Short-term variations in the galactic environment of the sun

Abstract

The galactic environment of the Sun varies over short timescales as the Sun and interstellar clouds travel through space. Small variations in the dynamics, ionization, density, and magnetic field strength in the interstellar medium (ISM) surrounding the Sun can lead to pronounced changes in the properties of the heliosphere. The ISM within ∼30 pc consists of a group of cloudlets that flow through the local standard of rest with a bulk velocity of ∼17-19 km s-1, and an upwind direction suggesting an origin associated with stellar activity in the Scorpius-Centaurus association. The Sun is situated in the leading edge of this flow, in a partially ionized warm cloud with a density of ∼0.3 cm-3. Radiative transfer models of this tenuous ISM show that the fractional ionization of the ISM, and therefore the boundary conditions of the heliosphere, will change from radiative transfer effects alone as the Sun traverses a tenuous interstellar cloud. Ionization equilibrium is achieved for a range of ionization levels, depending on the ISM parameters. Fractional ionization ranges of χ(H) = 0.16-0.27 and χ(He) = 0.19-0.34 are found for tenuous clouds in equilibrium. In addition, both temperature and velocity vary between clouds. Cloud densities derived from these models permit primitive estimates of the cloud morphology, and the timeline for the Sun's passage through interstellar clouds for the past and future ∼105 years. The most predictable transitions happen when the Sun emerged from the near vacuum of the Local Bubble interior and entered the cluster of local interstellar clouds flowing past the Sun, which appears to have occurred sometime during the past 44,000-150,000 years, and again when the Sun entered the local interstellar cloud now surrounding and inside of the solar system, which occurred sometime within the past 45,000 years, possibly a 1000 years ago. Prior to ∼150,000 years ago, no interstellar neutrals would have entered the solar system, so the pickup ion and anomalous cosmic ray populations would have been absent. The tenuous ISM within 30 pc is similar to low column density material observed globally. In this chapter, we review the factors important to understanding short-term variations in the galactic environment of the Sun. Most ISM within 40 pc is partially ionized warm material, but an intriguing possibility is that tiny cold structures may be present. © 2006 Springer. All Rights Reserved.