**FULL TITLE** ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION** **NAMES OF EDITORS** The Local Interstellar Medium Seth Redfield Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX, USA 6 0 Abstract. The Local Interstellar Medium (LISM) is a unique environment 0 that presents an opportunity to study general interstellar phenomena in great 2 detail and in three dimensions. In particular, high resolution optical and ultra- n violetspectroscopyhaveproventobepowerfultoolsforaddressingfundamental a questions concerning the physical conditions and three-dimensional (3D) mor- J phologyofthis localmaterial. After reviewingourcurrentunderstandingof the 6 structure of gas in the solar neighborhood, I will discuss the influence that the LISMcanhaveonstellarandplanetarysystems,includingLISMdustdeposition 1 ontoplanetaryatmospheresandthe modulationofgalacticcosmicraysthrough v the astrosphere — the balancing interface between the outward pressure of the 7 magnetized stellar wind and the inward pressure of the surrounding interstellar 1 1 medium. On Earth, galactic cosmic rays may play a role as contributors to 1 ozone layer chemistry, planetary electrical discharge frequency, biologicalmuta- 0 tion rates, and climate. Since the LISM shares the same volume as practically 6 all known extrasolar planets, the prototypical debris disks systems, and nearby 0 low-massstar-formationsites, it will be important to understandthe structures / of the LISM and how they may influence planetary atmospheres. h p - o r t s 1. Introduction a : v The interstellar medium (ISM) is a critical component of galactic structure. Xi Its role in the lifecycle of stars, mediating the transition from stellar death to stellar birth, evokes a sense of a “galactic ecology” (Burton 2004). The r a ISM provides a platform for the recycling of stellar material, by transferring and mixing the remnants of stellar nucleosynthesis and creating environments conducive for the creation of future generations of stars and planets. It also transfers energy and momentum, absorbing flows from supernovae blasts and strongwindsfromyoungstars andcouplingthesepeculiar motions with galactic rotation and turbulence. Ultimately, when a dense interstellar cloud collapses, it is the conservation of momentum from the parent cloud that leads to the formation of a protostellar disk from which stars and planets are formed. The local interstellar medium (LISM) is the interstellar material that re- sides in close (.100pc) proximity to the Sun. For a discussion on more distant ISM structures, see McClure-Griffiths, in this volume. Proximity is a special characteristic that drives much of the interest in the LISM. First, proximity provides an opportunity to observe general ISM phenomena in great detail, and inthreedimensions. ISMstructuresandprocessesarerepeatedalmostad infini- tum in our own galaxy (Dickey & Lockman 1990), and beyond in other galaxies (McCray & Kafatos 1987), even at high redshift (Rauch et al. 1999). Knowl- 1 2 Redfield edge of general ISM phenomena in our local corner of the galaxy, discussed in §2, can be applied to more distant and difficult to observe parts of the universe. Second, proximity implies an interconnectedness. The relationship between starsandtheir surroundinginterstellar environmentwillbediscussedin§3, with particular attention paid to the interaction of the Sun with the LISM. In §4, the consequences of the relationship between stars and the LISM on planetary atmospheres are discussed, and the LISM-Earth, or more generally, the ISM- planet connection is explored in more detail. This manuscript should not be considered a comprehensive review of the subject of the LISM, but an individual, and biased, thread through a rich re- search area. I certainly will not be able to explore many LISM topics to the level they deserve, nor will I be able to highlight all the work done by the large number of researchers in this area. Hopefully, this short review will introduce you to some new ideas, and the references provided can escort you to even more work that was not specifically mentioned in this manuscript. 2. Properties of the Local Interstellar Medium Measuringthemorphologicalandphysicalcharacteristicsofthenearestinterstel- largashaslongbeenofinteresttoastronomers. ObservationsofthegeneralISM via interstellar extinction of background stars (Neckel et al. 1980), the 21cm HI hyperfine transition (Lockman 2002), or foreground interstellar absorption in optical resonance lines (Cowie & Songaila 1986) typically focus on more distant ISMenvironments dueto theobservational challenges inherent in measuringthe properties of the LISM (see §2.2). Recent reviews that focus specifically on the LISM by Ferlet (1999) and Frisch (1995, 2004) also provide some discussion on the history of the field. 