magnetic field is generated by the effects of rotation-induced Coriolis forces acting on a thermally driven convection that permeates the outermost layer, spanning 30% of the solar radius. That constantly changing field has profound effects: It causes temperatures in the solar atmosphere to rise to millions of degrees to form the x-ray-emitting corona, powers the persistent outflow of charged particles known as the solar wind, and occasionally causes gigantic explosions that drive space weather around all the planets. (See the article by Gordon Holman in Physics Today, Deep inside the Sun, ais generated by the effects of rotation-induced Coriolis forces acting on a thermally driven convection that permeates the outermost layer, spanning 30% of the solar radius. That constantly changing field has profound effects: It causes temperatures in theto rise to millions of degrees to form the x-ray-emittingpowers the persistent outflow of charged particles known as theand occasionally causes gigantic explosions that drive space weather around all the planets. (See the article by Gordon Holman in April 2012, page 56 .)

NASA/SDO AND THE AIA, EVE, AND HMI SCIENCE TEAMS

heliosphere, the region of space through which the solar wind extends, remain poorly understood because they occur in a domain from which light is hardly emitted and into which spacecraft cannot go. Fortunately, nature recently offered two unexpected probes 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 up in the extreme UV. 1 et al. , Science 335, 324 (2012). 1. C. J. Schrijver, Science, 324 (2012). https://doi.org/10.1126/science.1211688 comet C/2011 W3, known as Lovejoy, was traced to within 135 000 km of the solar surface, and the comet’s tail was seen moving in response to forces exerted by both the Sun’s magnetic field and its atmosphere. 2 et al. , Science 340, 1196 (2013). 2. C. Downs, Science, 1196 (2013). https://doi.org/10.1126/science.1236550 comets leave a trail of gas that, after it heats up and begins to glow, provides a new way to study the local magnetic field and to compare it to state-of-the-art models. The processes that determine which field lines loop back onto the Sun and can hold in the million-degree gases and which ones are forced open into thethe region of space through which theextends, remain poorly understood because they occur in a domain from which light is hardly emitted and into whichcannot go. Fortunately, nature recently offered two unexpected probes by which to study that domain: In July 2011, scientists made the first observations of adubbed C/2011 N3 (sometimes abbreviated N3), moving through thewith its tail lit up in the extreme UV.Only half a year later, the even more spectacularC/2011 W3, known as Lovejoy, was traced to within 135 000 km of the solar surface, and the comet’s tail was seen moving in response to forces exerted by both the Sun’sand its atmosphere.Such sungrazingleave a trail of gas that, after it heats up and begins to glow, provides a new way to study the localand to compare it to state-of-the-art models.

The Sun’s complex dynamo Section: Choose Top of page ABSTRACT The Sun’s complex dynamo << Sungrazing comets Leaving a trail Falling through the atmos... Probing the corona Opportunities REFERENCES Buoyant bundles of magnetic field float from deep within the solar interior to the surface—or more accurately, the photosphere, the outermost region dense enough to be opaque and thereby resemble a surface. There, they appear in sizes that range from at least a few hundred kilometers in diameter—the smallest scale currently observable—to over 100 000 km across. The largest bundles can form sunspots, in which particularly concentrated fields suppress the near-surface convection and cause the surface to be relatively cool and therefore dark. The smallest field bundles evolve over tens of minutes in the overturning plasma. The largest ones can resist disintegration for up to several weeks. Eventually, however, all magnetic bundles break up and disperse, as they are subject to random convection processes, large-scale winds, and collisions in which magnetic field may be removed from the surface. Meanwhile, new bundles breach the surface elsewhere at rates modulated by the Sun’s 11-year cycle. The dynamic ensemble of all those field bundles shapes the Sun’s large-scale electromagnetic field as it reaches into interplanetary space. solar atmosphere. Dissipation of that energy heats the outer atmosphere to 1–3 MK, some 300 times hotter than the 5780-K solar surface (see the Quick Study by Charles Kankelborg in Physics Today, solar corona, pictured in the center of figure 1 The overturning plasma and small-scale field bundles entrained in it supply nonradiative energy to theDissipation of that energy heats the outer atmosphere to 1–3 MK, some 300 times hotter than the 5780-K solar surface (see the Quick Study by Charles Kankelborg in April 2012, page 72 ). This outer atmosphere thereby forms