New paper: on the nature of the time lag in cosmic-ray modulation

My new paper is out. It’s entitled “Temporal evolution and rigidity dependence of the solar modulation lag of Galactic cosmic rays”. It’s about the connection between the 11-year solar activity and the Galactic cosmic-rays reaching Earth.

Schematic view of the Heliospheric modulation of Galactic cosmic rays.

The paper is available open access here: Tomassetti et al. PRD 103022 (2022).
A short summary is given here:

✪ In this paper, we use data collected over 5 solar cycles, 1965 to 2020 to investigate the association between solar activity and cosmic rays in the heliosphere. The variation of the cosmic ray intensity is known to be well correlated with the monthly number of sunspot number. The latter is a good proxy for the Sun’s magnetic activity. However, there is a time lag of few months between sunspots and cosmic ray fluxes. Moreover, the lag is cycle dependent.
✪ We found that the lag is subjected itself to a quasi-periodical modulation. Here is the reconstruction of the lag using neutron monitor data from Rome (IT) and data from the ACE spacecraft. The time-series can be fit with a sinusoidal function of period T~ 22 years.
✪ So, there is a quasi-periodic change of the lag, clearly connected with the Sun’s magnetic activity and its effect in space. But we also found that its mean value (i.e.g the cycle-averaged lag) depends on the energy of the cosmic rays. Higher is their energy, and lower is the lag.
✪ These results can be interpreted in terms of transport of high-energy particles through heliospheric turbulence. The 22-yrs periodicity of the lag arises from charge-sign dependent drift. It’s dependence on particle energy, from Kolmogorov-type diffusion of cosmic rays.
✪ An important aspect, to interpret our results, is to consider the latitudinal profile of the solar wind. A radially symmetric wind cannot work. A latitudinal profile 𝑽(𝜽) for the solar wind speed explains well the observed periodicity in the modulation lag. Such a profile is also based on the data from the Ulysses spacecraft.
✪ If our interpretation is correct, the observed effect is charge-sign dependent. If we could measure the time-lag using cosmic antiprotons instead of protons, we should have a somehow opposite behavior. This prediction can be resolutely tested by the AMS-02 experiment.
✪ Finally, we also provide a simple formula to describe the temporal & energy dependence of the lag over the solar cycle. It can be used as an effective input for our effective & predictive model of solar modulation, but this will be done in a next paper.