Interstellar gas that approaches the Sun is ionized and accelerated by the convective electric field to solar wind speed, yielding interstellar pickup ions. These ions move with the solar wind bulk speed when averaged over a gyro orbit in the solar wind frame. Their instantaneous speed as they gyrate varies from zero to two times the solar wind speed, which is why their distribution function shows a cutoff at twice the bulk speed in the inertial frame.
The charge-exchange and photoionization process can ionize the interstellar neutral atoms in the inner heliosphere and form the pick-up ions.
\[ p^+(\mathbf v_{sw}) +H( \mathbf v_{ISM} \to H( \mathbf v_{sw}) + p^+(\mathbf v_{ISM}) \]
$\mathbf v_{ISM}$ has a magnitude of only 25 km/s. In the solar wind frame, a proton near zero velocity is suddenly replaced by a proton (from ionization) which is born with an initial velocity approximately equal to $-\mathbf v_{sw}$.
The new born proton gyrates around the magnetic field. In the case of solar wind flow perpendicular to $\mathbf B_{sw}$, the initial gyrospeed is $v_{sw}$ and so the picked-up proton moves on a circle. In the Sun's frame, the energy of the pick-up ion varies between zero and 4 times the solar wind proton energy. The extra energy comes from the convection electric field when it accelerates the pickup ion into its gyromotion. The consequence is that the solar wind flow must slow down to accommodate this energy flow into pickup ions.
Reference:
supra-thermal ions in the outer heliosphere
http://www.physics.usyd.edu.au/~cairns/teaching/lecture12/node6.html