Physics Today Comets as solar probesCarolus J. Schrijver, Carey M. Lisse, and Cooper Downs Citation: Physics Today 66(10), 27 (2013); doi: 10.1063/PT.3.2146 View online: http://dx.doi.org/10.1063/PT.3.2146 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/66/10?ver=pdfcov Published by the AIP Publishing

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Deep inside the Sun, a magnetic field isgenerated by the effects of rotation-induced Coriolis forces acting on a ther-mally driven convection that permeatesthe outermost layer, spanning 30% of

the solar radius. That constantly changing field hasprofound effects: It causes temperatures in the solaratmosphere to rise to millions of degrees to form thex-ray-emitting corona, powers the persistent out-flow of charged particles known as the solar wind,and occasionally causes gigantic explosions thatdrive space weather around all the planets. (See thearticle by Gordon Holman in PHYSICS TODAY, April2012, page 56.)

The processes that determine which field linesloop back onto the Sun and can hold in the million-degree gases and which ones are forced open intothe heliosphere, the region of space through which

the solar wind extends, remain poorly understoodbecause they occur in a domain from which light ishardly emitted and into which spacecraft cannot go.Fortunately, nature recently offered two unexpectedprobes by which to study that domain: In July 2011,scientists made the first observations of a comet,dubbed C/2011 N3 (sometimes abbreviated N3),moving through the solar corona with its tail lit upin the extreme UV.1 Only half a year later, the evenmore spectacular comet C/2011 W3, known asLovejoy, was traced to within 135 000 km of the

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Karel Schrijver is a senior fellow at the Lockheed Martin Advanced Technology Center in Palo Alto, California. Carey Lisse is a senior researchscientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Cooper Downs is a research scientist at Predictive Science Inc in San Diego, California.

By rapidly evaporating as they fly through the Sun’s hot outer atmosphere,sungrazing comets reveal an otherwise invisible magnetic field thatshapes the very beginnings of the solar wind.

Carolus J. Schrijver, Carey M. Lisse,

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solar surface, and the comet’s tail was seen movingin response to forces exerted by both the Sun’s mag-netic field and its atmosphere.2 Such sungrazingcomets leave a trail of gas that, after it heats up andbegins to glow, provides a new way to study thelocal magnetic field and to compare it to state-of-the-art models.

The Sun’s complex dynamoBuoyant bundles of magnetic field float from deepwithin the solar interior to the surface—or more accurately, the photosphere, the outermost regiondense enough to be opaque and thereby resemble asurface. There, they appear in sizes that range fromat least a few hundred kilometers in diameter—thesmallest scale currently observable—to over 100 000km across. The largest bundles can form sunspots,in which particularly concentrated fields suppressthe near-surface convection and cause the surface tobe relatively cool and therefore dark.

The smallest field bundles evolve over tens ofminutes in the overturning plasma. The largest onescan resist disintegration for up to several weeks.Eventually, however, all magnetic bundles break up

and disperse, as they are subject to random convec-tion processes, large-scale winds, and collisions inwhich magnetic field may be removed from the sur-face. Meanwhile, new bundles breach the surfaceelsewhere at rates modulated by the Sun’s 11-yearcycle. The dynamic ensemble of all those field bun-dles shapes the Sun’s large-scale electromagneticfield as it reaches into interplanetary space.

The overturning plasma and small-scale fieldbundles entrained in it supply nonradiative energyto the solar atmosphere. Dissipation of that energyheats the outer atmosphere to 1–3 MK, some 300times hotter than the 5780-K solar surface (see theQuick Study by Charles Kankelborg in PHYSICSTODAY, April 2012, page 72). This outer atmospherethereby forms the solar corona, pictured in the cen-ter of figure 1, which radiates in the x-ray and EUVparts of the electromagnetic spectrum.

The glow from the almost fully ionized coronalplasma traces the magnetic field from numerous“coronal loops,” as light-emitting ions and the heat-conducting electrons that excite them are locked ontothe lines of force. The multi-megakelvin temperatureof the plasma produces a substantial pressure high in

the multipolar magnetic field. That pres-sure, somehow aided by the effective pres-sure of ubiquitous magnetohydrodynamicwaves, forces some of the field lines to bulgeinto the far reaches of the planetary system.The result is an ever-flowing supersonicsolar wind of mostly electrons and ionizedhydrogen and helium, moving at speedsfrom 300 to 1000 km/s.

