Properties of Cosmic Helium Isotopes Measured by the Alpha Magnetic Spectrometer
M Aguilar1, L Ali Cavasonza2, G Ambrosi3
1Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), 28040 Madrid, Spain.
Physical Review Letters
|November 26, 2019
Summary
The Alpha Magnetic Spectrometer measured helium-3 and helium-4 fluxes, finding their variations over time decrease with increasing rigidity. The helium-3 to helium-4 flux ratio is time-independent above 4 GV, matching other cosmic ray data.
Area of Science:
- Cosmic ray physics
- Particle astrophysics
- Space exploration
Background:
- Precise measurements of cosmic ray isotopes are crucial for understanding their origin and propagation in the galaxy.
- The Alpha Magnetic Spectrometer (AMS) on the International Space Station provides high-statistics data on cosmic ray composition.
- Previous studies have focused on individual isotope fluxes, but the ratio of helium-3 to helium-4 and its rigidity dependence require further investigation.
Purpose of the Study:
- To present precision measurements of helium-3 and helium-4 fluxes and their ratio using AMS data.
- To investigate the time variations and rigidity dependence of these fluxes.
- To compare the measured helium-3/helium-4 flux ratio with predictions and other cosmic ray spectral indices.
Main Methods:
- Utilized data from the Alpha Magnetic Spectrometer (AMS) on the International Space Station.
- Analyzed approximately 100 million helium-4 nuclei and 18 million helium-3 nuclei.
- Measured fluxes and ratios in specific rigidity ranges (2.1-21 GV for helium-4, 1.9-15 GV for helium-3).
Main Results:
- Observed nearly identical time variations for helium-3 and helium-4 fluxes, with decreasing relative magnitude at higher rigidities.
- Measured the rigidity dependence of the helium-3/helium-4 flux ratio for the first time.
- Found the ratio to be time-independent above 4 GV, described by a power law ∝R^{-0.294±0.004}.
- This spectral index agrees with high-energy Boron/Oxygen and Boron/Carbon spectral indices.
Conclusions:
- The observed time variations suggest common modulation mechanisms for helium isotopes.
- The time-independent ratio above 4 GV provides a crucial constraint for cosmic ray propagation models.
- The agreement with B/O and B/C spectral indices hints at a common origin or propagation effect for different cosmic ray species.
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