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Jupiter Encounter
Pluto-bound New Horizons' Jupiter flyby generates a flurry of discoveries
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| The New Horizons spacecraft rode atop a booster rocket from a Florida launch complex in 2006 on a 10-year voyage to Pluto and the Kuiper Belt. |
New theory: Jovian magnetosphere differs from Earth’s
Space physicists have long assumed that the magnetosphere at Jupiter circulates that planet’s magnetic field in the same way as Earth. At Earth, this circulation drives the aurora and the magnetic storms that cause space weather.
However, researchers from SwRI and the University of Colorado at Boulder have developed a new model that postulates the structure and magnetospheric processes at Jupiter are significantly different from those at Earth.
The invisible area of space around a planet controlled by its magnetic field, called the magnetosphere, interacts with the high-speed solar wind in a complex way, particularly in the area where the magnetic field in the solar wind interconnects with the planetary field, through a process called magnetic reconnection.
The Dungey cycle, developed by British scientist Jim Dungey in 1961, is the scientifically accepted paradigm for explaining how magnetic reconnection circulates the Earth’s magnetic field. During this cycle, magnetic field lines are brought up near the nose of the magnetosphere where they interconnect, becoming “open” and coupling the energy from the motion of the solar wind into the magnetosphere.
That interconnection allows vast energy from the million mile-per-hour solar wind into the magnetosphere, which is the driving force behind geomagnetic storms, or space weather, that can seriously damage or destroy probes and satellites. Subsequent motion of the solar wind around the Earth’s magnetosphere drags the interconnected field lines back over its magnetic poles where they drift down into the center of the magnetotail and reconnect again, but this time with similar field lines from the opposite hemisphere so that they are “closed” or connected to the planet at both ends. Finally, the Dungey cycle completes as the newly closed field lines circulate back toward Earth, around to its dayside and back to its starting position at the nose of the magnetosphere.
“For years space
physicists have considered the Dungey cycle to be the dominant circulation
process in magnetospheres throughout the solar system, even though observations
from the largest magnetosphere in the solar system — Jupiter’s — didn’t add up,”
says Dr. David McComas, senior executive director of the Space Science and
Engineering Division at Southwest Research Institute.
“There are three key ways that the magnetosphere of Jupiter differs from that of Earth,” argues Dr. Fran Bagenal, a professor of Astrophysical and Planetary Sciences at CU. “It’s much bigger, it spins faster and it has a powerful source of material.”
The time it takes for material that reconnects in the magnetotail and moves back up to Earth is only about 10 hours, less than half a day. However, the process at Jupiter takes 750 to 1,000 hours.
“Consider that a Jupiter day is only about 10 hours,” says McComas. “That means it would take as many as 100 Jovian days for reconnected field lines to move back up to Jupiter - a staggering difference.”
Furthermore, the magnetosphere of Jupiter is coupled to the spinning planet. “Imagine stirring up a bowl of spaghetti,” says Bagenal. “The fast, 10-hour spin of the Jovian magnetic field complicates the topology of flux tubes that are connected to the planet on one end while the other, open end is swept away by the solar wind.”
Another difference is that Jupiter has an active volcanic moon, Io, which spews out roughly a ton of material, mostly sulfur and oxygen, every second. Half of that material is lost through a process called charge exchange, but the other half moves down the Jovian magnetotail as ions dragging the planetary magnetic field tailward. Earth has no such counterpart to impede the return flow back toward the planet.
The new theory suggests a different geometry for closing off the magnetic field that has become interconnected with the solar wind — additional magnetic reconnection with other solar wind field lines that produce closed planetary field lines by reconnecting with open lines anchored back to both magnetic poles. This geometry at Jupiter allows for tailward flow everywhere in the tail and doesn’t require a planetward flow, as at Earth. This explains why the polar aurora at Jupiter doesn’t look like the terrestrial aurora. It also explains why observations from Ulysses showed that open flux occurs at low latitudes, not at the high latitudes required at Earth. At Jupiter the material is dragged farther down the sides of the magnetosphere so that they occur at lower latitudes.