thepictured in the center of figure, which radiates in the x-ray and EUV parts of the electromagnetic spectrum. The glow from the almost fully ionized coronal plasma traces the magnetic field from numerous “coronal loops,” as light-emitting ions and the heat-conducting electrons that excite them are locked onto the lines of force. The multi-megakelvin temperature of the plasma produces a substantial pressure high in the multipolar magnetic field. That pressure, somehow aided by the effective pressure of ubiquitous magnetohydrodynamic waves, forces some of the field lines to bulge into the far reaches of the planetary system. The result is an ever-flowing supersonic solar wind of mostly electrons and ionized hydrogen and helium, moving at speeds from 300 to 1000 km/s. The processes that heat and drive the fluctuating solar wind and its entrained magnetic field are among the primary puzzles of heliophysics. Observational access is difficult. The glow of the innermost, closed-field corona can be measured out to several tenths of a solar radius using space-based x-ray and EUV telescopes. And the solar wind has been measured in situ from some 50 solar radii out to distances far beyond Pluto with particle sensors flown on the occasional interplanetary spacecraft. But the region in between, which straddles the top of the corona and the base of the wind, is very difficult to probe. coronagraphic telescopes block out the bright solar disk and can routinely view the striations in the solar wind down to about half a solar radius (about 350 000 km) above the solar surface; figure 1 solar atmosphere, even close to the Sun’s surface. To map the diversity of coupled physical processes between the corona and the outflowing wind, scientists are increasingly turning to numerical computation. Current models are subject to a multitude of simplifying assumptions, however, which makes observational validation essential. That validation is what makes observations of two comets passing to within 20% of the Sun’s radius in 2011 so valuable. Space-basedtelescopes block out the bright solar disk and can routinely view the striations in thedown to about half a solar radius (about 350 000 km) above the solar surface; figureshows such striations in the visible-light portion (red). And occasional solar eclipses provide brief glimpses into theeven close to the Sun’s surface. To map the diversity of coupled physical processes between theand the outflowing wind, scientists are increasingly turning to numerical computation. Current models are subject to a multitude of simplifying assumptions, however, which makes observational validation essential. That validation is what makes observations of twopassing to within 20% of the Sun’s radius in 2011 so valuable.

Sungrazing comets Section: Choose Top of page ABSTRACT The Sun’s complex dynamo Sungrazing comets << Leaving a trail Falling through the atmos... Probing the corona Opportunities REFERENCES comet is created by sunlight scattering off the dust and gas that sublimate from an irregular kilometer-sized chunk of ice-cold primordial solar-system matter. Comets whose orbits bring them relatively close to the Sun are readily visible because large amounts of cometary ices evaporate off their surfaces and release embedded dust as they do. As figure 2 comet tails stand out against the starry sky. Indeed, the tails stand out even close to the Sun, provided the Sun’s bright surface is efficiently blocked out, as it is by coronagraphs flown on the SoHO and STEREO spacecraft, which routinely observe comets at great distances from the Sun. Over the past decade, coronagraphs have discovered more than 1600 members of a family of comets named after its discoverer, Heinrich Kreutz. The celestial phenomenon that people see as ais created by sunlight scattering off the dust and gas thatfrom an irregular kilometer-sized chunk of ice-cold primordial solar-system matter.whose orbits bring them relatively close to the Sun are readily visible because large amounts of cometary ices evaporate off their surfaces and release embedded dust as they do. As figureshows, scattered sunlight makes thetails stand out against the starry sky. Indeed, the tails stand out even close to the Sun, provided the Sun’s bright surface is efficiently blocked out, as it is byflown on theandwhich routinely observeat great distances from the Sun. Over the past decade,have discovered more than 1600 members of a family ofnamed after its discoverer, Heinrich Kreutz. comets, including C/2011 N3 and Lovejoy, have closely aligned common orbits with a propensity for perihelion distances of less than a few solar radii. Thought to be the remnants of a giant parent comet that fragmented upon its near-Sun passage several thousand years ago, the Kreutz family has been the subject of intense study. 3 et al. , Astron. J. 139, 926 (2010). 