Precision Measurement of the monthly cosmic Ray fluxes e−, e+, p, He) with the Alpha Magnetic Spectrometer on the ISS

The precision measurements of the monthly cosmic ray fluxes with Alpha Magnetic Spectrometer on the International Space Station are presented. Individual electron, positron, proton and helium spectra have been measured for each Bartel’s rotation period (27 days) in the time range from May 2011 to May 2017. This period covers the ascending phase of solar cycle #24 together with the reversal of the Sun’s magnetic field polarity through the minimum. The fluxes reveal a characteristic time dependence below 20 GeV. The data show a strong charge-sign dependent effects corresponding to the the polarity reversal of the solar magnetic field.


Introduction
The study of Cosmic Rays (CR) constitutes a unique instrument for understanding our universe. By means of the CR hadronic component, the knowledge about CR propagation through the galaxy and the Interstellar Medium can be improved. With the CR electromagnetic component (as well as the other rare components of CR like antiprotons) we can investigate the local CR sources, looking for indirect signs of Dark Matter. However, CR spectra, when measured near Earth, are significantly affected by the solar activity. The solar activity has a cycle of ∼11 years, during which it increases reaching a maximum and then decreases again. The intensity of cosmic ray radiation is anti-correlated with the activity of the sun [1], and this is the so called Solar Modulation (SM) effect. In order to have a correct understanding of CR spectra out of the heliosphere, the SM should be well known and taken into account. A detailed study of the CR fluxes evolution with time is needed in order to develop and test different models of the SM effects based on the interaction of cosmic rays with the Heliosphere. The simultaneous measurements of e − and e + (or p andp) over a complete solar activity cycle can represent a sound test of the current charge-sign dependent modulation models. AMS-02 can provide the most accurate measurements of the time dependence of particle and anti-particle fluxes since 2011 thanks to its high acceptance and the excellent performance of the detector. In this letter, the time variation of CR electron (e − ), positron (e + ), proton (p) and Helium (He) fluxes during the first 6 years of data taking will be presented.

The AMS-0detector
The AMS-02 is a large acceptance CR detector which has been installed during the STS-134 NASA Endeavour Space Shuttle mission in May 2011 on the International Space Station, where it will collect

Flux measurements
The AMS data from May 2011 to May 2017 have been analysed for the measurement of e − , e + , p and He fluxes in time. The results have been published in [3,4]. The applied analysis follows the formula used for the measurement of the time-averaged electron and positron fluxes [5], improving low-energy effective acceptance. The fluxes have been measured in 79 different time interval, each one corresponding to a different Bartel's rotation, in the rigidity range from 1 to 60 GV for p, from 1.9 to 60 GV for He, and in the energy range from 1 to 50 GeV for e − and e + .

Electron and positron fluxes in time
To study the time behavior in more detail, the fluxes are own in Fig. 2 as a function of time for five characteristic ergy bins. We find a clear evolution of the fluxes with time low energies that gradually diminishes towards high ergies. At the lowest energies, the amplitudes of both the ctron flux and the positron flux change by a factor of 3. th fluxes exhibit profound short-and long-term variations. , which is sensitive to charge-sign dependent effects in the solar modulation process of galactic cosmic rays. Therefore, restricted to the time interval covered here, we use a modelindependent approach to extract the energy dependence of the quantities that characterize the observed transition in Re. With a set of four parameters, the 3871 independent Re measurements as a function of energy and time can be described well with a logistic function, At a given energy E, parameters in the func the midpoint of the t transition Δt. We ch time it takes for the tra the change in magnit each energy bin are sh for all fits.
The parameters t1 at low energies, whe large, see Fig. 3. A duration Δt is indepe of 830 AE 30 days.  The e + /e − ratio, R e , as a function of time with the statistical uncertainties. The red curves show a best-fit parametrisation obtained with an analytical function described in [3]. The polarity of the heliospheric magnetic field is denoted by A < 0 and A > 0. The period without well-defined polarity is marked by the shaded area.
In Figure 1a, the time dependence of the e − and e + fluxes is reported for 5 characteristic energy bins (for more details see [6]). Both e − flux and e + flux show a significant time dependence with a The main reason for this to occur is that when the solar magnetic field reverses its polarity, the galactic CRs of opposite charge will reach Earth from different heliospheric directions due to the magnetic drift. The charge-sign dependencies of solar modulation, can be clearly observed in Figure 1b in which the e + /e − ratio (R e ) is shown as a function of time for all energy bins up to 5 GeV. In R e , the important, newly discovered short-term variations in the fluxes largely cancel, and a clear overall long-term trend appears. At low energies, R e is flat at first, then smoothly increases after the time of the solar magnetic field reversal, to reach a plateau at a higher amplitude.