2.1. Our Cosmic Neighborhood The LISM is a diverse collection of gas. The outer bound of what I consider to be “local” in the context of the LISM, is the edge of Local Bubble (LB), coinci- dent, by definition, with the location of the nearby dense molecular clouds, such as Taurus and Ophiuchus (Lallement et al. 2003). Figure1 is a schematic illus- tration of the volume populated by the LISM, adapted from a similar figure by Mewaldt & Liewer (2001). Within the LISM volume, our cosmic neighborhood sotospeak,residesthenearest104 to105 stars,includingalmostallknownplan- etary systems (see Ford, in this volume) and the prototypical debris disk stars (see Chen, in this volume). Among these stars drifts interstellar gas known as the LISM. Several warm partially ionized clouds, such as the Local Interstellar Cloud (LIC) and the Galactic (G) Cloud, are observed within the Local Bubble. The LIC is the material that directly surrounds our solar system. The in- ternalpressureandrampressureoftheLIC,functionsof itsdensity andvelocity relative to theSun,balance theforce of theoutward-moving solar windto define theboundaryandshapeoftheheliosphere(see§3.1). Theheliosphericstructure is not unique to the Sun, but is observed around other stars, including the near- est stellar system, αCen (Wood et al. 2001). Nor is the heliospheric structure static. The heliosphere will expand and contract, in response to the density of the LISM material surrounding the Sun (see §3.2). The Local Interstellar Medium 3 Figure1. Ourcosmicneighborhood,shownonalogarithmicscale. TheSo- larSystemincludesthemajorplanets,theasteroidbelt(AB),theKuiperbelt (KB), and the Oort cloud (OC). As of October 2005, Voyager 1 was 97.0AU from the Sun. The heliospheric structure consists of the termination shock (TS), the heliopause (HP), and the bow shock (BS). The Local Interstellar Cloud (LIC) is the warm, partially ionized cloud that directly surrounds our solar system and currently determines the size and shape of the heliosphere. Our nearest neighboring cloud is the Galactic (G) Cloud which directly sur- rounds the αCen stellar system. Analogous to the heliosphere, the αCen system includes an astrosphere. The LISM resides in a larger ISM structure, theLocalBubble(LB).WithintheLocalBubble,whichextends∼100pcfrom the Sun, lie the nearest104–105 stars. This figure is inspired by a diagramin Mewaldt & Liewer (2001). The boundary between the LISM and the solar system is not an obvious one. The Oort cloud contains the most distant objects that are gravitationally bound to the Sun, which reside at a third of the distance to αCen. The Oort cloud is completely enclosed by LISM material, and due to the short extent of the LIC in the direction of αCen (Redfield & Linsky 2000), Oort cloud objects in that direction may be surrounded by a different collection of gas (G Cloud material) than surrounds our planetary system (LIC material). Currently, even much of the Kuiper belt (KB) extends beyond the bow shock (BS) into pristine LISMmaterial (see Sheppard,in this volume). Someneutral LISMmaterial can penetrate well into the solar system. This material is utilized to make in situ measurements of propertiesof the LIC (see §2.2). Theinterconnectedness of our solar system with our surrounding interstellar medium could have important consequences for planetary atmospheres (see §4). Hot Gas: Local Bubble The Local Bubble refers to the apparent lack of dense cold material within approximately 100pc of the Sun. Therefore, the distance to the edge of the Local Bubble cavity is equal to the distance at which cold dense gas is first observed. Lallement et al. (2003) were able to map out the contours of the Local Bubble by tracing the onset of the detection of foreground NaI absorption lines in the spectra of ∼1000 early type stars. In general, the interstellar material within the Local Bubble is too hot for neutral sodium, and therefore none is detected until the edge of the Local Bubble is reached. The 4 Redfield edge of the Local Bubble can be as close as 60pc, and as far as ∼250pc, or even unbound, as toward the north and south galactic poles (Lallement et al. 2003). The hot gas within the Local