One of the most striking and interesting developments in the last few years has been the detection of extra-solar ions picked up by the solar wind and entrained in its flow. These ions result from the ionization in the interplanetary medium of atoms and molecules, whose source is overwhelmingly the local interstellar medium (LISM). Their presence not only allows us to study many aspects of the LISM from the solar system, but it appears that these pickup ions produce the dominant part of the particle pressure in the outer heliosphere. In an examination of pressure balanced structures observed by Voyager 2 near 35 AU, Burlaga et al. (1994), show that the contributions of the magnetic pressure and the pressure due to pickup ions begin to dominate; beyond this distance the pressure of the solar wind ions and electrons becomes negligible. Thus, in the outer part of the heliosphere, near its interface with the interstellar medium, pickup ions must be of great importance in establishing the location and structure of this interface.
The first observations of interstellar pickup ions (He
) in
the solar wind were made by Möbius et al. (1985), and were
explained in terms of an earlier theory developed for the
scattering of Lyman alpha radiation from interstellar neutrals.
Largely because neutrals are subjected only to gravitational
force and radiation pressure, their trajectories can be
calculated and their densities in the heliosphere determined.
After ionization, the particles gyrate at the solar wind velocity
about the magnetic field, with their guiding center moving at the
solar wind velocity. The ions form a ring velocity distribution,
which pitch angle scattering deforms into a spherical shell. As
they move outward from the sun with the solar wind they undergo
adiabatic cooling, which reduces the radius of the shell, but
they do not become completely incorporated into the solar wind.
The velocity distribution function f(v) for pickup ions has been
derived by Vasyliunas and Siscoe (1976) in the solar wind frame.
A sharp discontinuity in the distribution function observed in
the spacecraft frame at a velocity of twice that of the solar
wind is a clear signature of pickup ions even in the presence of
another distribution.
Pickup ions are predominantly singly charged, with a small
amount of He
being formed by charge exchange with He
in
the solar wind. Subsequent interactions of these ions with, for
example, propagating or co-rotating interplanetary shocks,
further accelerates the ions, and observation of the results of
this process may perhaps be used to check models of shock
acceleration. Protons and alpha particles from the solar wind
are much less efficiently accelerated than He
pickup ions
(Gloeckler and Geiss 1994), which strengthens the identification
of pickup ions as the source of (usually singly charged)
anomalous cosmic rays. Particle acceleration by shocks is, of
course, a ubiquitous and important process in this and other
galaxies.
The measurement and analysis of pickup ions has been made
possible by the development of low background of time-of-flight
instrumentation, which uses double and triple coincidence
techniques. Because the flux of pickup ions, especially protons,
increases with distance from the sun, while that of the solar
wind falls, observations at heliocentric distances of several AU
are well suited to pickup studies. Recently, Gloeckler and Geiss
(1993) have reported measurements of the pick-up ions H
,
He
, He
, N
, O
, and Ne
obtained during the
Ulysses cruise to Jupiter. They have shown that abundances of
these species in the interstellar medium can be deduced from the
measurements, and that this method can also be extended to
isotopic abundances. This is discussed in detail in Gloeckler
and Geiss (1994) and some resulting abundances are shown in Table
3. Because interstellar matter is partly in form of grains and
may be partly ionized, these abundances are lower limits, which
probably explains the quite large differences from solar system
abundances, also shown in Table 3. In particular, the He/H ratio
might have to be amended to reflect better knowledge of the
production rate of He
. Clearly, however, the measurement of
pickup ions can provide useful new information on the
constitution of the local interstellar medium.
Finally, we should note that neutrals from planets and comets are ionized and picked up by the solar wind. Although solar system bodies form quite strong local sources---especially comets such as Halley---the LISM is the largest source, and probably the only one which can affect the structure of the heliosphere (Ogilvie et al. 1994, Luhmann 1994).