The processes that heat and drive thefluctuating solar wind and its entrainedmagnetic field are among the primary puz-zles of heliophysics. Observational access isdifficult. The glow of the innermost, closed-field corona can be measured out to severaltenths of a solar radius using space-basedx-ray and EUV telescopes. And the solarwind has been measured in situ from some50 solar radii out to distances far beyondPluto with particle sensors flown on the oc-casional interplanetary spacecraft. But theregion in between, which straddles the topof the corona and the base of the wind, isvery difficult to probe.

Space-based coronagraphic telescopesblock out the bright solar disk and can rou-tinely view the striations in the solar winddown to about half a solar radius (about350 000 km) above the solar surface; figure 1shows such striations in the visible-lightportion (red). And occasional solar eclipsesprovide brief glimpses into the solar atmos-phere, even close to the Sun’s surface. Tomap the diversity of coupled physicalprocesses between the corona and the out-flowing wind, scientists are increasinglyturning to numerical computation. Currentmodels are subject to a multitude of simpli-fying assumptions, however, which makesobservational validation essential. That val-idation is what makes observations of two

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Comets

Figure 1. A composite photograph of the Sun and inner heliosphere taken at01:30 universal time on 16 December 2011. The center exposure, taken by NASA’sSolar Dynamics Observatory spacecraft, shows a false-color image of extreme UVradiation from gases in the corona at about 1.5 million K. Surrounding that centralimage are images made using the Large-Angle and Spectrometric Coronagraph(LASCO) aboard the European Space Agency’s SoHO spacecraft. Two differentLASCO telescopes (with image segments shown in red and blue) reveal structuresknown as streamers, which outline relatively dense coronal regions shaped by themagnetic field, against a backdrop of stars. The arc reaching toward the Sun fromthe lower left is comet Lovejoy’s tail as it approached perihelion. The bright marknear 2 o’clock at the inner edge of the red image shows post-perihelion Lovejoyjust beginning to regrow a dust and gas tail after having lost it while closer to theSun. Within three days from these images, Lovejoy’s nucleus completely subli-mated and ceased to exist.

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comets passing to within 20% of the Sun’s radius in2011 so valuable.

Sungrazing cometsThe celestial phenomenon that people see as acomet is created by sunlight scattering off the dustand gas that sublimate from an irregular kilometer-sized chunk of ice-cold primordial solar-systemmatter. Comets whose orbits bring them relativelyclose to the Sun are readily visible because largeamounts of cometary ices evaporate off their sur-faces and release embedded dust as they do. As fig-ure 2 shows, scattered sunlight makes the comettails stand out against the starry sky. Indeed, thetails stand out even close to the Sun, provided theSun’s bright surface is efficiently blocked out, as itis by coronagraphs flown on the SoHO and STEREOspacecraft, which routinely observe comets at greatdistances from the Sun. Over the past decade, coro-nagraphs have discovered more than 1600 membersof a family of comets named after its discoverer,Heinrich Kreutz.

The Kreutz comets, including C/2011 N3 andLovejoy, have closely aligned common orbits with apropensity for perihelion distances of less than afew solar radii. Thought to be the remnants of agiant parent comet that fragmented upon its near-Sun passage several thousand years ago, the Kreutzfamily has been the subject of intense study.3 Someof the brightest comets ever seen were large Kreutzsungrazers. But a steady stream of smaller frag-ments also arrive more or less continuously with estimated radii3 between about 10 m and 1000 m.4,5

Before the sightings of C/2011 N3 and Lovejoy,though, none had been observed to fly through thesolar corona.

Leaving a trailUnderstanding the physical construction of comets—how micron-sized specks of dust and gas moleculesaccreted into large ice- and rock-rich bodies—is oneof the great mysteries of planetary science. Usingknown physical parameters such as bulk modulus,porosity, surface cohesion, and dielectric constant,most models of the aggregation of gas and dustshow that particles should build up to centimeter-sized objects quite easily in the plane of the solarsystem. But such studies also suggest that larger-sized particles should disintegrate when collidingwith each other, which happens at speeds of a fewkilometers per second or more. Thus we face what’sknown as an aggregational barrier to the formationof planetesimals.6,7

Past that barrier, accretion into the knownplanet-sized objects is relatively straightforward tounderstand (see the article by Robin Canup inPHYSICS TODAY, April 2004, page 56). The size distri-bution of fragments of sungrazing comets can tellus about the sizes of the bodies that formed the par-ent comet. Moreover, such sungrazers are probes toa temperature regime, on the order of 500–2000 K,that is not otherwise encountered in the solar sys-tem. In that regime, comets emit material throughsublimation and thermal desorption. Thus, remote-sensing spectroscopy of sungrazers can yield in-

sight about the least volatile components that makeup comets and presumably the rest of the bodies inthe solar system (see the article by Don Brownlee inPHYSICS TODAY, June 2008, page 30).

