Inhomogeneity in the Local ISM and Its Relation to the Heliosphere

Abstract

This paper reviews past research and new studies underway of the local interstellar environment and its changing influence on the heliosphere. The size, shape, and physical properties of the heliosphere outside of the heliopause are determined by the surrounding environment – now the outer region of the Local Interstellar Cloud (LIC). The temperature, turbulence, and velocity vector of neutral atoms and ions in the LIC and other partially ionized interstellar clouds are measured from high-resolution spectra of interstellar absorption lines observed with the STIS instrument on the HST. Analysis of such spectra led to a kinematic model with many interstellar clouds defined by velocity vectors derived from radial velocity measurements. This analysis identified fifteen clouds located within about 10 pc of the Sun and their mean temperatures, turbulence, and velocity vectors. With the increasing number of sight lines now being analyzed, we find that temperatures and turbulent velocities have spatial variations within the LIC and other nearby clouds much larger than measurement uncertainties, and that these spatial variations appear to be randomly distributed and can be fit by Gaussians. The inhomogeneous length scale is less than 4,000 AU, a distance that the heliosphere will traverse in less than 600 years. The temperatures and turbulent velocities do not show significant trends with stellar distance or angle from the LIC center. If/when the Sun enters an inter-cloud medium, the physical properties of the future heliosphere will be very different from the present. For the heliosheath and the very local interstellar medium (VLISM) just outside of the heliopause, the total pressures are approximately equal to the gravitational pressure of overlying material in the Galaxy. The internal pressure in the LIC is far below that in the VLISM, but there is an uncertain ram pressure term produced by the flow of the LIC with respect to its environment.

Inhomogeneity within Local Interstellar Clouds*

“…The heliosphere is now located near the edge of the LIC and will likely leave the LIC in less than 2000 yr (Redfield & Linsky 2004b;Frisch et al 2011;Linsky et al 2022). The trajectory is in the direction of the G Cloud, but whether the heliosphere will directly enter the G Cloud or an intercloud boundary consisting of Local Cavity gas or compressed partially ionized gas has been a matter of speculation.…”

“…This paper investigates the inhomogeneity of the LISM: the range of temperatures and turbulent velocity variations inside the LIC and the nearby partially ionized clouds in the Cluster of Interstellar Clouds (CLIC), possible trends in these inhomogeneities, possible causes for these inhomogeneities, and whether the LISM consists of warm clouds that partially or completely fill the LISM. A review paper, entitled "Inhomogeneity in the local ISM and its relation to the heliosphere" (Linsky et al 2022), contains a preliminary presentation and discussion of these issues based upon a smaller data set. Other previous reviews include Redfield (2006) and Frisch et al (2011).…”

Galactic Cosmic Rays Throughout the Heliosphere and in the Very Local Interstellar Medium

“… 2022 ; Linsky et al. 2022 ; Richardson et al. 2022 ), but two topics reviewed briefly here are as follows: (i) interactions of TeV to PeV GCRs with the structure of the heliosphere, and (ii) GCR acceleration and propagation processes in the Local Interstellar Medium (LISM).…”

The Heliosphere and Local Interstellar Medium from Neutral Atom Observations at Energies Below 10 keV

“…This paper complements the paper by in the same topical collection that discusses observations of high-energy ENAs (above 6 keV) and their implications for heliospheric science. Further articles directly related to this paper are: Theory and modeling of pick-up ions (PUI) and ENAs by Sokół et al (2022), observations of interstellar PUI by Zirnstein et al (2022), in-situ observations of the outer heliosphere by , heliosphere models by , the observation of backscattered Ly-α emissions by and the article on the Local Interstellar Medium by Linsky et al (2022).…”