JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A8, PAGES 15,767-15,781, AUGUST 1, 2001 Energeticneutral atom imaging of the heliospheric boundary region Mike Gruntman,• Edmond C. Roelof,2 Donald G. Mitchell,2 Hans J. Fahr,3 HerbertO. Funsten,4 and David J. McComass Abstract.Energetic neutral atom (ENA) imagingis a powerfultechnique, which can remotely probethe properties of distanthot plasmas. Hot plasmas are abundant at theheliospheric boundary,the region where the expanding solar wind meets the surrounding local interstellar cloud.Here we presenta newconcept for imaging this boundary in ENA fluxes.Heliospheric ENAsare born from charge exchange between energetic protons and the background interstellaratomic hydrogen gas. The technique is idealfor studyingthe asymmetric three- dimensionalheliospheric interface region remotely, from 1 AU. We showthat ENA imagingin the0.2-6 keV energyrange will establishthe nature of thetelTnination shock and properties of hotproton populations in theheliosheath. We alsoexamine how the evolution of pickupproton populationsat andbeyond the shock can be explored.Global heliosphere ENA imageswill distinguishamong the competingmodels of the interactionbetween the Sunand the local interstellarmedium, and they will revealthe physics of importantprocesses in theinterface region.Heliospheric ENA fluxesare exceptionally weak, which makes imaging implementation difficult.Nonetheless, we showhow single-pixel ENA sensorscan image the heliosphere from a spicmingspacecraft on a typicalmission near 1 AU. Therequired instrumentation is briefly discussed. 1. Heliosphere equal the externalLISM pressureof the interstellarwind and magnetic field. The solar wind expansiontransitions to a The interactionof the expandingsolar wind plasmaand local subsonicflow at thetermination shock. There the kinetic energy interstellarmedium (LISM) createsthe heliosphere[Davis, of the supersonicflow is largelyconverted into thermalenergy 1955; Parker, 1963; Dessler, 1967; Axford, 1972]. The in the subsonicplasma beyond the shock. This subsonic heliosphereis a complexphenomenon where the solarwind and postshocksolar wind plasma flows around the termination interstellar plasmas, interstellar gas, magnetic fields, and shock and down the heliospherictail. The solar plasma energeticparticles all play importantroles. eventuallymixes with the interstellargalactic plasma in the tail The Sun's interactionwith the nearbyinterstellar medium is at distances of 1000-2000 AU. of considerableastrophysical interest. Our Sun is typicalof its Interstellarplasma does not penetrate through the heliopause, classof stars.Thus the hellosphereoffers a uniqueopportunity the discontinuoustransition boundary separating the solarand to study in detail the only accessible example of a galacticplasmas, but flows aroundthe hellopause.The region commonplaceand fundamentalastrophysical phenomenon, the betweenthe terminationshock and the hellopauseis calledthe formation of an astrosphere.Wood and Linsky [1998] heliosphericsheath (heliosheath). Other hellosphericmodels demonstratedthe possibility of remotely observing the make somewhatdifferent assumptions,including a subsonic astrospheresof nearby stars,which opensthe possibilitiesof interstellarwind, a weak terminationshock, an asymmetric comparativeastrosphere studies. solarwind, and may includean interstellarmagnetic field and A possible two-shock Sun-LISM interaction scenario cosmicrays [e.g., Fahr and Fichtner, 1991; Suess,1993; Linde [Baranov,1990; Baranovand Malama, 1993, 1995] illustrates et al., 1998; Zank, 1999]. In all the models,the hot shocked the main featuresof the hellosphere(Figure 1). The interstellar solar wind plasma fills the heliosheathand flows down the wind approachesthe hellosphere with a supersonicvelocity and hellospherictail. formsthe bow shock.The dynamicpressure of the expanding, Three major factors make the heliosphere essentially highly supersonicsolar wind decreaseswith the heliocenthc asymmetric.First, the partially ionized interstellargas of the distance.At a certaindistance from the Sunthis pressure would [.ISM moveswith a 26 km s-• speed(Figure 1) withrespect to the sun (interstellarwind). Second,the interstellarmagnetic field couldbe pointedin any direction.Third, the solarwind is anisotropic with the properties over the Sun's poles of lDepartmentSouthernCalifornia, ofAerospace Los Angeles, andMechanicalCalifornia. Engineering, Universitysignificantly differentfrom those closeto the ecliptic plane. 