3. M. M. Knight, Astron. J., 926 (2010). https://doi.org/10.1088/0004-6256/139/3/926 comets ever seen were large Kreutz sungrazers. But a steady stream of smaller fragments also arrive more or less continuously with estimated radii 3 et al. , Astron. J. 139, 926 (2010). 3. M. M. Knight, Astron. J., 926 (2010). https://doi.org/10.1088/0004-6256/139/3/926 4 et al. , Science 292, 1329 (2001). 4. H. A. Weaver, Science, 1329 (2001). https://doi.org/10.1126/science.1058606 , 5 et al. , Icarus 203, 571 (2009). 5. W. T. Reach, Icarus, 571 (2009). https://doi.org/10.1016/j.icarus.2009.05.027 solar corona. The Kreutzincluding C/2011 N3 and Lovejoy, have closely aligned common orbits with a propensity for perihelion distances of less than a few solar radii. Thought to be the remnants of a giant parentthat fragmented upon its near-Sun passage several thousand years ago, the Kreutz family has been the subject of intense study.Some of the brightestever seen were large Kreutz sungrazers. But a steady stream of smaller fragments also arrive more or less continuously with estimated radiibetween about 10 m and 1000 m.Before the sightings of C/2011 N3 and Lovejoy, though, none had been observed to fly through the

Leaving a trail Section: Choose Top of page ABSTRACT The Sun’s complex dynamo Sungrazing comets Leaving a trail << Falling through the atmos... Probing the corona Opportunities REFERENCES 6 et al. , Astrophys. J. 702, 1490 (2009). 6. K. Wada, Astrophys. J., 1490 (2009). https://doi.org/10.1088/0004-637X/702/2/1490 , 7 et al. , Astron. Astrophys. 513, A56 (2010). 7. A. Zsom, Astron. Astrophys., A56 (2010). https://doi.org/10.1051/0004-6361/200912852 Understanding the physical construction of comets—how micron-sized specks of dust and gas molecules accreted into large ice- and rock-rich bodies—is one of the great mysteries of planetary science. Using known physical parameters such as bulk modulus, porosity, surface cohesion, and dielectric constant, most models of the aggregation of gas and dust show that particles should build up to centimeter-sized objects quite easily in the plane of the solar system. But such studies also suggest that larger-sized particles should disintegrate when colliding with each other, which happens at speeds of a few kilometers per second or more. Thus we face what’s known as an aggregational barrier to the formation of planetesimals. Physics Today, comets can tell us about the sizes of the bodies that formed the parent comet. Moreover, such sungrazers are probes to a temperature regime, on the order of 500–2000 K, that is not otherwise encountered in the solar system. In that regime, comets emit material through sublimation and thermal desorption. Thus, remote-sensing spectroscopy of sungrazers can yield insight about the least volatile components that make up comets and presumably the rest of the bodies in the solar system (see the article by Don Brownlee in Physics Today, Past that barrier, accretion into the known planet-sized objects is relatively straightforward to understand (see the article by Robin Canup in April 2004, page 56 ). The size distribution of fragments of sungrazingcan tell us about the sizes of the bodies that formed the parentMoreover, such sungrazers are probes to a temperature regime, on the order of 500–2000 K, that is not otherwise encountered in the solar system. In that regime,emit material throughand thermal desorption. Thus, remote-sensing spectroscopy of sungrazers can yield insight about the least volatile components that make upand presumably the rest of the bodies in the solar system (see the article by Don Brownlee in June 2008, page 30 ). comet nucleus. They’re known to actively sublimate and lose mass in sunlight. But as long as the chunks remain large enough to efficiently cool themselves by evaporation, they can maintain their surface near the sublimation temperature of water ice—about 200 K. When that cooling no longer suffices, they rapidly heat up to thousands of degrees and explode into tiny pieces of dust and ice. The pieces quickly evaporate into a gas of molecules that then rapidly dissociate in sunlight and through collisions with the coronal plasma. That fate happened to N3, whose nucleus, coma, debris tail, and path across the face of the Sun are shown in figure 3 No one has yet worked out the details of what happens to chunks of matter once they leave anucleus. They’re known to activelyand lose mass in sunlight. But as long as the chunks remain large enough to efficiently cool themselves by evaporation, they can maintain their surface near thetemperature of water ice—about 200 K. When that cooling no longer suffices, they rapidly heat up to thousands of degrees and explode into tiny pieces of dust and ice. The pieces quickly evaporate into a gas of molecules that then rapidly dissociate in sunlight and throughwith theplasma. That fate happened to N3, whose nucleus, coma, debris tail, and path across the face of the Sun are shown in figure comet mass loss near the Sun is large by human standards: It’s estimated 1 et al. , Science 335, 324 (2012). 