Pickup
ions originate from neutral atoms entering the heliosphere. Due to the combined effects of the Sun's
gravity and ionization, the neutral interstellar Helium flow through the inner
solar system forms a distinctly shaped gravitational focusing
cone on the downwind side of the Sun. Untangling spatial from
temporal variations requires a three-dimensional constellation of heliospheric
spacecraft. STEREO provides two additional simultaneous in-ecliptic
measurements separated in solar longitude.
Earth
passes through this focusing
cone in December of each year.
Reflecting
their different solar longitudes from Earth, STEREO A's first passage through the cone was
seen in November of 2007.
STEREO B lagged behind, passing through the same cone in January
2008.