Bubble is notoriously difficult to observe. Early direct detections of million degree gas came from diffuse soft X-ray emis- sion (Snowden et al. 1998), although part of this emission is now thought to be caused by charge exchange reactions in the heliosphere, the same process that causes comets to emit X-rays (Lallement 2004). Absorption lines of highly ionized elements are generally weak or not detected, as Oegerle et al. (2005) found when looking for OVI absorption toward nearby white dwarfs. Detecting emission from highly ionized atoms has also been more difficult than expected. UsingtheCosmic Hot Interstellar Plasma Spectrometer (CHIPS),Hurwitz et al. (2005) do not detect the array of extreme ultraviolet emission lines that are pre- dicted from the“standard” LocalBubbletemperatureanddensity. Canonically, itisthoughtthattheLocalBubbleisfilledwithT ∼ 106K,n ∼ 5×10−3cm−3 gas that extends about R ∼ 100pc. However, much work remains to be done to understand the nature of the hot Local Bubble gas. Warm Gas: Local Interstellar Clouds Within the hot Local Bubble substrate aredozensofindividualaccumulations ofdiffusegas thatarewarmandpartially ionized (T ∼ 7000K, n ∼ 0.3cm−3, R ∼ 0.5–5pc). It is most commonly this material that is being referenced with the term “local interstellar medium.” It is warm, partially ionized material that directly surroundsour solar system, and which can bemeasured with in situ observations and high resolution optical and ultraviolet (UV) spectroscopy (see §2.2). The warm LISM will dominate the remainder of this review, because it is the best studied of the different phases of LISM material, and the most significant with regards to interaction with stars. Typical properties of the local warm interstellar clouds are given in Table1. Absorption linespectroscopy and in situ observations often provideindependent measurementsofthesamequantity(e.g., T,v ,l ,b ). Inaddition,thetwotech- 0 0 0 niques are often complementary, as when parameters derived from absorption spectraofnearbystars(e.g., N /N )arecombinedwithin situmeasurements HI HeI (e.g., n ) to determine a third physical quantity that would be difficult or im- HeI possible to determine using either technique alone (e.g., n ). The LISM is a HI diverse collection of gas. For example, individualtemperature determinations of LISM material can be made to the precision of ±200K, although temperatures ranging from 2000–11000K are observed (Redfield & Linsky 2004b). The value given for the LISM in Table1 is a weighted LISM mean. Despitethediversity,thereareseveralobservationalcluesthatindicateaco- herence in the LISM. First, LISM absorption features in high resolution spectra can almost always be fit by one to three individual, symmetric, well-separated, Gaussian profiles, as opposed to a broad asymmetric feature that would result from multiple absorption features with a gradient of velocities. Second, the pro- jected velocities of LISM features toward nearby stars can be characterized by a single bulk flow (Lallement & Bertin 1992; Frisch et al. 2002) that matches the observed ISM flow into our solar system (Witte 2004). However, small devia- tions from this bulk velocity vector are observed in the direction of the leading edge of the LIC, where the gas appears to be decelerated, possibly due to a collision of LIC material with neighboring LISM material (Redfield & Linsky 2001). Third, chaotic small scale structure in the LISM has not been detected. The Local Interstellar Medium 5 Table 1. Properties of Warm Local Interstellar Clouds Property Value Ref. Commentsa Temperature(T) 6680±1490K 1 LISM,AL 6300±340K 2 LIC,IS TurbulentVelocity(ξ) 2.24±1.03kms−1 1 LISM,AL VelocityMagnitude(v0) 25.7±0.5kms−1 3 LIC,AL 28.1±4.6kms−1 4 LISM,AL 26.3±0.4kms−1 2 LIC,IS VelocityDirection(l0,b0) 186◦.1,–16◦.4 3 LIC,AL 192◦.4,–11◦.6 4 LISM,AL 183◦.3±0◦.5,–15◦.9±0◦.2 2 LIC,IS HIColumnDensity(logNHI) 17.18±0.70cm−2 5 LISM,AL HeIVolumeDensity(nHeI) 0.0151±0.0015cm−3 6 LIC,IS HIandHeIRatio(NHI/NHeI) 14.7±2.0 7 LISM,AL ElectronVolumeDensity(ne) 0.11+−00..1026 cm−3 8 LIC,AL HIVolumeDensity(nHI) 0.222±0.037cm−3 6,7 nHI = nHeI × NHI/NHeI ThermalPressure(PT/k) 3180+−11815300 Kcm−3 1,6,7 PT = nkT TurbulentPressure(Pξ/k) 140+−114300 Kcm−3 1,6,7 Pξ = 0.5ρξ2 HydrogenIonization (XH) 0.33+−00..2143 6,7,8 XH = nHII/(nHI + nHII) Absorberspersightline 1.8 9 LISM,AL Cloudsize(r) ∼2.3pc 10 LIC,AL Cloudmass(M) ∼0.32M⊙ 10 LIC,AL aLISM=average of several LISM sightlines; LIC=quantity for LIC material only; AL=derived fromabsorptionlineobservations; IS=derivedfromin situobservations. References.