Not all solar wind protons reach the interstellar medium as protons. Instead, a small fraction undergo a collisional charge-exchange interaction with neutral hydrogen atoms from the interstellar medium. This interaction can be written as
Here 
represents the solar wind velocity and 
represents the
velocity of the interstellar medium relative to our solar system, which has a magnitude of
only 25 km s 
. From the viewpoint of the solar wind (Figure 12.12), then, a proton
near zero velocity is suddenly replaced by a (interstellar) proton which is ``born''
with an initial velocity approximately equal to 
.
Figure 12.12: The top portion shows the pickup ring, solar wind flow, and the directions of the
solar wind velocity, electric, and magnetic fields in the solar frame: pickup ions
are born at zero velocity but are then sped up by the convetion electric field to develop
a gyrospeed equal to 
. The bottom portion is a contour plot of the ion velocity
distribution in the solar wind frame, showing the solar wind protons near zero velocity
and the pickup ions in a ring distribution.
This new proton, however, must also start to
gyrate around the magnetic field. In the case of solar wind flow perpendicular to
this means that the initial gyrospeed is and so the ``picked-up''
proton moves on a circle of radius centered on the solar wind ions. In the Sun's
frame then the energy of the pick-up ion varies between zero and 4 times the solar wind
proton energy. In general, the maximum energy is
where 
is the angle between the magnetic field direction and .
It may be asked where this extra energy has come from? The answer is ``from
the convection electric field when it accelerates the pickup ion into its gyromotion''. The
consequence is, of course, that the solar wind flow must slow down to accommodate this energy
flow into pick-up ions. Furthermore, the interstellar pick-up ions will appear as a
heated component with ``thermal'' energy well above the thermal energy of the solar wind ions,
thereby appearing to increase the temperature of the solar wind ions. Finally, the ring
distribution of pickup ions has sizeable gradients 
and
and is theoretically unstable to the growth of MHD waves with
specific ranges of frequencies and wavevectors.
In the last 5 years detection of interstellar pickup ions has become routine using advanced
detectors on the Ulysses spacecraft. Figure 12.13 shows the energy distribution
of interstellar pickup protons and He 
ions [Gloeckler et al. 1993].
Figure 12.13: The phase space density of interstellar pickup protons as a function of 
observed in the spacecraft frame by Ulysses at 4.2 AU [Gloeckler et al., 1993].
Theoretically pickup protons are expected to be important for the evolution of the solar wind beyond about 5 AU. Experimental evidence for pickup ions quantitatively affecting the solar wind is provided in Figures 12.14 [Williams et al., 1995]
Figure 12.14: Proton temperature data observed by the Voyager 2
spacecraft (jagged curve) are compared with the predictions of adiabatic
cooling (dotted curve and a turbulent heating model including heating by CIRs alone (solid
curve) and by pickup ions and CIR heating (dashed curve) [Williams et al., 1995].
and 12.15 [Richardson et al., 1995], which show the apparent heating and slowing of the solar wind. In both cases the effects of pickup ions are semi-quantitatively consistent with the observations.
Figure 12.15: The speed, density-weighted speed, and number flux for solar wind protons
measured by the Voyager 2 spacecraft in the outer solar system (solid curve) and
by the IMP 8 spacecraft at 1 AU (dotted curve) [Richardson et al., 1995]. Note that
the solar wind speed
is approximately constant with heliocentric distance but with the suggestion
of a small decrease beyond about 15 AU.