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HomeSciencePhysicsDisentangling the Sun’s Impact on Cosmic Rays

Disentangling the Sun’s Impact on Cosmic Rays

• Physics 16, 62

An instrument on the International Space Station has revealed new details about how the Sun’s magnetic discipline impacts cosmic rays on their option to Earth.

Figure 1: An illustration of the predominantly diffusive movement of Galactic-cosmic-ray protons (pink) and electrons (blue) within the heliosphere for various magnetic-polarity cycles (high: A > 0, backside: A < 0, the place A denotes the polarity of the cycle). The grey areas signify the heliospheric present sheet.An illustration of the predominantly diffusive movement of Galactic-cosmic-ray protons (pink) and electrons (blue) within the heliosphere for various magnetic-polarity cycles (high: A > 0, backside: A < 0, the place A denotes the polarity of the cycle). The grey… Show more

Galactic cosmic rays (GCRs) are extremely energetic charged particles which can be produced by way of numerous acceleration mechanisms in astrophysical objects similar to supernova remnants. These particles propagate by way of the Galaxy and may attain the heliosphere, a area dominated by plasma originating from the Sun. Within the heliosphere, GCRs work together with the turbulent plasma atmosphere in a manner that decreases their flux, inflicting them to diffuse in area and to lose power [1]. Most of the affect of this “solar modulation” on GCRs is unbiased of particle cost. But GCR drift can be influenced by large-scale gradients in, and curvatures of, the heliospheric magnetic discipline and by the present sheet—a tenuous construction that separates the heliosphere into areas of reverse magnetic-field polarity [2]. These results are cost dependent and result in variations in how GCR electrons and protons propagate on their option to Earth and all through the Solar System (Fig. 1). The Alpha Magnetic Spectrometer (AMS) Collaboration has now measured these variations with unprecedented accuracy [4], permitting scientists to probe the basic physics of GCR transport within the turbulent heliosphere.

The photo voltaic modulation of GCRs adjustments over time due to an 11-year solar-activity cycle and a 22-year cycle within the polarity of the heliospheric magnetic discipline (Fig. 2). The development of the exercise cycle could be noticed by way of the sunspot quantity—an index that quantifies the abundance of darkish spots related to areas of excessive magnetic-field energy on the Sun’s floor. Higher sunspot numbers point out more-intense photo voltaic exercise. These two cycles have an effect on the variety of GCRs detected on Earth by devices known as neutron screens.

R. D. Strauss and N. E. Engelbrecht

Figure 2: The high panel exhibits the solar-activity cycle as noticed by way of the sunspot quantity. The shaded areas signify instances of peak exercise. The AMS Collaboration [4] used information taken in the course of the indicated interval. The center panel exhibits the GCR rely fee recorded on the Hermanus neutron monitor (NM) in South Africa, with the Sun’s magnetic-polarity cycles (A < 0 and A > 0) indicated. The backside panel exhibits the signal and magnitude of the northern (N) and southern (S) photo voltaic magnetic fields that outline the polarity cycles. Data sources: high panel, Royal Observatory of Belgium; center panel, South African Neutron Monitor Program; backside panel, Wilcox Solar Observatory.The high panel exhibits the solar-activity cycle as noticed by way of the sunspot quantity. The shaded areas signify instances of peak exercise. The AMS Collaboration [4] used information taken in the course of the indicated interval. The center panel exhibits the GCR rely fee… Show more

The polarity cycle is outlined as optimistic (denoted by A > 0) when the northern photo voltaic magnetic discipline is directed away from the Sun, and as detrimental (A < 0) when this discipline is pointed towards the Sun. During the A > 0 cycle, positively charged particles drift towards the Sun alongside the heliospheric polar areas, whereas electrons primarily drift alongside the heliospheric present sheet within the equatorial areas (Fig. 1). When the polarity cycle switches, nonetheless, these behaviors are swapped [5]. Astrophysicists perceive this international drift image qualitatively, however many unanswered questions stay on the quantitative drift results. One key query is how turbulence within the heliospheric magnetic discipline disrupts the drift course of [6].