No one has yet worked out the details of whathappens to chunks of matter once they leave a cometnucleus. They’re known to actively sublimate andlose mass in sunlight. But as long as the chunks re-main large enough to efficiently cool themselves byevaporation, they can maintain their surface nearthe sublimation temperature of water ice—about200 K. When that cooling no longer suffices, theyrapidly heat up to thousands of degrees and ex-plode into tiny pieces of dust and ice. The piecesquickly evaporate into a gas of molecules that thenrapidly dissociate in sunlight and through collisionswith the coronal plasma. That fate happened to N3,whose nucleus, coma, debris tail, and path acrossthe face of the Sun are shown in figure 3.

The rate of comet mass loss near the Sun is largeby human standards: It’s estimated1 at 1–100 tons/sfor N3. Nonetheless, the telescopes used to imageN3 and Lovejoy—the Atmospheric Imaging Assem-bly8 aboard NASA’s Solar Dynamics Observatory(SDO) for N3 and the AIA and the SECCHI tele-scopes aboard NASA’s STEREO spacecraft for Love-joy—detected gases escaping from debris fragmentsno more than about 400 m in diameter against abright star with a diameter some two million timesas large as the fragments.

Detecting that signal against the coronal glowis possible because the solar corona is made up ofover 99.9% hydrogen and helium ions by number,but a comet, having lost almost all of those volatilespecies, consists predominantly of water ice and

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Ion-gas tail

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Figure 2. An image of comet Hale–Bopp in March 1997 when thecomet was about 1.01 AU from the Sun. The nucleus of Hale–Bopp wasunusually large, with a radius of 25–50 km; by comparison, a typicalcomet nucleus is 1–10 km in radius. The Hale–Bopp nucleus is here surrounded by a 100 000-km haze known as the coma. Two tails stretchaway from the haze. The gray dust tail, pushed back from the coma bythe pressure of sunlight, stretches over 33 million km. The bluish ion tail is pushed away from the Sun by the solar wind and its embeddedmagnetic field.

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rock, with more than 40% oxygen atoms and about5% iron atoms by number. Consequently, an ablat-ing comet locally enriches the solar coronal plasmawith first neutral and then ionized O and Fe atoms.Those ions’ subsequent glow from collisions withelectrons adds measurably to the characteristiccoronal EUV photons to which the state-of-the-artinstruments on SDO and STEREO are tuned.

Falling through the atmosphereFor any cometary nucleus that survives to within25 000 km of the solar surface, the solar atmospherewould be dense enough—exceeding 1011 cm−3—thatdrag and the stresses of deceleration would behuge.9 Those forces can create an exploding airburstfollowed by a fireball that spreads and rises throughthe atmosphere, just as comet Shoemaker–Levy 9did when it fell into Jupiter10 in 1994 (see PHYSICSTODAY, February 1995, page 17). The Sun itself pro-vided a scaled-down view of such impacts on 7 June2011. On that day, dense clouds of cool gas that wereejected from the solar surface during an unusuallylarge filament eruption fell back onto the Sun, reach-ing impact velocities up to 450 km/s.11 Within some10 seconds of their descent from hot corona to solarsurface, the falling clouds experienced a billionfoldincrease in atmospheric density. The resulting ex-plosion was clearly visible in the UV and EUV andproduced a spray of matter heated in excess of a mil-lion kelvin.

Comets N3 and Lovejoy did not come that closeto the solar surface, however. They reached only toabout 110 000 km and 135 000 km, respectively. Thefate of comets at those distances is dominated bysublimation,9 even though they are moving in freefall at nearly the escape velocity of 650 km/s, or 0.002times the speed of light. The sublimated atoms and

small particles quickly decelerate behind the nu-cleus into the coronal rest frame in collisions withthe atmosphere. They thus lose their kinetic energyand momentum in tens of seconds and therebywarm to EUV-emitting temperatures at densitieshigh enough to be detectable against the back-ground coronal emission.

The free-fall velocity of a sungrazing cometnear perihelion lies in the range of typical solar-wind speeds (300–800 km/s) that comets encounterfar into the heliosphere. Hence, the relative velocityof the solar plasma for a sungrazing comet near per-ihelion is comparable to that for a comet much moredistant in the heliosphere. What mainly distin-guishes comets probing the two environments arethe rate of molecular dissociation following subli-mation and the rate at which atoms collide with thesurrounding medium. The density of the solar windnear Earth’s orbit, for example, is 3–10 atoms/cm3.Within the corona near the perihelions of N3 andLovejoy, in contrast, the density is on the order of108 atoms/cm3.

