Elsevier

Physics Letters B

Volume 472, Issues 1–2, 13 January 2000, Pages 215-226
Physics Letters B

Protons in near earth orbit

https://doi.org/10.1016/S0370-2693(99)01427-6Get rights and content

Abstract

The proton spectrum in the kinetic energy range 0.1 to 200 GeV was measured by the Alpha Magnetic Spectrometer (AMS) during space shuttle flight STS-91 at an altitude of 380 km. Above the geomagnetic cutoff the observed spectrum is parameterized by a power law. Below the geomagnetic cutoff a substantial second spectrum was observed concentrated at equatorial latitudes with a flux ∼70 m−2 s−1 sr−1. Most of these second spectrum protons follow a complicated trajectory and originate from a restricted geographic region.

Introduction

Protons are the most abundant charged particles in space. The study of cosmic ray protons improves the understanding of the interstellar propagation and acceleration of cosmic rays.

There are three distinct regions in space where protons have been studied by different means:

  • The altitudes of 30–40 km above the Earth's surface. This region has been studied with balloons for several decades. Balloon experiments have made important contributions to the understanding of the primary cosmic ray spectrum of protrons and the behavior of atmospheric secondary particles in the upper layer of the atmosphere.

  • The inner and outer radiation belts, which extend from altitudes of about 1000 km up to the boundary of the magnetosphere. Small size detectors on satellites have been sufficient to study the high intensities in the radiation belts.

  • A region intermediate between the top of the atmosphere and the inner radiation belt. The radiation levels are normally not very high, so satellite-based detectors used so far, i.e. before AMS, have not been sensitive enough to systematically study the proton spectrum in this region over a broad energy range.

Ref. [1] includes some of the previous studies. The primary feature in the proton spectrum observed near Earth is a low energy drop off in the flux, known as the geomagnetic cutoff. This cutoff occurs at kinetic energies ranging from ∼10 MeV to ∼10 GeV depending on the latitude and longitude. Above cutoff, from ∼10 to ∼100 GeV, numerous measurements indicate the spectrum falls off according to a power law.

The Alpha Magnetic Spectrometer (AMS) [2] is a high energy physics experiment scheduled for installation on the International Space Station. In preparation for this long duration mission, AMS flew a precursor mission on board the space shuttle Discovery during flight STS-91 in June 1998. In this report we use the data collected during the flight to study the cosmic ray proton spectrum from kinetic energies of 0.1 to 200 GeV, taking advantage of the large acceptance, the accurate momentum resolution, the precise trajectory reconstruction and the good particle identification capabilities of AMS.

The high statistics (∼107) available allow the variation of the spectrum with position to be measured both above and below the geomagnetic cutoff. Because the incident particle direction and momentum were accurately measured in AMS, it is possible to investigate the origin of protons below cutoff by tracking them in the Earth's magnetic field.

Section snippets

The AMS detector

The major elements of AMS as flown on STS-91 consisted of a permanent magnet, a tracker, time of flight hodoscopes, a Cerenkov counter and anticoincidence counters [3]. The permanent magnet had the shape of a cylindrical shell with inner diameter 1.1 m, length 0.8 m and provided a central dipole field of 0.14 Tesla across the magnet bore and an analysing power, BL2, of 0.14 Tm2 parallel to the magnet, or z, axis. The six layers of double sided silicon tracker were arrayed transverse to the magnet

Analysis

Reconstruction of the incident particle type, energy and direction started with a track finding procedure which included cluster finding, cluster coordinate transformation and pattern recognition. The track was then fit using two independent algorithms 5, 6. For a track to be accepted the fit was required to include at least 4 hits in the bending plane and at least 3 hits in the non-bending plane.

The track was then extrapolated to each time of flight plane and matched with the nearest hit if it

Results and interpretation

The differential spectra in terms of kinetic energy for downward and upward going protons integrated over incident angles within 32° of the AMS z-axis, which was within 1° of the zenith or nadir, are presented in Fig. 2 and Table 2, Table 3, Table 4. The results have been separated according to the absolute value of the corrected geomagnetic latitude [9], ΘM (radians), at which they were observed. Fig. 2a, b and c clearly show the effect of the geomagnetic cutoff and the decrease in this cutoff

Acknowledgements

The support of INFN, Italy, ETH–Zürich, the University of Geneva, the Chinese Academy of Sciences, Academia Sinica and National Central University, Taiwan, the RWTH–Aachen, Germany, the University of Turku, the University of Technology of Helsinki, Finland, the US DOE and M.I.T., CIEMAT, Spain, LIP, Portugal and IN2P3, France, is gratefully acknowledged.

We thank Professors S. Ahlen, C. Canizares, A. De Rujula, J. Ellis, A. Guth, M. Jacob, L. Maiani, R. Mewaldt, R. Orava, J. F. Ormes and M.

References (11)

  • S. Ahlen

    Nucl. Instr. Meth. A

    (1994)
  • J.C. Hart et al.

    Nucl. Instr. Meth.

    (1984)
  • S.C. Freden, R.S. White, Phys. Rev. Lett. 3 (1959) 9; S.D. Verma, J. Geophys. Res. 72 (1967) 915; B.J. Teegarden, J....
  • AMS Collaboration, The Construction of the Alpha Magnetic Spectrometer, Nucl. Instr. Meth., in preperation; G.M....
  • R. Brun et al., GEANT 3, CERN DD/EE/84-1, revised 1987; P.A. Aamio et al., FLUKA Users Guide, CERN TIS-RP-190,...
There are more references available in the full text version of this article.

Cited by (0)

6

Supported by the Deutsches Zentrum für Luft- und Raumfahrt, DLR.

7

Supported by the National Natural Science Foundation of China.

8

Supported also by the Comisión Interministerial de Ciencia y Tecnologı́a.

9

Also supported by the Italian Space Agency.

1

Permanent address: HEPPG, Univ. of Bucharest, Romania.

2

Permanent address: Nuclear Physics Institute, St. Petersburg, Russia.

3

Now at European Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland.

4

Now at National Institute for High Energy Physics, NIKHEF, NL-1009 DB Amsterdam, The Netherlands.

5

Supported by ETH Zürich.

View full text