Observing these charge-dependent results is problematic as a result of the fluxes of oppositely charged particles have to be measured concurrently and with excessive precision. Previous research relied totally on evaluating totally different polarity cycles. But this technique led to ambiguous outcomes as a result of GCR transport can be influenced by time-dependent adjustments in heliospheric plasma. Essentially, no two photo voltaic cycles are precisely alike, and a significant comparability of cosmic-ray transport between them would require similar solar-modulation circumstances.

The AMS Collaboration used a detector onboard the International Space Station to exactly measure each day fluxes of GCR electrons and protons between 2011 and 2021. The researchers analyzed each long- and short-term adjustments within the relationship between these two fluxes. They found that on lengthy timescales, this relationship exhibits hysteresis, which, as traditional, signifies a system with reminiscence. Drift results result in variations within the transport pace and path of electrons and protons by way of the heliosphere as a result of these particles propagate on totally different timescales. As a consequence, oppositely charged particles are saved in a different way from one another inside the heliosphere [7]. The long-term adjustments within the flux relationship could be understood by way of the solar-activity and magnetic-polarity cycles. But the short-term adjustments are most likely associated to transient photo voltaic phenomena, similar to coronal mass ejections, that should be investigated in additional element.

These outcomes will enable GCR drift results—and particularly the turbulence-induced disruption of such results—to be investigated with unprecedented accuracy. Additionally, some points of the findings problem up to date understanding of GCR transport. For instance, these outcomes present, for a while intervals, recurrent 27-day flux variations which can be bigger at greater particle energies. In distinction, idea predicts that these variations ought to disappear at such energies. Furthermore, the recurrent electron-flux variations on quick timescales signify a powerful observational constraint on fashions for the time-dependent photo voltaic modulation of GCRs.

Reproducing these precision measurements for each GCR electrons and protons utilizing solar-modulation fashions will result in worthwhile insights into the mechanisms governing the transport of those particles. Once these transport processes are totally understood, progress could be made on reaching the “holy grail” of solar-modulation research: the flexibility to precisely predict the GCR flux and its related radiation ranges with the intention to safeguard human exploration of the Solar System.


  1. H. Moraal, “Cosmic-ray modulation equations,” Space Sci. Rev. 176, 299 (2011).
  2. O. Khabarova et al., “Current sheets, plasmoids and flux ropes in the heliosphere,” Space Sci. Rev. 217, 38 (2021).
  3. R. D. Strauss et al., “The heliospheric transport of protons and anti-protons: A stochastic modelling approach to Pamela observations,” Astroparticle, Particle, Space Physics and Detectors for Physics Applications – Proceedings of the thirteenth ICATPP Conference, edited by G. Simone et al. (World Scientific Publishing, New Jersey, 2012), p. 288[Amazon][WorldCat].
  4. M. Aguilar et al. (AMS Collaboration), “Temporal structures in electron spectra and charge sign effects in galactic cosmic rays,” Phys. Rev. Lett. 130, 161001 (2023).
  5. J. R. Jokipii et al., “Effects of particle drift on cosmic-ray transport. I – General properties, application to solar modulation,” Astrophys. J. 213, 861 (1977).
  6. N. E. Engelbrecht et al., “Toward a greater understanding of the reduction of drift coefficients in the presence of turbulence,” Astrophys. J. 841, 107 (2017).
  7. R. D. Strauss et al., “On the propagation times and energy losses of cosmic rays in the heliosphere,” J. Geophys. Res.: Space Phys. 116 (2011).

About the Authors

Image of R. Du Toit Strauss

R. Du Toit Strauss acquired his PhD in physics from the North-West University, Potchefstroom, South Africa, the place he’s at present a professor in physics. He is an alumnus of the Fulbright and Alexander von Humboldt associations and holds an adjunct place on the Department of Space Science on the University of Alabama in Huntsville. His major analysis pursuits contain the modeling of cosmic-ray propagation by way of the turbulent heliosphere.

Image of N. Eugene Engelbrecht

N. Eugene Engelbrecht is at present a professor in physics on the North-West University, Potchefstroom, South Africa, the place he acquired his PhD. in physics. His analysis pursuits embrace modeling the transport of cosmic rays all through the heliosphere from first rules, turbulence and its transport, and the diffusion of charged particles in turbulent plasmas.

Subject Areas

AstrophysicsParticles and Fields

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