Standard Parker-type solar wind theories predict that the radial and 
components of
the magnetic field should vary approximately as 
and 
for constant solar wind
speed, where R is the heliocentric distance (see Eqs. (7.20) and (7.24)). Accordingly, the
component dominates in the outer heliosphere and the magnetic field becomes increasingly
perpendicular to the solar wind velocity, on average. In addition, the magnetic field energy
density falls off as at large R, similar to the ram kinetic energy of the solar
wind. These variations are consistent with Figure 12.10. They also mean that the Alfven Mach number
is essentially constant for large R. On the
other hand, the fast mode and sonic Mach numbers decrease at large R as the heating by pickup
ions causes the sound speed for the overall fluid to increase with R.
The solar wind plasma density decreases as (Figure 12.9),
as expected for global mass conservation in radial flows and from standard Parker theory, whence
the plasma frequency 
on average. The solar wind speed is observed
and predicted to be essentially constant with heliocentric distance, at least until mass-loading
by pick-up ions becomes important and the flow is slowed (Figure 12.15). The ordinary solar wind
ions and electrons are expected to cool adiabatically (they are
moving into an expanding volume), although heating by CIR shocks is important from a few to about
20 AU, and heating by MHD turbulence is likely important too.
As discussed in Chapter 12, interstellar pick-up ions come from interstellar neutrals which enter
the heliosphere, charge-exchange with solar wind protons, start off essentially at zero velocity
relative to the Sun, and are then accelerated by the solar wind convection electric field to reach
a large gyrospeed 
( 
is the angle between 
and
), forming a ring-beam distribution with very large effective temperature relative to
the solar wind ions. The pick-up ions then 
drift out of the heliosphere
with the solar wind, as well as moving parallel to (with very low speed) and
performing their gyromotion. The energy required to create this gyromotion comes from the convection
electric field and so from the solar wind flow itself. Accordingly, the solar wind is effectively
heated and slowed by the charge-exchange and pick-up of these interstellar neutrals. Evidence
for this heating and slowing is provided by Figures 12.14 and 12.15, respectively.
Pick-up ions are predicted theoretically to have significant effects on the details of the global outer heliosphere, affecting for instance the locations and perhaps structures of the characteristic plasma boundaries like the termination shock and the heliopause, as reviewed by Zank [1999]. Additionally, these pick-up ions are accelerated at the termination shock, forming the so-called ``anomalous cosmic ray component'' observed at Earth and in the heliosphere.