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The two Voyager spacecraft have been traveling away from Earth for more than 33 years and
they are finally in the outer edge of the solar system. This boundary is marked by the
outer reaches of the sun's magnetic field and solar wind, which form an enormous expanse
called the heliosphere.
As the solar wind travels out from the sun, it pushes against the galactic medium and
abruptly slows down. This is called the termination shock. Outside this is the heliosheath, where
the solar wind slows to a stop and the magnetic field is bent back by the ionized interstellar
wind.
The sun's magnetic field spins opposite directions on the north and south poles, creating a sheet
where the two spins meet. This sheet gently ripples as it travels outward and the ripples
get bigger as they go. When this sheet reaches the termination shock, it starts to compress,
like water waves hitting a wall. The Voyager spacecraft have now found that after the termination
shock, these stacked-up ripples of magnetic field form bubbles, shown here as a computer
simulation.
This discovery has prompted a complete revision of what the heliosheath region looks like.
The smooth streamlined look is gone, replaced with a bubbly, frothy outer layer. This new
layer also changes our understanding of how extremely fast-moving particles called cosmic
rays enter our solar system. When they arrive at the bubble region, they slowly move from
bubble to bubble until they can reach smooth magnetic field lines and follow them toward
the sun.
The nature of the bubble region explains why Voyager II has been seeing variations in the
number of energetic particles compared to Voyager I. Because of its path, Voyager II
has been passing in and out of the bubble region. When it is in the region Voyager II
sees many trapped cosmic rays and electrons. When it is out of the region the spacecraft
sees fewer.
Even as the Voyagers answer questions about our solar system, they raise others. For example,
scientists aren't clear yet how the bubbly heliosheath is linked to the ribbon feature
discovered by IBEX and Cassini. This ribbon shows the emission of energetic particles
and seems to indicate some interaction with interstellar space. In the meantime, the Voyager
spacecraft continue sending back data, and, after three decades, they still have a unique
perspective to offer about the universe we live in.