Cosmic rays are constantly passing through the Solar System, but the cosmic radiation environment around us is far from constant.
The Sun continuously changes the magnetic environment through which cosmic rays travel. As a result, the intensity of cosmic radiation can vary on timescales ranging from hours to days and weeks, and eventually over the entire approximately 11-year solar activity cycle.
By continuously measuring cosmic rays in space, the Alpha Magnetic Spectrometer (AMS) on the International Space Station allows us to follow these changes in remarkable detail and to compare how different particles respond to the same changing environment.
From hours to an entire solar cycle
The fastest changes can occur during solar energetic particle events. Solar flares and other solar activity can accelerate particles and produce sudden increases in the flux of energetic particles reaching near-Earth space. Over its long-term operation, AMS has observed dozens of such events, allowing solar particles to be studied together with the much larger background of Galactic cosmic rays.
The opposite can also happen. When coronal mass ejections disturb the heliosphere, the flux of Galactic cosmic rays can suddenly decrease. These events are known as Forbush decreases. AMS can follow these changes for different particle species and at different rigidities, providing a detailed view of how cosmic rays respond to disturbances propagating through the heliosphere.
On somewhat longer timescales, high-speed solar-wind streams originating from coronal holes interact with slower solar wind. Together with the Sun’s approximately 27-day rotation, these structures produce recurrent variations in the cosmic-ray flux. AMS has observed variations with periods of approximately 27, 13.5 and 9 days.
Interestingly, the shorter-period variations do not always behave as expected. The strength of the 9- and 13.5-day variations increases with rigidity over part of the measured range, contrary to the behavior anticipated in standard descriptions. These observations provide new constraints on models of cosmic-ray transport. (PRL 127, 271102)
Over years, an even larger pattern appears. The intensity of low-rigidity Galactic cosmic rays follows the solar activity cycle, decreasing when solar activity increases and recovering as the Sun becomes quieter. Around the maximum of solar activity, the Sun’s global magnetic field also reverses its polarity, adding another important change to the environment through which cosmic rays propagate.
Different particles, different behavior
One of the most interesting aspects of these measurements is that cosmic rays do not all respond to the changing heliosphere in the same way.
Protons and helium nuclei both have positive electric charge, yet their temporal variations are not identical. At lower rigidities, helium shows stronger solar modulation than protons. AMS has also found systematic differences between helium and heavier nuclei such as lithium, beryllium and boron. (PRL 128, 231102; PRL 134, 051001)
Comparing particles with different masses and charge signs provides an even more powerful test.
Electrons and positrons have the same mass but opposite electric charges, and their temporal behavior is distinctly different. Their measurements reveal hysteresis effects associated with charge-sign-dependent transport in the heliosphere. Positrons, in contrast, show temporal variations closely related to those of protons, despite their very different masses. (PRL 130, 161001; PRL 131, 151002)
These comparisons show that the response of cosmic rays to the magnetic environment of the Solar System cannot be understood simply in terms of one particle property. Rigidity, charge, mass and the shape of the cosmic-ray spectrum all play a role.
The Curious Case of Antiprotons
Antiprotons provide a particularly interesting comparison.
They have the same mass as protons but the opposite electric charge. Yet their temporal variations over the solar cycle are significantly smaller than those observed for protons, electrons and positrons.
This is particularly intriguing because it shows that the difference cannot simply be attributed to particle mass or charge sign. The AMS measurements indicate that the shape of the cosmic-ray spectrum also plays an important role.
The connection between spectral shape and temporal variation provides a new way to study how cosmic rays are transported through the heliosphere. (PRL 134, 051002)
Why does it matter?
These observations are important for two reasons.
First, they provide increasingly precise tests of models describing the propagation of cosmic rays through the heliosphere. Some of the structures observed by AMS are not fully reproduced by existing expectations, giving us new information about the physical processes that control cosmic-ray transport.
Second, this is directly relevant to the space environment near Earth and throughout the Solar System.
Astronauts and spacecraft are exposed to both Galactic cosmic rays and energetic particles produced by the Sun. The radiation environment changes with solar activity, and future missions to the Moon and Mars will require a better understanding of both its long-term evolution and short-term variations.
AMS therefore provides more than a measurement of cosmic-ray fluxes. By continuously watching cosmic radiation change, we can study how particles move through the heliosphere, improve our models of the space environment, and better understand the radiation that future explorers will encounter.
Thanks to my AMS colleague Dr. Miguel Orcinha for his useful comments during the preparation of this text.