1. C. J. Schrijver, Science, 324 (2012). https://doi.org/10.1126/science.1211688 8 et al. , Sol. Phys. 275, 17 (2012). 8. J. R. Lemen, Sol. Phys., 17 (2012). https://doi.org/10.1007/s11207-011-9776-8 Solar Dynamics Observatory (SDO) for N3 and the AIA and the SECCHI telescopes aboard NASA’s STEREO spacecraft for Lovejoy—detected gases escaping from debris fragments no more than about 400 m in diameter against a bright star with a diameter some two million times as large as the fragments. The rate ofmass loss near the Sun is large by human standards: It’s estimatedat 1–100 tons/s for N3. Nonetheless, the telescopes used to image N3 and Lovejoy—the Atmospheric Imaging Assemblyaboard NASA’sfor N3 and the AIA and the SECCHI telescopes aboard NASA’sfor Lovejoy—detected gases escaping from debris fragments no more than about 400 m in diameter against a bright star with a diameter some two million times as large as the fragments. Detecting that signal against the coronal glow is possible because the solar corona is made up of over 99.9% hydrogen and helium ions by number, but a comet, having lost almost all of those volatile species, consists predominantly of water ice and rock, with more than 40% oxygen atoms and about 5% iron atoms by number. Consequently, an ablating comet locally enriches the solar coronal plasma with first neutral and then ionized O and Fe atoms. Those ions’ subsequent glow from collisions with electrons adds measurably to the characteristic coronal EUV photons to which the state-of-the-art instruments on SDO and STEREO are tuned.

Falling through the atmosphere Section: Choose Top of page ABSTRACT The Sun’s complex dynamo Sungrazing comets Leaving a trail Falling through the atmos... << Probing the corona Opportunities REFERENCES solar atmosphere would be dense enough—exceeding 1011 cm−3—that drag and the stresses of deceleration would be huge. 9 et al. , Astron. Astrophys. 535, A71 (2011). 9. J. C. Brown, Astron. Astrophys., A71 (2011). https://doi.org/10.1051/0004-6361/201015660 comet Shoemaker–Levy 9 did when it fell into Jupiter 10 et al. , Icarus 128, 251 (1997). 10. R. W. Carlson, Icarus, 251 (1997). https://doi.org/10.1006/icar.1997.5756 Physics Today, 11 et al. , Science 341, 251 (2013). 11. F. Reale, Science, 251 (2013). https://doi.org/10.1126/science.1235692 corona to solar surface, the falling clouds experienced a billionfold increase in atmospheric density. The resulting explosion was clearly visible in the UV and EUV and produced a spray of matter heated in excess of a million kelvin. For any cometary nucleus that survives to within 25 000 km of the solar surface, thewould be dense enough—exceeding 10cm—that drag and the stresses of deceleration would be huge.Those forces can create an exploding airburst followed by a fireball that spreads and rises through the atmosphere, just asShoemaker–Levy 9 did when it fell into Jupiterin 1994 (see February 1995, page 17 ). The Sun itself provided a scaled-down view of such impacts on 7 June 2011. On that day, dense clouds of cool gas that were ejected from the solar surface during an unusually large filament eruption fell back onto the Sun, reaching impact velocities up to 450 km/s.Within some 10 seconds of their descent from hotto solar surface, the falling clouds experienced a billionfold increase in atmospheric density. The resulting explosion was clearly visible in the UV and EUV and produced a spray of matter heated in excess of a million kelvin. Comets N3 and Lovejoy did not come that close to the solar surface, however. They reached only to about 110 000 km and 135 000 km, respectively. The fate of comets at those distances is dominated by sublimation, 9 et al. , Astron. Astrophys. 535, A71 (2011). 9. J. C. Brown, Astron. Astrophys., A71 (2011). https://doi.org/10.1051/0004-6361/201015660 coronal rest frame in collisions with the atmosphere. They thus lose their kinetic energy and momentum in tens of seconds and thereby warm to EUV-emitting temperatures at densities high enough to be detectable against the background coronal emission. N3 and Lovejoy did not come that close to the solar surface, however. They reached only to about 110 000 km and 135 000 km, respectively. The fate ofat those distances is dominated byeven though they are moving in free fall at nearly the escape velocity of 650 km/s, or 0.002 times the speed of light. The sublimated atoms and small particles quickly decelerate behind the nucleus into therest frame inwith the atmosphere. They thus lose their kinetic energy and momentum in tens of seconds and thereby warm to EUV-emitting temperatures at densities high enough to be detectable against the backgroundemission. The free-fall velocity of a sungrazing comet near perihelion lies in the range of typical solar-wind speeds (300–800 km/s) that comets encounter far into the heliosphere. Hence, the relative velocity of the solar plasma for a sungrazing comet near perihelion is comparable