From AAS NOVA: “A Shifting Shield Provides Protection Against Cosmic Rays”

AASNOVA

AAS NOVA

1 December 2017
Susanna Kohler

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Artist’s impression of the shower of particles caused when a cosmic ray, a charged particle often produced by a distant astrophysical source, hits Earth’s upper atmosphere. [J. Yang/NSF]

Magnetosphere of Earth, original bitmap from NASA. SVG rendering by Aaron Kaase

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Spacecraft outside of Earth’s atmosphere and magnetic field are at risk of damage from cosmic rays. [ESA]

The Sun plays an important role in protecting us from cosmic rays, energetic particles that pelt us from outside our solar system. But can we predict when and how it will provide the most protection, and use this to minimize the damage to both piloted and robotic space missions?

The Challenge of Cosmic Rays

Galactic cosmic rays are high-energy, charged particles that originate from astrophysical processes — like supernovae or even distant active galactic nuclei — outside of our solar system.

One reason to care about the cosmic rays arriving near Earth is because these particles can provide a significant challenge for space missions traveling above Earth’s protective atmosphere and magnetic field. Since impacts from cosmic rays can damage human DNA, this risk poses a major barrier to plans for interplanetary travel by crewed spacecraft. And robotic missions aren’t safe either: cosmic rays can flip bits, wreaking havoc on spacecraft electronics as well.

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The magnetic field carried by the solar wind provides a protective shield, deflecting galactic cosmic rays from our solar system. [Walt Feimer/NASA GSFC’s Conceptual Image Lab]

Shielded by the Sun

Conveniently, we do have some broader protection against galactic cosmic rays: a built-in shield provided by the Sun. The interplanetary magnetic field, which is embedded in the solar wind, deflects low-energy cosmic rays from us at the outer reaches of our solar system, decreasing the flux of these cosmic rays that reach us at Earth.

This shield, however, isn’t stationary; instead, it moves and changes as the strength and direction of the solar wind moves and changes. This results in a much lower cosmic-ray flux at Earth when solar activity is high — i.e., at the peak of the 11-year solar cycle — than when solar activity is low. This visible change in local cosmic-ray flux with solar activity is known as “solar modulation” of the cosmic ray flux at Earth.

In a new study, a team of scientists led by Nicola Tomassetti (University of Perugia, Italy) has modeled this solar modulation to better understand the process by which the Sun’s changing activity influences the cosmic ray flux that reaches us at Earth.

Modeling a Lag

Tomassetti and collaborators’ model uses two solar-activity observables as inputs: the number of sunspots and the tilt angle of the heliospheric current sheet. By modeling basic transport processes in the heliosphere, the authors then track the impact that the changing solar properties have on incoming galactic cosmic rays. In particular, the team explores the time lag between when solar activity changes and when we see the responding change in the cosmic-ray flux.

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Cosmic-ray flux observations are best fit by the authors’ model when an 8-month lag is included (red bold line). A comparison model with no lag (black dashed line) is included. [Tomassetti et al. 2017]

By comparing their model outputs to the large collection of time-dependent observations of cosmic-ray fluxes, Tomassetti and collaborators show that the best fit to data occurs with an ~8-month lag between changing solar activity and local cosmic-ray flux modulation.

This is an important outcome for studying the processes that affect the cosmic-ray flux that reaches Earth. But there’s an additional intriguing consequence of this result: knowledge of the current solar activity could allow us to predict the modulation that will occur for cosmic rays near Earth an entire 8 months from now! If this model is correct, it brings us one step closer to being able to plan safer space missions for the future.

Citation

Nicola Tomassetti et al 2017 ApJL 849 L32. doi:10.3847/2041-8213/aa9373

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