A Major Variation on a Theme

Saturn has been recognized as a standout since Galileo first pointed his telescope at it. The beauty of the rings immediately draws attention. But Saturn’s invisible radiation belts are special too. Trapped in Saturn’s magnetosphere, the volume of space controlled by the planet’s magnetic field, the radiation belts are only present between zones limited by the orbital distances of the planet’s moons. The radiation disappears where the moons orbit because the particles collide with the moons. The radiation belts themselves should have disappeared long ago because the particles slowly spiral in towards the planet. But the belts are still there!

A recent paper in the Journal of Geophysical Research by Cassini scientists working with an energetic particle detector on the spacecraft’s magnetospheric imaging instrument provides an explanation: Saturn’s radiation belts are regularly replenished through the collision of galactic cosmic rays coming from outside the solar system with atoms in Saturn’s atmosphere and its rings. This phenomenon, known as cosmic ray albedo neutron decay, makes Saturn unique in the solar system and gives scientists a better understanding of the behavior of Saturn’s belts and their differences from Earth’s.

Saturn’s magnetosphere, like those of the other planets, contains radiation belts. These belts consist of ions (atoms that have lost one or more electrons, primarily by collisions or excitation by ultraviolet or x-ray radiation from the sun) and electrons that cycle back and forth along the field lines connecting the north and south magnetic poles. (The ions are mostly the nuclei – single protons – of hydrogen atoms.) After visits by NASA’s Pioneer 11 and Voyager spacecraft, scientists observed that Saturn’s rings and inner satellites absorb these ions and electrons as they orbit around Saturn, clearing zones in the radiation belts. How, then, do the belts maintain themselves over time when the particles slowly drift in, and therefore across the orbits of the satellites?

Roussos and colleagues have found that the rain of local radiation on the moons is maintained by the combination of cosmic rays streaming in from our Milky Way galaxy and the nuclear physics of the cosmic ray collisions at Saturn. Although these kinds of cosmic ray collisions also occur at Earth (and possibly at Jupiter, Uranus and Neptune), the radiation belts of those planets may receive energetic particles through many different mechanisms. At Saturn, cosmic ray collisions appear to be the only process providing high-energy particles, making its radiation belts a unique, filtered reservoir of cosmic ray products from our galaxy.

These cosmic rays are ions themselves and must have enough energy, first, to get inside Saturn’s magnetosphere (which is a barrier to them). The higher the energy of the cosmic rays, the deeper they can penetrate in Saturn’s magnetosphere. Some of these cosmic rays may even make it in as close as the distance of the planet’s main rings or atmosphere. There the collision of a galactic cosmic ray with an atom can shatter the nucleus and release a high energy neutron, one of the constituents of the nucleus of an atom. The important physics in the process is that (1) neutrons are not affected by magnetic fields (so they can travel in any direction) and that (2) neutrons have a limited lifetime: they decay into a lower energy proton, electron, and an anti-neutrino. The proton (and electron) can then become constituents of Saturn’s radiation belts, replenishing particles lost to collisions with Saturn’s moons and rings.

This is a nice picture, but is it correct? Other researchers propose that the now-familiar process of coronal mass ejections from the sun could replenish the radiation belts. Roussos and colleagues point out that three intense solar events collided with Saturn’s magnetosphere and actually generated a temporary new radiation belt outside Tethys’ orbit, a moon orbiting at the outer edge of Saturn’s permanent ion radiation belts. However, no changes were detected in those inner radiation belts throughout this period. Tethys’ absorption of particles apparently isolates the inner radiation belts from events outside. Occasional injections of particles just don’t work to maintain or intensify the inner radiation belts.

Except for some simulations that match the particle energy spectrum observed, there was not observational support for either cosmic ray collisions or an internal acceleration process at work. The Cassini researchers took advantage of Cassini’s long stay studying the Saturn system and the fortuitous coincidence of the extended sunspot minimum just now ending to study any correlation between the behavior of the radiation belts’ intensities over time.

