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.