Helium and proton fluxes in time
improve the accuracy and the sensitivity of the time dependent proton and helium measurements and this provides information for detailed studies of the correlation between sunspot number and the fluxes of protons and helium. For illustration, Fig. SM 2 in the Supplemental Material [32] shows the relative variation of the AMS proton flux integrated with different minimum rigidities as a function of time together with the relative variation of the rate reported by the Oulu, Finland neutron monitor. As seen, the relative variation of this neutron monitor rate matches the AMS proton flux only when the flux is integrated over R ≥ 6.47 GV. Figure 4 shows the AMS p=He flux ratio, see Supplemental Material [32], as a function of time for 9 rigidity bins. As seen, depending on the rigidity range, the p=He flux ratio shows two different behaviors in time. Above ∼3 GV the ratio is time independent. Below ∼3 GV the ratio has a long-term time dependence. To assess the transition between these two behaviors, we performed a fit of the p=He flux ratio r i for each rigidity bin ias a function of time t, with where a i is the average p=He flux ratio from May 2011 to t i , t i is the time when the p=He flux ratio deviates from the average a i , and b i is the slope of the time variation. Above In Figure 2a, the time dependence of the p and He fluxes is reported for 5 characteristic rigidity bins (for more details see [7]). Both the p and He fluxes exhibit large variations with time at low rigidities which decrease with increasing rigidity. The structures in the p flux and He flux are nearly identical in both time and relative amplitude (indicated by the green shading) that decrease progressively with rigidity. The five red vertical dashed lines in the figure indicate the structure that have been also observed in the electron flux and the positron flux. After one year from the solar maximum (April 2014 for solar cycle 24), the amplitudes of the structures are considerably reduced and the proton and helium fluxes steadily increase at rigidities less than 40 GV. Figure 2b shows the AMS p/He flux ratio as a function of time for 9 rigidity bins. We can observe that, depending on the rigidity range, the p/He flux ratio shows two different behaviours in time. Above ∼3 GV the ratio is time independent. Below ∼3 GV the ratio has a long-term time dependence. An analytic fit on p/He ratio was performed and has shown that above 3.29 GV the p/He flux ratio is consistent with a constant value at the 95% confidence level. This shows the universality of the solar modulation of cosmic ray nuclei at relativistic rigidities. Below 3.29 GV, the observed p/He flux ratio is steadily decreasing with time.

Conclusion
The fluxes for p, He, e + and e − as a function of time have been measured by AMS during the ascending phase of solar cycle 24 through its maximum and toward its minimum. The unique performance of AMS-02 provides measurment of both e + and e − fluxes as a function of time with an unprecedented high time granularity. Based on 23.5×10 6 events, we report the observation of short-term structures on the timescale of months coincident in both the e − flux and the e + flux. These structures are not visible in the e + /e − flux ratio. The precision measurements across the solar polarity reversal show that the ratio exhibits a smooth transition over 830±30 days from one value to another. The midpoint of the transition shows an energy dependent delay relative to the reversal and changes by 260±30 days from 1 to 6 GeV.
The precision p flux and the He flux observed by AMS have fine time structures nearly identical in both time and relative amplitude. The amplitudes of the flux structures decrease with increasing rigidity and vanish above 40 GV. The amplitudes of the structures are reduced during the time period, which started one year after solar maximum, when the proton and helium fluxes steadily increase. In addition, above ∼3 GV the p/He flux ratio is time independent. Below ∼3 GV the ratio has a long-term decrease coinciding with the period during which the fluxes start to rise. Before AMS data, several effects had been proposed that lead to a time dependence of the p/He flux ratio at low rigidities, such as velocity dependence of the diffusion tensor, differences in the interstellar spectra of p and He, and the 3 He and 4 He isotopic composition [8][9][10].
AMS is measuring solar effects for all nuclei, particle and anti-particle fluxes in the present and next solar cycle providing information for the development of refined solar modulation models.