Properties of Daily Helium Fluxes.

Area of Science:

• Cosmic Ray Physics
• Heliophysics
• Particle Astrophysics

Background:

• Cosmic rays, including helium nuclei, are modulated by solar activity as they travel through space.
• Understanding these variations is crucial for interpreting astrophysical observations and space weather.
• Previous studies have shown solar cycle dependencies in cosmic ray fluxes.

Purpose of the Study:

• To present a precision measurement of daily helium fluxes in cosmic rays.
• To investigate the time variations and rigidity dependence of helium flux and its ratio to proton flux.
• To explore short-term periodicities and hysteresis effects in helium flux modulation.

Main Methods:

• Analysis of 7.6×10^8 helium nuclei data collected by the Alpha Magnetic Spectrometer (AMS) on the International Space Station.
• Measurement of daily helium fluxes over a 8.5-year period (May 2011 - October 2019).
• Rigidity interval studied: 1.71 to 100 GV.

Main Results:

• Observed recurrent 27-day helium flux variations from 2014-2018, with shorter 9-day and 13.5-day periods in 2016.
• Helium flux shows larger time variations than proton flux below ~7 GV.
• A significant hysteresis (>7σ) between helium to proton flux ratio and helium flux was found below 2.4 GV, differing before and after the 2014 solar maximum.

Conclusions:

• The study reveals complex temporal variations in cosmic ray helium flux, influenced by solar activity on multiple timescales.
• The observed hysteresis below 2.4 GV highlights distinct modulation mechanisms for helium nuclei at low rigidities.
• These findings provide crucial data for refining models of cosmic ray propagation and solar modulation.