2AppliedPhysics Laboratory, Johns Hopkins University, Laurel, The hellospherehas been extensivelystudied both Maryland. theoreticallyand by variousexperimental techniques since the Universitiit3InstitutBonn, far AstrophysikBonn, Germany. und Extraterrestrische Forschung,early 1960s. Experimental dataon the region between the 4LosAlamos National Laboratory, Los Alamos, New Mexico. supersonicsolar wind and the surroundingLISM, the 5SouthwestResearch Institute, San Antonio, Texas. hellosphericinterface region, are scarce,mostly indirect, and often ambiguous.Although significantprogress was achieved Copyright2001 by theAmerican Geophysical Union in understandingthe Sun-LISM interaction,many important Papernumber 2000JA000328. questionsare still unanswered,for example:Is the solar wind 0148-0227/01/2000JA000328509.00 expansionterminated by a shock?What is the nature of the 15,767 15,768 GRUNTMAN ET AL.: ENERGETIC NEUTRAL ATOM IMAGING OF THE HELIOSPHERE BS Becausethe terminationshock and the interfaceregion beyond are so huge (hundreds of astronomicalunits), only remote techniquescan provide a global view of the time-varyingthree- dimensionalhellosphere on a continuousbasis. We argue that the technique for obtaining our first informationon the globalstructure of the heliosphereis already ISG at hand. This technique, energetic neutral atom (ENA) imaging, is sensitiveto the details of the structureof the s hellosphericinterface, to the nature of the terminationshock, M and to the evolutionof the pickup proton propertiesat and beyondthe terminationshock. The scientific promise of ENA imaging has been increasinglyargued for since the early 1980s and has led to extensivedevelopment of the conceptand instrumentation.The conceptwas validated by imaging the magnetosphericion Figure 1. Two-shockmodel of the interactionof the solarwind populationsusing ad hoc techniqueswith existingnon-ENA with the local interstellarmedium. Angle © is measuredfrom instruments[Roelof, 1987; Henderson et al., 1997] and the upwind direction. The hatched region between the dedicatedENA instruments[Orsini et al., 1992; Mitchell et al., terminationshock and the heliopausecontains the postshock 1993, 2000; Roelofet al., 1999;Brandt et al., 1999;Krimigis et solar wind plasma and pickup protons.The energeticneutral atoms (ENAs) producedin this region are the focus of this al., 2001; Pollock et al., 2000; Moore et al., 2000]. Williamset work. LISM, local interstellar medium; TS, termination shock; al. [1992] reviewed the conceptof magnetosphericimaging, HP, heliopause; BS, bow shock; CR, cosmicrays; ISP(G), and Gruntman [1997] reviewed ENA instrumentationand interstellarplasma (gas); B, magneticfield. presenteda historyof ENA experimentalstudy. Imaging of the hellospherein ENA fluxes has been advocatedfor sometime [Gruntmanet al., 1990; Gruntman, 1992, 1997; Hsieh and Gruntman,1993] as a way of exploringremotely properties of termination shock? What is the distance to the shock and its the hellosphericsheath plasma. The first dedicatedspace shape?Is the natureof the shockthe samein all directionsfrom experimentto detectthe heliosphericENAs wasprepared in the the upwind (with respect to the interstellarwind) to the mid-1980s[Gruntman et al., 1990] but has neverbeen flown. downwind?What happensto the solarwind pickupions at and ComplementaryEUV mapping of the hellopauseis being beyond the shock? We also know very little about the currently evaluated [Gruntman and Fahr, 1998, 2000; heliopauseand the bow shock. Gruntman,2001 a, 2001b]. Inferring the properties of the heliosphericinterface is In this articlewe presentthe conceptof heliosphereimaging complicatedby the complexity of the Sun-LISM interaction. using naturally occurring ENA fluxes. ENA imaging will The plasmaand neutral gas flows aremultifluid with oftennon- distinguish among the competing Sun-LISM interaction Maxwellian ion velocity distributions.Plasmas and neutral models,radically constrain the models,and explorethe physics atompopulations are not in thermodynamicequilibrium, many of the criticaland often superimposedprocesses in the interface processesare nonstationary,energetic ions substantiallymodify region.Heliospheric ENA fluxesare exceptionallyweak, which the shocks,and galacticand anomalouscosmic rays contribute makes imaging implementationnontrivial. Thereforewe also importantpressure gradients. The mean free path of neutral describehow the hellospherecan be imagedon a realisticspace atoms is comparableto the size of the heliosphere,which missionbased on the availableinstrumentation technology. strongly limits applicability of gasdynamical methods. Consequently,a self-consistentmodel of the stationary 2. Heliospheric Interface heliospherehas yet to be developed. The lack of the direct experimentaldata severelylimits our The propertiesof the LISM determineimportant features of abilityto understandthe detailsof the hellosphericinteractions. the Sun-LISM interaction.The physicsof the LISM is far from Furtherprogress in studyingthe Sun-LISM interactioncritically