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Interstellar pickup ions

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

equation104

Here tex2html_wrap_inline508 represents the solar wind velocity and tex2html_wrap_inline510 represents the velocity of the interstellar medium relative to our solar system, which has a magnitude of only 25 km s tex2html_wrap_inline402 . 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 tex2html_wrap_inline516 .

  figure123
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 tex2html_wrap_inline404 . 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 tex2html_wrap_inline520 this means that the initial gyrospeed is tex2html_wrap_inline404 and so the ``picked-up'' proton moves on a circle of radius tex2html_wrap_inline404 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

equation132

where tex2html_wrap_inline528 is the angle between the magnetic field direction and tex2html_wrap_inline508 . 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 tex2html_wrap_inline532 and tex2html_wrap_inline534 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 tex2html_wrap_inline536 ions [Gloeckler et al. 1993].

  figure147
Figure 12.13: The phase space density of interstellar pickup protons as a function of tex2html_wrap_inline406 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]

  figure151
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.

  figure155
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.


next up previous
Next: Pickup ions at comets Up: Kinetic and Small Scale Previous: Arguments for kinetic physics

Iver Cairns
Wed Sep 8 09:24:55 EST 1999