At first glance, cosmic ray collisions and a lack of sunspots would not seem to be related at all. But the galactic cosmic rays that are the engine of cosmic ray albedo neutron decay are modulated by the sun’s behavior. The cosmic rays need to have enough energy to enter the heliosphere (the sun’s version of a planet’s magnetosphere, but with more phenomena). The energy necessary to enter is lower when the sun is less active: this occurs when there are fewer sunspots and fewer solar flares and a weaker magnetic field carried by the solar wind (which is really more like a breeze during sunspot minimum). As a result, the number of galactic cosmic rays reaching Saturn’s magnetosphere is higher during sunspot minimum.

This effect occurs also at our planet and was first established by Scott Forbush in 1937. It is now recognized that there can be long-term Forbush effects tied to the sunspot cycle and short-term Forbush effects due to solar activity like coronal mass ejections. The researchers found evidence of both long- and short-term Forbush effects in their studies of the radiation belts.

In the paper led by Roussos, data from the low energy magnetospheric measurement system on Cassini’s magnetospheric imaging instrument (MIMI/LEMMS) showed that radiation belt intensity rose from the time Cassini arrived at Saturn (June 2004) through the first months of 2010 in step with the rise of the galactic cosmic ray intensity getting into the heliosphere. Then the sun started showing signs of renewed sunspot activity. This finding was in agreement with expectations for radiation belts generated by cosmic rays and represents long term Forbush effects in Saturn’s magnetosphere.

Short-term effects were less obvious but more intriguing. As sunspot activity was falling during the 15-month period spanning the last quarter of 2004 to the start of 2006, three “Solar Energetic Particle” (SEP) events were observed at Saturn. These are enhancements of cosmic rays, but their origin is the sun (solar cosmic rays). Solar energetic particle events can happen anytime but are more frequent when sunspot numbers are high.

Studies at Earth show that these solar cosmic rays supply other planets’ magnetospheres and their radiation belts with a significant number of high energy particles. This is what one would naturally expect to happen.

This is not, however, the case for Saturn, in part because Tethys isolates the inner Saturnian belts from particles coming from outside. Solar cosmic ray events “carry” with them a stronger magnetic field, which excludes galactic cosmic rays from their volume. Galactic cosmic ray intensity is therefore reduced when solar cosmic ray events are moving across Saturn’s magnetosphere (these reductions are called “Forbush decreases”). Such Forbush decreases were identified in the MIMI/LEMMS dataset during the “active” 2004 to 2006 period.

Interestingly, during this same period increases in radiation belt intensity were lower (or even absent) compared to the increasing intensities seen between 2006 and 2010. This indicates that Forbush decreases may actually limit the high-energy particle content of Saturn’s radiation belts and even cause a reduction in their intensity (since fewer galactic cosmic rays could enter and have collisions). If this finding is verified by subsequent observations, it may mean that Saturn’s radiation belts are the only ones known to us whose intensity is reduced, instead of being enhanced, after a solar energetic particle event.

Still, some mysteries remain. While cosmic ray collisions may explain the presence of high-energy ions at Saturn, the radiation belts also contain lower energy products. The research team also found that, if the data were separated into lower and higher energy particles in the radiation belts, both behaved the same way. This suggests that both have the same origin or that both might be tied together by some cause-effect relationship, but scientists have not yet understood where the low-energy population comes from or how the high- and low-energy products are related.

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publication:

”Long- and short- term variability of Saturn’s ionic radiation belts,” Roussos, E. (Max Planck Institute for Solar System Research, Katlenburg?Lindau, Germany), N. Krupp (Max Planck Institute for Solar System Research, Katlenburg?Lindau, Germany), C. P. Paranicas (John Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA.), P. Kollmann (Max Planck Institute for Solar System Research, Katlenburg?Lindau, Germany), D. G. Mitchell (John Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA.), S. M. Krimigis (John Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. & Office of Space Research and Technology, Academy of Athens, Athens, Greece), T. P. Armstrong (Fundamental Technologies, Lawrence, Kansas, USA), D. R. Went (Blackett Laboratory, Imperial College London, London, UK), M. K. Dougherty (Blackett Laboratory, Imperial College London, London, UK), and G. H. Jones (Mullard Space Science Laboratory, UCL, Dorking, UK & Centre for Planetary Sciences, UCL/Birkbeck, London, UK), 2011, J. Geophys. Res., 32 116, A02217 , doi:10.1029/2010JA015954.

-- Stephen J. Edberg, Cassini science communication coordinator