The local interstellar medium

2.2 The local interstellar medium

The solar wind does not expand forever. Eventually it runs into the local interstellar medium (LISM). The interaction region between the solar wind and LISM is the subject of Section 2.3, and it is through analogous interaction regions around other stars that solar-like stellar winds can be detected (see Section 4), but before the wind/ISM interaction regions are discussed, it is necessary to review the properties of the undisturbed LISM.

The principle method by which one studies the ISM is by observing absorption lines that interstellar material produces in spectra of distant stars. These studies reveal that ISM column densities remain rather low within about 100 pc of the Sun in most directions and then increase dramatically (Sfeir et al., 1999 ). This low density region is called the Local Bubble. Figure 2 shows a map of the Local Bubble in the Galactic plane (Lallement et al., 2003 ). The hot plasma within the

Figure 2:

Map of the Local Bubble in the Galactic plane, where the contours indicate 20mÅ and 50mÅ equivalent widths for the Na I D2 line (Lallement et al., 2003 ). The distance scale is in parsecs.

Bubble has been detected directly from observations of the soft X-ray background (Snowden et al., 1995 ). Most locations within the Local Bubble are very hot ( $6 T ∼ 10 K$ ) and rarified ( $ne ∼ 10 −3cm −3$ ), and therefore completely ionized.

Although most of the volume of the Local Bubble consists of this hot material, the absorption line studies clearly demonstrate that there are cooler, partially neutral clouds embedded within the Local Bubble. Furthermore, since even the shortest lines of sight show absorption from H I and other low temperature species (Linsky, 1998 ; Linsky et al., 2000 ), the Sun must be located within one of these clouds. The cloud immediately surrounding the Sun has been called the Local Interstellar Cloud (LIC), which is roughly 5 – 7 pc across and has a total mass of about $0.32M ⊙$ (Redfield and Linsky, 2000 ). There are similar clouds that are apparently adjacent to the LIC (e.g. the “G” cloud and “Hyades” clouds, see Lallement and Bertin, 1992 ; Redfield and Linsky, 2001 ), although it is debatable whether the LIC is truly distinct from these clouds. Velocity gradients within a single cloud could in principle create the appearance of multiple clouds in absorption line studies.

The first evidence that the LIC is not entirely ionized came not from absorption line studies but from observations of solar Ly $α$ emission scattering off interstellar H I gas flowing into the heliosphere (Bertaux and Blamont, 1971 ; Quémerais et al., 1999 , 2000 ). Interstellar atoms have also been observed directly with particle detectors on board spacecraft such as Ulysses (Witte et al., 1993 , 1996 ). Both measurements of LIC material flowing through the heliosphere and LISM absorption line studies have been used to estimate the direction and magnitude of the LIC vector, and the resulting vectors are in good agreement. The heliocentric vector derived from absorption lines has a magnitude of $−1 25.7km s$ directed towards Galactic coordinates $∘ l = 186.1$ and $∘ b = − 16.4$ (Lallement and Bertin, 1992 ; Lallement et al., 1995 ).

Other properties of the undisturbed LISM just beyond the heliosphere are less precisely known. Absorption line studies are hampered by probable variations of densities, temperatures, and ionization states within the LIC (see Cheng and Bruhweiler, 1990 ; Slavin and Frisch, 2002 ; Wood et al., 2003b ), meaning that line-of-sight averages of these properties towards even the nearest stars are potentially different from the actual circumsolar LISM properties. Studies of LISM particles streaming through the heliosphere are hampered by the fact that the properties of these particles are often altered in the outer heliosphere, thereby requiring the assistance of models to extrapolate back to undisturbed LISM conditions (see Izmodenov et al., 2004 ). In any case, typical temperatures measured for the LIC are $T = 6000 – 8000 K$ , typical hydrogen densities are $n (H I) = 0.1– 0.2cm −3$ , and typical proton and electron densities are $n (H+ ) ≈ ne = 0.04 –0.2 cm− 3$ (Witte et al., 1993 , 1996 ; Wood and Linsky, 1997 , 1998 ; Izmodenov et al., 1999a ; Redfield and Linsky, 2000 ; Frisch and Slavin, 2003 ).