In the distant heliosphere, radiation pressureon the gas and dust tail is the dominant force, withsome ionization of cometary atoms producing a sec-ond, windswept tail, as shown in figure 2. Near theSun, however, the collisions of the monoatomicgases with the solar atmosphere dominate. The re-sult is that the comet’s tail becomes ionized plasmaand thus feels the force of the solar magnetic field.Dust and molecular gas survive too briefly to be vis-ible, and the ion tail quickly decelerates into the restframe of the coronal plasma and its all-permeatingmagnetic field. For Lovejoy, no dust survived to beblown out into the heliosphere for about 2 days (or0.17 astronomical unit) on either side of its perihe-lion passage. Even gas molecules were quickly bro-

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Figure 3. Extreme UV imagesof comet C/2011 N3 as itpassed in front of the Sun on 6 July 2011. The images weretaken by NASA’s Solar DynamicsObservatory using its 171 Åchannel, which is most sensi-tive to emission from coronalplasma near 106 K. The insetsshow running-difference images—each subtracted froman exposure in the same regiontaken several seconds earlier—to isolate the comet’s tail fromthe background solar corona.The appearance of fragments isattributable to variable out-gassing rates rather than tosubstantial separation of nu-clear fragments. (Courtesy ofWei Liu, Stanford University.)

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ken up: The dissociation of water molecules, for ex-ample, would have taken only 3 seconds, followedby ionization of its atoms in less than 0.1 second.12

Probing the coronaAll known Kreutz comets were discovered as theywere falling toward the Sun. But before Lovejoy ap-proached it in late December 2011, no Kreutz comethad been observed to survive close perihelion pas-sage. Even Lovejoy lasted only 2–3 days after pass-ing through the Sun’s corona.13 But before Lovejoy’s4.5-billion-year history ended, both the descent andascent phases of its path were visible to spacecraftlooking from three very different perspectives.2

The idea of using comets to learn about the Sunand its surroundings is not new. Observations of lin-ear tails that pointed away from the Sun and gloweddue to emission from ionized gases led to the firstinklings of what since the late 1950s and early 1960has become known as the solar wind. The dust tailsof comets follow parabolic trajectories consistentwith a gravitational pull that is counteracted, if notovercome, by outward radiation pressure. But thetrajectories of the linear ion-plasma tails depend onthe collisional ionization of sublimating gases in thecomet’s coma and how they are channelled by themagnetic field blown along with the solar wind.14

Nowadays, the main puzzles about the solarwind concern the largely unobservable region inwhich it forms. In situ data is nonexistent becausethe deep corona is simply too harsh an environmentfor spacecraft. Though the environment is also tooharsh for sungrazing comets, their much larger ini-tial masses enable them to survive longer. Observa-tions of sungrazers close to perihelion thus enableus to probe the coronal medium along the comets’well-defined trajectories.

The Lorentz force acts on ionized cometary ma-terial, and the resulting ion motions reveal the localorientation of the coronal magnetic field even as thecomet’s ions decelerate and settle into the coronalplasma. The ratio of the energy density of the coro-nal magnetic field to the kinetic energy density ofthe plasma in the comet’s ion tail is likely to influ-ence the tail evolution. In N3’s case it appears thatthe comet’s inertia dominated: As the cometaryplasma decelerated during collisions with thecorona’s atmosphere, the corona’s magnetic field be-came strongly deformed. In Lovejoy’s case, in con-trast, the solar magnetic field appeared to largelyhold its own.

From their observations of comet Lovejoy, theSDO and STEREO science teams immediately rec-ognized that the dynamical evolution of the tail con-tained information about the coronal magnetic field.Tail motions observed during ingress and egressfrom perihelion, as shown in figure 4, correspondedto neither the radial direction nor a direction tan-gential to the orbit; the radial direction would be ex-pected if the solar wind were fully developed, anda tangential direction would be expected if the coro-nal medium had no influence on the comet at all. In-stead, the SDO and STEREO imagers revealed wig-gles in the tail about the comet’s orbital paththrough the inner corona.

The varying deflections of Lovejoy’s tail indi-cated a highly inhomogeneous medium. Applica-tion of a state-of-the-art computer model of the solarcorona2 revealed a striking consistency between theobserved tail motions and the orientation of themagnetic field. The result provides a unique valida-tion of the model, particularly important given thatcreating one is daunting.