—(1) Redfield&Linsky 2004b; (2) Witte 2004; (3) Lallement&Bertin 1992; (4) Frisch,Grodnicki,&Welty 2002; (5) Redfield&Linsky 2004a; (6) Gloeckleretal. 2004; (7) Dupuisetal.1995;(8)Wood&Linsky1997;(9)Redfield&Linsky2002;(10)Redfield&Linsky2000; ◦ ◦ In one example, a collection of 18 Hyades stars, separated by only 1 –10 , shows a smooth slowly varying gradient in column density with angular distance, as opposed to a chaotic, filamentary geometry. However, an extensive database of observations is required to fully study the three dimensional structure of the LISM (see §2.3). Several properties of warm LISM clouds are not well known. Except for the material currently streaming into the solar system, volume densities are very difficult to measure. Inherent in absorption line observations are: (1) the ignorance of a length scale to the absorbing material, other than the limit set by the distance of the background star, and (2) the ignorance of density variations within a single collection of gas, since only the total column density is observed. Measurements of the magnetic field in the LISM have also been difficult to make. For lack of better measurements, observations of distant (several kpc) pulsars give a “local” galactic magnetic field strength of ∼1.4µG (Rand & Lyne 1994), although this value may have little to do with the magnetic fields en- trained in the LISM. Polarization measurements of nearby (<35 pc) stars are weak,butseemtoindicateanorientationparalleltothegalacticplane(Tinbergen 1982; Frisch 2004). The same orientation is derived from heliospheric obser- vations of 1.78–3.11kHz radio emission by Voyager1 (Kurth & Gurnett 2003). Theorientation andstrengthofthelocalmagnetic fieldwillhave importantcon- sequences on the structure and shape of the heliosphere (e.g., Gloeckler et al. 1997; Pogorelov et al. 2004; Florinski et al. 2004; Lallement et al. 2005). Ad- ditionally, since the magnetic pressure goes as B2, the strength of the LISM 6 Redfield magnetic field (B) could have important consequences for the relationship be- tween the hot Local Bubble gas and the warm LISM clouds. The apparent pressure imbalance between the warm (P /k ∼ 3300K cm−3, see Table1) and tot hot (P /k ∼ 10000K cm−3, see “Hot Gas” section above) components of the T LISM has been a persistent topic concerning the structure of our local inter- stellar environment (Jenkins 2002). Possible scenarios include: (1) the presence of a LISM magnetic field of ∼4.8µG to match the hot Local Bubble pressure, (2) refinement of Local Bubble observations and models that may reduce the Local Bubble pressure, in particular refining the X-ray and extreme ultraviolet observations of nearby hot gas (Hurwitz et al. 2005; Lallement 2004), or (3) the hot and warm components of the LISM are not in pressure equilibrium. It will beimportanttoresolve thisissueinordertounderstandtheinteraction between the hot Local Bubble gas and warm local interstellar clouds. ColdGas? AlthoughthevolumeoftheLocalBubbleisdefinedbythescarcityof cold dense gas, there are some indications that collections of cold gas may reside within the Local Bubble. Observations of NaI by Lallement et al. (2003), which define the morphology of the Local Bubble, also identify a number of isolated dense clouds, just inside the Local Bubble boundary. Magnani et al. (1996) identify several small molecular clouds that are thought to be relatively nearby (≤200pc), although their precise distances are not often known. These clouds have T ∼ 20K, n ∼ 30cm−3, and sizes of about R ∼ 1.4pc. One such nearby molecularcloud,MBM40(∼100pc),containsmolecularcores,althoughtheyare notmassiveenoughforstarformation(Chol Minh et al.2003). MBM40islikely an example of a dense cloud that resides within the Local Bubble boundary. 