to that for a comet much more distant in the heliosphere. What mainly distinguishes comets probing the two environments are the rate of molecular dissociation following sublimation and the rate at which atoms collide with the surrounding medium. The density of the solar wind near Earth’s orbit, for example, is 3–10 atoms/cm3. Within the corona near the perihelions of N3 and Lovejoy, in contrast, the density is on the order of 108 atoms/cm3. heliosphere, radiation pressure on the gas and dust tail is the dominant force, with some ionization of cometary atoms producing a second, windswept tail, as shown in figure 2 collisions of the monoatomic gases with the solar atmosphere dominate. The result is that the comet’s tail becomes ionized plasma and thus feels the force of the solar magnetic field. Dust and molecular gas survive too briefly to be visible, and the ion tail quickly decelerates into the rest frame of the coronal plasma and its all-permeating magnetic field. For Lovejoy, no dust survived to be blown out into the heliosphere for about 2 days (or 0.17 astronomical unit) on either side of its perihelion passage. Even gas molecules were quickly broken up: The dissociation of water molecules, for example, would have taken only 3 seconds, followed by ionization of its atoms in less than 0.1 second. 12 760, 18 (2012). 12. P. Bryans, W. D. Pesnell, Astrophys. J., 18 (2012). https://doi.org/10.1088/0004-637X/760/1/18 In the distantradiation pressure on the gas and dust tail is the dominant force, with some ionization of cometary atoms producing a second, windswept tail, as shown in figure. Near the Sun, however, theof the monoatomic gases with thedominate. The result is that the comet’s tail becomes ionized plasma and thus feels the force of theDust and molecular gas survive too briefly to be visible, and the ion tail quickly decelerates into the rest frame of theplasma and its all-permeatingFor Lovejoy, no dust survived to be blown out into thefor about 2 days (or 0.17 astronomical unit) on either side of its perihelion passage. Even gas molecules were quickly broken up: The dissociation of water molecules, for example, would have taken only 3 seconds, followed by ionization of its atoms in less than 0.1 second.

Probing the corona Section: Choose Top of page ABSTRACT The Sun’s complex dynamo Sungrazing comets Leaving a trail Falling through the atmos... Probing the corona << Opportunities REFERENCES comets were discovered as they were falling toward the Sun. But before Lovejoy approached it in late December 2011, no Kreutz comet had been observed to survive close perihelion passage. Even Lovejoy lasted only 2–3 days after passing through the Sun’s corona. 13 757, 127 (2012). 13. Z. Sekanina, P. W. Chodas, Astrophys. J., 127 (2012). https://doi.org/10.1088/0004-637X/757/2/127 spacecraft looking from three very different perspectives. 2 et al. , Science 340, 1196 (2013). 2. C. Downs, Science, 1196 (2013). https://doi.org/10.1126/science.1236550 All known Kreutzwere discovered as they were falling toward the Sun. But before Lovejoy approached it in late December 2011, no Kreutzhad been observed to survive close perihelion passage. Even Lovejoy lasted only 2–3 days after passing through the Sun’sBut before Lovejoy’s 4.5-billion-year history ended, both the descent and ascent phases of its path were visible tolooking from three very different perspectives. comets to learn about the Sun and its surroundings is not new. Observations of linear tails that pointed away from the Sun and glowed due to emission from ionized gases led to the first inklings of what since the late 1950s and early 1960 has become known as the solar wind. The dust tails of comets follow parabolic trajectories consistent with a gravitational pull that is counteracted, if not overcome, by outward radiation pressure. But the trajectories of the linear ion-plasma tails depend on the collisional ionization of sublimating gases in the comet’s coma and how they are channelled by the magnetic field blown along with the solar wind. 14 128, 664 (1958). 14. E. N. Parker, Astrophys. J., 664 (1958). https://doi.org/10.1086/146579 The idea of usingto learn about the Sun and its surroundings is not new. Observations of linear tails that pointed away from the Sun and glowed due to emission from ionized gases led to the first inklings of what since the late 1950s and early 1960 has become known as theThe dust tails offollow parabolic trajectories consistent with a gravitational pull that is counteracted, if not overcome, by outward radiation pressure. But the trajectories of the linear ion-plasma tails depend on the collisional ionization of sublimating gases in the comet’s coma and how they are channelled by theblown along with the Nowadays, the main puzzles about the solar wind concern the largely unobservable region in which it forms. In situ data is nonexistent because the deep corona is simply too harsh an environment for spacecraft. Though