Creating a global model of the corona startswith the need for a full-sphere map of the field. Butcurrently, only the field in front of the Sun can be re-liably measured and only for latitudes up to some70°. Latitude-dependent solar rotation—with oneturn per month, on average—allows researchers toobserve the entire low- and mid-latitude belts inter-mittently from Earth’s perspective. Even so, the fieldevolves significantly in the more than two weeksduring which observational access is limited orblocked altogether before the region spins back intoview. The field in the polar caps, which generallycontribute strongly to the large-scale dipolar field,is always subject to substantial uncertainty.

Because of those observational difficulties, onlyabout one-quarter of the solar surface can be accu-rately mapped from observations of its magnetic

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Figure 4. Comet Lovejoy’s orbit, shown as the pink arc through theSun’s magnetic field. In the center is the model Sun with magnetic fieldpatches of opposite polarities, red and blue. The insets show snapshotsof Lovejoy’s tail (white streaks) at different locations, as imaged at 171 Åby the Solar Dynamics Observatory (SDO) and STEREO spacecraft. Scien-tists have connected the apparent motion of the tail to the simulatedmagnetic field, shown here as it would appear from above the orbitalplane. Orange field lines are open to interplanetary space; blue lines areclosed between two points on the solar surface. Ionized comet plasmabecomes entrained in the magnetic field and thus provides a naturalprobe of the corona and a test of magnetic field models. The relative orientation of the field with respect to the comet’s path accounts for thenonradial and nonorbital motions that were observed by SDO in regionsa and c. (For a movie of Lovejoy’s approach, see the online version of thisarticle.) The rapidly changing loop orientation of the magnetic field linesnear perihelion is consistent with the wiggling motion observed in re-gion b by extreme UV telescopes on the two STEREO spacecraft lookingfrom different sides of the Sun.

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field. The rest is subject to guesswork or approxi -mations using various assimilative and modelingprocedures.

The best-effort global surface map of the mag-netic field is used as the foundation for a magneto-hydrodynamic (MHD) corona. Disregarding hard-to-model and hard-to-validate large-scale currentsystems, one such MHD model2 varies the parame-terizations of atmospheric energy deposition untilthe forces of the field–plasma interactions balance,selecting the solution in which the computed coro-nal-brightness patterns best resemble observations.Models like that15 have been made for some time,and their complexity has increased over the yearsas computer processing speeds have risen. Love-joy’s passage through the inner corona provides thefirst detailed test of such models at altitudes wherethe Sun’s corona and nascent solar wind alternateside by side—basically by using the tail motions aswind vanes.

OpportunitiesA new comet, dubbed C/2012 S1 or ISON when itwas discovered last year, is now approaching theSun. Its perihelion is anticipated for 28 November2013 and expected to occur at about 1.2 million kilo-meters from the solar surface—about 10 timeshigher in the corona than N3 and Lovejoy. Becausedensity drops rapidly with altitude in the corona,the interactions experienced by a comet at that dis-tance may be dominated by waves and turbulence

rather than by collisions and Lorentz forces. ISONthus promises to probe yet another key region in thenascent solar wind—if indeed it lights up adequatelyalong its orbit to be seen close to the Sun.

Nature provides comets free of charge. Thetrick is to catch them approaching the Sun. NASA is now building, at considerable expense in man-power and resources, a spacecraft called Solar ProbePlus that will fly late this decade to the very outerreaches of the corona, some 7 million kilometers dis-tant from the Sun’s surface. In the meantime, we cantake advantage of the free probes to learn as muchas possible about the interface between the coronaand the enveloping heliosphere at distances farcloser than human probes can currently reach.

References1. C. J. Schrijver et al., Science 335, 324 (2012).2. C. Downs et al., Science 340, 1196 (2013).3. M. M. Knight et al., Astron. J. 139, 926 (2010).4. H. A. Weaver et al., Science 292, 1329 (2001).5. W. T. Reach et al., Icarus 203, 571 (2009).6. K. Wada et al., Astrophys. J. 702, 1490 (2009).7. A. Zsom et al., Astron. Astrophys. 513, A56 (2010).8. J. R. Lemen et al., Sol. Phys. 275, 17 (2012).9. J. C. Brown et al., Astron. Astrophys. 535, A71 (2011).

10. R. W. Carlson et al., Icarus 128, 251 (1997).11. F. Reale et al., Science 341, 251 (2013).12. P. Bryans, W. D. Pesnell, Astrophys. J. 760, 18 (2012).13. Z. Sekanina, P. W. Chodas, Astrophys. J. 757, 127 (2012).14. E. N. Parker, Astrophys. J. 128, 664 (1958).15. Z. Mikić et al., Phys. Plasmas 6, 2217 (1999). ■

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