next up previous
Next: Remotely determined properties of Up: The Outer Heliosphere Previous: The Outer Heliosphere

Recap of solar wind physics and interstellar pick-up ions

Standard Parker-type solar wind theories predict that the radial and tex2html_wrap_inline358 components of the magnetic field should vary approximately as tex2html_wrap_inline360 and tex2html_wrap_inline362 for constant solar wind speed, where R is the heliocentric distance (see Eqs. (7.20) and (7.24)). Accordingly, the tex2html_wrap_inline358 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 tex2html_wrap_inline360 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 tex2html_wrap_inline372 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 tex2html_wrap_inline360 (Figure 12.9), as expected for global mass conservation in radial flows and from standard Parker theory, whence the plasma frequency tex2html_wrap_inline382 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 tex2html_wrap_inline384 ( tex2html_wrap_inline386 is the angle between tex2html_wrap_inline388 and tex2html_wrap_inline390 ), forming a ring-beam distribution with very large effective temperature relative to the solar wind ions. The pick-up ions then tex2html_wrap_inline392 drift out of the heliosphere with the solar wind, as well as moving parallel to tex2html_wrap_inline390 (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.


next up previous
Next: Remotely determined properties of Up: The Outer Heliosphere Previous: The Outer Heliosphere

Iver Cairns
Wed Oct 20 15:39:59 EST 1999