomenon by means of several independent methods. The methods included the primary proton and helium nucleon interactions in the atmosphere with tracing all trajectories in the Earth's magnetic ÿeld.…”

Measurements of electron and positron fluxes below the geomagnetic cutoff by the PAMELA magnetic spectrometer

“…Results for albedo and quasitrapped particles are in agreement with AMS-01 data. Calculations in paper [4,3] shows that well-known East -West effect leads…”

“…More modern Monte-Carlo simulation also were undertaken to explain results of AMS-01 experiments [2], e.g. [4,3]. It was shown that well-known East -West effect leads to positron excess in near equatorial region for quasitrapped and trapped components from this source.…”

Measurements of quasi‐trapped electron and positron fluxes with PAMELA

“…A positron excess is therefore observed. Simulations performed in the work of Derome et al [2001], Plyaskin [2008], and Lipari [2002] elucidated the nature of particle propagation in near‐Earth space and included important effects due to the East‐West asymmetry of the geomagnetic cutoff. Gusev et al [1985] discuss this effect in more detail.…”