2.2. How do we Measure the Properties of the LISM? The proximity of the LISM presents unique challenges and opportunities for measuring the properties of nearby interstellar gas, which is too sparse to cause measurable reddening or be detected in atomic hydrogen 21cm emission. One unique observational technique is in situ measurements of ISM particles, which stream directly into the inner solar system. These observations complement the traditional ISM observational technique of high resolution absorption line spectroscopy. Observing the closest ISM provides many advantages, such as simple absorption spectra, well known distances to background stars, and large projected areas that allow multiple observations through different parts of a single cloud, enabling a probe of its properties in three dimensions. In Situ Measurements A powerful observational technique, absent in the vast majorityofastrophysicalresearch,istheabilitytosendinstrumentstophysically interact with, collect, and measure the properties of the material of interest di- rectly, instead of relying on photons. Due to the close proximity of the LISM, interstellar particles are continually streaming into the interplanetary medium. Neutral helium atoms, and helium “pick-up” ions (neutral helium atoms ion- ized as they approach the Sun and are “picked-up,” or entrained, in the solar wind plasma), are observed by mass spectrometers onboard the Ulysses space- craft. M¨obius et al. (2004) review these measurements, together with HeI UV- backscattered emission, collected and analyzed by several groups over many years. These observations give consistent measurements of the temperature, velocity, and density of the interstellar medium directly surrounding the solar The Local Interstellar Medium 7 system(seeTable1). LISMdustparticlescollectedintheinterplanetarymedium areamongthesampleonboardtheStardustmission,scheduledtoreturntoEarth in January 2006 (Brownlee et al. 2003). Laboratories will beable to analyze the collected particles indetail. Notonlywillthisprovideinformationaboutthena- tureofdustintheLISM,butmayanswerquestionsabouttheorigin ofISMdust and its role in circumstellar and disk environments (see Chen, in this volume). Both Voyager spacecraft, launched in 1977, are still functioning and returning data as part of the Voyager Interstellar Mission (VIM). On 16 December 2004, Voyager1 provided a long sought-after measurement of the distance to the ter- mination shock, at 94.01AU. With enough power to last until 2020, Voyager1 should provide measurements of its encounter with pristine interstellar material once it crosses the heliopause around 2015 (Stone et al. 2005). High Resolution Absorption Line Spectroscopy A standard technique used to measure the physical properties of foreground interstellar material along the line of sight toward a background star is high resolution absorption line spec- troscopy. This kind of work has a long and rich history, but has typically been dominated by more distant ISM environments, with large column densities and strongabsorptionsignatures (Cowie & Songaila 1986;Savage & Sembach 1996). The challenge inherent in absorption line spectroscopy of the LISM is the low column density along sightlines to nearby stars. This limits the number of diag- nostic lines to only the strongest resonance line transitions. In Figure2, the hy- drogen column density sensitivities are shown for the100 strongest ground-state transitions at wavelengths from the far-ultraviolet (FUV), through the ultravi- olet (UV), to the optical. The lower sensitivity limit indicates a 3σ detection in a high signal-to-noise observation with modern high resolution instruments. The upper sensitivity limit marks the column density at which the transition becomes optically thick and leaves the linear part of the curve of growth, where, although absorption is detected, limited information can be obtained from the saturated absorption profile. Ionization structure typical for warm LISM clouds isincorporated(Slavin & Frisch2002;Wood et al.2002),althoughtypicalLISM depletion is not; only solar abundances are assumed (Asplund et al. 2005). The range of LISM absorbers (16.0 ≤ log N (cm−2)≤ 17.7) is such that less than H 100 transitions are available to study the LISM. Taking into account such issues as blending or continuum placement, which can limit the diagnostic value of an individual transition, reduces the number of useful transitions even more. Most of thetransitions lieintheFUV andUV,withonly afewtransitions available in the optical, most importantly CaII resonance lines at ∼3950˚A. (Other notable optical transitions, such as NaI and KI, probe more distant and higher col- umn density ISM environments, see Lallement et al. 2003 and Welty & Hobbs 2001.) Recent LISM absorption line observations of these transitions have been made in the FUV with the Far Ultraviolet Spectroscopic Explorer (FUSE) (e.g., Lehner et al. 2003; Wood et al. 2002), in the UV with the Hubble Space Tele- scope (HST)(e.g.,Redfield & Linsky2004a,2002),andintheoptical withultra- high resolution spectrographs, such as those at McDonald Observatory and the Anglo-Australian Observatory (e.g., Crawford 2001; Welty et al. 1994). 