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Over the course of its motion through the Galaxy, our solar system hasencountered many interstellar environments of varying characteristics.Interstellar medium (ISM) density variations spanning seven orders of magnitudeare commonly seen throughout the general Galactic environment, and asufficiently dense cloud within this range has the potential to dramaticallyalter the structure of the heliosphere. We present observations of the ISMenvironments the Sun has most recently encountered based on high-resolutionoptical spectra toward nearby stars in the direction of the historical solartrajectory. The data were obtained with the highest-resolution spectrographsavailable, including the Tull Spectrograph on the Harlan J. Smith Telescope atMcDonald Observatory and the Ultra-High-Resolution Facility on theAnglo-Australian Telescope at the Anglo-Australian Observatory. Observationswere made of interstellar Na I and Ca II doublet absorption toward 43 brightstars within about 500 pc. No absorption is seen out to a distance of 120 pc(consistent with the outer boundary of the Local Bubble), but a complexcollection of absorbers is seen in stars beyond 120 pc. While common absorbersare consistently seen in most sight lines, significant spatial variation isalso detected, even between closely spaced sight lines. This pervasive evidenceof small-scale structure not only speaks to the complexity of the morphology orphysical properties of the gas in the ISM, but also emphasizes that dramaticstructural changes to the heliosphere are common and it is important tounderstand the implications of such changes, such as the modulation in thecosmic ray flux, on planets.

Probing Our Heliospheric History I: High-Resolution Observations of Na I and Ca II Along the Solar Historical Trajectory