Is the Sun Embedded in a Typical Interstellar Cloud?

Abstract

The physical properties and kinematics of the partially ionized interstellar material (ISM) near the Sun are typical of warm diffuse clouds in the solar vicinity. The direction of the interstellar magnetic field at the heliosphere, the polarization of light from nearby stars, and the kinematics of nearby clouds are naturally explained in terms of the S1 superbubble shell. The interstellar radiation field at the Sun appears to be harder than the field ionizing ambient diffuse gas, which may be a consequence of the low opacity of the tiny cloud surrounding the heliosphere. The spatial context of the Local Bubble is consistent with our location in the Orion spur.

Project Icarus: A review of local interstellar medium properties of relevance for space missions to the nearest stars

“…For completeness, Table 4 also lists the CHISM magnetic field strength (B); there are a number of estimates for this value in the literature, and Table 4 gives two values. By assuming that magnetic and thermal pressures are balanced, Slavin and Frisch [29] obtained B~2.7μG, which is similar to previous estimates and comparable to astronomical measurements of typical magnetic field strengths (summarised by Frisch [14]). However, using Voyager 2 measurements of the deflection of the solar wind in the heliosheath, Opher et al [31] have recently argued that the local interstellar magnetic field strength actually lies in the range 3.7 to 5.5 μG which, if confirmed, would imply that the magnetic energy density dominates the overall LIC energy budget.…”

“…Here, I have adopted the average of the hydrogen densities recommended by Bzowski et al [28] (n HI = 0.18±0.03 cm -3 ) and Frisch [14] (n HI = 0.195±0.005 cm -3 ), but have retained the larger (~ 20%) errors recommended by Bzowski et al [28]. The CHISM electron density (n e = 0.07±0.02 cm -3 ) and temperature (T = 6300±400 K) are taken from Slavin and Frisch [29] and Frisch [14], and references cited by these authors. Within the quoted errors, these values are also consistent with early models which reproduce measurements of interstellar energetic neutral atoms in the Earth's vicinity by the Interstellar Boundary Explorer (IBEX) spacecraft (with the caveat that these models adopt a neutral hydrogen density at the lower end of the range: i.e.…”

“…Our only direct knowledge of the physical properties of the LISM comes from measurements of interstellar gas and dust entering the Solar System, allowing for whatever disturbances may occur as the material enters the heliosphere and becomes influenced by the Sun's magnetic field and solar wind [e.g. 14,15,26,27]. Over recent years a consensus has emerged regarding the physical properties of the interstellar medium immediately adjacent to the heliosphere (sometimes referred to as the Circum-Heliospheric Interstellar Medium: CHISM), and these properties are listed in Table 4.…”

“…In the years since the pioneering Daedalus study [10] our knowledge of the LISM has improved significantly (e.g. [11][12][13][14][15]). It is generally accepted that the Sun is currently located close to the boundary of a small (spatial extent ≤ 10 light-years); low density (n H ~ 0.1-0.2 cm -3 , where n H is the density of hydrogen nuclei), warm (T ~ 7500 K), partially ionised interstellar cloud known as the Local Interstellar Cloud (LIC).…”

“…Whether the Sun lies just within, or just outside, the LIC is currently a matter of debate (e.g. [13][14][15]. The LIC is only one of a several broadly similar interstellar clouds within a few light-years of the Sun: Redfield & Linsky [13] identify six within 15 light-years (4.6 parsecs).…”