the environment is also too harsh for sungrazing comets, their much larger initial masses enable them to survive longer. Observations of sungrazers close to perihelion thus enable us to probe the coronal medium along the comets’ well-defined trajectories. The Lorentz force acts on ionized cometary material, and the resulting ion motions reveal the local orientation of the coronal magnetic field even as the comet’s ions decelerate and settle into the coronal plasma. The ratio of the energy density of the coronal magnetic field to the kinetic energy density of the plasma in the comet’s ion tail is likely to influence the tail evolution. In N3’s case it appears that the comet’s inertia dominated: As the cometary plasma decelerated during collisions with the corona’s atmosphere, the corona’s magnetic field became strongly deformed. In Lovejoy’s case, in contrast, the solar magnetic field appeared to largely hold its own. comet Lovejoy, the SDO and STEREO science teams immediately recognized that the dynamical evolution of the tail contained information about the coronal magnetic field. Tail motions observed during ingress and egress from perihelion, as shown in figure 4 solar wind were fully developed, and a tangential direction would be expected if the coronal medium had no influence on the comet at all. Instead, the SDO and STEREO imagers revealed wiggles in the tail about the comet’s orbital path through the inner corona. From their observations ofLovejoy, theandscience teams immediately recognized that the dynamical evolution of the tail contained information about theTail motions observed during ingress and egress from perihelion, as shown in figure, corresponded to neither the radial direction nor a direction tangential to the orbit; the radial direction would be expected if thewere fully developed, and a tangential direction would be expected if themedium had no influence on theat all. Instead, theandimagers revealed wiggles in the tail about the comet’s orbital path through the inner solar corona 2 et al. , Science 340, 1196 (2013). 2. C. Downs, Science, 1196 (2013). https://doi.org/10.1126/science.1236550 magnetic field. The result provides a unique validation of the model, particularly important given that creating one is daunting. The varying deflections of Lovejoy’s tail indicated a highly inhomogeneous medium. Application of a state-of-the-art computer model of therevealed a striking consistency between the observed tail motions and the orientation of theThe result provides a unique validation of the model, particularly important given that creating one is daunting. Creating a global model of the corona starts with the need for a full-sphere map of the field. But currently, only the field in front of the Sun can be reliably measured and only for latitudes up to some 70°. Latitude-dependent solar rotation—with one turn per month, on average—allows researchers to observe the entire low- and mid-latitude belts intermittently from Earth’s perspective. Even so, the field evolves significantly in the more than two weeks during which observational access is limited or blocked altogether before the region spins back into view. The field in the polar caps, which generally contribute strongly to the large-scale dipolar field, is always subject to substantial uncertainty. Because of those observational difficulties, only about one-quarter of the solar surface can be accurately mapped from observations of its magnetic field. The rest is subject to guesswork or approximations using various assimilative and modeling procedures. magnetic field is used as the foundation for a magnetohydrodynamic (MHD) corona. Disregarding hard-to-model and hard-to-validate large-scale current systems, one such MHD model 2 et al. , Science 340, 1196 (2013). 2. C. Downs, Science, 1196 (2013). https://doi.org/10.1126/science.1236550 coronal-brightness patterns best resemble observations. Models like that 15 et al. , Phys. Plasmas 6, 2217 (1999). 15. Z. Mikić, Phys. Plasmas, 2217 (1999). https://doi.org/10.1063/1.873474 corona provides the first detailed test of such models at altitudes where the Sun’s corona and nascent solar wind alternate side by side—basically by using the tail motions as wind vanes. The best-effort global surface map of theis used as the foundation for a magnetohydrodynamic (MHD)Disregarding hard-to-model and hard-to-validate large-scale current systems, one such MHD modelvaries the parameterizations of atmospheric energy deposition until the forces of the field–plasma interactions balance, selecting the solution in which the computedpatterns best resemble observations. Models like thathave been made for some time, and their complexity has increased over the years as computer processing speeds have risen. Lovejoy’s passage through the innerprovides the first detailed test of such models at altitudes where the Sun’sand nascentalternate side by side—basically by using the tail motions as wind vanes.