Estimates for the average properties of the VLISM can be obtained remotely using a number of methods: (1) the Doppler shift and intensity of solar radiation that is backscattered to observers near 1 AU or in the outer heliosphere, (2) radiation from other stars that has superposed absorption or emission lines, (3) interstellar pick-up ions, and (4) interstellar neutral atoms detected directly. The properties of the local interstellar medium on a larger scale (several parsecs) can also be estimated by studying the absorption and emission features of light from nearby stars.
Figure 20.1 [Linsky et al., 1992; Lallement, 1993] is a Hubble Space Telescope GHRS Lyman-alpha spectrum of Capella (12.5 pc distant) showing the absorption line due to the local interstellar medium between the Sun and Capella.
Figure 20.1: Hubble GHRS spectrum of Capella [Linsky et al., 1992; Lallement, 1992].
The spectrum indicates that there is a single well-defined ``cloud'' of interstellar
material between the Sun and Capella, having a line-of-sight velocity difference of about
23 km s 
. This interpretation is consistent with absorption features along different
lines of sight to other nearby stars [Lallement, 1992].
Backscattering of solar Lyman-alpha radiation by hydrogen and helium atoms leads to the estimates in Table 1 for the number density, temperature, and relative speeds of these species. The table also contains estimates derived from direct detection of neutral helium atoms [Witte et al., 1996] and of pick-up ions, as summarised by Zank [1999].
Table 20.1: The number densities, flow speeds, and temperatures inferred for neutral hydrogen and
helium atoms in the VLISM using various techniques. ``UV'' refers to techniques using UV radiation either
in the VLISM or solar wind (SW), while ``direct'' refers to Witte et al.'s [1996] direct detection method
and ``pick-up'' refers to interstellar pick-up ions detected in the solar wind.
Note that the velocity of the VLISM relative to our solar system is well determined,
that hydrogen atoms appear to be slowed preferentially on entering the heliosphere,
that the temperatures of the interstellar hydrogen and helium atoms are consistently
K, and that the total neutral number densities are 
cm 
. Calculations using versions of the Saha equation (which relates together the ionization potential,
thermal temperature, and number densities of plasma constituents and neutrals) then yield the expected
electron number density in the VLISM: 
cm . The corresponding limits
on the plasma frequency are then
These constraints on 
will be important in understanding the radio emissions
observed and generated in the outer heliosphere (Section 20.5).