“Our model matches up with the observations — further evidence that the magnetospheric structure and processes at Earth and Jupiter are quite different,” says McComas.
McComas and Bagenal determined these processes for Jupiter, yet they could aid in understanding the magnetospheres of the other outer planets, as well as in other astrophysical environments where magnetic fields play an important role.
The paper, “Jupiter: A Fundamentally Different Magnetospheric Interaction with the Solar Wind,” by David J. McComas and Fran Bagenal was published in the Oct. 24 issue of Geophysical Research Letters.
New Horizons’ instrument reveals structure, plasma in Jupiter’s magnetotail
During the first traversal nearly straight down Jupiter’s magnetotail, the Solar Wind Around Pluto (SWAP) instrument aboard New Horizons gathered remarkable new data on the magnetospheric bubble that surrounds Jupiter. The encounter, a bonus science mission for the Pluto-bound spacecraft, occurred as it rounded the planet in February 2007 for a gravity assist to help speed its journey to the edge of the solar system.
During the flyby, SWAP measured plasma populations inside the planet’s magnetosphere on an orbit that has never been traveled before. That orbit carried the spacecraft from the planet back a hundred million miles deep into the magnetotail, the portion of the magnetosphere dragged away from the Sun by the flow of the million mile-per-hour solar wind. Previous examinations of Jupiter’s magnetotail were limited to measurements very close to the planet and few very brief encounters at even greater distances.
“This was an absolutely fabulous trajectory for doing new science; the spacecraft went almost straight down the middle of the largest cohesive structure in the solar system,” says Dr. David J. McComas, who is SWAP principal investigator as well as the senior executive director of the Space Science and Engineering Division at Southwest Research Institute. “We could actually see the structure of the magnetotail and watch its evolution with distance for the first time.”
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SWAP gathered these plasma observations just after New Horizons’ inbound crossing of Jupiter’s magnetopause, through closest approach and back down the magnetotail. The schematic at top shows the plasma disk near Jupiter and large plasmoids (colored) moving down the magnetotail past New Horizons. The five spectrograms below it cover the five intervals numbered in the schematic. In general, the magnetotail becomes more disturbed with increasing variability in ion flux and flow speed with greater distances. |
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Observations revealed an extremely complicated structure in the magnetotail with large blobs, or plasmoids, of magnetically influenced plasma drifting down the tail at a relatively slow rate of speed. As the distance from the planet increased, the magnetotail became more highly structured with gradual variations in the plasma and sharp boundaries (discontinuities) between plasma regimes.
SWAP gathered data from solar wind observations upstream of Jupiter, through the closest approach encounter, and back to about 2,500 Jovian radii (about 100 million miles) at the boundary of the magnetotail, called the magnetopause. Data show that the inner magnetotail contains very hot ions — hotter than the top of SWAP's 7.5-kilovolt energy per charge range — that evolved to cooler and slower flows down the tail, beginning at about 100 Jovian radii; these flows were highly variable in flux and energy.
SWAP observations also revealed an unexpected component in the material flowing away from Jupiter. In addition to the volcanic material released from Io and material entering the magnetotail from the solar wind, the team found intense bursts of ionospheric hydrogen and H3+, which could only be coming from Jupiter's atmosphere.
“It’s clear there's significant escape of material from the planet because the brightest burst we see turns out to be material that's largely from Jupiter, not from the solar wind or Io,” says McComas.
New Horizons’ encounter with Jupiter also raised some new questions. “In addition to seeing flows move down the magnetotail, we saw them sometimes move across it,” says McComas. “Because Jupiter has the largest and most powerful magnetosphere in the solar system, everything we can learn about this and other mysteries could have implications for the other planets.” Additional questions center on the unexpected variability of the energy and speed of the plasma flows, as well as the multi-day periodicities that were consistent with plasmoids expanding as they move down the tail.
Following New Horizons’ arrival at Pluto in 2015, SWAP's primary mission will be to measure particles from the solar wind near the planet to determine if it has an atmosphere and how fast that atmosphere might be escaping.