8 Redfield Figure 2. The 100 strongest resonance lines, ranked in order of their hy- drogen column density sensitivity. White bars indicate those that fall in the optical (3000-10000˚A), gray for ultraviolet (1200-3000˚A), and black for far- ultraviolet (900-1200˚A). The top 15 lines are HI transitions from Lyman-α to Lyman-o, only Lyman series lines to Lyman-ω are shown. The vertical dashedlinesindicatethe typicalrangeofhydrogencolumndensities observed for warm LISM clouds. Those transitions with sensitivities left of this range will be optically-thick and saturated, whereas those transitions to the right will not be sensitive enough to detect absorption from warm LISM clouds. 2.3. Future Directions Absorption line analyses, supported by in situ observations of the LIC, have re- sulted in numerous single sightline measurements of projected velocity, column density,andlinewidthforseveraldozensofsightlines, leadingtoindividualmea- surementsofvariousphysicalpropertiesoftheLISM,includingtemperatureand turbulence (Redfield & Linsky 2004b), electron density (Wood & Linsky 1997), ionization (Jenkins et al. 2000), depletion (Lehner et al. 2003), and small scale structure (Redfield & Linsky 2001). However, with the recent accumulation of significant numbers of observations, it is possible to go beyond the single sight- lineanalysisanddevelopaglobalmorphologicalandphysicalmodeloftheLISM. Initial steps have been made toward this end with global bulk flow kinematic models of the LIC and G Clouds produced by Lallement & Bertin (1992), and a global morphological model of the LIC by Redfield & Linsky (2000). Future LISM research will synthesize the growing database of LISM ob- servations, taking advantage of the information contained in the comparison of numerous individual sightlines. A global morphological model would enable the development of global models of various physical properties, such as kinematics, ionization, depletion, density, etc. Ultimately, these global models are required to tackle larger issues that cannot be fully addressed by single sightline analy- ses, such as the interactions of clouds, the interaction of the warm LISM clouds with the surrounding hot Local Bubble substrate, the strength and orientation The Local Interstellar Medium 9 of magnetic fields, and the origin, evolution, and ages of clouds in the LISM. Such work will not only be important in understanding the structure of gas in our local environment, butwill beapplicable to other, more distant and difficult to observe interstellar environments in our galaxy and beyond. CLOSE: CaII LISM Optical Survey of our Environment A global morpholog- ical model of the LISM requires high spatial and distance sampling. The de- velopment of a morphological model for the LIC by Redfield & Linsky (2000) was possible because LIC absorption is detected in practically every direction, since the material surrounds the solar system. More distant LISM clouds will subtend smaller angles on the sky, and will require higher density sampling of LISM observations in order to be morphologically characterized. The CLOSE (CaII LISM Optical Survey of our Environment) project is a large scale, ultra- high resolution survey of ∼500 nearby stars that will enable a global morpho- logical model of the LISM (work in collaboration with M.S. Sahu, B.K. Gib- son, C. Thom, A. Hughes, N. McClure-Griffiths, P. Palunas). Previous CaII surveys (Vallerga et al. 1993; Lallement et al. 1986; Lallement & Bertin 1992; Welty et al. 1996, and see summary, Redfield & Linsky 2002) only accumulated ∼50nearby sightlines, butdetected LISMCaIIabsorptionin 80% ofthetargets. Our survey will provide extensive coverage. The maximum angular distance be- ◦ tween anytwoadjacenttargets willbe<10 ,includingmorethan45targetpairs ◦ that will be <1 apart, providing an interesting study of small scale structure in the LISM. When combined with past and future observations, this survey will provide a significant baseline with which to search for long-term LISM ab- sorption variation, as the sightlines to these high proper motion nearby stars vary over timescales of decades. Ultimately, the CLOSE project will provide a valuable database for the development of a global morphological model of the LISM. 3. Relationship Between Stars and their Local Interstellar Medium As discussed in reference to Figure1, the interaction between the LISM and solar/stellar winds is mediated by the heliospheric/astrospheric interface. This interface is definedby the balance between the solar/stellar wind and the LISM. Reviews of heliospheric modeling include Zank (1999) and Baranov (1990), and the detection of astrospheres around nearby stars is reviewed by Wood (2004). 