New Horizons is the first mission in NASA’s New Frontiers program. The Johns Hopkins University Applied Physics Laboratory manages the mission and operates the spacecraft for the NASA Science Mission Directorate. Southwest Research Institute leads the SWAP instrument and hosts the Tombaugh Science Operations Center.
The paper, “Diverse Plasma Populations and Structures in Jupiter's Magnetotail,” by D.J. McComas, F. Allegrini, F. Bagenal, F.J. Crary, R.W. Ebert, H.A. Elliott, S.A. Stern and P.W. Valek, was published in the Oct. 12 issue of Science.
New Horizons’ UV spectrograph sees Io’s atmosphere change in response to eclipse events
A Dramatic changes in the atmospheric density of Jupiter’s moon Io and its interaction with Jupiter’s magnetosphere during solar eclipse were observed through Io's aurora on four occasions this past spring as the New Horizons spacecraft rounded Jupiter for a gravity assist on its way to Pluto.
Scientists using New Horizons’ SwRI-developed Alice ultraviolet spectrograph, which is designed to image ultraviolet emissions, noted auroral brightness and morphology variations as the spacecraft entered and then exited the eclipse zone revealing changes in the relative contribution of sublimation and volcanic sources to the atmosphere. The findings were supported by concurrent Hubble Space Telescope ultraviolet imaging.
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Auroral emissions from the East Girru plume were observed in both this UV image by the Hubble Space Telescope and simultaneous New Horizons images of Io in solar eclipse by Jupiter. |
FUV (far-ultraviolet) aurora morphology also reveals the plumes effect on Io’s electrodynamic interaction with Jupiter's magnetosphere. Comparisons to simulations of Io’s aurora indicate that volcanoes supply 1 percent to 3 percent of Io’s dayside atmosphere.
Aurora observations, particularly while Io is in solar eclipse by Jupiter, can provide information on both Io’s atmosphere and its interaction with Jupiter, the paper states.
An aurora is a luminous phenomenon in the upper atmosphere of a planet caused by the emission of light from atoms excited by electrons accelerated along magnetic field lines. Most planetary aurorae occur in the polar regions, but Io's aurora is brightest near its equator as well as in volcanic plumes distributed across the satellite.
“Io is volcanically active, and that volcanism ultimately is the source material for Io’s sulfur-dioxide atmosphere, but the relative contributions of volcanic plumes and sublimation of frosts deposited near the plumes have remained a question for almost 30 years,” said Dr. Kurt Retherford, a senior research scientist in the Space Science and Engineering Division at the Institute.
The interaction between Io’s atmosphere and the Io plasma torus produces displays of auroral emissions on Io, supplies plasma to Jupiter's magnetosphere and physically links Io to Jupiter, according to the paper.
“When Io goes into solar eclipse, and during the night, its surface temperature drops significantly, causing diminished sublimation of surface material into the atmosphere. The atmosphere at that point collapses down so that all that is left supplying the atmosphere are the volcanoes,” Retherford said.
A dramatic difference between Io’s dayside and nightside atmospheric density best explains the aurorae observations, he added.
Alice provides spectral images in the extreme- and far-ultraviolet (EUV and FUV) passbands. S. Alan Stern of NASA Headquarters, a former executive director of the Space Science and Engineering Division at SwRI, is the principal investigator of New Horizons, Alice and Ralph, a visible and infrared camera onboard the spacecraft. Prof. Joachim Saur at the University of Cologne, Germany, conducted the simulations.
The paper, titled, “Io’s Atmospheric Response to
Eclipse: UV Aurorae Observations,” by K.D. Retherford, J.R. Spencer, S.A. Stern,
J. Saur, D.F. Strobel, A.J. Steffl, G.R. Gladstone, H.A. Weaver, A.F. Cheng,
J.Wm. Parker, D.C. Slater, M.H. Versteeg, M.W. Davis, F. Bagenal, H.B. Throop,
R.M.C. Lopes, D.C. Reuter, A. Lunsford, S.J. Conard, L.A. Young and J.M. Moore,
was published in the October 12 issue of Science.
Published in the Winter 2007 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.