3.1. The Heliosphere and Astrospheres In the standard picture of the heliosphere, discussed by Zank (1999), Baranov (1990)andWood(2004),themagnetizedsolarwindisshockedtosubsonicspeeds (“termination shock”), as is the ionized LISM material (“bow shock”). The in- terface in between (“heliopause”) is where the plasma flows of the solar wind and LISM are deflected from each other. It was originally thought that neutral atoms from the LISM pass through the heliosphere unimpeded and therefore have a negligible influence on the structure of the heliosphere. However, charge exchange reactions between ionized hydrogen in the solar wind and neutral hy- drogen in the LISM act to heat and decelerate LISM hydrogen atoms just prior 10 Redfield to the heliopause. The resulting structure, referred to as the “hydrogen wall,” is an accumulation of hot hydrogen between the heliopause and the bow shock. At the same time that heliospheric simulations were indicating an enhance- ment of hydrogen just beyond the heliopause, it was becoming clear that ob- servations of LISM absorption in HI Lymanα were discrepant with other LISM absorption lines along thesame line of sight. Inparticular, excess HI absorption was required on the red and blue sides of the LISM HI absorption feature, in order to be consistent with the optically thin DI absorption profile, which is only 82kms−1 to the blueof the HI absorption. Themodels predicted a column density for the “hydrogen wall” of log N (cm−2)∼14.5. From Figure2, it is H clear that only the Lyman series hydrogen lines are sensitive enough to detect these low column densities. Indeed, heliospheric HI absorption was first de- tected usingthe Lyman-α profile, when Linsky & Wood (1996) measured excess absorption redshifted with respect to the LISM absorption. The blueshifted excess absorption is associated with an astrosphere. Because we observe the decelerated heliospheric hydrogen from the inside, the heliospheric absorption is redshifted, whereas the decelerated hydrogen in an astrosphere is observed exterior to the astrosphere, and therefore is blueshifted, see Figure6 of Wood (2004). It should be noted that if the interstellar absorption gets to be too large, log N (cm−2)≥18.7, the saturated ISM absorption will obliterate any sign of H a slightly offset heliospheric or astrospheric absorption. So, these measurements are only possible within the low column density volume of the LISM. Among a sample of nearby stars, heliospheric and astrospheric absorption has been detected for many stars (Wood et al. 2005b). Multiple heliospheric detections samplethestructureofourheliosphereinthreedimensions,whilemultipleastro- spheric detections provide measurements of weak solar-like winds around other stars. Morethan50%ofstarswithin10pcthathavehighresolutionUVLyman- α spectra show signs of astrospheric absorption (Wood et al. 2005a). 3.2. Heliospheric Variability Short Term The solar wind strength and distribution fluctuate with the 11- year solar cycle (Richardson 1997). These variations slowly propagate out to the heliospheric boundary and it is expected that the heliopause will expand and contract on a comparable timescale. The stochastic injection of energy into the solar wind in the form of flares and mass ejections leads to variability on evenshortertimescales. Thisdynamicwindisconstantly buffetingthemagneto- spheresofplanetsinitspathaswellastheheliosphericboundary. Voyager1may have detected such short-term variability when over a 7-month period in 2003 the termination shock contracted inward, over Voyager1, and then expanded back outward over Voyager1 yet again, a year before Voyager1 unambiguously crossedtheterminationshock(Krimigis et al.2003;McDonald et al.2003). Due to the immensity of ISM clouds, even the smallest of LISM structures are not expected to contribute to short-term (∼yrs) heliospheric variability. Long Term Long-term variations inthesolar windstrength arenotwellknown, but observations of astrospheres around young solar analogs provide clues as to what kind of wind the Sun had in its distant past. The solar wind, 3.5billion years ago, may have been ∼35× stronger than it is today (Wood et al. 2005a).